From 3a0619ea8ec3f7265e64aaa1839c1f40677d1687 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Thu, 26 Sep 2024 13:43:30 -0700
Subject: [PATCH 01/41] Add option for partially heterogeneous 1D XS model.

---
 .../neutronics/crossSectionSettings.py        |  8 +++
 .../latticePhysics/latticePhysicsWriter.py    |  1 +
 armi/reactor/converters/blockConverters.py    | 60 +++++++++++++++----
 3 files changed, 59 insertions(+), 10 deletions(-)

diff --git a/armi/physics/neutronics/crossSectionSettings.py b/armi/physics/neutronics/crossSectionSettings.py
index 22a58cc14..b2725041e 100644
--- a/armi/physics/neutronics/crossSectionSettings.py
+++ b/armi/physics/neutronics/crossSectionSettings.py
@@ -57,6 +57,7 @@
 CONF_COMPONENT_AVERAGING = "averageByComponent"
 CONF_XS_MAX_ATOM_NUMBER = "xsMaxAtomNumber"
 CONF_MIN_DRIVER_DENSITY = "minDriverDensity"
+CONF_PARTIALLY_HETEROGENEOUS = "partiallyHeterogeneous"
 
 
 class XSGeometryTypes(Enum):
@@ -149,6 +150,7 @@ def getStr(cls, typeSpec: Enum):
         CONF_XS_PRIORITY,
         CONF_XS_MAX_ATOM_NUMBER,
         CONF_MIN_DRIVER_DENSITY,
+        CONF_PARTIALLY_HETEROGENEOUS,
     },
     XSGeometryTypes.getStr(XSGeometryTypes.TWO_DIMENSIONAL_HEX): {
         CONF_XSID,
@@ -433,6 +435,11 @@ class XSModelingOptions:
         The minimum number density for nuclides included in driver material for a 1D
         lattice physics model.
 
+    partiallyHeterogeneous : bool
+        This is a lattice physics configuration option used to enable a partially
+        heterogeneous approximation for a 1D cylindrical model. Everything inside of the
+        duct will be treated as homogeneous.
+
     Notes
     -----
     Not all default attributes may be useful for your specific application and you may
@@ -674,6 +681,7 @@ def setDefaults(self, blockRepresentation, validBlockTypes):
                 CONF_HOMOGBLOCK: False,
                 CONF_BLOCK_REPRESENTATION: crossSectionGroupManager.CYLINDRICAL_COMPONENTS_BLOCK_COLLECTION,
                 CONF_BLOCKTYPES: validBlockTypes,
+                CONF_PARTIALLY_HETEROGENEOUS: False,
             }
         elif self.geometry == XSGeometryTypes.getStr(
             XSGeometryTypes.TWO_DIMENSIONAL_HEX
diff --git a/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py b/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py
index 85c2fe3ef..f57e7e187 100644
--- a/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py
+++ b/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py
@@ -113,6 +113,7 @@ def __init__(
         self.driverXsID = self.xsSettings.driverID
         self.numExternalRings = self.xsSettings.numExternalRings
         self.criticalBucklingSearchActive = self.xsSettings.criticalBuckling
+        self.partiallyHeterogeneous = self.xsSettings.partiallyHeterogeneous
 
         self.executeExclusive = self.xsSettings.xsExecuteExclusive
         self.priority = self.xsSettings.xsPriority
diff --git a/armi/reactor/converters/blockConverters.py b/armi/reactor/converters/blockConverters.py
index d78d1c48d..56cccd420 100644
--- a/armi/reactor/converters/blockConverters.py
+++ b/armi/reactor/converters/blockConverters.py
@@ -507,6 +507,11 @@ class HexComponentsToCylConverter(BlockAvgToCylConverter):
     duct/intercoolant pinComponentsRing1 | coolant | pinComponentsRing2 | coolant | ... |
     nonpins ...
 
+    The ``partiallyHeterogeneous`` option allows the user to treat everything inside the duct
+    as a single homogenized composition. This will significantly reduce the memory and runtime
+    required for MC2, and also provide an alternative approximation for the spatial self-shielding
+    effect on microscopic cross sections.
+
     This converter expects the ``sourceBlock`` and ``driverFuelBlock`` to defined and for
     the ``sourceBlock`` to have a spatial grid defined. Additionally, both the ``sourceBlock``
     and ``driverFuelBlock`` must be instances of HexBlocks.
@@ -518,6 +523,7 @@ def __init__(
         driverFuelBlock=None,
         numExternalRings=None,
         mergeIntoClad=None,
+        partiallyHeterogeneous=False,
     ):
         BlockAvgToCylConverter.__init__(
             self,
@@ -547,6 +553,7 @@ def __init__(
                 )
         self.pinPitch = sourceBlock.getPinPitch()
         self.mergeIntoClad = mergeIntoClad or []
+        self.partiallyHeterogeneous = partiallyHeterogeneous
         self.interRingComponent = sourceBlock.getComponent(Flags.COOLANT, exact=True)
         self._remainingCoolantFillArea = self.interRingComponent.getArea()
         if not self.interRingComponent:
@@ -581,10 +588,13 @@ def convert(self):
         numRings = self._sourceBlock.spatialGrid.getMinimumRings(
             self._sourceBlock.getNumPins()
         )
-        pinComponents, nonPins = self._classifyComponents()
-        self._buildFirstRing(pinComponents)
-        for ring in range(2, numRings + 1):
-            self._buildNthRing(pinComponents, ring)
+        if self.partiallyHeterogeneous:
+            self._buildInsideDuct()
+        else:
+            pinComponents, nonPins = self._classifyComponents()
+            self._buildFirstRing(pinComponents)
+            for ring in range(2, numRings + 1):
+                self._buildNthRing(pinComponents, ring)
         self._buildNonPinRings(nonPins)
         self._addDriverFuelRings()
 
@@ -644,6 +654,35 @@ def _classifyComponents(self):
 
         return list(sorted(pinComponents)), nonPins
 
+    def _buildInsideDuct(self):
+        """Build a homogenized material of the components inside the duct."""
+        blockType = self._sourceBlock.getType()
+        blockName = f"Homogenized {blockType}"
+        newBlock = copy.deepcopy(self._sourceBlock)
+
+        removeComponents = False
+        for c in sorted(newBlock.getComponents()):
+            if c.hasFlags(Flags.DUCT):
+                removeComponents = True
+            if removeComponents:
+                newBlock.remove(c)
+            else:
+                compFlags = compFlags | c.p.flags
+
+        outerDiam = getOuterDiamFromIDAndArea(0.0, newBlock.getArea())
+
+        circle = components.Circle(
+            blockName,
+            "_Mixture",
+            newBlock.getAverageTempInC(),
+            newBlock.getAverageTempInC(),
+            id=0.0,
+            od=outerDiam,
+            mult=1,
+        )
+        circle.setNumberDensities(newBlock.getNumberDensities())
+        circle.p.flags = compFlags
+
     def _buildFirstRing(self, pinComponents):
         """Add first ring of components to new block."""
         for oldC in pinComponents:
@@ -692,12 +731,13 @@ def _buildNonPinRings(self, nonPins):
         Also needs to add final coolant layer between the outer pins and the non-pins.
         Will crash if there are things that are not circles or hexes.
         """
-        # fill in the last ring of coolant using the rest
-        coolInnerDiam = self.convertedBlock[-1].getDimension("od")
-        coolantOD = getOuterDiamFromIDAndArea(
-            coolInnerDiam, self._remainingCoolantFillArea
-        )
-        self._addCoolantRing(coolantOD, " outer")
+        if not self.partiallyHeterogeneous:
+            # fill in the last ring of coolant using the rest
+            coolInnerDiam = self.convertedBlock[-1].getDimension("od")
+            coolantOD = getOuterDiamFromIDAndArea(
+                coolInnerDiam, self._remainingCoolantFillArea
+            )
+            self._addCoolantRing(coolantOD, " outer")
 
         innerDiameter = coolantOD
         for i, hexagon in enumerate(sorted(nonPins)):

From 3f5225352523874203a54296f1fb1c26c1c0f9f8 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Thu, 26 Sep 2024 15:00:45 -0700
Subject: [PATCH 02/41] Add partiallyHeterogeneous to settings validator

---
 armi/physics/neutronics/crossSectionSettings.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/armi/physics/neutronics/crossSectionSettings.py b/armi/physics/neutronics/crossSectionSettings.py
index b2725041e..3ccfaa7df 100644
--- a/armi/physics/neutronics/crossSectionSettings.py
+++ b/armi/physics/neutronics/crossSectionSettings.py
@@ -196,6 +196,7 @@ def getStr(cls, typeSpec: Enum):
         vol.Optional(CONF_XS_MAX_ATOM_NUMBER): vol.Coerce(int),
         vol.Optional(CONF_MIN_DRIVER_DENSITY): vol.Coerce(float),
         vol.Optional(CONF_COMPONENT_AVERAGING): bool,
+        vol.Optional(CONF_PARTIALLY_HETEROGENEOUS): bool,
     }
 )
 

From 2da66ce60196874e6d64623f751f0e8bfb640b5b Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Thu, 26 Sep 2024 15:05:17 -0700
Subject: [PATCH 03/41] Add partiallyHeterogeneous to XSModelingOptions

---
 armi/physics/neutronics/crossSectionSettings.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/armi/physics/neutronics/crossSectionSettings.py b/armi/physics/neutronics/crossSectionSettings.py
index 3ccfaa7df..5b5b422c0 100644
--- a/armi/physics/neutronics/crossSectionSettings.py
+++ b/armi/physics/neutronics/crossSectionSettings.py
@@ -472,6 +472,7 @@ def __init__(
         xsMaxAtomNumber=None,
         averageByComponent=False,
         minDriverDensity=0.0,
+        partiallyHeterogeneous=False,
     ):
         self.xsID = xsID
         self.geometry = geometry
@@ -494,6 +495,7 @@ def __init__(
         self.xsMaxAtomNumber = xsMaxAtomNumber
         self.minDriverDensity = minDriverDensity
         self.averageByComponent = averageByComponent
+        self.partiallyHeterogeneous = partiallyHeterogeneous
         # these are related to execution
         self.xsExecuteExclusive = xsExecuteExclusive
         self.xsPriority = xsPriority

From 7336b6c3c0f1918de1627602555a7e84178077b2 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Thu, 26 Sep 2024 16:02:59 -0700
Subject: [PATCH 04/41] Update homogenization of inside duct area.

---
 armi/reactor/converters/blockConverters.py | 32 +++++++++++++++-------
 1 file changed, 22 insertions(+), 10 deletions(-)

diff --git a/armi/reactor/converters/blockConverters.py b/armi/reactor/converters/blockConverters.py
index 56cccd420..c784e20fa 100644
--- a/armi/reactor/converters/blockConverters.py
+++ b/armi/reactor/converters/blockConverters.py
@@ -588,10 +588,10 @@ def convert(self):
         numRings = self._sourceBlock.spatialGrid.getMinimumRings(
             self._sourceBlock.getNumPins()
         )
+        pinComponents, nonPins = self._classifyComponents()
         if self.partiallyHeterogeneous:
             self._buildInsideDuct()
         else:
-            pinComponents, nonPins = self._classifyComponents()
             self._buildFirstRing(pinComponents)
             for ring in range(2, numRings + 1):
                 self._buildNthRing(pinComponents, ring)
@@ -660,17 +660,26 @@ def _buildInsideDuct(self):
         blockName = f"Homogenized {blockType}"
         newBlock = copy.deepcopy(self._sourceBlock)
 
-        removeComponents = False
-        for c in sorted(newBlock.getComponents()):
+        compFlags = newBlock.geFirstComponent().p.flags
+        for c in sorted(newBlock.getComponents(), reverse=True):
+            newBlock.remove(c, recomputeAreaFractions=False)
             if c.hasFlags(Flags.DUCT):
-                removeComponents = True
-            if removeComponents:
-                newBlock.remove(c)
-            else:
-                compFlags = compFlags | c.p.flags
+                ductIP = c.getDimension("ip")
+                break
+
+        # add pitch defining component
+        newBlock.add(
+            components.Hexagon(
+                "pitchComponent",
+                "Void",
+                self._sourceBlock.getAverageTempInC(),
+                self._sourceBlock.getAverageTempInC(),
+                ip=ductIP,
+                op=ductIP,
+            )
+        )
 
         outerDiam = getOuterDiamFromIDAndArea(0.0, newBlock.getArea())
-
         circle = components.Circle(
             blockName,
             "_Mixture",
@@ -682,6 +691,7 @@ def _buildInsideDuct(self):
         )
         circle.setNumberDensities(newBlock.getNumberDensities())
         circle.p.flags = compFlags
+        self.convertedBlock.add(circle)
 
     def _buildFirstRing(self, pinComponents):
         """Add first ring of components to new block."""
@@ -738,8 +748,10 @@ def _buildNonPinRings(self, nonPins):
                 coolInnerDiam, self._remainingCoolantFillArea
             )
             self._addCoolantRing(coolantOD, " outer")
+            innerDiameter = coolantOD
+        else:
+            innerDiameter = self.convertedBlock[-1].getDimension("od")
 
-        innerDiameter = coolantOD
         for i, hexagon in enumerate(sorted(nonPins)):
             outerDiam = getOuterDiamFromIDAndArea(
                 innerDiameter, hexagon.getArea()

From 18dbb7cf598e3ee12267eb1cdd91e4f8031081ca Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Fri, 27 Sep 2024 14:10:06 -0700
Subject: [PATCH 05/41] Add partiallyHeterogeneous to unit tests.

---
 .../neutronics/tests/test_crossSectionSettings.py        | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/armi/physics/neutronics/tests/test_crossSectionSettings.py b/armi/physics/neutronics/tests/test_crossSectionSettings.py
index 631af79c1..ddcc923ab 100644
--- a/armi/physics/neutronics/tests/test_crossSectionSettings.py
+++ b/armi/physics/neutronics/tests/test_crossSectionSettings.py
@@ -118,6 +118,7 @@ def test_homogeneousXsDefaultSettingAssignment(self):
         self.assertNotIn("YA", xsModel)
         self.assertEqual(xsModel["YA"].geometry, "0D")
         self.assertEqual(xsModel["YA"].criticalBuckling, True)
+        self.assertEqual(xsModel["YA"].partiallyHeterogeneous, False)
 
     def test_setDefaultSettingsByLowestBuGroupHomogeneous(self):
         # Initialize some micro suffix in the cross sections
@@ -177,7 +178,12 @@ def test_optionalKey(self):
         """Test that optional key shows up with default value."""
         cs = settings.Settings()
         xsModel = XSSettings()
-        da = XSModelingOptions("DA", geometry="1D cylinder", meshSubdivisionsPerCm=1.0)
+        da = XSModelingOptions(
+            "DA",
+            geometry="1D cylinder",
+            meshSubdivisionsPerCm=1.0,
+            partiallyHeterogeneous=True,
+        )
         xsModel["DA"] = da
         xsModel.setDefaults(
             cs[CONF_XS_BLOCK_REPRESENTATION],
@@ -185,6 +191,7 @@ def test_optionalKey(self):
         )
         self.assertEqual(xsModel["DA"].mergeIntoClad, ["gap"])
         self.assertEqual(xsModel["DA"].meshSubdivisionsPerCm, 1.0)
+        self.assertEqual(xsModel["DA"].partiallyHeterogeneous, True)
 
     def test_badCrossSections(self):
         with self.assertRaises(TypeError):

From 8c9b098296b28d02a710ca2259a18fe45e44d160 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Fri, 27 Sep 2024 14:32:17 -0700
Subject: [PATCH 06/41] Fix typo / bugs

---
 armi/reactor/converters/blockConverters.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/armi/reactor/converters/blockConverters.py b/armi/reactor/converters/blockConverters.py
index c784e20fa..6580c9535 100644
--- a/armi/reactor/converters/blockConverters.py
+++ b/armi/reactor/converters/blockConverters.py
@@ -660,14 +660,14 @@ def _buildInsideDuct(self):
         blockName = f"Homogenized {blockType}"
         newBlock = copy.deepcopy(self._sourceBlock)
 
-        compFlags = newBlock.geFirstComponent().p.flags
+        compFlags = newBlock.getComponents()[0].p.flags
         for c in sorted(newBlock.getComponents(), reverse=True):
             newBlock.remove(c, recomputeAreaFractions=False)
             if c.hasFlags(Flags.DUCT):
                 ductIP = c.getDimension("ip")
                 break
 
-        # add pitch defining component
+        # add pitch defining component with no area
         newBlock.add(
             components.Hexagon(
                 "pitchComponent",

From def556daa9e52405b07d5f90123479df2143ff05 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Mon, 30 Sep 2024 14:19:06 -0700
Subject: [PATCH 07/41] Assign flags from all components inside duct to
 homogeneous mixture.

---
 armi/reactor/converters/blockConverters.py | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/armi/reactor/converters/blockConverters.py b/armi/reactor/converters/blockConverters.py
index 6580c9535..db441c6cd 100644
--- a/armi/reactor/converters/blockConverters.py
+++ b/armi/reactor/converters/blockConverters.py
@@ -660,12 +660,16 @@ def _buildInsideDuct(self):
         blockName = f"Homogenized {blockType}"
         newBlock = copy.deepcopy(self._sourceBlock)
 
-        compFlags = newBlock.getComponents()[0].p.flags
+        compFlags = newBlock.getComponent(Flags.COOLANT).p.flags
+        outsideDuct = True
         for c in sorted(newBlock.getComponents(), reverse=True):
-            newBlock.remove(c, recomputeAreaFractions=False)
-            if c.hasFlags(Flags.DUCT):
-                ductIP = c.getDimension("ip")
-                break
+            if outsideDuct:
+                newBlock.remove(c, recomputeAreaFractions=False)
+                if c.hasFlags(Flags.DUCT):
+                    ductIP = c.getDimension("ip")
+                    outsideDuct = False
+            else:
+                compFlags = compFlags | c.p.flags
 
         # add pitch defining component with no area
         newBlock.add(

From e8544822ae584fca0553c533f9bcef371f48c3e4 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Tue, 1 Oct 2024 10:46:06 -0700
Subject: [PATCH 08/41] Fix typo

---
 armi/nuclearDataIO/nuclearFileMetadata.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/armi/nuclearDataIO/nuclearFileMetadata.py b/armi/nuclearDataIO/nuclearFileMetadata.py
index 3726c305b..2e1ddb89a 100644
--- a/armi/nuclearDataIO/nuclearFileMetadata.py
+++ b/armi/nuclearDataIO/nuclearFileMetadata.py
@@ -252,4 +252,4 @@ def _getSkippedKeys(self, other, selfContainer, otherContainer, mergedData):
 
 
 class NuclideMetadata(_Metadata):
-    """Simple dictionary for providing metadata about how to read/write a nuclde to/from a file."""
+    """Simple dictionary for providing metadata about how to read/write a nuclide to/from a file."""

From f1018d4c0c355e6c3b8f8fb00102807a364d82a5 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Thu, 26 Sep 2024 23:01:44 +0000
Subject: [PATCH 09/41] Update partially heterogeneous homogenization.

---
 armi/reactor/converters/blockConverters.py | 32 +++++++++++++++-------
 1 file changed, 22 insertions(+), 10 deletions(-)

diff --git a/armi/reactor/converters/blockConverters.py b/armi/reactor/converters/blockConverters.py
index 56cccd420..6580c9535 100644
--- a/armi/reactor/converters/blockConverters.py
+++ b/armi/reactor/converters/blockConverters.py
@@ -588,10 +588,10 @@ def convert(self):
         numRings = self._sourceBlock.spatialGrid.getMinimumRings(
             self._sourceBlock.getNumPins()
         )
+        pinComponents, nonPins = self._classifyComponents()
         if self.partiallyHeterogeneous:
             self._buildInsideDuct()
         else:
-            pinComponents, nonPins = self._classifyComponents()
             self._buildFirstRing(pinComponents)
             for ring in range(2, numRings + 1):
                 self._buildNthRing(pinComponents, ring)
@@ -660,17 +660,26 @@ def _buildInsideDuct(self):
         blockName = f"Homogenized {blockType}"
         newBlock = copy.deepcopy(self._sourceBlock)
 
-        removeComponents = False
-        for c in sorted(newBlock.getComponents()):
+        compFlags = newBlock.getComponents()[0].p.flags
+        for c in sorted(newBlock.getComponents(), reverse=True):
+            newBlock.remove(c, recomputeAreaFractions=False)
             if c.hasFlags(Flags.DUCT):
-                removeComponents = True
-            if removeComponents:
-                newBlock.remove(c)
-            else:
-                compFlags = compFlags | c.p.flags
+                ductIP = c.getDimension("ip")
+                break
+
+        # add pitch defining component with no area
+        newBlock.add(
+            components.Hexagon(
+                "pitchComponent",
+                "Void",
+                self._sourceBlock.getAverageTempInC(),
+                self._sourceBlock.getAverageTempInC(),
+                ip=ductIP,
+                op=ductIP,
+            )
+        )
 
         outerDiam = getOuterDiamFromIDAndArea(0.0, newBlock.getArea())
-
         circle = components.Circle(
             blockName,
             "_Mixture",
@@ -682,6 +691,7 @@ def _buildInsideDuct(self):
         )
         circle.setNumberDensities(newBlock.getNumberDensities())
         circle.p.flags = compFlags
+        self.convertedBlock.add(circle)
 
     def _buildFirstRing(self, pinComponents):
         """Add first ring of components to new block."""
@@ -738,8 +748,10 @@ def _buildNonPinRings(self, nonPins):
                 coolInnerDiam, self._remainingCoolantFillArea
             )
             self._addCoolantRing(coolantOD, " outer")
+            innerDiameter = coolantOD
+        else:
+            innerDiameter = self.convertedBlock[-1].getDimension("od")
 
-        innerDiameter = coolantOD
         for i, hexagon in enumerate(sorted(nonPins)):
             outerDiam = getOuterDiamFromIDAndArea(
                 innerDiameter, hexagon.getArea()

From 43c3e2b43db80938c2026f6d2ca84db65a3cc8b5 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Mon, 30 Sep 2024 21:15:46 +0000
Subject: [PATCH 10/41] Put all flags into homogenized inside duct component.

---
 armi/reactor/converters/blockConverters.py | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/armi/reactor/converters/blockConverters.py b/armi/reactor/converters/blockConverters.py
index 6580c9535..72cee6038 100644
--- a/armi/reactor/converters/blockConverters.py
+++ b/armi/reactor/converters/blockConverters.py
@@ -660,13 +660,16 @@ def _buildInsideDuct(self):
         blockName = f"Homogenized {blockType}"
         newBlock = copy.deepcopy(self._sourceBlock)
 
-        compFlags = newBlock.getComponents()[0].p.flags
+        compFlags = newBlock.getComponent(Flags.COOLANT).p.flags
+        outsideDuct = True
         for c in sorted(newBlock.getComponents(), reverse=True):
-            newBlock.remove(c, recomputeAreaFractions=False)
-            if c.hasFlags(Flags.DUCT):
-                ductIP = c.getDimension("ip")
-                break
-
+            if outsideDuct:
+                newBlock.remove(c, recomputeAreaFractions=False)
+                if c.hasFlags(Flags.DUCT):
+                    ductIP = c.getDimension("ip")
+                    outsideDuct = False
+            else:
+                compFlags = compFlags | c.p.flags
         # add pitch defining component with no area
         newBlock.add(
             components.Hexagon(

From c2417f66ad992267efb8ad95f708d4c3f2d0fe47 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Tue, 1 Oct 2024 23:28:44 +0000
Subject: [PATCH 11/41] Introduce new Cylindrical block collection for
 partially het models

---
 .../neutronics/crossSectionGroupManager.py    | 55 +++++++++++++
 .../latticePhysics/latticePhysicsWriter.py    |  8 +-
 armi/reactor/converters/blockConverters.py    | 82 +++++++++++++------
 3 files changed, 118 insertions(+), 27 deletions(-)

diff --git a/armi/physics/neutronics/crossSectionGroupManager.py b/armi/physics/neutronics/crossSectionGroupManager.py
index 0131d625a..e8f82f683 100644
--- a/armi/physics/neutronics/crossSectionGroupManager.py
+++ b/armi/physics/neutronics/crossSectionGroupManager.py
@@ -63,6 +63,7 @@
 from armi.physics.neutronics.const import CONF_CROSS_SECTION
 from armi.reactor import flags
 from armi.reactor.components import basicShapes
+from armi.reactor.geometryConverters.blockConverters import stripComponents
 from armi.reactor.flags import Flags
 from armi.utils.units import TRACE_NUMBER_DENSITY
 
@@ -648,6 +649,52 @@ def _getNucTempHelper(self):
         return nvt, nv
 
 
+class CylindricalComponentsPartiallyHeterogeneousAverageBlockCollection(
+    CylindricalComponentsAverageBlockCollection
+):
+    """
+    Creates a representative block for the purpose of cross section generation with a one-
+    dimensional cylindrical model where all material inside the duct is homogenized.
+
+    .. impl:: Create partially heterogeneous representative blocks.
+        :id: I_ARMI_XSGM_CREATE_REPR_BLOCKS2
+        :implements: R_ARMI_XSGM_CREATE_REPR_BLOCKS
+
+        This class constructs representative blocks based on a volume-weighted average using
+        cylindrical blocks from an existing block list. Inheriting functionality from the abstract
+        :py:class:`Reactor <armi.physics.neutronics.crossSectionGroupManager.BlockCollection>`
+        object, this class will construct representative blocks using averaged parameters of all
+        blocks in the given collection. Number density averages are computed at a component level.
+        Nuclide temperatures from a median block-average temperature are used and the average burnup
+        is evaluated across all blocks in the block list.
+
+        The average nuclide temperatures are calculated only for the homogenized region inside of
+        the duct. For the non-homogenized regions, the MC2 writer uses the component temperatures.
+
+    Notes
+    -----
+    The representative block for this collection is the same as the parent. The only difference between
+    the two collection types is that this collection calculates average nuclide temperatures based only
+    on the components that are inside of the duct.
+    """
+
+    def _getNucTempHelper(self):
+        """All candidate blocks are used in the average."""
+        nvt = np.zeros(len(self.allNuclidesInProblem))
+        nv = np.zeros(len(self.allNuclidesInProblem))
+        for block in self.getCandidateBlocks():
+            wt = self.getWeight(block)
+            # remove the duct and intercoolant from the block before
+            # calculating average nuclide temps
+            newBlock, _mixtureFlags = stripComponents(block, Flags.DUCT)
+            nvtBlock, nvBlock = getBlockNuclideTemperatureAvgTerms(
+                newBlock, self.allNuclidesInProblem
+            )
+            nvt += nvtBlock * wt
+            nv += nvBlock * wt
+        return nvt, nv
+
+
 class SlabComponentsAverageBlockCollection(BlockCollection):
     """
     Creates a representative 1D slab block.
@@ -1520,6 +1567,9 @@ def updateNuclideTemperatures(self, blockCollectionByXsGroup=None):
 FLUX_WEIGHTED_AVERAGE_BLOCK_COLLECTION = "FluxWeightedAverage"
 SLAB_COMPONENTS_BLOCK_COLLECTION = "ComponentAverage1DSlab"
 CYLINDRICAL_COMPONENTS_BLOCK_COLLECTION = "ComponentAverage1DCylinder"
+CYLINDRICAL_COMPONENTS_PARTIALLY_HET_BLOCK_COLLECTION = (
+    "ComponentAverage1DCylinderPartiallyHeterogeneous"
+)
 
 # Mapping between block collection string constants and their
 # respective block collection classes.
@@ -1529,12 +1579,17 @@ def updateNuclideTemperatures(self, blockCollectionByXsGroup=None):
     FLUX_WEIGHTED_AVERAGE_BLOCK_COLLECTION: FluxWeightedAverageBlockCollection,
     SLAB_COMPONENTS_BLOCK_COLLECTION: SlabComponentsAverageBlockCollection,
     CYLINDRICAL_COMPONENTS_BLOCK_COLLECTION: CylindricalComponentsAverageBlockCollection,
+    CYLINDRICAL_COMPONENTS_PARTIALLY_HET_BLOCK_COLLECTION: CylindricalComponentsPartiallyHeterogeneousAverageBlockCollection,
 }
 
 
 def blockCollectionFactory(xsSettings, allNuclidesInProblem):
     """Build a block collection based on user settings and input."""
     blockRepresentation = xsSettings.blockRepresentation
+    if (
+        blockRepresentation == CYLINDRICAL_COMPONENTS_BLOCK_COLLECTION
+    ) and xsSettings.partiallyHeterogeneous:
+        blockRepresentation = CYLINDRICAL_COMPONENTS_PARTIALLY_HET_BLOCK_COLLECTION
     validBlockTypes = xsSettings.validBlockTypes
     averageByComponent = xsSettings.averageByComponent
     return BLOCK_COLLECTIONS[blockRepresentation](
diff --git a/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py b/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py
index f57e7e187..5c56c573c 100644
--- a/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py
+++ b/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py
@@ -267,8 +267,12 @@ def _getAllNuclidesByCategory(self, component=None):
                 continue  # skip LFPs here but add individual FPs below.
 
             if isinstance(subjectObject, components.Component):
-                # Heterogeneous number densities and temperatures
-                nucTemperatureInC = subjectObject.temperatureInC
+                if self.partiallyHeterogeneous and "Homogenized" in subjectObject.name:
+                    # Nuclide temperatures representing heterogeneous model component temperatures
+                    nucTemperatureInC = self._getAvgNuclideTemperatureInC(nucName)
+                else:
+                    # Heterogeneous number densities and temperatures
+                    nucTemperatureInC = subjectObject.temperatureInC
             else:
                 # Homogeneous number densities and temperatures
                 nucTemperatureInC = self._getAvgNuclideTemperatureInC(nucName)
diff --git a/armi/reactor/converters/blockConverters.py b/armi/reactor/converters/blockConverters.py
index 72cee6038..6fc3b2155 100644
--- a/armi/reactor/converters/blockConverters.py
+++ b/armi/reactor/converters/blockConverters.py
@@ -658,30 +658,7 @@ def _buildInsideDuct(self):
         """Build a homogenized material of the components inside the duct."""
         blockType = self._sourceBlock.getType()
         blockName = f"Homogenized {blockType}"
-        newBlock = copy.deepcopy(self._sourceBlock)
-
-        compFlags = newBlock.getComponent(Flags.COOLANT).p.flags
-        outsideDuct = True
-        for c in sorted(newBlock.getComponents(), reverse=True):
-            if outsideDuct:
-                newBlock.remove(c, recomputeAreaFractions=False)
-                if c.hasFlags(Flags.DUCT):
-                    ductIP = c.getDimension("ip")
-                    outsideDuct = False
-            else:
-                compFlags = compFlags | c.p.flags
-        # add pitch defining component with no area
-        newBlock.add(
-            components.Hexagon(
-                "pitchComponent",
-                "Void",
-                self._sourceBlock.getAverageTempInC(),
-                self._sourceBlock.getAverageTempInC(),
-                ip=ductIP,
-                op=ductIP,
-            )
-        )
-
+        newBlock, mixtureFlags = stripComponents(self._sourceBlock, Flags.DUCT)
         outerDiam = getOuterDiamFromIDAndArea(0.0, newBlock.getArea())
         circle = components.Circle(
             blockName,
@@ -693,7 +670,7 @@ def _buildInsideDuct(self):
             mult=1,
         )
         circle.setNumberDensities(newBlock.getNumberDensities())
-        circle.p.flags = compFlags
+        circle.p.flags = mixtureFlags
         self.convertedBlock.add(circle)
 
     def _buildFirstRing(self, pinComponents):
@@ -878,3 +855,58 @@ def radiiFromRingOfRods(distToRodCenter, numRods, rodRadii, layout="hexagon"):
         rLast, bigRLast = rodRadius, distFromCenterComp
 
     return sorted(radiiFromRodCenter)
+
+
+def stripComponents(block, compFlags):
+    """
+    Remove all components from a block outside of the first component that matches compFlags.
+
+    Parameters
+    ----------
+    block : armi.reactor.blocks.Block
+        Source block from which to produce a modified copy
+    compFlags : armi.reactor.flags.Flags
+        Component flags to indicate which components to strip from the
+        block. All components outside of the first one that matches
+        compFlags are stripped.
+
+    Returns
+    -------
+    newBlock : armi.reactor.blocks.Block
+        Copy of source block with specified components stripped off
+
+    Notes
+    -----
+    This is often used for creating a partially heterogeneous representation
+    of a block. For example, one might want to treat everything inside of a
+    specific component (such as the duct) as homogenized, while keeping a
+    heterogeneous representation of the remaining components.
+    """
+    newBlock = copy.deepcopy(block)
+    mixtureFlags = newBlock.getComponent(Flags.COOLANT).p.flags
+    innerMostComp = next(
+        i for i, c in enumerate(block.getComponents()) if c.hasFlags(compFlags)
+    )
+    outsideComp = True
+    indexedComponents = [(i, c) for i, c in enumerate(sorted(block.getComponents()))]
+    for i, c in sorted(indexedComponents, reverse=True):
+        if outsideComp:
+            block.remove(c, recomputeAreaFractions=False)
+            if i == innerMostCompt:
+                ductIP = c.getDimension("ip")
+                outsideComp = False
+        else:
+            mixtureFlags = mixtureFlags | c.p.flags
+
+    # add pitch defining component with no area
+    newBlock.add(
+        components.Hexagon(
+            "pitchComponent",
+            "Void",
+            self._sourceBlock.getAverageTempInC(),
+            self._sourceBlock.getAverageTempInC(),
+            ip=ductIP,
+            op=ductIP,
+        )
+    )
+    return newBlock, mixtureFlags

From 9b2088e7418cd7b424eeb3517c66641cee1432ba Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Tue, 1 Oct 2024 23:39:28 +0000
Subject: [PATCH 12/41] Fix import.

---
 .../neutronics/crossSectionGroupManager.py    | 23 +++++++------------
 1 file changed, 8 insertions(+), 15 deletions(-)

diff --git a/armi/physics/neutronics/crossSectionGroupManager.py b/armi/physics/neutronics/crossSectionGroupManager.py
index e8f82f683..854834fce 100644
--- a/armi/physics/neutronics/crossSectionGroupManager.py
+++ b/armi/physics/neutronics/crossSectionGroupManager.py
@@ -63,7 +63,7 @@
 from armi.physics.neutronics.const import CONF_CROSS_SECTION
 from armi.reactor import flags
 from armi.reactor.components import basicShapes
-from armi.reactor.geometryConverters.blockConverters import stripComponents
+from armi.reactor.converters.blockConverters import stripComponents
 from armi.reactor.flags import Flags
 from armi.utils.units import TRACE_NUMBER_DENSITY
 
@@ -648,10 +648,7 @@ def _getNucTempHelper(self):
             nv += nvBlock * wt
         return nvt, nv
 
-
-class CylindricalComponentsPartiallyHeterogeneousAverageBlockCollection(
-    CylindricalComponentsAverageBlockCollection
-):
+class CylindricalComponentsPartiallyHeterogeneousAverageBlockCollection(CylindricalComponentsAverageBlockCollection):
     """
     Creates a representative block for the purpose of cross section generation with a one-
     dimensional cylindrical model where all material inside the duct is homogenized.
@@ -668,13 +665,13 @@ class CylindricalComponentsPartiallyHeterogeneousAverageBlockCollection(
         Nuclide temperatures from a median block-average temperature are used and the average burnup
         is evaluated across all blocks in the block list.
 
-        The average nuclide temperatures are calculated only for the homogenized region inside of
+        The average nuclide temperatures are calculated only for the homogenized region inside of 
         the duct. For the non-homogenized regions, the MC2 writer uses the component temperatures.
 
     Notes
     -----
     The representative block for this collection is the same as the parent. The only difference between
-    the two collection types is that this collection calculates average nuclide temperatures based only
+    the two collection types is that this collection calculates average nuclide temperatures based only 
     on the components that are inside of the duct.
     """
 
@@ -684,7 +681,7 @@ def _getNucTempHelper(self):
         nv = np.zeros(len(self.allNuclidesInProblem))
         for block in self.getCandidateBlocks():
             wt = self.getWeight(block)
-            # remove the duct and intercoolant from the block before
+            # remove the duct and intercoolant from the block before 
             # calculating average nuclide temps
             newBlock, _mixtureFlags = stripComponents(block, Flags.DUCT)
             nvtBlock, nvBlock = getBlockNuclideTemperatureAvgTerms(
@@ -1567,9 +1564,7 @@ def updateNuclideTemperatures(self, blockCollectionByXsGroup=None):
 FLUX_WEIGHTED_AVERAGE_BLOCK_COLLECTION = "FluxWeightedAverage"
 SLAB_COMPONENTS_BLOCK_COLLECTION = "ComponentAverage1DSlab"
 CYLINDRICAL_COMPONENTS_BLOCK_COLLECTION = "ComponentAverage1DCylinder"
-CYLINDRICAL_COMPONENTS_PARTIALLY_HET_BLOCK_COLLECTION = (
-    "ComponentAverage1DCylinderPartiallyHeterogeneous"
-)
+CYLINDRICAL_COMPONENTS_PARTIALLY_HET_BLOCK_COLLECTION = "ComponentAverage1DCylinderPartiallyHeterogeneous"
 
 # Mapping between block collection string constants and their
 # respective block collection classes.
@@ -1586,10 +1581,8 @@ def updateNuclideTemperatures(self, blockCollectionByXsGroup=None):
 def blockCollectionFactory(xsSettings, allNuclidesInProblem):
     """Build a block collection based on user settings and input."""
     blockRepresentation = xsSettings.blockRepresentation
-    if (
-        blockRepresentation == CYLINDRICAL_COMPONENTS_BLOCK_COLLECTION
-    ) and xsSettings.partiallyHeterogeneous:
-        blockRepresentation = CYLINDRICAL_COMPONENTS_PARTIALLY_HET_BLOCK_COLLECTION
+    if (blockRepresentation == CYLINDRICAL_COMPONENTS_BLOCK_COLLECTION) and xsSettings.partiallyHeterogeneous:
+        blockRepresentation = CYLINDRICAL_COMPONENTS_PARTIALLY_HET_BLOCK_COLLECTION 
     validBlockTypes = xsSettings.validBlockTypes
     averageByComponent = xsSettings.averageByComponent
     return BLOCK_COLLECTIONS[blockRepresentation](

From c87c3d5d7675b63300eb98c95cbd9fbdfbf82065 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Tue, 1 Oct 2024 23:43:44 +0000
Subject: [PATCH 13/41] Fix a typo.

---
 armi/reactor/converters/blockConverters.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/armi/reactor/converters/blockConverters.py b/armi/reactor/converters/blockConverters.py
index 6fc3b2155..418134fa9 100644
--- a/armi/reactor/converters/blockConverters.py
+++ b/armi/reactor/converters/blockConverters.py
@@ -892,7 +892,7 @@ def stripComponents(block, compFlags):
     for i, c in sorted(indexedComponents, reverse=True):
         if outsideComp:
             block.remove(c, recomputeAreaFractions=False)
-            if i == innerMostCompt:
+            if i == innerMostComp:
                 ductIP = c.getDimension("ip")
                 outsideComp = False
         else:

From 102e7c12e8d111376e59adf30892cfdd327193a0 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Tue, 1 Oct 2024 23:45:49 +0000
Subject: [PATCH 14/41] Fix one more bug.

---
 armi/reactor/converters/blockConverters.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/armi/reactor/converters/blockConverters.py b/armi/reactor/converters/blockConverters.py
index 418134fa9..e8ceb8044 100644
--- a/armi/reactor/converters/blockConverters.py
+++ b/armi/reactor/converters/blockConverters.py
@@ -903,8 +903,8 @@ def stripComponents(block, compFlags):
         components.Hexagon(
             "pitchComponent",
             "Void",
-            self._sourceBlock.getAverageTempInC(),
-            self._sourceBlock.getAverageTempInC(),
+            block.getAverageTempInC(),
+            block.getAverageTempInC(),
             ip=ductIP,
             op=ductIP,
         )

From ab062f9a550c5a6914296b588f9b2aa91fe57ba2 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Tue, 1 Oct 2024 16:53:22 -0700
Subject: [PATCH 15/41] Black formatting.

---
 .../neutronics/crossSectionGroupManager.py    | 21 ++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/armi/physics/neutronics/crossSectionGroupManager.py b/armi/physics/neutronics/crossSectionGroupManager.py
index 854834fce..ea91b9d00 100644
--- a/armi/physics/neutronics/crossSectionGroupManager.py
+++ b/armi/physics/neutronics/crossSectionGroupManager.py
@@ -648,7 +648,10 @@ def _getNucTempHelper(self):
             nv += nvBlock * wt
         return nvt, nv
 
-class CylindricalComponentsPartiallyHeterogeneousAverageBlockCollection(CylindricalComponentsAverageBlockCollection):
+
+class CylindricalComponentsPartiallyHeterogeneousAverageBlockCollection(
+    CylindricalComponentsAverageBlockCollection
+):
     """
     Creates a representative block for the purpose of cross section generation with a one-
     dimensional cylindrical model where all material inside the duct is homogenized.
@@ -665,13 +668,13 @@ class CylindricalComponentsPartiallyHeterogeneousAverageBlockCollection(Cylindri
         Nuclide temperatures from a median block-average temperature are used and the average burnup
         is evaluated across all blocks in the block list.
 
-        The average nuclide temperatures are calculated only for the homogenized region inside of 
+        The average nuclide temperatures are calculated only for the homogenized region inside of
         the duct. For the non-homogenized regions, the MC2 writer uses the component temperatures.
 
     Notes
     -----
     The representative block for this collection is the same as the parent. The only difference between
-    the two collection types is that this collection calculates average nuclide temperatures based only 
+    the two collection types is that this collection calculates average nuclide temperatures based only
     on the components that are inside of the duct.
     """
 
@@ -681,7 +684,7 @@ def _getNucTempHelper(self):
         nv = np.zeros(len(self.allNuclidesInProblem))
         for block in self.getCandidateBlocks():
             wt = self.getWeight(block)
-            # remove the duct and intercoolant from the block before 
+            # remove the duct and intercoolant from the block before
             # calculating average nuclide temps
             newBlock, _mixtureFlags = stripComponents(block, Flags.DUCT)
             nvtBlock, nvBlock = getBlockNuclideTemperatureAvgTerms(
@@ -1564,7 +1567,9 @@ def updateNuclideTemperatures(self, blockCollectionByXsGroup=None):
 FLUX_WEIGHTED_AVERAGE_BLOCK_COLLECTION = "FluxWeightedAverage"
 SLAB_COMPONENTS_BLOCK_COLLECTION = "ComponentAverage1DSlab"
 CYLINDRICAL_COMPONENTS_BLOCK_COLLECTION = "ComponentAverage1DCylinder"
-CYLINDRICAL_COMPONENTS_PARTIALLY_HET_BLOCK_COLLECTION = "ComponentAverage1DCylinderPartiallyHeterogeneous"
+CYLINDRICAL_COMPONENTS_PARTIALLY_HET_BLOCK_COLLECTION = (
+    "ComponentAverage1DCylinderPartiallyHeterogeneous"
+)
 
 # Mapping between block collection string constants and their
 # respective block collection classes.
@@ -1581,8 +1586,10 @@ def updateNuclideTemperatures(self, blockCollectionByXsGroup=None):
 def blockCollectionFactory(xsSettings, allNuclidesInProblem):
     """Build a block collection based on user settings and input."""
     blockRepresentation = xsSettings.blockRepresentation
-    if (blockRepresentation == CYLINDRICAL_COMPONENTS_BLOCK_COLLECTION) and xsSettings.partiallyHeterogeneous:
-        blockRepresentation = CYLINDRICAL_COMPONENTS_PARTIALLY_HET_BLOCK_COLLECTION 
+    if (
+        blockRepresentation == CYLINDRICAL_COMPONENTS_BLOCK_COLLECTION
+    ) and xsSettings.partiallyHeterogeneous:
+        blockRepresentation = CYLINDRICAL_COMPONENTS_PARTIALLY_HET_BLOCK_COLLECTION
     validBlockTypes = xsSettings.validBlockTypes
     averageByComponent = xsSettings.averageByComponent
     return BLOCK_COLLECTIONS[blockRepresentation](

From fdb2772899b86fd2cc21326251930382a0eb5e42 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Tue, 1 Oct 2024 16:56:53 -0700
Subject: [PATCH 16/41] Fix a bug.

---
 armi/reactor/converters/blockConverters.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/armi/reactor/converters/blockConverters.py b/armi/reactor/converters/blockConverters.py
index e8ceb8044..8de4ffdbb 100644
--- a/armi/reactor/converters/blockConverters.py
+++ b/armi/reactor/converters/blockConverters.py
@@ -891,7 +891,7 @@ def stripComponents(block, compFlags):
     indexedComponents = [(i, c) for i, c in enumerate(sorted(block.getComponents()))]
     for i, c in sorted(indexedComponents, reverse=True):
         if outsideComp:
-            block.remove(c, recomputeAreaFractions=False)
+            newBlock.remove(c, recomputeAreaFractions=False)
             if i == innerMostComp:
                 ductIP = c.getDimension("ip")
                 outsideComp = False

From 716a7e211e7be1a818907041273074a14d262ad2 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Tue, 1 Oct 2024 23:45:49 +0000
Subject: [PATCH 17/41] Fix one more bug.

---
 .../neutronics/crossSectionGroupManager.py    | 21 ++++++++++++-------
 armi/reactor/converters/blockConverters.py    | 11 +++++-----
 2 files changed, 20 insertions(+), 12 deletions(-)

diff --git a/armi/physics/neutronics/crossSectionGroupManager.py b/armi/physics/neutronics/crossSectionGroupManager.py
index 854834fce..ea91b9d00 100644
--- a/armi/physics/neutronics/crossSectionGroupManager.py
+++ b/armi/physics/neutronics/crossSectionGroupManager.py
@@ -648,7 +648,10 @@ def _getNucTempHelper(self):
             nv += nvBlock * wt
         return nvt, nv
 
-class CylindricalComponentsPartiallyHeterogeneousAverageBlockCollection(CylindricalComponentsAverageBlockCollection):
+
+class CylindricalComponentsPartiallyHeterogeneousAverageBlockCollection(
+    CylindricalComponentsAverageBlockCollection
+):
     """
     Creates a representative block for the purpose of cross section generation with a one-
     dimensional cylindrical model where all material inside the duct is homogenized.
@@ -665,13 +668,13 @@ class CylindricalComponentsPartiallyHeterogeneousAverageBlockCollection(Cylindri
         Nuclide temperatures from a median block-average temperature are used and the average burnup
         is evaluated across all blocks in the block list.
 
-        The average nuclide temperatures are calculated only for the homogenized region inside of 
+        The average nuclide temperatures are calculated only for the homogenized region inside of
         the duct. For the non-homogenized regions, the MC2 writer uses the component temperatures.
 
     Notes
     -----
     The representative block for this collection is the same as the parent. The only difference between
-    the two collection types is that this collection calculates average nuclide temperatures based only 
+    the two collection types is that this collection calculates average nuclide temperatures based only
     on the components that are inside of the duct.
     """
 
@@ -681,7 +684,7 @@ def _getNucTempHelper(self):
         nv = np.zeros(len(self.allNuclidesInProblem))
         for block in self.getCandidateBlocks():
             wt = self.getWeight(block)
-            # remove the duct and intercoolant from the block before 
+            # remove the duct and intercoolant from the block before
             # calculating average nuclide temps
             newBlock, _mixtureFlags = stripComponents(block, Flags.DUCT)
             nvtBlock, nvBlock = getBlockNuclideTemperatureAvgTerms(
@@ -1564,7 +1567,9 @@ def updateNuclideTemperatures(self, blockCollectionByXsGroup=None):
 FLUX_WEIGHTED_AVERAGE_BLOCK_COLLECTION = "FluxWeightedAverage"
 SLAB_COMPONENTS_BLOCK_COLLECTION = "ComponentAverage1DSlab"
 CYLINDRICAL_COMPONENTS_BLOCK_COLLECTION = "ComponentAverage1DCylinder"
-CYLINDRICAL_COMPONENTS_PARTIALLY_HET_BLOCK_COLLECTION = "ComponentAverage1DCylinderPartiallyHeterogeneous"
+CYLINDRICAL_COMPONENTS_PARTIALLY_HET_BLOCK_COLLECTION = (
+    "ComponentAverage1DCylinderPartiallyHeterogeneous"
+)
 
 # Mapping between block collection string constants and their
 # respective block collection classes.
@@ -1581,8 +1586,10 @@ def updateNuclideTemperatures(self, blockCollectionByXsGroup=None):
 def blockCollectionFactory(xsSettings, allNuclidesInProblem):
     """Build a block collection based on user settings and input."""
     blockRepresentation = xsSettings.blockRepresentation
-    if (blockRepresentation == CYLINDRICAL_COMPONENTS_BLOCK_COLLECTION) and xsSettings.partiallyHeterogeneous:
-        blockRepresentation = CYLINDRICAL_COMPONENTS_PARTIALLY_HET_BLOCK_COLLECTION 
+    if (
+        blockRepresentation == CYLINDRICAL_COMPONENTS_BLOCK_COLLECTION
+    ) and xsSettings.partiallyHeterogeneous:
+        blockRepresentation = CYLINDRICAL_COMPONENTS_PARTIALLY_HET_BLOCK_COLLECTION
     validBlockTypes = xsSettings.validBlockTypes
     averageByComponent = xsSettings.averageByComponent
     return BLOCK_COLLECTIONS[blockRepresentation](
diff --git a/armi/reactor/converters/blockConverters.py b/armi/reactor/converters/blockConverters.py
index 418134fa9..c102c398a 100644
--- a/armi/reactor/converters/blockConverters.py
+++ b/armi/reactor/converters/blockConverters.py
@@ -883,15 +883,16 @@ def stripComponents(block, compFlags):
     heterogeneous representation of the remaining components.
     """
     newBlock = copy.deepcopy(block)
+    avgBlockTemp = block.getAverageTempInC()
     mixtureFlags = newBlock.getComponent(Flags.COOLANT).p.flags
     innerMostComp = next(
-        i for i, c in enumerate(block.getComponents()) if c.hasFlags(compFlags)
+        i for i, c in enumerate(newBlock.getComponents()) if c.hasFlags(compFlags)
     )
     outsideComp = True
-    indexedComponents = [(i, c) for i, c in enumerate(sorted(block.getComponents()))]
+    indexedComponents = [(i, c) for i, c in enumerate(sorted(newBlock.getComponents()))]
     for i, c in sorted(indexedComponents, reverse=True):
         if outsideComp:
-            block.remove(c, recomputeAreaFractions=False)
+            newBlock.remove(c, recomputeAreaFractions=False)
             if i == innerMostComp:
                 ductIP = c.getDimension("ip")
                 outsideComp = False
@@ -903,8 +904,8 @@ def stripComponents(block, compFlags):
         components.Hexagon(
             "pitchComponent",
             "Void",
-            self._sourceBlock.getAverageTempInC(),
-            self._sourceBlock.getAverageTempInC(),
+            avgBlockTemp,
+            avgBlockTemp,
             ip=ductIP,
             op=ductIP,
         )

From 752b256ad17e214d1eeec416daaeb74748998bf5 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Wed, 2 Oct 2024 00:01:33 +0000
Subject: [PATCH 18/41] Fix one more bug.

---
 armi/reactor/converters/blockConverters.py | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/armi/reactor/converters/blockConverters.py b/armi/reactor/converters/blockConverters.py
index 8de4ffdbb..c102c398a 100644
--- a/armi/reactor/converters/blockConverters.py
+++ b/armi/reactor/converters/blockConverters.py
@@ -883,12 +883,13 @@ def stripComponents(block, compFlags):
     heterogeneous representation of the remaining components.
     """
     newBlock = copy.deepcopy(block)
+    avgBlockTemp = block.getAverageTempInC()
     mixtureFlags = newBlock.getComponent(Flags.COOLANT).p.flags
     innerMostComp = next(
-        i for i, c in enumerate(block.getComponents()) if c.hasFlags(compFlags)
+        i for i, c in enumerate(newBlock.getComponents()) if c.hasFlags(compFlags)
     )
     outsideComp = True
-    indexedComponents = [(i, c) for i, c in enumerate(sorted(block.getComponents()))]
+    indexedComponents = [(i, c) for i, c in enumerate(sorted(newBlock.getComponents()))]
     for i, c in sorted(indexedComponents, reverse=True):
         if outsideComp:
             newBlock.remove(c, recomputeAreaFractions=False)
@@ -903,8 +904,8 @@ def stripComponents(block, compFlags):
         components.Hexagon(
             "pitchComponent",
             "Void",
-            block.getAverageTempInC(),
-            block.getAverageTempInC(),
+            avgBlockTemp,
+            avgBlockTemp,
             ip=ductIP,
             op=ductIP,
         )

From 9806e7876ec4154e5209a7ae7d2f269814967152 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Wed, 2 Oct 2024 00:45:30 +0000
Subject: [PATCH 19/41] Calculate average nuclide temperatures for partially
 het cylindrical block collection

---
 armi/physics/neutronics/crossSectionGroupManager.py | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/armi/physics/neutronics/crossSectionGroupManager.py b/armi/physics/neutronics/crossSectionGroupManager.py
index ea91b9d00..0a5d51deb 100644
--- a/armi/physics/neutronics/crossSectionGroupManager.py
+++ b/armi/physics/neutronics/crossSectionGroupManager.py
@@ -678,6 +678,16 @@ class CylindricalComponentsPartiallyHeterogeneousAverageBlockCollection(
     on the components that are inside of the duct.
     """
 
+    def _getNewBlock(self):
+        newBlock = copy.deepcopy(self._selectCandidateBlock())
+        newBlock.name = "1D_CYL_PART_HET_AVG_" + newBlock.getMicroSuffix()
+        return newBlock
+
+    def _makeRepresentativeBlock(self):
+        """Build a representative fuel block based on component number densities."""
+        self.calcAvgNuclideTemperatures()
+        return CylindricalComponentsAverageBlockCollection._makeRepresentativeBlock(self)
+
     def _getNucTempHelper(self):
         """All candidate blocks are used in the average."""
         nvt = np.zeros(len(self.allNuclidesInProblem))

From 69d5acfb79bbf91f1bda246a0b60f6e698dd16c3 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Wed, 2 Oct 2024 00:45:30 +0000
Subject: [PATCH 20/41] Calculate average nuclide temperatures for partially
 het cylindrical block collection

---
 armi/physics/neutronics/crossSectionGroupManager.py | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/armi/physics/neutronics/crossSectionGroupManager.py b/armi/physics/neutronics/crossSectionGroupManager.py
index ea91b9d00..0a5d51deb 100644
--- a/armi/physics/neutronics/crossSectionGroupManager.py
+++ b/armi/physics/neutronics/crossSectionGroupManager.py
@@ -678,6 +678,16 @@ class CylindricalComponentsPartiallyHeterogeneousAverageBlockCollection(
     on the components that are inside of the duct.
     """
 
+    def _getNewBlock(self):
+        newBlock = copy.deepcopy(self._selectCandidateBlock())
+        newBlock.name = "1D_CYL_PART_HET_AVG_" + newBlock.getMicroSuffix()
+        return newBlock
+
+    def _makeRepresentativeBlock(self):
+        """Build a representative fuel block based on component number densities."""
+        self.calcAvgNuclideTemperatures()
+        return CylindricalComponentsAverageBlockCollection._makeRepresentativeBlock(self)
+
     def _getNucTempHelper(self):
         """All candidate blocks are used in the average."""
         nvt = np.zeros(len(self.allNuclidesInProblem))

From 391901fed16d1f927a973779a4c123559070faea Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Thu, 3 Oct 2024 03:29:24 +0000
Subject: [PATCH 21/41] Add mergeIntoFuel option.

---
 armi/physics/neutronics/crossSectionSettings.py      | 12 ++++++++++++
 .../latticePhysics/latticePhysicsWriter.py           |  1 +
 armi/reactor/converters/blockConverters.py           |  6 ++++++
 3 files changed, 19 insertions(+)

diff --git a/armi/physics/neutronics/crossSectionSettings.py b/armi/physics/neutronics/crossSectionSettings.py
index 5b5b422c0..887381865 100644
--- a/armi/physics/neutronics/crossSectionSettings.py
+++ b/armi/physics/neutronics/crossSectionSettings.py
@@ -49,6 +49,7 @@
 CONF_HOMOGBLOCK = "useHomogenizedBlockComposition"
 CONF_INTERNAL_RINGS = "numInternalRings"
 CONF_MERGE_INTO_CLAD = "mergeIntoClad"
+CONF_MERGE_INTO_FUEL = "mergeIntoFuel"
 CONF_MESH_PER_CM = "meshSubdivisionsPerCm"
 CONF_REACTION_DRIVER = "nuclideReactionDriver"
 CONF_XSID = "xsID"
@@ -137,6 +138,7 @@ def getStr(cls, typeSpec: Enum):
         CONF_XSID,
         CONF_GEOM,
         CONF_MERGE_INTO_CLAD,
+        CONF_MERGE_INTO_FUEL,
         CONF_DRIVER,
         CONF_HOMOGBLOCK,
         CONF_INTERNAL_RINGS,
@@ -188,6 +190,7 @@ def getStr(cls, typeSpec: Enum):
         vol.Optional(CONF_INTERNAL_RINGS): vol.Coerce(int),
         vol.Optional(CONF_EXTERNAL_RINGS): vol.Coerce(int),
         vol.Optional(CONF_MERGE_INTO_CLAD): [str],
+        vol.Optional(CONF_MERGE_INTO_FUEL): [str],
         vol.Optional(CONF_XS_FILE_LOCATION): [str],
         vol.Optional(CONF_EXTERNAL_FLUX_FILE_LOCATION): str,
         vol.Optional(CONF_MESH_PER_CM): vol.Coerce(float),
@@ -413,6 +416,12 @@ class XSModelingOptions:
         and is sometimes used to merge a "gap" or low-density region into
         a "clad" region to avoid numerical issues.
 
+    mergeIntoFuel : list of str
+        This is a lattice physics configuration option that is a list of component
+        names to merge into a "fuel" component. This is highly-design specific
+        and is sometimes used to merge a "gap" or low-density region into
+        a "fuel" region to avoid numerical issues.
+
     meshSubdivisionsPerCm : float
         This is a lattice physics configuration option that can be used to control
         subregion meshing of the representative block in 1D problems.
@@ -466,6 +475,7 @@ def __init__(
         numInternalRings=None,
         numExternalRings=None,
         mergeIntoClad=None,
+        mergeIntoFuel=None,
         meshSubdivisionsPerCm=None,
         xsExecuteExclusive=None,
         xsPriority=None,
@@ -491,6 +501,7 @@ def __init__(
         self.numInternalRings = numInternalRings
         self.numExternalRings = numExternalRings
         self.mergeIntoClad = mergeIntoClad
+        self.mergeIntoFuel = mergeIntoFuel
         self.meshSubdivisionsPerCm = meshSubdivisionsPerCm
         self.xsMaxAtomNumber = xsMaxAtomNumber
         self.minDriverDensity = minDriverDensity
@@ -678,6 +689,7 @@ def setDefaults(self, blockRepresentation, validBlockTypes):
                 CONF_GEOM: self.geometry,
                 CONF_DRIVER: "",
                 CONF_MERGE_INTO_CLAD: ["gap"],
+                CONF_MERGE_INTO_FUEL: [],
                 CONF_MESH_PER_CM: 1.0,
                 CONF_INTERNAL_RINGS: 0,
                 CONF_EXTERNAL_RINGS: 1,
diff --git a/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py b/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py
index 5c56c573c..8e0ead4f3 100644
--- a/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py
+++ b/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py
@@ -110,6 +110,7 @@ def __init__(
         self.xsId = representativeBlock.getMicroSuffix()
         self.xsSettings = self.cs[CONF_CROSS_SECTION][self.xsId]
         self.mergeIntoClad = self.xsSettings.mergeIntoClad
+        self.mergeIntoFuel = self.xsSettings.mergeIntoFuel
         self.driverXsID = self.xsSettings.driverID
         self.numExternalRings = self.xsSettings.numExternalRings
         self.criticalBucklingSearchActive = self.xsSettings.criticalBuckling
diff --git a/armi/reactor/converters/blockConverters.py b/armi/reactor/converters/blockConverters.py
index c102c398a..97de38ca9 100644
--- a/armi/reactor/converters/blockConverters.py
+++ b/armi/reactor/converters/blockConverters.py
@@ -523,6 +523,7 @@ def __init__(
         driverFuelBlock=None,
         numExternalRings=None,
         mergeIntoClad=None,
+        mergeIntoFuel=None,
         partiallyHeterogeneous=False,
     ):
         BlockAvgToCylConverter.__init__(
@@ -553,6 +554,7 @@ def __init__(
                 )
         self.pinPitch = sourceBlock.getPinPitch()
         self.mergeIntoClad = mergeIntoClad or []
+        self.mergeIntoFuel = mergeIntoFuel or []
         self.partiallyHeterogeneous = partiallyHeterogeneous
         self.interRingComponent = sourceBlock.getComponent(Flags.COOLANT, exact=True)
         self._remainingCoolantFillArea = self.interRingComponent.getArea()
@@ -619,6 +621,10 @@ def _dissolveComponents(self):
         for componentName in self.mergeIntoClad:
             self.dissolveComponentIntoComponent(componentName, "clad")
 
+        # do user-input merges
+        for componentName in self.mergeIntoFuel:
+            self.dissolveComponentIntoComponent(componentName, "fuel")
+
     def _classifyComponents(self):
         """
         Figure out which components are in each pin ring and which are not.

From c979b40f1134d550547fcb34faf8ea7529d84d21 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Thu, 3 Oct 2024 04:44:09 +0000
Subject: [PATCH 22/41] Update XSGM to work when components have zero area.

---
 armi/physics/neutronics/crossSectionGroupManager.py | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/armi/physics/neutronics/crossSectionGroupManager.py b/armi/physics/neutronics/crossSectionGroupManager.py
index 0a5d51deb..fcba86d1d 100644
--- a/armi/physics/neutronics/crossSectionGroupManager.py
+++ b/armi/physics/neutronics/crossSectionGroupManager.py
@@ -624,7 +624,11 @@ def _getAverageComponentNucs(self, components, bWeights):
             weight = bWeight * c.getArea()
             totalWeight += weight
             densities += weight * np.array(c.getNuclideNumberDensities(allNucNames))
-        return allNucNames, densities / totalWeight
+        if totalWeight > 0.0:
+            weightedDensities = densities / totalWeight
+        else:
+            weightedDensities = np.zeros_like(densities)
+        return allNucNames, weightedDensities
 
     def _orderComponentsInGroup(self, repBlock):
         """Order the components based on dimension and material type within the representative
@@ -846,7 +850,11 @@ def _getAverageComponentNucs(self, components, bWeights):
             weight = bWeight * c.getArea()
             totalWeight += weight
             densities += weight * np.array(c.getNuclideNumberDensities(allNucNames))
-        return allNucNames, densities / totalWeight
+        if totalWeight > 0.0:
+            weightedDensities = densities / totalWeight
+        else:
+            weightedDensities = np.zeros_like(densities)
+        return allNucNames, weightedDensities
 
     def _orderComponentsInGroup(self, repBlock):
         """Order the components based on dimension and material type within the representative block."""

From f511b5b50d29735570758e8dd74bac08c2f3ae3f Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Thu, 3 Oct 2024 21:11:03 +0000
Subject: [PATCH 23/41] Fix a component sorting issue.

---
 armi/reactor/converters/blockConverters.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/armi/reactor/converters/blockConverters.py b/armi/reactor/converters/blockConverters.py
index 97de38ca9..669339f0e 100644
--- a/armi/reactor/converters/blockConverters.py
+++ b/armi/reactor/converters/blockConverters.py
@@ -892,16 +892,16 @@ def stripComponents(block, compFlags):
     avgBlockTemp = block.getAverageTempInC()
     mixtureFlags = newBlock.getComponent(Flags.COOLANT).p.flags
     innerMostComp = next(
-        i for i, c in enumerate(newBlock.getComponents()) if c.hasFlags(compFlags)
+        i for i, c in enumerate(sorted(newBlock.getComponents())) if c.hasFlags(compFlags)
     )
     outsideComp = True
     indexedComponents = [(i, c) for i, c in enumerate(sorted(newBlock.getComponents()))]
     for i, c in sorted(indexedComponents, reverse=True):
         if outsideComp:
-            newBlock.remove(c, recomputeAreaFractions=False)
             if i == innerMostComp:
                 ductIP = c.getDimension("ip")
                 outsideComp = False
+            newBlock.remove(c, recomputeAreaFractions=False)
         else:
             mixtureFlags = mixtureFlags | c.p.flags
 

From cb39d04505388de43fedc487418e2f222d83f40f Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Thu, 3 Oct 2024 15:03:05 -0700
Subject: [PATCH 24/41] Fix a component sorting issue

---
 armi/reactor/converters/blockConverters.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/armi/reactor/converters/blockConverters.py b/armi/reactor/converters/blockConverters.py
index c102c398a..022aa0dc1 100644
--- a/armi/reactor/converters/blockConverters.py
+++ b/armi/reactor/converters/blockConverters.py
@@ -886,7 +886,7 @@ def stripComponents(block, compFlags):
     avgBlockTemp = block.getAverageTempInC()
     mixtureFlags = newBlock.getComponent(Flags.COOLANT).p.flags
     innerMostComp = next(
-        i for i, c in enumerate(newBlock.getComponents()) if c.hasFlags(compFlags)
+        i for i, c in enumerate(sorted(newBlock.getComponents())) if c.hasFlags(compFlags)
     )
     outsideComp = True
     indexedComponents = [(i, c) for i, c in enumerate(sorted(newBlock.getComponents()))]

From 9b7165071e3f1175aad575fb67cb6a33419138a5 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Thu, 3 Oct 2024 15:03:36 -0700
Subject: [PATCH 25/41] Black formatting

---
 armi/physics/neutronics/crossSectionGroupManager.py | 4 +++-
 armi/reactor/converters/blockConverters.py          | 4 +++-
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/armi/physics/neutronics/crossSectionGroupManager.py b/armi/physics/neutronics/crossSectionGroupManager.py
index 0a5d51deb..dd46f8fbd 100644
--- a/armi/physics/neutronics/crossSectionGroupManager.py
+++ b/armi/physics/neutronics/crossSectionGroupManager.py
@@ -686,7 +686,9 @@ def _getNewBlock(self):
     def _makeRepresentativeBlock(self):
         """Build a representative fuel block based on component number densities."""
         self.calcAvgNuclideTemperatures()
-        return CylindricalComponentsAverageBlockCollection._makeRepresentativeBlock(self)
+        return CylindricalComponentsAverageBlockCollection._makeRepresentativeBlock(
+            self
+        )
 
     def _getNucTempHelper(self):
         """All candidate blocks are used in the average."""
diff --git a/armi/reactor/converters/blockConverters.py b/armi/reactor/converters/blockConverters.py
index 022aa0dc1..2ff76e9b2 100644
--- a/armi/reactor/converters/blockConverters.py
+++ b/armi/reactor/converters/blockConverters.py
@@ -886,7 +886,9 @@ def stripComponents(block, compFlags):
     avgBlockTemp = block.getAverageTempInC()
     mixtureFlags = newBlock.getComponent(Flags.COOLANT).p.flags
     innerMostComp = next(
-        i for i, c in enumerate(sorted(newBlock.getComponents())) if c.hasFlags(compFlags)
+        i
+        for i, c in enumerate(sorted(newBlock.getComponents()))
+        if c.hasFlags(compFlags)
     )
     outsideComp = True
     indexedComponents = [(i, c) for i, c in enumerate(sorted(newBlock.getComponents()))]

From f641447a8a8ec9a81c3085735eac1203a73ee1b0 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Mon, 7 Oct 2024 16:00:26 -0700
Subject: [PATCH 26/41] Fix linting error.

---
 armi/physics/neutronics/crossSectionGroupManager.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/armi/physics/neutronics/crossSectionGroupManager.py b/armi/physics/neutronics/crossSectionGroupManager.py
index 09d005062..e139d3f39 100644
--- a/armi/physics/neutronics/crossSectionGroupManager.py
+++ b/armi/physics/neutronics/crossSectionGroupManager.py
@@ -653,7 +653,7 @@ def _getNucTempHelper(self):
         return nvt, nv
 
 
-class CylindricalComponentsPartiallyHeterogeneousAverageBlockCollection(
+class CylindricalComponentsPartHetAverageBlockCollection(
     CylindricalComponentsAverageBlockCollection
 ):
     """
@@ -1599,7 +1599,7 @@ def updateNuclideTemperatures(self, blockCollectionByXsGroup=None):
     FLUX_WEIGHTED_AVERAGE_BLOCK_COLLECTION: FluxWeightedAverageBlockCollection,
     SLAB_COMPONENTS_BLOCK_COLLECTION: SlabComponentsAverageBlockCollection,
     CYLINDRICAL_COMPONENTS_BLOCK_COLLECTION: CylindricalComponentsAverageBlockCollection,
-    CYLINDRICAL_COMPONENTS_PARTIALLY_HET_BLOCK_COLLECTION: CylindricalComponentsPartiallyHeterogeneousAverageBlockCollection,
+    CYLINDRICAL_COMPONENTS_PARTIALLY_HET_BLOCK_COLLECTION: CylindricalComponentsPartHetAverageBlockCollection,
 }
 
 

From bc357664d8192b242a384fe2a95caa2101bba527 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Mon, 7 Oct 2024 16:07:15 -0700
Subject: [PATCH 27/41] Update comments and docstrings.

---
 armi/reactor/converters/blockConverters.py | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/armi/reactor/converters/blockConverters.py b/armi/reactor/converters/blockConverters.py
index 67876613b..603b8e17c 100644
--- a/armi/reactor/converters/blockConverters.py
+++ b/armi/reactor/converters/blockConverters.py
@@ -509,9 +509,9 @@ class HexComponentsToCylConverter(BlockAvgToCylConverter):
     nonpins ...
 
     The ``partiallyHeterogeneous`` option allows the user to treat everything inside the duct
-    as a single homogenized composition. This will significantly reduce the memory and runtime
-    required for MC2, and also provide an alternative approximation for the spatial self-shielding
-    effect on microscopic cross sections.
+    as a single homogenized composition. This could significantly reduce the memory and runtime
+    required for the lattice physics solver, and also provide an alternative approximation for 
+    the spatial self-shielding effect on microscopic cross sections.
 
     This converter expects the ``sourceBlock`` and ``driverFuelBlock`` to defined and for
     the ``sourceBlock`` to have a spatial grid defined. Additionally, both the ``sourceBlock``
@@ -618,11 +618,11 @@ def _dissolveComponents(self):
         )
         self._remainingCoolantFillArea = self.interRingComponent.getArea()
 
-        # do user-input merges
+        # do user-input merges into cladding
         for componentName in self.mergeIntoClad:
             self.dissolveComponentIntoComponent(componentName, "clad")
 
-        # do user-input merges
+        # do user-input merges into fuel
         for componentName in self.mergeIntoFuel:
             self.dissolveComponentIntoComponent(componentName, "fuel")
 

From 2211c3b28f2074de5eb3acfaaf75a8c03b127208 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Mon, 7 Oct 2024 16:09:57 -0700
Subject: [PATCH 28/41] Update a unit test.

---
 armi/physics/neutronics/tests/test_crossSectionSettings.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/armi/physics/neutronics/tests/test_crossSectionSettings.py b/armi/physics/neutronics/tests/test_crossSectionSettings.py
index ddcc923ab..d7578df5f 100644
--- a/armi/physics/neutronics/tests/test_crossSectionSettings.py
+++ b/armi/physics/neutronics/tests/test_crossSectionSettings.py
@@ -192,6 +192,7 @@ def test_optionalKey(self):
         self.assertEqual(xsModel["DA"].mergeIntoClad, ["gap"])
         self.assertEqual(xsModel["DA"].meshSubdivisionsPerCm, 1.0)
         self.assertEqual(xsModel["DA"].partiallyHeterogeneous, True)
+        self.assertEqual(xsModel["DA"].mergeIntoFuel, [])
 
     def test_badCrossSections(self):
         with self.assertRaises(TypeError):

From 3fd64bf1e66b53dc7c93a33ffee0efdd9b3f686c Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Wed, 9 Oct 2024 16:05:55 -0700
Subject: [PATCH 29/41] Rename variable.

---
 armi/reactor/converters/blockConverters.py | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/armi/reactor/converters/blockConverters.py b/armi/reactor/converters/blockConverters.py
index 603b8e17c..99ff3d666 100644
--- a/armi/reactor/converters/blockConverters.py
+++ b/armi/reactor/converters/blockConverters.py
@@ -510,7 +510,7 @@ class HexComponentsToCylConverter(BlockAvgToCylConverter):
 
     The ``partiallyHeterogeneous`` option allows the user to treat everything inside the duct
     as a single homogenized composition. This could significantly reduce the memory and runtime
-    required for the lattice physics solver, and also provide an alternative approximation for 
+    required for the lattice physics solver, and also provide an alternative approximation for
     the spatial self-shielding effect on microscopic cross sections.
 
     This converter expects the ``sourceBlock`` and ``driverFuelBlock`` to defined and for
@@ -902,7 +902,7 @@ def stripComponents(block, compFlags):
     for i, c in sorted(indexedComponents, reverse=True):
         if outsideComp:
             if i == innerMostComp:
-                ductIP = c.getDimension("ip")
+                compIP = c.getDimension("ip")
                 outsideComp = False
             newBlock.remove(c, recomputeAreaFractions=False)
         else:
@@ -915,8 +915,8 @@ def stripComponents(block, compFlags):
             "Void",
             avgBlockTemp,
             avgBlockTemp,
-            ip=ductIP,
-            op=ductIP,
+            ip=compIP,
+            op=compIP,
         )
     )
     return newBlock, mixtureFlags

From 5bf38fe73f230f62469fd311829ad3b013fe0cc7 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Thu, 10 Oct 2024 12:06:37 -0700
Subject: [PATCH 30/41] Add option to split trace isotopes from 1D to 0D.

---
 armi/physics/neutronics/crossSectionSettings.py        | 10 ++++++++++
 .../neutronics/latticePhysics/latticePhysicsWriter.py  |  1 +
 2 files changed, 11 insertions(+)

diff --git a/armi/physics/neutronics/crossSectionSettings.py b/armi/physics/neutronics/crossSectionSettings.py
index 887381865..8ea7d5dc5 100644
--- a/armi/physics/neutronics/crossSectionSettings.py
+++ b/armi/physics/neutronics/crossSectionSettings.py
@@ -59,6 +59,7 @@
 CONF_XS_MAX_ATOM_NUMBER = "xsMaxAtomNumber"
 CONF_MIN_DRIVER_DENSITY = "minDriverDensity"
 CONF_PARTIALLY_HETEROGENEOUS = "partiallyHeterogeneous"
+CONF_SPLIT_TRACE_ISOTOPES = "splitTraceIsotopes"
 
 
 class XSGeometryTypes(Enum):
@@ -450,6 +451,12 @@ class XSModelingOptions:
         heterogeneous approximation for a 1D cylindrical model. Everything inside of the
         duct will be treated as homogeneous.
 
+    splitTraceIsotopes : bool
+        This is a lattice physics configuration option used to enable a separate 0D fuel
+        cross section calculation for trace fission products when using a 1D cross section
+        model. This can significantly reduce the memory and run time required for the 1D
+        model.
+
     Notes
     -----
     Not all default attributes may be useful for your specific application and you may
@@ -483,6 +490,7 @@ def __init__(
         averageByComponent=False,
         minDriverDensity=0.0,
         partiallyHeterogeneous=False,
+        splitTraceIsotopes=False,
     ):
         self.xsID = xsID
         self.geometry = geometry
@@ -507,6 +515,7 @@ def __init__(
         self.minDriverDensity = minDriverDensity
         self.averageByComponent = averageByComponent
         self.partiallyHeterogeneous = partiallyHeterogeneous
+        self.splitTraceIsotopes = splitTraceIsotopes
         # these are related to execution
         self.xsExecuteExclusive = xsExecuteExclusive
         self.xsPriority = xsPriority
@@ -697,6 +706,7 @@ def setDefaults(self, blockRepresentation, validBlockTypes):
                 CONF_BLOCK_REPRESENTATION: crossSectionGroupManager.CYLINDRICAL_COMPONENTS_BLOCK_COLLECTION,
                 CONF_BLOCKTYPES: validBlockTypes,
                 CONF_PARTIALLY_HETEROGENEOUS: False,
+                CONF_SPLIT_TRACE_ISOTOPES: False,
             }
         elif self.geometry == XSGeometryTypes.getStr(
             XSGeometryTypes.TWO_DIMENSIONAL_HEX
diff --git a/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py b/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py
index 8e0ead4f3..3699c2f4c 100644
--- a/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py
+++ b/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py
@@ -115,6 +115,7 @@ def __init__(
         self.numExternalRings = self.xsSettings.numExternalRings
         self.criticalBucklingSearchActive = self.xsSettings.criticalBuckling
         self.partiallyHeterogeneous = self.xsSettings.partiallyHeterogeneous
+        self.splitTraceIsotopes = self.xsSettings.splitTraceIsotopes
 
         self.executeExclusive = self.xsSettings.xsExecuteExclusive
         self.priority = self.xsSettings.xsPriority

From be1fd4e8f075e6334ca782da893dcd01bed4e4a3 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Thu, 10 Oct 2024 12:17:44 -0700
Subject: [PATCH 31/41] Add splitTraceIsotopes as a valid input option.

---
 armi/physics/neutronics/crossSectionSettings.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/armi/physics/neutronics/crossSectionSettings.py b/armi/physics/neutronics/crossSectionSettings.py
index 8ea7d5dc5..a86771a07 100644
--- a/armi/physics/neutronics/crossSectionSettings.py
+++ b/armi/physics/neutronics/crossSectionSettings.py
@@ -154,6 +154,7 @@ def getStr(cls, typeSpec: Enum):
         CONF_XS_MAX_ATOM_NUMBER,
         CONF_MIN_DRIVER_DENSITY,
         CONF_PARTIALLY_HETEROGENEOUS,
+        CONF_SPLIT_TRACE_ISOTOPES,
     },
     XSGeometryTypes.getStr(XSGeometryTypes.TWO_DIMENSIONAL_HEX): {
         CONF_XSID,
@@ -201,6 +202,7 @@ def getStr(cls, typeSpec: Enum):
         vol.Optional(CONF_MIN_DRIVER_DENSITY): vol.Coerce(float),
         vol.Optional(CONF_COMPONENT_AVERAGING): bool,
         vol.Optional(CONF_PARTIALLY_HETEROGENEOUS): bool,
+        vol.Optional(CONF_SPLIT_TRACE_ISOTOPES): bool,
     }
 )
 

From 4c50fac249b39596ba7d3f2552e48cd57e59a892 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Thu, 10 Oct 2024 14:15:13 -0700
Subject: [PATCH 32/41] Add splitTraceIsotopes to unit test

---
 armi/physics/neutronics/tests/test_crossSectionSettings.py | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/armi/physics/neutronics/tests/test_crossSectionSettings.py b/armi/physics/neutronics/tests/test_crossSectionSettings.py
index d7578df5f..61df3f238 100644
--- a/armi/physics/neutronics/tests/test_crossSectionSettings.py
+++ b/armi/physics/neutronics/tests/test_crossSectionSettings.py
@@ -119,6 +119,7 @@ def test_homogeneousXsDefaultSettingAssignment(self):
         self.assertEqual(xsModel["YA"].geometry, "0D")
         self.assertEqual(xsModel["YA"].criticalBuckling, True)
         self.assertEqual(xsModel["YA"].partiallyHeterogeneous, False)
+        self.assertEqual(xsModel["YA"].splitTraceIsotopes, False)
 
     def test_setDefaultSettingsByLowestBuGroupHomogeneous(self):
         # Initialize some micro suffix in the cross sections
@@ -183,6 +184,7 @@ def test_optionalKey(self):
             geometry="1D cylinder",
             meshSubdivisionsPerCm=1.0,
             partiallyHeterogeneous=True,
+            splitTraceIsotopes=True,
         )
         xsModel["DA"] = da
         xsModel.setDefaults(
@@ -192,6 +194,7 @@ def test_optionalKey(self):
         self.assertEqual(xsModel["DA"].mergeIntoClad, ["gap"])
         self.assertEqual(xsModel["DA"].meshSubdivisionsPerCm, 1.0)
         self.assertEqual(xsModel["DA"].partiallyHeterogeneous, True)
+        self.assertEqual(xsModel["DA"].splitTraceIsotopes, True)
         self.assertEqual(xsModel["DA"].mergeIntoFuel, [])
 
     def test_badCrossSections(self):

From e8b0efc1b803f516822096dcf808215447075f3f Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Fri, 11 Oct 2024 10:46:33 -0700
Subject: [PATCH 33/41] Check for lower-case XS IDs

---
 armi/nuclearDataIO/xsLibraries.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/armi/nuclearDataIO/xsLibraries.py b/armi/nuclearDataIO/xsLibraries.py
index 126437761..f38bbaa65 100644
--- a/armi/nuclearDataIO/xsLibraries.py
+++ b/armi/nuclearDataIO/xsLibraries.py
@@ -127,7 +127,7 @@ def getISOTXSLibrariesToMerge(xsLibrarySuffix, xsLibFileNames):
         isosWithSuffix = [
             iso
             for iso in isosToMerge
-            if re.match(f".*ISO[A-Z]{{2}}F?{xsLibrarySuffix}$", iso)
+            if re.match(f".*ISO[A-Za-z]{{2}}F?{xsLibrarySuffix}$", iso)
         ]
         isosToMerge = [
             iso
@@ -193,7 +193,7 @@ def mergeXSLibrariesInWorkingDirectory(
     for xsLibFilePath in sorted(xsLibFiles):
         try:
             # get XS ID from the cross section library name
-            xsID = re.search("ISO([A-Z0-9]{2})", xsLibFilePath).group(1)
+            xsID = re.search("ISO([A-Z0-9a-z]{2})", xsLibFilePath).group(1)
         except AttributeError:
             # if glob has matched something that is not actually an ISOXX file,
             # the .group() call will fail

From 92fe1a67cece9469f585cde39f9e346e521c6743 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Fri, 11 Oct 2024 16:17:13 -0700
Subject: [PATCH 34/41] Update unit test.

---
 armi/nuclearDataIO/tests/test_xsLibraries.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/armi/nuclearDataIO/tests/test_xsLibraries.py b/armi/nuclearDataIO/tests/test_xsLibraries.py
index 324206e9e..ae4b55229 100644
--- a/armi/nuclearDataIO/tests/test_xsLibraries.py
+++ b/armi/nuclearDataIO/tests/test_xsLibraries.py
@@ -127,7 +127,7 @@ def test_mergeFailsWithNonIsotxsFiles(self):
             os.remove(dummyFileName)
 
         with TemporaryDirectoryChanger():
-            dummyFileName = "ISOtopics.txt"
+            dummyFileName = "ISO[]"
             with open(dummyFileName, "w") as file:
                 file.write(
                     "This is a file that starts with the letters 'ISO' but will"

From 0bcfa76359a485fc87cb587be3989c95fb47c2de Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Mon, 14 Oct 2024 12:00:29 -0700
Subject: [PATCH 35/41] Add missing parameter to XSModelingOptions docstring.

---
 armi/physics/neutronics/crossSectionSettings.py | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/armi/physics/neutronics/crossSectionSettings.py b/armi/physics/neutronics/crossSectionSettings.py
index a86771a07..9cf602834 100644
--- a/armi/physics/neutronics/crossSectionSettings.py
+++ b/armi/physics/neutronics/crossSectionSettings.py
@@ -434,7 +434,7 @@ class XSModelingOptions:
         no others will allocate to it. This is useful for time balancing when you
         have one task that takes much longer than the others.
 
-    xsPriority:
+    xsPriority: int
         The priority of the mpi tasks that results from this xsID. Lower priority
         will execute first. starting longer jobs first is generally more efficient.
 
@@ -444,6 +444,15 @@ class XSModelingOptions:
         (e.g., fission products) as a depletion product of an isotope with a much
         smaller atomic number.
 
+    averageByComponent: bool
+        Controls whether the representative block averaging is performed on a
+        component-by-component basis or on the block as a whole. If True, the
+        resulting representative block will have component compositions that
+        largely reflect those of the underlying blocks in the collection. If
+        False, the number densities of some nuclides in the individual
+        components may not be reflective of those of the underlying components
+        due to the block number density "dehomogenization".
+
     minDriverDensity: float
         The minimum number density for nuclides included in driver material for a 1D
         lattice physics model.

From e592fa0f01194a92824c71e33fb7c11e90706576 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Wed, 16 Oct 2024 14:52:38 -0700
Subject: [PATCH 36/41] Add partially heterogeneous example to gallery.

---
 doc/gallery-src/analysis/run_hexBlockToRZConversion.py | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/doc/gallery-src/analysis/run_hexBlockToRZConversion.py b/doc/gallery-src/analysis/run_hexBlockToRZConversion.py
index 78b7cebe5..fd4649930 100644
--- a/doc/gallery-src/analysis/run_hexBlockToRZConversion.py
+++ b/doc/gallery-src/analysis/run_hexBlockToRZConversion.py
@@ -50,6 +50,7 @@
 
 _o, r = test_reactors.loadTestReactor()
 
+# fully heterogeneous
 bFuel = r.core.getBlocks(Flags.FUEL)[0]
 bControl = r.core.getBlocks(Flags.CONTROL)[0]
 converter = blockConverters.HexComponentsToCylConverter(
@@ -57,3 +58,10 @@
 )
 converter.convert()
 converter.plotConvertedBlock()
+
+# partially heterogeneous
+converter = blockConverters.HexComponentsToCylConverter(
+    sourceBlock=bFuel, partiallyHeterogeneous=True
+)
+converter.convert()
+converter.plotConvertedBlock()

From 1726569bd175616bc1c2a1bf3ddc10d2de16d6ec Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Thu, 17 Oct 2024 14:59:45 -0700
Subject: [PATCH 37/41] Change setting name.

---
 armi/physics/neutronics/crossSectionSettings.py | 17 +++++++++--------
 .../latticePhysics/latticePhysicsWriter.py      |  2 +-
 .../tests/test_crossSectionSettings.py          |  6 +++---
 3 files changed, 13 insertions(+), 12 deletions(-)

diff --git a/armi/physics/neutronics/crossSectionSettings.py b/armi/physics/neutronics/crossSectionSettings.py
index 9cf602834..0dee5c06e 100644
--- a/armi/physics/neutronics/crossSectionSettings.py
+++ b/armi/physics/neutronics/crossSectionSettings.py
@@ -59,7 +59,7 @@
 CONF_XS_MAX_ATOM_NUMBER = "xsMaxAtomNumber"
 CONF_MIN_DRIVER_DENSITY = "minDriverDensity"
 CONF_PARTIALLY_HETEROGENEOUS = "partiallyHeterogeneous"
-CONF_SPLIT_TRACE_ISOTOPES = "splitTraceIsotopes"
+CONF_TRACE_ISOTOPE_THRESHOLD = "traceIsotopeThreshold"
 
 
 class XSGeometryTypes(Enum):
@@ -154,7 +154,7 @@ def getStr(cls, typeSpec: Enum):
         CONF_XS_MAX_ATOM_NUMBER,
         CONF_MIN_DRIVER_DENSITY,
         CONF_PARTIALLY_HETEROGENEOUS,
-        CONF_SPLIT_TRACE_ISOTOPES,
+        CONF_TRACE_ISOTOPE_THRESHOLD,
     },
     XSGeometryTypes.getStr(XSGeometryTypes.TWO_DIMENSIONAL_HEX): {
         CONF_XSID,
@@ -202,7 +202,7 @@ def getStr(cls, typeSpec: Enum):
         vol.Optional(CONF_MIN_DRIVER_DENSITY): vol.Coerce(float),
         vol.Optional(CONF_COMPONENT_AVERAGING): bool,
         vol.Optional(CONF_PARTIALLY_HETEROGENEOUS): bool,
-        vol.Optional(CONF_SPLIT_TRACE_ISOTOPES): bool,
+        vol.Optional(CONF_TRACE_ISOTOPE_THRESHOLD): vol.Coerce(float),
     }
 )
 
@@ -462,11 +462,12 @@ class XSModelingOptions:
         heterogeneous approximation for a 1D cylindrical model. Everything inside of the
         duct will be treated as homogeneous.
 
-    splitTraceIsotopes : bool
+    traceIsotopeThreshold : float
         This is a lattice physics configuration option used to enable a separate 0D fuel
         cross section calculation for trace fission products when using a 1D cross section
         model. This can significantly reduce the memory and run time required for the 1D
-        model.
+        model. The setting takes a float value that represents the number density cutoff
+        for isotopes to be considered "trace". If no value is provided, the default is 0.0.
 
     Notes
     -----
@@ -501,7 +502,7 @@ def __init__(
         averageByComponent=False,
         minDriverDensity=0.0,
         partiallyHeterogeneous=False,
-        splitTraceIsotopes=False,
+        traceIsotopeThreshold=0.0,
     ):
         self.xsID = xsID
         self.geometry = geometry
@@ -526,7 +527,7 @@ def __init__(
         self.minDriverDensity = minDriverDensity
         self.averageByComponent = averageByComponent
         self.partiallyHeterogeneous = partiallyHeterogeneous
-        self.splitTraceIsotopes = splitTraceIsotopes
+        self.traceIsotopeThreshold = traceIsotopeThreshold
         # these are related to execution
         self.xsExecuteExclusive = xsExecuteExclusive
         self.xsPriority = xsPriority
@@ -717,7 +718,7 @@ def setDefaults(self, blockRepresentation, validBlockTypes):
                 CONF_BLOCK_REPRESENTATION: crossSectionGroupManager.CYLINDRICAL_COMPONENTS_BLOCK_COLLECTION,
                 CONF_BLOCKTYPES: validBlockTypes,
                 CONF_PARTIALLY_HETEROGENEOUS: False,
-                CONF_SPLIT_TRACE_ISOTOPES: False,
+                CONF_TRACE_ISOTOPE_THRESHOLD: 0.0,
             }
         elif self.geometry == XSGeometryTypes.getStr(
             XSGeometryTypes.TWO_DIMENSIONAL_HEX
diff --git a/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py b/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py
index 3699c2f4c..0009eae2e 100644
--- a/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py
+++ b/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py
@@ -115,7 +115,7 @@ def __init__(
         self.numExternalRings = self.xsSettings.numExternalRings
         self.criticalBucklingSearchActive = self.xsSettings.criticalBuckling
         self.partiallyHeterogeneous = self.xsSettings.partiallyHeterogeneous
-        self.splitTraceIsotopes = self.xsSettings.splitTraceIsotopes
+        self.traceIsotopeThreshold = self.xsSettings.traceIsotopeThreshold
 
         self.executeExclusive = self.xsSettings.xsExecuteExclusive
         self.priority = self.xsSettings.xsPriority
diff --git a/armi/physics/neutronics/tests/test_crossSectionSettings.py b/armi/physics/neutronics/tests/test_crossSectionSettings.py
index 61df3f238..4022f6aff 100644
--- a/armi/physics/neutronics/tests/test_crossSectionSettings.py
+++ b/armi/physics/neutronics/tests/test_crossSectionSettings.py
@@ -119,7 +119,7 @@ def test_homogeneousXsDefaultSettingAssignment(self):
         self.assertEqual(xsModel["YA"].geometry, "0D")
         self.assertEqual(xsModel["YA"].criticalBuckling, True)
         self.assertEqual(xsModel["YA"].partiallyHeterogeneous, False)
-        self.assertEqual(xsModel["YA"].splitTraceIsotopes, False)
+        self.assertEqual(xsModel["YA"].traceIsotopeThreshold, 0.0)
 
     def test_setDefaultSettingsByLowestBuGroupHomogeneous(self):
         # Initialize some micro suffix in the cross sections
@@ -184,7 +184,7 @@ def test_optionalKey(self):
             geometry="1D cylinder",
             meshSubdivisionsPerCm=1.0,
             partiallyHeterogeneous=True,
-            splitTraceIsotopes=True,
+            traceIsotopeThreshold=1.0e-5,
         )
         xsModel["DA"] = da
         xsModel.setDefaults(
@@ -194,7 +194,7 @@ def test_optionalKey(self):
         self.assertEqual(xsModel["DA"].mergeIntoClad, ["gap"])
         self.assertEqual(xsModel["DA"].meshSubdivisionsPerCm, 1.0)
         self.assertEqual(xsModel["DA"].partiallyHeterogeneous, True)
-        self.assertEqual(xsModel["DA"].splitTraceIsotopes, True)
+        self.assertEqual(xsModel["DA"].traceIsotopeThreshold, 1.0e-5)
         self.assertEqual(xsModel["DA"].mergeIntoFuel, [])
 
     def test_badCrossSections(self):

From ed2f318c3808f32d1663d90a07b79e177efbae9e Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Thu, 17 Oct 2024 15:14:27 -0700
Subject: [PATCH 38/41] Change another setting name.

---
 .../physics/neutronics/crossSectionGroupManager.py | 14 +++++++-------
 armi/physics/neutronics/crossSectionSettings.py    | 14 +++++++-------
 .../latticePhysics/latticePhysicsWriter.py         |  4 ++--
 .../neutronics/tests/test_crossSectionSettings.py  |  6 +++---
 armi/reactor/converters/blockConverters.py         | 10 +++++-----
 .../analysis/run_hexBlockToRZConversion.py         |  2 +-
 6 files changed, 25 insertions(+), 25 deletions(-)

diff --git a/armi/physics/neutronics/crossSectionGroupManager.py b/armi/physics/neutronics/crossSectionGroupManager.py
index e139d3f39..9e8972c6f 100644
--- a/armi/physics/neutronics/crossSectionGroupManager.py
+++ b/armi/physics/neutronics/crossSectionGroupManager.py
@@ -653,7 +653,7 @@ def _getNucTempHelper(self):
         return nvt, nv
 
 
-class CylindricalComponentsPartHetAverageBlockCollection(
+class CylindricalComponentsDuctHetAverageBlockCollection(
     CylindricalComponentsAverageBlockCollection
 ):
     """
@@ -684,7 +684,7 @@ class CylindricalComponentsPartHetAverageBlockCollection(
 
     def _getNewBlock(self):
         newBlock = copy.deepcopy(self._selectCandidateBlock())
-        newBlock.name = "1D_CYL_PART_HET_AVG_" + newBlock.getMicroSuffix()
+        newBlock.name = "1D_CYL_DUCT_HET_AVG_" + newBlock.getMicroSuffix()
         return newBlock
 
     def _makeRepresentativeBlock(self):
@@ -1587,8 +1587,8 @@ def updateNuclideTemperatures(self, blockCollectionByXsGroup=None):
 FLUX_WEIGHTED_AVERAGE_BLOCK_COLLECTION = "FluxWeightedAverage"
 SLAB_COMPONENTS_BLOCK_COLLECTION = "ComponentAverage1DSlab"
 CYLINDRICAL_COMPONENTS_BLOCK_COLLECTION = "ComponentAverage1DCylinder"
-CYLINDRICAL_COMPONENTS_PARTIALLY_HET_BLOCK_COLLECTION = (
-    "ComponentAverage1DCylinderPartiallyHeterogeneous"
+CYLINDRICAL_COMPONENTS_DUCT_HET_BLOCK_COLLECTION = (
+    "ComponentAverage1DCylinderDuctHeterogeneous"
 )
 
 # Mapping between block collection string constants and their
@@ -1599,7 +1599,7 @@ def updateNuclideTemperatures(self, blockCollectionByXsGroup=None):
     FLUX_WEIGHTED_AVERAGE_BLOCK_COLLECTION: FluxWeightedAverageBlockCollection,
     SLAB_COMPONENTS_BLOCK_COLLECTION: SlabComponentsAverageBlockCollection,
     CYLINDRICAL_COMPONENTS_BLOCK_COLLECTION: CylindricalComponentsAverageBlockCollection,
-    CYLINDRICAL_COMPONENTS_PARTIALLY_HET_BLOCK_COLLECTION: CylindricalComponentsPartHetAverageBlockCollection,
+    CYLINDRICAL_COMPONENTS_DUCT_HET_BLOCK_COLLECTION: CylindricalComponentsDuctHetAverageBlockCollection,
 }
 
 
@@ -1608,8 +1608,8 @@ def blockCollectionFactory(xsSettings, allNuclidesInProblem):
     blockRepresentation = xsSettings.blockRepresentation
     if (
         blockRepresentation == CYLINDRICAL_COMPONENTS_BLOCK_COLLECTION
-    ) and xsSettings.partiallyHeterogeneous:
-        blockRepresentation = CYLINDRICAL_COMPONENTS_PARTIALLY_HET_BLOCK_COLLECTION
+    ) and xsSettings.ductHeterogeneous:
+        blockRepresentation = CYLINDRICAL_COMPONENTS_DUCT_HET_BLOCK_COLLECTION
     validBlockTypes = xsSettings.validBlockTypes
     averageByComponent = xsSettings.averageByComponent
     return BLOCK_COLLECTIONS[blockRepresentation](
diff --git a/armi/physics/neutronics/crossSectionSettings.py b/armi/physics/neutronics/crossSectionSettings.py
index 0dee5c06e..14a4bc537 100644
--- a/armi/physics/neutronics/crossSectionSettings.py
+++ b/armi/physics/neutronics/crossSectionSettings.py
@@ -58,7 +58,7 @@
 CONF_COMPONENT_AVERAGING = "averageByComponent"
 CONF_XS_MAX_ATOM_NUMBER = "xsMaxAtomNumber"
 CONF_MIN_DRIVER_DENSITY = "minDriverDensity"
-CONF_PARTIALLY_HETEROGENEOUS = "partiallyHeterogeneous"
+CONF_DUCT_HETEROGENEOUS = "ductHeterogeneous"
 CONF_TRACE_ISOTOPE_THRESHOLD = "traceIsotopeThreshold"
 
 
@@ -153,7 +153,7 @@ def getStr(cls, typeSpec: Enum):
         CONF_XS_PRIORITY,
         CONF_XS_MAX_ATOM_NUMBER,
         CONF_MIN_DRIVER_DENSITY,
-        CONF_PARTIALLY_HETEROGENEOUS,
+        CONF_DUCT_HETEROGENEOUS,
         CONF_TRACE_ISOTOPE_THRESHOLD,
     },
     XSGeometryTypes.getStr(XSGeometryTypes.TWO_DIMENSIONAL_HEX): {
@@ -201,7 +201,7 @@ def getStr(cls, typeSpec: Enum):
         vol.Optional(CONF_XS_MAX_ATOM_NUMBER): vol.Coerce(int),
         vol.Optional(CONF_MIN_DRIVER_DENSITY): vol.Coerce(float),
         vol.Optional(CONF_COMPONENT_AVERAGING): bool,
-        vol.Optional(CONF_PARTIALLY_HETEROGENEOUS): bool,
+        vol.Optional(CONF_DUCT_HETEROGENEOUS): bool,
         vol.Optional(CONF_TRACE_ISOTOPE_THRESHOLD): vol.Coerce(float),
     }
 )
@@ -457,7 +457,7 @@ class XSModelingOptions:
         The minimum number density for nuclides included in driver material for a 1D
         lattice physics model.
 
-    partiallyHeterogeneous : bool
+    ductHeterogeneous : bool
         This is a lattice physics configuration option used to enable a partially
         heterogeneous approximation for a 1D cylindrical model. Everything inside of the
         duct will be treated as homogeneous.
@@ -501,7 +501,7 @@ def __init__(
         xsMaxAtomNumber=None,
         averageByComponent=False,
         minDriverDensity=0.0,
-        partiallyHeterogeneous=False,
+        ductHeterogeneous=False,
         traceIsotopeThreshold=0.0,
     ):
         self.xsID = xsID
@@ -526,7 +526,7 @@ def __init__(
         self.xsMaxAtomNumber = xsMaxAtomNumber
         self.minDriverDensity = minDriverDensity
         self.averageByComponent = averageByComponent
-        self.partiallyHeterogeneous = partiallyHeterogeneous
+        self.ductHeterogeneous = ductHeterogeneous
         self.traceIsotopeThreshold = traceIsotopeThreshold
         # these are related to execution
         self.xsExecuteExclusive = xsExecuteExclusive
@@ -717,7 +717,7 @@ def setDefaults(self, blockRepresentation, validBlockTypes):
                 CONF_HOMOGBLOCK: False,
                 CONF_BLOCK_REPRESENTATION: crossSectionGroupManager.CYLINDRICAL_COMPONENTS_BLOCK_COLLECTION,
                 CONF_BLOCKTYPES: validBlockTypes,
-                CONF_PARTIALLY_HETEROGENEOUS: False,
+                CONF_DUCT_HETEROGENEOUS: False,
                 CONF_TRACE_ISOTOPE_THRESHOLD: 0.0,
             }
         elif self.geometry == XSGeometryTypes.getStr(
diff --git a/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py b/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py
index 0009eae2e..68da4be98 100644
--- a/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py
+++ b/armi/physics/neutronics/latticePhysics/latticePhysicsWriter.py
@@ -114,7 +114,7 @@ def __init__(
         self.driverXsID = self.xsSettings.driverID
         self.numExternalRings = self.xsSettings.numExternalRings
         self.criticalBucklingSearchActive = self.xsSettings.criticalBuckling
-        self.partiallyHeterogeneous = self.xsSettings.partiallyHeterogeneous
+        self.ductHeterogeneous = self.xsSettings.ductHeterogeneous
         self.traceIsotopeThreshold = self.xsSettings.traceIsotopeThreshold
 
         self.executeExclusive = self.xsSettings.xsExecuteExclusive
@@ -269,7 +269,7 @@ def _getAllNuclidesByCategory(self, component=None):
                 continue  # skip LFPs here but add individual FPs below.
 
             if isinstance(subjectObject, components.Component):
-                if self.partiallyHeterogeneous and "Homogenized" in subjectObject.name:
+                if self.ductHeterogeneous and "Homogenized" in subjectObject.name:
                     # Nuclide temperatures representing heterogeneous model component temperatures
                     nucTemperatureInC = self._getAvgNuclideTemperatureInC(nucName)
                 else:
diff --git a/armi/physics/neutronics/tests/test_crossSectionSettings.py b/armi/physics/neutronics/tests/test_crossSectionSettings.py
index 4022f6aff..2106ba9b7 100644
--- a/armi/physics/neutronics/tests/test_crossSectionSettings.py
+++ b/armi/physics/neutronics/tests/test_crossSectionSettings.py
@@ -118,7 +118,7 @@ def test_homogeneousXsDefaultSettingAssignment(self):
         self.assertNotIn("YA", xsModel)
         self.assertEqual(xsModel["YA"].geometry, "0D")
         self.assertEqual(xsModel["YA"].criticalBuckling, True)
-        self.assertEqual(xsModel["YA"].partiallyHeterogeneous, False)
+        self.assertEqual(xsModel["YA"].ductHeterogeneous, False)
         self.assertEqual(xsModel["YA"].traceIsotopeThreshold, 0.0)
 
     def test_setDefaultSettingsByLowestBuGroupHomogeneous(self):
@@ -183,7 +183,7 @@ def test_optionalKey(self):
             "DA",
             geometry="1D cylinder",
             meshSubdivisionsPerCm=1.0,
-            partiallyHeterogeneous=True,
+            ductHeterogeneous=True,
             traceIsotopeThreshold=1.0e-5,
         )
         xsModel["DA"] = da
@@ -193,7 +193,7 @@ def test_optionalKey(self):
         )
         self.assertEqual(xsModel["DA"].mergeIntoClad, ["gap"])
         self.assertEqual(xsModel["DA"].meshSubdivisionsPerCm, 1.0)
-        self.assertEqual(xsModel["DA"].partiallyHeterogeneous, True)
+        self.assertEqual(xsModel["DA"].ductHeterogeneous, True)
         self.assertEqual(xsModel["DA"].traceIsotopeThreshold, 1.0e-5)
         self.assertEqual(xsModel["DA"].mergeIntoFuel, [])
 
diff --git a/armi/reactor/converters/blockConverters.py b/armi/reactor/converters/blockConverters.py
index 99ff3d666..0e6205a48 100644
--- a/armi/reactor/converters/blockConverters.py
+++ b/armi/reactor/converters/blockConverters.py
@@ -508,7 +508,7 @@ class HexComponentsToCylConverter(BlockAvgToCylConverter):
     duct/intercoolant pinComponentsRing1 | coolant | pinComponentsRing2 | coolant | ... |
     nonpins ...
 
-    The ``partiallyHeterogeneous`` option allows the user to treat everything inside the duct
+    The ``ductHeterogeneous`` option allows the user to treat everything inside the duct
     as a single homogenized composition. This could significantly reduce the memory and runtime
     required for the lattice physics solver, and also provide an alternative approximation for
     the spatial self-shielding effect on microscopic cross sections.
@@ -525,7 +525,7 @@ def __init__(
         numExternalRings=None,
         mergeIntoClad=None,
         mergeIntoFuel=None,
-        partiallyHeterogeneous=False,
+        ductHeterogeneous=False,
     ):
         BlockAvgToCylConverter.__init__(
             self,
@@ -556,7 +556,7 @@ def __init__(
         self.pinPitch = sourceBlock.getPinPitch()
         self.mergeIntoClad = mergeIntoClad or []
         self.mergeIntoFuel = mergeIntoFuel or []
-        self.partiallyHeterogeneous = partiallyHeterogeneous
+        self.ductHeterogeneous = ductHeterogeneous
         self.interRingComponent = sourceBlock.getComponent(Flags.COOLANT, exact=True)
         self._remainingCoolantFillArea = self.interRingComponent.getArea()
         if not self.interRingComponent:
@@ -592,7 +592,7 @@ def convert(self):
             self._sourceBlock.getNumPins()
         )
         pinComponents, nonPins = self._classifyComponents()
-        if self.partiallyHeterogeneous:
+        if self.ductHeterogeneous:
             self._buildInsideDuct()
         else:
             self._buildFirstRing(pinComponents)
@@ -728,7 +728,7 @@ def _buildNonPinRings(self, nonPins):
         Also needs to add final coolant layer between the outer pins and the non-pins.
         Will crash if there are things that are not circles or hexes.
         """
-        if not self.partiallyHeterogeneous:
+        if not self.ductHeterogeneous:
             # fill in the last ring of coolant using the rest
             coolInnerDiam = self.convertedBlock[-1].getDimension("od")
             coolantOD = getOuterDiamFromIDAndArea(
diff --git a/doc/gallery-src/analysis/run_hexBlockToRZConversion.py b/doc/gallery-src/analysis/run_hexBlockToRZConversion.py
index fd4649930..2f06455a8 100644
--- a/doc/gallery-src/analysis/run_hexBlockToRZConversion.py
+++ b/doc/gallery-src/analysis/run_hexBlockToRZConversion.py
@@ -61,7 +61,7 @@
 
 # partially heterogeneous
 converter = blockConverters.HexComponentsToCylConverter(
-    sourceBlock=bFuel, partiallyHeterogeneous=True
+    sourceBlock=bFuel, ductHeterogeneous=True
 )
 converter.convert()
 converter.plotConvertedBlock()

From 72151b6dfec5a5f90e3c9398a19c0c3c8f21b326 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Thu, 24 Oct 2024 09:06:42 -0700
Subject: [PATCH 39/41] Update docstring.

---
 armi/reactor/converters/blockConverters.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/armi/reactor/converters/blockConverters.py b/armi/reactor/converters/blockConverters.py
index 0e6205a48..b72804f2f 100644
--- a/armi/reactor/converters/blockConverters.py
+++ b/armi/reactor/converters/blockConverters.py
@@ -880,7 +880,9 @@ def stripComponents(block, compFlags):
     Returns
     -------
     newBlock : armi.reactor.blocks.Block
-        Copy of source block with specified components stripped off
+        Copy of source block with specified components stripped off.
+    mixtureFlags : TypeSpec
+        Combination of all component flags within newBlock.
 
     Notes
     -----

From c9dcecc7bfceef9b1fb3e4157067592a8e9eefd0 Mon Sep 17 00:00:00 2001
From: Michael Jarrett <mjarrett@terrapower.com>
Date: Wed, 30 Oct 2024 11:57:10 -0700
Subject: [PATCH 40/41] Update release notes.

---
 doc/release/0.4.rst | 1 +
 1 file changed, 1 insertion(+)

diff --git a/doc/release/0.4.rst b/doc/release/0.4.rst
index 1269218d7..5d7e862c5 100644
--- a/doc/release/0.4.rst
+++ b/doc/release/0.4.rst
@@ -25,6 +25,7 @@ New Features
 #. Provide ``Block.getInputHeight`` for determining the height of a block from blueprints. (`PR#1927 <https://github.com/terrapower/armi/pull/1927>`_)
 #. Improve performance by changing the lattice physics interface so that cross sections are not updated on ``everyNode`` calls during coupled calculations (`PR#1963 <https://github.com/terrapower/armi/pull/1963>`_)
 #. Improve efficiency of reaction rate calculations. (`PR#1887 <https://github.com/terrapower/armi/pull/1887>`_)
+#. Add some new options for simplification of 1D cross section modeling. (`PR#1949 <https://github.com/terrapower/armi/pull/1949>`_)
 #. TBD
 
 API Changes

From 4d5631be61d741bae5dea20ccfda8fa99d346575 Mon Sep 17 00:00:00 2001
From: John Stilley <1831479+john-science@users.noreply.github.com>
Date: Wed, 30 Oct 2024 14:02:22 -0700
Subject: [PATCH 41/41] Update doc/release/0.4.rst

---
 doc/release/0.4.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/release/0.4.rst b/doc/release/0.4.rst
index 5d7e862c5..84bd6f38a 100644
--- a/doc/release/0.4.rst
+++ b/doc/release/0.4.rst
@@ -25,7 +25,7 @@ New Features
 #. Provide ``Block.getInputHeight`` for determining the height of a block from blueprints. (`PR#1927 <https://github.com/terrapower/armi/pull/1927>`_)
 #. Improve performance by changing the lattice physics interface so that cross sections are not updated on ``everyNode`` calls during coupled calculations (`PR#1963 <https://github.com/terrapower/armi/pull/1963>`_)
 #. Improve efficiency of reaction rate calculations. (`PR#1887 <https://github.com/terrapower/armi/pull/1887>`_)
-#. Add some new options for simplification of 1D cross section modeling. (`PR#1949 <https://github.com/terrapower/armi/pull/1949>`_)
+#. Adding new options for simplifying 1D cross section modeling. (`PR#1949 <https://github.com/terrapower/armi/pull/1949>`_)
 #. TBD
 
 API Changes