Stabilize Adjoint Source (#3476)

Author: jtramm

src/random_ray/flat_source_domain.cpp

@@ -1242,6 +1242,30 @@ void FlatSourceDomain::set_adjoint_sources(const vector<double>& forward_flux)
     }
   }
 
+  // "Small" source regions in OpenMC are defined as those that are hit by
+  // MIN_HITS_PER_BATCH rays or fewer each batch. These regions typically have
+  // very small volumes combined with a low aspect ratio, and are often
+  // generated when applying a source region mesh that clips the edge of a
+  // curved surface. As perhaps only a few rays will visit these regions over
+  // the entire forward simulation, the forward flux estimates are extremely
+  // noisy and unreliable. In some cases, the noise may make the forward fluxes
+  // extremely low, leading to unphysically large adjoint source terms,
+  // resulting in weight windows that aggressively try to drive particles
+  // towards these regions. To fix this, we simply filter out any "small" source
+  // regions from consideration. If a source region is "small", we
+  // set its adjoint source to zero. This adds negligible bias to the adjoint
+  // flux solution, as the true total adjoint source contribution from small
+  // regions is likely to be negligible.
+#pragma omp parallel for
+  for (int64_t sr = 0; sr < n_source_regions(); sr++) {
+    if (source_regions_.is_small(sr)) {
+      for (int g = 0; g < negroups_; g++) {
+        source_regions_.external_source(sr, g) = 0.0;
+      }
+      source_regions_.external_source_present(sr) = 0;
+    }
+  }
+
   // Divide the fixed source term by sigma t (to save time when applying each
   // iteration)
 #pragma omp parallel for