openmc-dev · paulromano · Feb 14, 2026 · Feb 3, 2026 · Feb 3, 2026 · Feb 3, 2026
diff --git a/include/openmc/atomic_mass.h b/include/openmc/atomic_mass.h
diff --git a/include/openmc/constants.h b/include/openmc/constants.h
@@ -9,6 +9,7 @@
 #include <limits>
 
 #include "openmc/array.h"
+#include "openmc/atomic_mass.h"
 #include "openmc/vector.h"
 #include "openmc/version.h"
 
@@ -86,12 +87,12 @@ constexpr double INFTY {std::numeric_limits<double>::max()};
 // (CODATA) 2018 recommendation (https://physics.nist.gov/cuu/Constants/).
 
 // Physical constants
-constexpr double MASS_NEUTRON {1.00866491595}; // mass of a neutron in amu
+constexpr double AMU_EV {
+  9.3149410242e8}; // atomic mass unit energy equivalent in eV/c^2
 constexpr double MASS_NEUTRON_EV {
-  939.56542052e6};                             // mass of a neutron in eV/c^2
-constexpr double MASS_PROTON {1.007276466621}; // mass of a proton in amu
+  MASS_NEUTRON * AMU_EV}; // neutron mass energy equivalent in eV/c^2
 constexpr double MASS_ELECTRON_EV {
-  0.51099895000e6}; // electron mass energy equivalent in eV/c^2
+  MASS_ELECTRON * AMU_EV}; // electron mass energy equivalent in eV/c^2
 constexpr double FINE_STRUCTURE {
   137.035999084}; // inverse fine structure constant
 constexpr double PLANCK_C {

diff --git a/include/openmc/particle_type.h b/include/openmc/particle_type.h
@@ -12,6 +12,7 @@
 #include <type_traits>
 
 #include "openmc/constants.h"
+#include "openmc/error.h"
 
 namespace openmc {
 
@@ -61,6 +62,17 @@ class ParticleType {
   //----------------------------------------------------------------------------
   // Methods
 
+  // Get particle mass
+  double mass() const
+  {
+    int32_t p = std::abs(pdg_number_);
+    if (atomic_mass.count(p)) {
+      return atomic_mass[p];
+    } else {
+      fatal_error("Unknown mass for particle " + str());
+    }
+  }
+
   // Convert to string representation
   std::string str() const;
 

diff --git a/src/particle.cpp b/src/particle.cpp
@@ -47,21 +47,7 @@ double Particle::speed() const
 {
   if (settings::run_CE) {
     // Determine mass in eV/c^2
-    double mass;
-    switch (this->type().pdg_number()) {
-    case PDG_NEUTRON:
-      mass = MASS_NEUTRON_EV;
-      break;
-    case PDG_PHOTON:
-      mass = 0.0;
-      break;
-    case PDG_ELECTRON:
-    case PDG_POSITRON:
-      mass = MASS_ELECTRON_EV;
-      break;
-    default:
-      fatal_error("Unsupported particle for speed calculation.");
-    }
+    double mass = this->type().mass() * AMU_EV;
     // Equivalent to C * sqrt(1-(m/(m+E))^2) without problem at E<<m:
     return C_LIGHT * std::sqrt(this->E() * (this->E() + 2 * mass)) /
            (this->E() + mass);

diff --git a/tests/regression_tests/cmfd_feed_2g/results_true.dat b/tests/regression_tests/cmfd_feed_2g/results_true.dat
@@ -1,5 +1,5 @@
 k-combined:
-1.027434E+00 6.509170E-03
+1.027434E+00 6.509157E-03
 tally 1:
 1.162758E+02
 1.352562E+03

diff --git a/tests/regression_tests/cmfd_feed_ng/results_true.dat b/tests/regression_tests/cmfd_feed_ng/results_true.dat
@@ -1,5 +1,5 @@
 k-combined:
-1.008852E+00 9.028695E-03
+1.008852E+00 9.028696E-03
 tally 1:
 1.151271E+02
 1.325871E+03

diff --git a/tests/regression_tests/deplete_no_transport/test_reference_coupled_days.h5 b/tests/regression_tests/deplete_no_transport/test_reference_coupled_days.h5
diff --git a/tests/regression_tests/deplete_no_transport/test_reference_coupled_hours.h5 b/tests/regression_tests/deplete_no_transport/test_reference_coupled_hours.h5
diff --git a/tests/regression_tests/deplete_no_transport/test_reference_coupled_minutes.h5 b/tests/regression_tests/deplete_no_transport/test_reference_coupled_minutes.h5
diff --git a/tests/regression_tests/deplete_no_transport/test_reference_coupled_months.h5 b/tests/regression_tests/deplete_no_transport/test_reference_coupled_months.h5
diff --git a/tests/regression_tests/deplete_no_transport/test_reference_fission_q.h5 b/tests/regression_tests/deplete_no_transport/test_reference_fission_q.h5
diff --git a/tests/regression_tests/deplete_no_transport/test_reference_source_rate.h5 b/tests/regression_tests/deplete_no_transport/test_reference_source_rate.h5
diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_ext_source.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_ext_source.h5
diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_feed.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_feed.h5
diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_redox.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_redox.h5
diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_removal.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_removal.h5
diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_removal_and_redox.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_removal_and_redox.h5
diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_transfer.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_transfer.h5
diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_transfer_and_redox.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_transfer_and_redox.h5
diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_only_feed.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_only_feed.h5
diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_only_removal.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_only_removal.h5
diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_with_ext_source.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_with_ext_source.h5
diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_with_transfer.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_with_transfer.h5
diff --git a/tests/regression_tests/deplete_with_transport/last_step_reference_materials.xml b/tests/regression_tests/deplete_with_transport/last_step_reference_materials.xml
diff --git a/tests/regression_tests/deplete_with_transport/test_reference.h5 b/tests/regression_tests/deplete_with_transport/test_reference.h5
diff --git a/tests/regression_tests/filter_mesh/results_true.dat b/tests/regression_tests/filter_mesh/results_true.dat
@@ -1 +1 @@
-e07ed2bc8893c69721abf61b123336f1f6128a3bee6ec63b84d1f549f31707a74a6ce885091ccc0eac6b7f16f7cab39ede4784584c08825829e108de878ea5fb
+749d980164f4b587cd4af455c8fb214d26dbe8c14a114dc5faa96ca95f9198199e55fb86b1eb0d4038b41bbbc9dfb6d754fb21b1b655ba396530edfacbe51a54
diff --git a/tests/regression_tests/microxs/test_reference_materials_direct.csv b/tests/regression_tests/microxs/test_reference_materials_direct.csv
@@ -1,7 +1,7 @@
 nuclides,reactions,groups,xs
-U235,"(n,gamma)",1,0.1475718536187164
-U235,fission,1,1.2504996049257149
-O16,"(n,gamma)",1,0.00010981236259441559
+U235,"(n,gamma)",1,0.14757185361871647
+U235,fission,1,1.2504996049257133
+O16,"(n,gamma)",1,0.00010981236259441548
 O16,fission,1,0.0
-Xe135,"(n,gamma)",1,0.014570546772870611
+Xe135,"(n,gamma)",1,0.014570546772870624
 Xe135,fission,1,0.0
diff --git a/tests/regression_tests/microxs/test_reference_materials_flux.csv b/tests/regression_tests/microxs/test_reference_materials_flux.csv
@@ -1,7 +1,7 @@
 nuclides,reactions,groups,xs
-U235,"(n,gamma)",1,0.15003016703758473
-U235,fission,1,1.2646269005413537
-O16,"(n,gamma)",1,0.00012069778439640301
+U235,"(n,gamma)",1,0.15003016703758476
+U235,fission,1,1.2646269005413535
+O16,"(n,gamma)",1,0.00012069778439640315
 O16,fission,1,0.0
 Xe135,"(n,gamma)",1,0.014820264774863562
 Xe135,fission,1,0.0
diff --git a/tests/regression_tests/microxs/test_reference_mesh_direct.csv b/tests/regression_tests/microxs/test_reference_mesh_direct.csv
@@ -1,7 +1,7 @@
 nuclides,reactions,groups,xs
-U235,"(n,gamma)",1,0.14757185361871633
-U235,fission,1,1.2504996049257142
-O16,"(n,gamma)",1,0.0001098123625944155
+U235,"(n,gamma)",1,0.14757185361871636
+U235,fission,1,1.250499604925713
+O16,"(n,gamma)",1,0.00010981236259441542
 O16,fission,1,0.0
-Xe135,"(n,gamma)",1,0.0145705467728706
+Xe135,"(n,gamma)",1,0.014570546772870606
 Xe135,fission,1,0.0
diff --git a/tests/regression_tests/microxs/test_reference_mesh_flux.csv b/tests/regression_tests/microxs/test_reference_mesh_flux.csv
@@ -1,7 +1,7 @@
 nuclides,reactions,groups,xs
 U235,"(n,gamma)",1,0.15003016703758473
-U235,fission,1,1.2646269005413537
-O16,"(n,gamma)",1,0.00012069778439640301
+U235,fission,1,1.2646269005413535
+O16,"(n,gamma)",1,0.00012069778439640312
 O16,fission,1,0.0
-Xe135,"(n,gamma)",1,0.014820264774863562
+Xe135,"(n,gamma)",1,0.01482026477486356
 Xe135,fission,1,0.0
diff --git a/tests/regression_tests/pulse_height/results_true.dat b/tests/regression_tests/pulse_height/results_true.dat
@@ -197,5 +197,5 @@ tally 1:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-2.000000E-01
-8.600000E-03
+1.900000E-01
+7.500000E-03
diff --git a/tests/regression_tests/random_ray_adjoint_k_eff/results_true.dat b/tests/regression_tests/random_ray_adjoint_k_eff/results_true.dat
@@ -79,7 +79,7 @@ tally 1:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-9.248400E-01
+9.248399E-01
 1.710699E-01
 0.000000E+00
 0.000000E+00
@@ -148,7 +148,7 @@ tally 1:
 9.272977E-01
 1.721077E-01
 3.161443E-02
-2.000179E-04
+2.000180E-04
 2.002789E-07
 8.027290E-15
 8.778794E-01

diff --git a/tests/regression_tests/random_ray_halton_samples/results_true.dat b/tests/regression_tests/random_ray_halton_samples/results_true.dat
@@ -24,7 +24,7 @@ tally 1:
 1.707007E-02
 5.933921E-05
 4.154514E-02
-3.514889E-04
+3.514888E-04
 1.513147E+00
 4.652938E-01
 1.810962E-02
@@ -34,7 +34,7 @@ tally 1:
 4.347893E+00
 3.798064E+00
 6.851785E-03
-9.430195E-06
+9.430194E-06
 1.695426E-02
 5.773923E-05
 2.809071E+00
@@ -152,7 +152,7 @@ tally 1:
 4.036663E-02
 3.323832E-04
 1.516801E+00
-4.675245E-01
+4.675244E-01
 1.752096E-02
 6.241641E-05
 4.264304E-02

diff --git a/tests/regression_tests/random_ray_k_eff/results_true.dat b/tests/regression_tests/random_ray_k_eff/results_true.dat
@@ -1,5 +1,5 @@
 k-combined:
-8.400321E-01 8.023357E-03
+8.400321E-01 8.023358E-03
 tally 1:
 1.075769E+00
 2.317354E-01
@@ -28,7 +28,7 @@ tally 1:
 1.509054E+00
 4.629325E-01
 1.749679E-02
-6.226587E-05
+6.226588E-05
 4.258421E-02
 3.688336E-04
 4.322054E+00
@@ -131,7 +131,7 @@ tally 1:
 2.304093E-01
 4.053505E-01
 3.291277E-02
-9.865420E-01
+9.865421E-01
 1.949549E-01
 6.047420E-01
 7.327008E-02

diff --git a/tests/regression_tests/random_ray_k_eff_mesh/results_true.dat b/tests/regression_tests/random_ray_k_eff_mesh/results_true.dat
@@ -1,5 +1,5 @@
 k-combined:
-8.379203E-01 8.057199E-03
+8.379203E-01 8.057198E-03
 tally 1:
 1.073897E+00
 2.309328E-01

diff --git a/tests/regression_tests/random_ray_linear/linear_xy/results_true.dat b/tests/regression_tests/random_ray_linear/linear_xy/results_true.dat
@@ -90,7 +90,7 @@ tally 1:
 1.746692E+00
 1.536860E-01
 4.251098E+00
-9.103405E-01
+9.103406E-01
 3.651202E+00
 6.682179E-01
 5.431675E-01

diff --git a/tests/regression_tests/surface_source/surface_source_true.h5 b/tests/regression_tests/surface_source/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source/surface_source_true.mcpl b/tests/regression_tests/surface_source/surface_source_true.mcpl
diff --git a/tests/regression_tests/surface_source_write/case-01/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-01/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-02/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-02/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-03/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-03/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-04/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-04/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-05/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-05/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-06/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-06/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-07/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-07/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-08/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-08/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-09/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-09/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-10/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-10/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-11/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-11/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-12/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-12/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-13/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-13/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-14/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-14/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-15/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-15/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-16/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-16/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-17/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-17/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-18/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-18/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-19/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-19/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-20/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-20/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-21/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-21/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-a01/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-a01/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-d01/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-d01/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-d02/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-d02/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-d03/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-d03/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-d04/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-d04/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-d05/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-d05/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-d06/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-d06/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-d07/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-d07/surface_source_true.h5
diff --git a/tests/regression_tests/surface_source_write/case-d08/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-d08/surface_source_true.h5
diff --git a/tests/regression_tests/triso/results_true.dat b/tests/regression_tests/triso/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-1.604832E+00 2.031393E-03
+1.604832E+00 2.031398E-03
Original file line number	Diff line number	Diff line change
		@@ -1 +1 @@
		e07ed2bc8893c69721abf61b123336f1f6128a3bee6ec63b84d1f549f31707a74a6ce885091ccc0eac6b7f16f7cab39ede4784584c08825829e108de878ea5fb
		749d980164f4b587cd4af455c8fb214d26dbe8c14a114dc5faa96ca95f9198199e55fb86b1eb0d4038b41bbbc9dfb6d754fb21b1b655ba396530edfacbe51a54