Fix IFP implementation (#3580)

include/openmc/ifp.h

@@ -68,15 +68,14 @@ vector<T> _ifp(const T& value, const vector<T>& data)
 //!
 //! Add the IFP information in the IFP banks using the same index
 //! as the one used to append the fission site to the fission bank.
+//! The information stored are the delayed group number and lifetime
+//! of the neutron that created the fission event.
 //! Multithreading protection is guaranteed by the index returned by the
 //! thread_safe_append call in physics.cpp.
 //!
-//! Needs to be done after the delayed group is found.
-//!
 //! \param[in] p Particle
-//! \param[in] site Fission site
 //! \param[in] idx Bank index from the thread_safe_append call in physics.cpp
-void ifp(const Particle& p, const SourceSite& site, int64_t idx);
+void ifp(const Particle& p, int64_t idx);
 
 //! Resize the IFP banks used in the simulation
 void resize_simulation_ifp_banks();

include/openmc/particle_data.h

@@ -631,6 +631,7 @@ public:
   int& event_mt() { return event_mt_; } // MT number of collision
   const int& event_mt() const { return event_mt_; }
   int& delayed_group() { return delayed_group_; } // delayed group
+  const int& delayed_group() const { return delayed_group_; }
   const int& parent_nuclide() const { return parent_nuclide_; }
   int& parent_nuclide() { return parent_nuclide_; } // Parent nuclide
 

src/ifp.cpp

@@ -28,13 +28,13 @@ bool is_generation_time_or_both()
   return false;
 }
 
-void ifp(const Particle& p, const SourceSite& site, int64_t idx)
+void ifp(const Particle& p, int64_t idx)
 {
   if (is_beta_effective_or_both()) {
     const auto& delayed_groups =
       simulation::ifp_source_delayed_group_bank[p.current_work() - 1];
     simulation::ifp_fission_delayed_group_bank[idx] =
-      _ifp(site.delayed_group, delayed_groups);
+      _ifp(p.delayed_group(), delayed_groups);
   }
   if (is_generation_time_or_both()) {
     const auto& lifetimes =

src/particle.cpp

@@ -144,6 +144,7 @@ void Particle::from_source(const SourceSite* src)
   time() = src->time;
   time_last() = src->time;
   parent_nuclide() = src->parent_nuclide;
+  delayed_group() = src->delayed_group;
 
   // Convert signed surface ID to signed index
   if (src->surf_id != SURFACE_NONE) {

src/physics.cpp

@@ -246,18 +246,15 @@ void create_fission_sites(Particle& p, int i_nuclide, const Reaction& rx)
       }
       // Iterated Fission Probability (IFP) method
       if (settings::ifp_on) {
-        ifp(p, site, idx);
+        ifp(p, idx);
       }
     } else {
       p.secondary_bank().push_back(site);
     }
 
-    // Set the delayed group on the particle as well
-    p.delayed_group() = site.delayed_group;
-
     // Increment the number of neutrons born delayed
-    if (p.delayed_group() > 0) {
-      nu_d[p.delayed_group() - 1]++;
+    if (site.delayed_group > 0) {
+      nu_d[site.delayed_group - 1]++;
     }
 
     // Write fission particles to nuBank

tests/regression_tests/ifp/results_true.dat

@@ -3,7 +3,7 @@ k-combined:
 tally 1:
 9.109384E-08
 5.667165E-16
-6.500000E-02
-6.710000E-04
+5.200000E-02
+5.420000E-04
 1.489000E+01
 1.480036E+01