From 5c38b1552849bdd2df0169219aff98d44b75a409 Mon Sep 17 00:00:00 2001
From: Patrick Shriwise <pshriwise@gmail.com>
Date: Tue, 7 Oct 2025 17:34:42 -0500
Subject: [PATCH 1/6] Reset DAGMC history when reviving from source.

---
 src/particle.cpp | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/particle.cpp b/src/particle.cpp
index 402af2498a5..d8967b55029 100644
--- a/src/particle.cpp
+++ b/src/particle.cpp
@@ -151,6 +151,10 @@ void Particle::from_source(const SourceSite* src)
     int index_plus_one = model::surface_map[std::abs(src->surf_id)] + 1;
     surface() = (src->surf_id > 0) ? index_plus_one : -index_plus_one;
   }
+
+#ifdef OPENMC_DAGMC_ENABLED
+  history().reset();
+#endif
 }
 
 void Particle::event_calculate_xs()

From 676a073485319bfc3a0a92234b18441098c0be5c Mon Sep 17 00:00:00 2001
From: Paul Romano <paul.k.romano@gmail.com>
Date: Tue, 14 Oct 2025 07:53:33 -0500
Subject: [PATCH 2/6] Move history reset up

---
 src/particle.cpp | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/src/particle.cpp b/src/particle.cpp
index d8967b55029..bf7d79d7db5 100644
--- a/src/particle.cpp
+++ b/src/particle.cpp
@@ -121,6 +121,9 @@ void Particle::from_source(const SourceSite* src)
   fission() = false;
   zero_flux_derivs();
   lifetime() = 0.0;
+#ifdef OPENMC_DAGMC_ENABLED
+  history().reset();
+#endif
 
   // Copy attributes from source bank site
   type() = src->particle;
@@ -151,10 +154,6 @@ void Particle::from_source(const SourceSite* src)
     int index_plus_one = model::surface_map[std::abs(src->surf_id)] + 1;
     surface() = (src->surf_id > 0) ? index_plus_one : -index_plus_one;
   }
-
-#ifdef OPENMC_DAGMC_ENABLED
-  history().reset();
-#endif
 }
 
 void Particle::event_calculate_xs()

From c951e1a101715b9f4faa0d4b5eab4cafeacb04f1 Mon Sep 17 00:00:00 2001
From: Patrick Shriwise <pshriwise@gmail.com>
Date: Mon, 20 Oct 2025 23:02:16 -0500
Subject: [PATCH 3/6] Adding simple test for ensuring particle splitting near a
 boundary doesn't cause a problem in DAGMC models

---
 .../dagmc/weight_windows/model.xml            |  58 ++++++++++++++++++
 .../weight_windows/nested_shell_geometry.h5m  | Bin 0 -> 58352 bytes
 .../dagmc/weight_windows/test.py              |  33 ++++++++++
 3 files changed, 91 insertions(+)
 create mode 100644 tests/regression_tests/dagmc/weight_windows/model.xml
 create mode 100644 tests/regression_tests/dagmc/weight_windows/nested_shell_geometry.h5m
 create mode 100644 tests/regression_tests/dagmc/weight_windows/test.py

diff --git a/tests/regression_tests/dagmc/weight_windows/model.xml b/tests/regression_tests/dagmc/weight_windows/model.xml
new file mode 100644
index 00000000000..2986a89a492
--- /dev/null
+++ b/tests/regression_tests/dagmc/weight_windows/model.xml
@@ -0,0 +1,58 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1" name="shell">
+      <density value="7.8" units="g/cc"/>
+      <nuclide name="W180" ao="0.0012"/>
+      <nuclide name="W182" ao="0.265"/>
+      <nuclide name="W183" ao="0.1431"/>
+      <nuclide name="W184" ao="0.3064"/>
+      <nuclide name="W186" ao="0.2843"/>
+    </material>
+  </materials>
+  <geometry>
+    <dagmc_universe id="1" filename="nested_shell_geometry.h5m"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>500</particles>
+    <batches>2</batches>
+    <max_lost_particles>1</max_lost_particles>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="point">
+        <parameters>0.0 0.0 0.0</parameters>
+      </space>
+      <angle type="isotropic"/>
+      <energy type="discrete">
+        <parameters>14100000.0 1.0</parameters>
+      </energy>
+    </source>
+    <output>
+      <tallies>false</tallies>
+      <summary>false</summary>
+    </output>
+    <survival_biasing>false</survival_biasing>
+    <weight_windows id="1">
+      <mesh>10003</mesh>
+      <particle_type>neutron</particle_type>
+      <energy_bounds>0.0 1.7976931348623157e+308</energy_bounds>
+      <lower_ww_bounds>0.030750733294361156 0.056110505674355333 0.08187875047968339 0.1101743496347699 0.13982370013053508 0.17443799246829372 0.21576286623367483 0.26416659508033646 0.318574932646899 0.3804031702117963 0.42899359749256355 0.4954283294279403 0.49999999999999994 0.43432341070872266 0.38302303850488206 0.32148375935490886 0.2637416945702018 0.21498369367288853 0.17163611765361744 0.13832102142074995 0.10717772257151495 0.07986176041282561 0.05499644859408233 0.03058023506703803</lower_ww_bounds>
+      <upper_ww_bounds>0.15375366647180577 0.2805525283717767 0.40939375239841697 0.5508717481738494 0.6991185006526754 0.8721899623414686 1.0788143311683742 1.3208329754016823 1.592874663234495 1.9020158510589815 2.1449679874628176 2.4771416471397014 2.4999999999999996 2.1716170535436135 1.9151151925244103 1.6074187967745444 1.318708472851009 1.0749184683644426 0.8581805882680872 0.6916051071037498 0.5358886128575747 0.39930880206412805 0.27498224297041163 0.15290117533519015</upper_ww_bounds>
+      <survival_ratio>3.0</survival_ratio>
+      <max_lower_bound_ratio>1.0</max_lower_bound_ratio>
+      <max_split>10</max_split>
+      <weight_cutoff>1e-38</weight_cutoff>
+    </weight_windows>
+    <mesh id="10003" name="ww_mesh" type="rectilinear">
+      <x_grid>-60.0 -55.0 -50.0 -45.0 -40.0 -35.0 -30.0 -25.0 -20.0 -15.0 -10.0 -5.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0 60.0</x_grid>
+      <y_grid>-60.0 60.0</y_grid>
+      <z_grid>-60.0 60.0</z_grid>
+    </mesh>
+    <weight_windows_on>true</weight_windows_on>
+    <weight_window_checkpoints>
+      <collision>true</collision>
+      <surface>true</surface>
+    </weight_window_checkpoints>
+    <max_history_splits>10000000</max_history_splits>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/dagmc/weight_windows/nested_shell_geometry.h5m b/tests/regression_tests/dagmc/weight_windows/nested_shell_geometry.h5m
new file mode 100644
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diff --git a/tests/regression_tests/dagmc/weight_windows/test.py b/tests/regression_tests/dagmc/weight_windows/test.py
new file mode 100644
index 00000000000..bb4cfd001a7
--- /dev/null
+++ b/tests/regression_tests/dagmc/weight_windows/test.py
@@ -0,0 +1,33 @@
+from tests.testing_harness import TestHarness
+
+import openmc
+
+class DagmcWeightWindowsTestHarness(TestHarness):
+
+    def __init__(self):
+        super().__init__('')
+
+    def execute_test(self):
+        # this test merely needs to run OpenMC without error
+        try:
+            self._run_openmc()
+        finally:
+            self._cleanup()
+
+
+def test_dagmc_weight_windows_near_boundary():
+    """A regression test that ensures particle splitting near a boundary does
+    not result in lost particles due to a stale DAGMC history on the particle
+    object"""
+
+    # This DAGMC model consists of three nested cubes. The innermost cube
+    # contains a fusion neutron source. The two outer cubes are filled with
+    # tungsten. The entire model has weight windows defined on a mesh such that
+    # particles will be split as they move outward from the source. The
+    # outermost cubes are very similary in size, the outer cube is just slightly
+    # larger than the inner cube. This means that particles moving outward will
+    # frequently cross the boundary between the two cubes right after being
+    # split. This test ensures that no particles are lost due to a stale DAGMC
+    # history after splitting.
+    harness = DagmcWeightWindowsTestHarness()
+    harness.main()

From 52bc49954783dd83437ae9a136a763c92706f3cf Mon Sep 17 00:00:00 2001
From: Patrick Shriwise <pshriwise@gmail.com>
Date: Mon, 20 Oct 2025 23:13:24 -0500
Subject: [PATCH 4/6] Adding init file for pytest

---
 tests/regression_tests/dagmc/weight_windows/__init__.py | 0
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 tests/regression_tests/dagmc/weight_windows/__init__.py

diff --git a/tests/regression_tests/dagmc/weight_windows/__init__.py b/tests/regression_tests/dagmc/weight_windows/__init__.py
new file mode 100644
index 00000000000..e69de29bb2d

From 2031255f5dff3c765734d36a28bae1effed399f9 Mon Sep 17 00:00:00 2001
From: Patrick Shriwise <pshriwise@gmail.com>
Date: Tue, 21 Oct 2025 09:35:08 -0500
Subject: [PATCH 5/6] Skip test if DAGMC is not enabled

---
 tests/regression_tests/dagmc/weight_windows/test.py | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/tests/regression_tests/dagmc/weight_windows/test.py b/tests/regression_tests/dagmc/weight_windows/test.py
index bb4cfd001a7..7f2fe00b7ef 100644
--- a/tests/regression_tests/dagmc/weight_windows/test.py
+++ b/tests/regression_tests/dagmc/weight_windows/test.py
@@ -2,6 +2,13 @@
 
 import openmc
 
+import pytest
+
+pytestmark = pytest.mark.skipif(
+    not openmc.lib._dagmc_enabled(),
+    reason="DAGMC CAD geometry is not enabled.")
+
+
 class DagmcWeightWindowsTestHarness(TestHarness):
 
     def __init__(self):

From bcc27f5eb6f5da4a70bd4e1d8d90b7b46ef22754 Mon Sep 17 00:00:00 2001
From: Patrick Shriwise <pshriwise@gmail.com>
Date: Fri, 7 Nov 2025 15:45:30 -0600
Subject: [PATCH 6/6] Updating DAGMC model to avoid a point in volume issue
 with it. The current set of weight windows still elicits the bug we're
 concerned with here.

---
 .../weight_windows/nested_shell_geometry.h5m  | Bin 58352 -> 58680 bytes
 1 file changed, 0 insertions(+), 0 deletions(-)

diff --git a/tests/regression_tests/dagmc/weight_windows/nested_shell_geometry.h5m b/tests/regression_tests/dagmc/weight_windows/nested_shell_geometry.h5m
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