Write new step-specific statepoint files at BOS, EOL

Closes #1283 by writing a statepoint file at the beginning
of every depletion step [BOS] and at the final transport simulation,
[EOL]. These files are named "openmc_simulation_n<N>.h5", where
<N> represents the current depletion step, including restart
steps. The new files are written using the C API.

It is worth noting that OpenMC will still write statepoint files
after each intermediate transport simulation used by various
depletion schemes. This commit makes no effort to move or delete
these files, nor stop their creation. It is not clear how,
using the python nor C API, one can stop OpenMC from
writing a statepoint file.

The source bank is not written to these files as this causes
a segmentation fault.

Comparisons of k_combined pull from all statepoints,
reference depletion_results file, and test depletion_results
file have been added to depletion regression test.

openmc/deplete/integrator/abc.py

@@ -4,6 +4,7 @@ from collections.abc import Iterable
 
 from uncertainties import ufloat
 
+from openmc.capi import statepoint_write
 from openmc.deplete import Results, OperatorResult
 
 
@@ -111,6 +112,7 @@ class Integrator(ABC):
     def _get_bos_data_from_openmc(self, step_index, step_power, bos_conc):
         x = deepcopy(bos_conc)
         res = self.operator(x, step_power)
+        self._write_statepoint(step_index)
         return x, res
 
     def _get_bos_data_from_restart(self, step_index, step_power, bos_conc):
@@ -160,17 +162,21 @@ class Integrator(ABC):
             res_list = [self.operator(conc, p)]
             self._save_results(
                 [conc], res_list, [t, t], p, self._ires + len(self))
+            self._write_statepoint(len(self))
 
     def _save_results(self, conc_list, results_list, time_list, power,
                       index, proc_time=None):
-        """Save the results at the end of of one step
-
-        Abstracted to support the predictor's unique save location
-        """
+        """Save the results at the end of of one step"""
         Results.save(
             self.operator, conc_list, results_list, time_list,
             power, index, proc_time)
 
+    def _write_statepoint(self, step_index):
+        """Use capi to write a statepoint for this index"""
+        statepoint_write(
+            "openmc_simulation_n{}.h5".format(step_index + self._ires),
+            write_source=False)
+
 
 class SI_Integrator(Integrator):
     """Abstract for the Stochastic Implicit Euler integrators
@@ -252,3 +258,4 @@ class SI_Integrator(Integrator):
             # No final simulation for SIE, use last iteration results
             self._save_results(
                 [conc], [res_list[-1]], [t, t], p, self._ires + len(self))
+            self._write_statepoint(len(self))

tests/regression_tests/deplete/test.py

@@ -39,7 +39,7 @@ def test_full(run_in_tmpdir):
     space = openmc.stats.Box(lower_left, upper_right)
     settings.source = openmc.Source(space=space)
     settings.seed = 1
-    settings.verbosity = 3
+    settings.verbosity = 1
 
     # Create operator
     chain_file = Path(__file__).parents[2] / 'chain_simple.xml'
@@ -101,3 +101,18 @@ def test_full(run_in_tmpdir):
 
             assert correct, "Discrepancy in mat {} and nuc {}\n{}\n{}".format(
                 mat, nuc, y_old, y_test)
+
+    # Compare statepoint files with depletion results
+
+    t_test, k_test = res_test.get_eigenvalue()
+    t_ref, k_ref = res_ref.get_eigenvalue()
+    k_state = np.empty_like(k_ref)
+
+    # Get statepoint files for all BOS points and EOL
+    for n in range(N + 1):
+        statepoint = openmc.StatePoint("openmc_simulation_n{}.h5".format(n))
+        k_n = statepoint.k_combined
+        k_state[n] = [k_n.nominal_value, k_n.std_dev]
+    # Look for exact match pulling from statepoint and depletion_results
+    assert np.all(k_state == k_test)
+    assert np.allclose(k_test, k_ref)