Add transformation boundary condition

include/openmc/boundary_condition.h

@@ -148,5 +148,24 @@ class RotationalPeriodicBC : public PeriodicBC {
   double angle_;
 };
 
+//==============================================================================
+//! A BC that transforms particle trajectory by a matrix.
+//==============================================================================
+
+class TransformationBC : public BoundaryCondition {
+public:
+  TransformationBC(vector<double> dir_trans, vector<double> pos_trans);
+
+  void handle_particle(Particle& p, const Surface& surf) const override;
+
+  std::string type() const override { return "transformation"; }
+
+protected:
+  //! Matrix by which particle directions are transformed
+  vector<double> dir_trans_;
+  //! Matrix by which particle positions are transformed
+  vector<double> pos_trans_;
+};
+
 } // namespace openmc
 #endif // OPENMC_BOUNDARY_CONDITION_H

include/openmc/particle.h

@@ -103,6 +103,15 @@ class Particle : public ParticleData {
   void cross_periodic_bc(
     const Surface& surf, Position new_r, Direction new_u, int new_surface);
 
+  //! Cross a transformation boundary condition.
+  //
+  //! \param surf The surface (with the transformation boundary condition) that
+  //! the particle struck.
+  //! \param new_r The position of the particle after transformation.
+  //! \param new_u The direction of the particle after transformation.
+  void cross_transformation_bc(
+    const Surface& surf, Position new_r, Direction new_u);
+
   //! mark a particle as lost and create a particle restart file
   //! \param message A warning message to display
   virtual void mark_as_lost(const char* message) override;

src/boundary_condition.cpp

@@ -290,4 +290,41 @@ void RotationalPeriodicBC::handle_particle(
   p.cross_periodic_bc(surf, new_r, new_u, new_surface);
 }
 
+//==============================================================================
+// TransformationBC implementation
+//==============================================================================
+
+TransformationBC::TransformationBC(
+  vector<double> dir_trans, vector<double> pos_trans)
+{
+  // Set the transformation matrices
+  dir_trans_ = dir_trans;
+  pos_trans_ = pos_trans;
+}
+
+void TransformationBC::handle_particle(Particle& p, const Surface& surf) const
+{
+  Position r = p.r();
+  Direction u = p.u();
+
+  Position new_r = {r.x * pos_trans_[0] + r.y * pos_trans_[1] +
+                      r.z * pos_trans_[2] + pos_trans_[3],
+    r.x * pos_trans_[4] + r.y * pos_trans_[5] + r.z * pos_trans_[6] +
+      pos_trans_[7],
+    r.x * pos_trans_[8] + r.y * pos_trans_[9] + r.z * pos_trans_[10] +
+      pos_trans_[11]};
+
+  Position new_u = {u.x * dir_trans_[0] + u.y * dir_trans_[1] +
+                      u.z * dir_trans_[2] + dir_trans_[3],
+    u.x * dir_trans_[4] + u.y * dir_trans_[5] + u.z * dir_trans_[6] +
+      dir_trans_[7],
+    u.x * dir_trans_[8] + u.y * dir_trans_[9] + u.z * dir_trans_[10] +
+      dir_trans_[11]};
+
+  // Handle the effects of the surface albedo on the particle's weight.
+  BoundaryCondition::handle_albedo(p, surf);
+
+  p.cross_transformation_bc(surf, new_r, new_u);
+}
+
 } // namespace openmc

src/particle.cpp

@@ -741,6 +741,63 @@ void Particle::cross_periodic_bc(
   }
 }
 
+void Particle::cross_transformation_bc(
+  const Surface& surf, Position new_r, Direction new_u)
+{
+  // Do not handle transformation boundary conditions on lower universes
+  if (n_coord() != 1) {
+    mark_as_lost("Cannot transform particle " + std::to_string(id()) +
+                 " off surface in a lower universe.");
+    return;
+  }
+
+  // Score surface currents since transformation causes the direction of the
+  // particle to change. For surface filters, we need to score the tallies
+  // twice, once before the particle's surface attribute has changed and
+  // once after. For mesh surface filters, we need to artificially move
+  // the particle slightly back in case the surface crossing is coincident
+  // with a mesh boundary
+
+  if (!model::active_surface_tallies.empty()) {
+    score_surface_tally(*this, model::active_surface_tallies);
+  }
+
+  if (!model::active_meshsurf_tallies.empty()) {
+    Position r {this->r()};
+    this->r() -= TINY_BIT * u();
+    score_surface_tally(*this, model::active_meshsurf_tallies);
+    this->r() = r;
+  }
+
+  // Adjust the particle's location and direction.
+  r() = new_r;
+  u() = new_u;
+
+  // Reassign particle's cell and surface
+  coord(0).cell = cell_last(0);
+  surface() = -surface();
+
+  // If a transformation surface is coincident with a lattice or universe
+  // boundary, it is necessary to redetermine the particle's coordinates in
+  // the lower universes.
+  // (unless we're using a dagmc model, which has exactly one universe)
+  n_coord() = 1;
+  if (surf.geom_type() != GeometryType::DAG &&
+      !neighbor_list_find_cell(*this)) {
+    mark_as_lost("Couldn't find particle after transforming from surface " +
+                 std::to_string(surf.id_) + ".");
+    return;
+  }
+
+  // Set previous coordinate going slightly past surface crossing
+  r_last_current() = r() + TINY_BIT * u();
+
+  // Diagnostic message
+  if (settings::verbosity >= 10 || trace()) {
+    write_message(1, "    Transformed from surface {}", surf.id_);
+  }
+}
+
 void Particle::mark_as_lost(const char* message)
 {
   // Print warning and write lost particle file

src/surface.cpp

@@ -88,6 +88,38 @@ Surface::Surface(pugi::xml_node surf_node)
       bc_ = make_unique<WhiteBC>();
     } else if (surf_bc == "periodic") {
       // Periodic BCs are handled separately
+    } else if (surf_bc == "transformation" || surf_bc == "transform") {
+      int vector_size_exp = 12;
+      vector<double> dir_trans(vector_size_exp);
+      vector<double> pos_trans(vector_size_exp);
+
+      if (check_for_node(surf_node, "direction_transformation")) {
+        dir_trans =
+          get_node_array<double>(surf_node, "direction_transformation");
+        if (dir_trans.size() != vector_size_exp) {
+          fatal_error(fmt::format(
+            "Transformation on surface {} expects direction matrix size {} "
+            "but was given size {}",
+            id_, vector_size_exp, dir_trans.size()));
+        }
+      } else {
+        dir_trans = {
+          1.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0};
+      }
+      if (check_for_node(surf_node, "position_transformation")) {
+        pos_trans =
+          get_node_array<double>(surf_node, "position_transformation");
+        if (pos_trans.size() != vector_size_exp) {
+          fatal_error(
+            fmt::format("Transformation on surface {} expects position matrix "
+                        "size {} but was given size {}",
+              id_, vector_size_exp, pos_trans.size()));
+        }
+      } else {
+        pos_trans = {
+          1.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0};
+      }
+      bc_ = make_unique<TransformationBC>(dir_trans, pos_trans);
     } else {
       fatal_error(fmt::format("Unknown boundary condition \"{}\" specified "
                               "on surface {}",

tests/regression_tests/transformation_plane/geometry.xml

@@ -0,0 +1,11 @@
+<?xml version='1.0' encoding='UTF-8'?>
+<geometry>
+  <!-- Reflective box using transformation BCs -->
+  <cell id="1" material="1" region="1 -2 3 -4 5 -6" universe="1"/>
+  <surface boundary="transformation" coeffs="-5.0" id="1" direction_transformation="-1.0 0.0 0.0 0.0 0.0 1.0 0.0 0.0 0.0 0.0 1.0 0.0" type="x-plane"/>
+  <surface boundary="transformation" coeffs="5.0" id="2" direction_transformation="-1.0 0.0 0.0 0.0 0.0 1.0 0.0 0.0 0.0 0.0 1.0 0.0" type="x-plane"/>
+  <surface boundary="transformation" coeffs="-5.0" id="3" direction_transformation="1.0 0.0 0.0 0.0 0.0 -1.0 0.0 0.0 0.0 0.0 1.0 0.0" type="y-plane"/>
+  <surface boundary="transformation" coeffs="5.0" id="4" direction_transformation="1.0 0.0 0.0 0.0 0.0 -1.0 0.0 0.0 0.0 0.0 1.0 0.0" type="y-plane"/>
+  <surface boundary="transformation" coeffs="-5.0" id="5" direction_transformation="1.0 0.0 0.0 0.0 0.0 1.0 0.0 0.0 0.0 0.0 -1.0 0.0" type="z-plane"/>
+  <surface boundary="transformation" coeffs="5.0" id="6" direction_transformation="1.0 0.0 0.0 0.0 0.0 1.0 0.0 0.0 0.0 0.0 -1.0 0.0" type="z-plane"/>
+</geometry>

tests/regression_tests/transformation_plane/materials.xml

@@ -0,0 +1,7 @@
+<?xml version='1.0' encoding='utf-8'?>
+<materials>
+  <material id="1">
+    <density units="g/cc" value="4.5"/>
+    <nuclide ao="1.0" name="U235"/>
+  </material>
+</materials>

tests/regression_tests/transformation_plane/results_true.dat

@@ -0,0 +1,2 @@
+k-combined:
+2.276857E+00 8.776678E-03

tests/regression_tests/transformation_plane/settings.xml

@@ -0,0 +1,12 @@
+<?xml version='1.0' encoding='UTF-8'?>
+<settings>
+  <run_mode>eigenvalue</run_mode>
+  <particles>1000</particles>
+  <batches>10</batches>
+  <inactive>5</inactive>
+  <source particle="neutron" strength="1.0" type="independent">
+    <space type="box">
+      <parameters>-4.0 -4.0 -4.0 4.0 4.0 4.0</parameters>
+    </space>
+  </source>
+</settings>

tests/regression_tests/transformation_plane/test.py

@@ -0,0 +1,6 @@
+from tests.testing_harness import TestHarness
+
+
+def test_reflective_plane():
+    harness = TestHarness('statepoint.10.h5')
+    harness.main()