Determine whether nuclides are fissionable in volume calc mode (#3047)

Co-authored-by: Patrick Shriwise <[email protected]>

src/nuclide.cpp

@@ -63,6 +63,19 @@ Nuclide::Nuclide(hid_t group, const vector<double>& temperature)
   read_attribute(group, "atomic_weight_ratio", awr_);
 
   if (settings::run_mode == RunMode::VOLUME) {
+    // Determine whether nuclide is fissionable and then exit
+    int mt;
+    hid_t rxs_group = open_group(group, "reactions");
+    for (auto name : group_names(rxs_group)) {
+      if (starts_with(name, "reaction_")) {
+        hid_t rx_group = open_group(rxs_group, name.c_str());
+        read_attribute(rx_group, "mt", mt);
+        if (is_fission(mt)) {
+          fissionable_ = true;
+          break;
+        }
+      }
+    }
     return;
   }
 

tests/unit_tests/test_lib.py

@@ -963,7 +963,8 @@ def test_sample_external_source(run_in_tmpdir, mpi_intracomm):
     model.settings.source = openmc.IndependentSource(
         space=openmc.stats.Box([-5., -5., -5.], [5., 5., 5.]),
         angle=openmc.stats.Monodirectional((0., 0., 1.)),
-        energy=openmc.stats.Discrete([1.0e5], [1.0])
+        energy=openmc.stats.Discrete([1.0e5], [1.0]),
+        constraints={'fissionable': True}
     )
     model.settings.particles = 1000
     model.settings.batches = 10
@@ -993,3 +994,8 @@ def test_sample_external_source(run_in_tmpdir, mpi_intracomm):
         assert p1.wgt == p2.wgt
 
     openmc.lib.finalize()
+
+    # Make sure sampling works in volume calculation mode
+    openmc.lib.init(["-c"])
+    openmc.lib.sample_external_source(100)
+    openmc.lib.finalize()