Clip negative atom densities that result from CRAM (#3879)

Co-authored-by: Paul Romano <paul.k.romano@gmail.com>

Author: yrrepy

openmc/deplete/abc.py

@@ -738,6 +738,11 @@ def _timed_deplete(self, n, rates, dt, i=None, matrix_func=None):
         results = deplete(
             self._solver, self.chain, n, rates, dt, i, matrix_func,
             self.transfer_rates, self.external_source_rates)
+
+        # Clip unphysical negative number densities
+        for r in results:
+            r.clip(min=0.0, out=r)
+
         return time.time() - start, results
 
     @abstractmethod

tests/dummy_operator.py

@@ -24,7 +24,7 @@
 
 predictor_solution = DepletionSolutionTuple(
     PredictorIntegrator, np.array([1.0, 2.46847546272295, 4.11525874568034]),
-    np.array([1.0, 0.986431226850467, -0.0581692232513460]))
+    np.array([1.0, 0.986431226850467, 0.0]))
 
 
 cecm_solution = DepletionSolutionTuple(