From 3247587d4920fa3c9a4e3e3da79adc20e23eb1f7 Mon Sep 17 00:00:00 2001
From: Paul Romano <paul.k.romano@gmail.com>
Date: Thu, 2 Jul 2026 11:54:12 -0500
Subject: [PATCH] Ensure photon cross sections are loaded when FileSource
 contains photons (#3988)

---
 src/source.cpp                       |  9 ++++++++-
 tests/unit_tests/test_source_file.py | 24 ++++++++++++++++++++++++
 2 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/src/source.cpp b/src/source.cpp
index bee20d5c3..1e783b623 100644
--- a/src/source.cpp
+++ b/src/source.cpp
@@ -523,9 +523,16 @@ void FileSource::load_sites_from_file(const std::string& path)
     file_close(file_id);
   }
 
-  // Make sure particles in source file have valid types
+  // Make sure particles in source file have valid types. If any particle is a
+  // photon, electron, or positron, enable photon transport so that the
+  // appropriate cross sections are loaded.
   for (const auto& site : this->sites_) {
     validate_particle_type(site.particle, "FileSource");
+    if (site.particle == ParticleType::photon() ||
+        site.particle == ParticleType::electron() ||
+        site.particle == ParticleType::positron()) {
+      settings::photon_transport = true;
+    }
   }
 }
 
diff --git a/tests/unit_tests/test_source_file.py b/tests/unit_tests/test_source_file.py
index 3e84fbe51..f19ea74a6 100644
--- a/tests/unit_tests/test_source_file.py
+++ b/tests/unit_tests/test_source_file.py
@@ -145,3 +145,27 @@ def test_source_file_transport(run_in_tmpdir):
 
     # Try running OpenMC
     model.run()
+
+
+def test_source_file_photon_transport(run_in_tmpdir):
+    # Create a source file containing a photon. Note that photon_transport is
+    # not explicitly enabled in the settings -- it should be turned on
+    # automatically because the source file contains a photon.
+    particle = openmc.SourceParticle(E=1.0e6, particle='photon')
+    openmc.write_source_file([particle], 'photon_source.h5')
+
+    # Create simple model to use the photon source file
+    model = openmc.Model()
+    al = openmc.Material()
+    al.add_element('Al', 1.0)
+    al.set_density('g/cm3', 2.7)
+    sph = openmc.Sphere(r=10.0, boundary_type='vacuum')
+    cell = openmc.Cell(fill=al, region=-sph)
+    model.geometry = openmc.Geometry([cell])
+    model.settings.source = openmc.FileSource(path='photon_source.h5')
+    model.settings.particles = 10
+    model.settings.batches = 3
+    model.settings.run_mode = 'fixed source'
+
+    # Running OpenMC should succeed
+    model.run()