From 7ef73f048d146a8de20892fdc6eb0f6d9ae18c64 Mon Sep 17 00:00:00 2001
From: Paul Romano <paul.k.romano@gmail.com>
Date: Fri, 13 Nov 2020 13:28:32 -0600
Subject: [PATCH] Remove special treatment of mu for fission

---
 include/openmc/secondary_uncorrelated.h |  2 --
 src/physics.cpp                         | 19 +++++++------------
 src/reaction.cpp                        | 19 -------------------
 src/secondary_uncorrelated.cpp          |  7 +------
 4 files changed, 8 insertions(+), 39 deletions(-)

diff --git a/include/openmc/secondary_uncorrelated.h b/include/openmc/secondary_uncorrelated.h
index 79b8b5031..673a299dc 100644
--- a/include/openmc/secondary_uncorrelated.h
+++ b/include/openmc/secondary_uncorrelated.h
@@ -35,11 +35,9 @@ public:
 
   // Accessors
   AngleDistribution& angle() { return angle_; }
-  bool& fission() { return fission_; }
 private:
   AngleDistribution angle_; //!< Angle distribution
   std::unique_ptr<EnergyDistribution> energy_; //!< Energy distribution
-  bool fission_ {false}; //!< Whether distribution is use for fission
 };
 
 } // namespace openmc
diff --git a/src/physics.cpp b/src/physics.cpp
index 7a8be4ee0..3b1e05a0f 100644
--- a/src/physics.cpp
+++ b/src/physics.cpp
@@ -1009,24 +1009,13 @@ sample_cxs_target_velocity(double awr, double E, Direction u, double kT, uint64_
 
 void sample_fission_neutron(int i_nuclide, const Reaction& rx, double E_in, Particle::Bank* site, uint64_t* seed)
 {
-  // Sample cosine of angle -- fission neutrons are always emitted
-  // isotropically. Sometimes in ACE data, fission reactions actually have
-  // an angular distribution listed, but for those that do, it's simply just
-  // a uniform distribution in mu
-  double mu = 2.0 * prn(seed) - 1.0;
-
-  // Sample azimuthal angle uniformly in [0,2*pi)
-  double phi = 2.0*PI*prn(seed);
-  site->u.x = mu;
-  site->u.y = std::sqrt(1.0 - mu*mu) * std::cos(phi);
-  site->u.z = std::sqrt(1.0 - mu*mu) * std::sin(phi);
-
   // Determine total nu, delayed nu, and delayed neutron fraction
   const auto& nuc {data::nuclides[i_nuclide]};
   double nu_t = nuc->nu(E_in, Nuclide::EmissionMode::total);
   double nu_d = nuc->nu(E_in, Nuclide::EmissionMode::delayed);
   double beta = nu_d / nu_t;
 
+  double mu;
   if (prn(seed) < beta) {
     // ====================================================================
     // DELAYED NEUTRON SAMPLED
@@ -1096,6 +1085,12 @@ void sample_fission_neutron(int i_nuclide, const Reaction& rx, double E_in, Part
       }
     }
   }
+
+  // Sample azimuthal angle uniformly in [0, 2*pi) and assign angle
+  double phi = 2.0*PI*prn(seed);
+  site->u.x = mu;
+  site->u.y = std::sqrt(1.0 - mu*mu) * std::cos(phi);
+  site->u.z = std::sqrt(1.0 - mu*mu) * std::sin(phi);
 }
 
 void inelastic_scatter(const Nuclide& nuc, const Reaction& rx, Particle& p)
diff --git a/src/reaction.cpp b/src/reaction.cpp
index 3e4d3b981..303bd3393 100644
--- a/src/reaction.cpp
+++ b/src/reaction.cpp
@@ -63,25 +63,6 @@ Reaction::Reaction(hid_t group, const std::vector<int>& temperatures)
       close_group(pgroup);
     }
   }
-
-  // <<<<<<<<<<<<<<<<<<<<<<<<<<<< REMOVE THIS <<<<<<<<<<<<<<<<<<<<<<<<<
-  // Before the secondary distribution refactor, when the angle/energy
-  // distribution was uncorrelated, no angle was actually sampled. With
-  // the refactor, an angle is always sampled for an uncorrelated
-  // distribution even when no angle distribution exists in the ACE file
-  // (isotropic is assumed). To preserve the RNG stream, we explicitly
-  // mark fission reactions so that we avoid the angle sampling.
-  if (is_fission(mt_)) {
-    for (auto& p : products_) {
-      if (p.particle_ == Particle::Type::neutron) {
-        for (auto& d : p.distribution_) {
-          auto d_ = dynamic_cast<UncorrelatedAngleEnergy*>(d.get());
-          if (d_) d_->fission() = true;
-        }
-      }
-    }
-  }
-  // <<<<<<<<<<<<<<<<<<<<<<<<<<<< REMOVE THIS <<<<<<<<<<<<<<<<<<<<<<<<<
 }
 
 double
diff --git a/src/secondary_uncorrelated.cpp b/src/secondary_uncorrelated.cpp
index a1aa8ca0b..9b3eb9d3d 100644
--- a/src/secondary_uncorrelated.cpp
+++ b/src/secondary_uncorrelated.cpp
@@ -55,12 +55,7 @@ UncorrelatedAngleEnergy::sample(double E_in, double& E_out, double& mu,
   uint64_t* seed) const
 {
   // Sample cosine of scattering angle
-  if (fission_) {
-    // <<<<<<<<<<<<<<<<<<<<<<<<<<<<<< REMOVE THIS <<<<<<<<<<<<<<<<<<<<<<<<<<<<<
-    // For fission, the angle is not used, so just assign a dummy value
-    mu = 1.0;
-    // <<<<<<<<<<<<<<<<<<<<<<<<<<<<<< REMOVE THIS <<<<<<<<<<<<<<<<<<<<<<<<<<<<<
-  } else if (!angle_.empty()) {
+  if (!angle_.empty()) {
     mu = angle_.sample(E_in, seed);
   } else {
     // no angle distribution given => assume isotropic for all energies