29109839871

Committed 10 Jul 2026 05:07PM UTC coverage: 81.355% (+0.06%) from 81.295%

Build # 29109839871

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Pull #3971

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Merge b03e8c77b into 7256d5046

Pull Request Pull Request #3971: Delta tracking

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85.98

/src/majorant.cpp

#include <fmt/core.h>

#include "openmc/capi.h"
#include "openmc/constants.h"
#include "openmc/geometry.h"
#include "openmc/interpolate.h"
#include "openmc/majorant.h"
#include "openmc/material.h"
#include "openmc/nuclide.h"
#include "openmc/photon.h"
#include "openmc/search.h"
#include "openmc/settings.h"
#include "openmc/simulation.h"
#include "openmc/thermal.h"
#include "openmc/timer.h"
#include "openmc/universe.h"

namespace openmc {

//==============================================================================
// Global variables
//==============================================================================

namespace data {
std::unique_ptr<NeutronMajorant> n_majorant;
std::unique_ptr<PhotonMajorant> p_majorant;

} // namespace data

//==============================================================================
// Majorant implementation
//==============================================================================

Majorant::Majorant(int i_universe) : maj_universe_(i_universe)
{
  if (maj_universe_ == C_NONE || maj_universe_ >= model::universes.size()) {
    fatal_error(fmt::format("Invalid majorant universe: {}", maj_universe_));
  }

  // First, find unique cells contained in this universe.
  std::unordered_set<int> unique_mat_cells;

  const auto& maj_uni = model::universes[maj_universe_];
  for (int i_cell : maj_uni->cells_) {
    const auto& uni_cell = model::cells[i_cell];

    // If the cell is filled with a material, it won't have any sub-cells.
    if (uni_cell->type_ == Fill::MATERIAL) {
      if (unique_mat_cells.count(i_cell) == 0) {
        unique_mat_cells.emplace(i_cell);
      }
    } else {
      // This cell is filled with a universe or lattice. Need to get the list of
      // cells and cell instances.
      const auto contained_cells = uni_cell->get_contained_cells();
      for (const auto& [i_con_cell, contained_instances] : contained_cells) {
        if (unique_mat_cells.count(i_con_cell) == 0) {
          unique_mat_cells.emplace(i_con_cell);
        }
      }
    }
  }

  // Next, find all materials contained in the majorant's universe. This also
  // obtains the maximum density multiplier applied to that material.
  std::unordered_set<int> unique_materials;
  for (int i_cell : unique_mat_cells) {
    auto& cell = model::cells[i_cell];

    for (int instance = 0; instance < cell->n_instances(); ++instance) {
      int i_material = cell->material(instance);
      // Skip over void materials.
      if (i_material == MATERIAL_VOID) {
        continue;
      }

      // Check to see if we've found the contained material yet. If not, add
      // to the set of materials discovered and add to the map of density
      // multipliers.
      if (unique_materials.count(i_material) == 0) {
        unique_materials.emplace(i_material);
        max_density_mult_[i_material] = cell->density_mult(instance);
      } else {
        // We've found this material already. Need to take the maximum density
        // multiplier.
        max_density_mult_.at(i_material) = std::max(
          max_density_mult_.at(i_material), cell->density_mult(instance));
      }
    }
  }

  // Insert the elements from the set.
  contained_materials_.assign(unique_materials.begin(), unique_materials.end());
}

void Majorant::compute_majorant()
{
  // Fill with zeros.
  xs_.resize(grid_.energy.size(), 0.0);

  std::vector<double> material_maj_xs;
  for (int i_material : contained_materials_) {
    // Populate the per-material majorant cross section. We pass in
    // 'material_maj_xs' instead of returning a vector with the per-material
    // majorant every time to avoid costly reallocations and copy operations,
    // which have a fairly large impact on the time it takes to build the
    // majorant. The function 'fill_material_maj_xs(...)' is responsible for
    // resizing 'material_maj_xs' and populating each value at a given energy
    // grid point.
    fill_material_maj_xs(i_material, max_density_mult_.at(i_material),
      grid_.energy, material_maj_xs);

    // Compute the full majorant by taking the max over each material cross
    // section.
    for (int i_energy = 0; i_energy < xs_.size(); ++i_energy) {
      xs_[i_energy] = std::max(xs_[i_energy], material_maj_xs[i_energy]);
    }
  }
}

void Majorant::post_process_grid(Nuclide::EnergyGrid& grid) const
{
  // Fetch the minimum and maximum transport energies from the superclass.
  const double E_min = min_transport_energy();
  const double E_max = max_transport_energy();

  std::sort(grid.energy.begin(), grid.energy.end());
  auto unique_end = std::unique(grid.energy.begin(), grid.energy.end());
  grid.energy.resize(std::distance(grid.energy.begin(), unique_end));

  // Remove all values below the minimum neutron energy.
  auto min_it = grid.energy.begin();
  while (*min_it < E_min) {
    min_it++;
  }
  grid.energy.erase(grid.energy.begin(), min_it + 1);

  // Insert the minimum neutron energy at the beginning.
  grid.energy.insert(grid.energy.begin(), E_min);

  // Remove all values above the maximum neutron energy.
  auto max_it = --grid.energy.end();
  while (*max_it > E_max) {
    max_it--;
  }
  grid.energy.erase(max_it - 1, grid.energy.end());

  // Insert the maximum neutron energy at the end.
  grid.energy.insert(grid.energy.end(), E_max);
}

//==============================================================================
// NeutronMajorant implementation
//==============================================================================

NeutronMajorant::NeutronMajorant(int i_universe) : Majorant(i_universe) {}

double NeutronMajorant::calculate_neutron_xs(double energy) const
{
  const int i_grid = get_i_grid(energy, grid_);
  return interpolate_lin_lin(grid_.energy[i_grid], grid_.energy[i_grid + 1],
    xs_[i_grid], xs_[i_grid + 1], energy);
}

void NeutronMajorant::compute_unionized_grid()
{
  // This function generates a unionized cross section grid between smooth cross
  // sections and URR probability table grids.
  std::unordered_set<int> processed_nuclides;
  for (int i_mat : contained_materials_) {
    const auto& mat = model::materials[i_mat];
    for (auto i_nuclide : mat->nuclide_) {
      // Only unionize nuclides we haven't checked yet.
      if (processed_nuclides.count(i_nuclide) > 0) {
        continue;
      }

      const auto& nuclide = data::nuclides[i_nuclide];
      // ======================================================================
      // Unionizing the URR energy grids.
      if (nuclide->urr_present_ && settings::urr_ptables_on) {
        for (const auto& nuc_urr : nuclide->urr_data_) {
          grid_.energy.insert(
            grid_.energy.end(), nuc_urr.energy_.begin(), nuc_urr.energy_.end());
        }
      }

      // ======================================================================
      // Unionize the smooth cross section energy grids.
      for (const auto& nuc_grid : nuclide->grid_) {
        grid_.energy.insert(
          grid_.energy.end(), nuc_grid.energy.begin(), nuc_grid.energy.end());
      }

      processed_nuclides.insert(i_nuclide);
    }
  }

  // Post-process the energy grid now that all points from nuclides are
  // included. This sorts the energy points, removes duplicates, and removes all
  // energies exceeding neutron transport bounds.
  post_process_grid(grid_);

  // Initialize the grid for fast lookups. This only applies to neutrons.
  grid_.init();
}

void NeutronMajorant::fill_material_maj_xs(int i_material,
  double max_density_mult, const std::vector<double>& to_grid,
  std::vector<double>& mat_maj) const
{
  const auto& mat = *model::materials[i_material];

  mat_maj.resize(to_grid.size());

#pragma omp parallel for
  for (int i_energy = 0; i_energy < to_grid.size(); ++i_energy) {
    mat_maj[i_energy] = 0.0;
    const double union_energy = to_grid[i_energy];

    int mat_sab_table_idx = 0;
    bool check_sab = (mat.thermal_tables_.size() > 0);

    for (int i = 0; i < mat.nuclide_.size(); ++i) {
      // ======================================================================
      // CHECK FOR S(A,B) TABLE
      int i_sab = C_NONE;
      double sab_frac = 0.0;

      // Check if this nuclide matches one of the S(a,b) tables specified.
      // This relies on thermal_tables_ being sorted by .index_nuclide
      if (check_sab) {
        const auto& sab {mat.thermal_tables_[mat_sab_table_idx]};
        if (i == sab.index_nuclide) {
          // Get index in sab_tables
          i_sab = sab.index_table;
          sab_frac = sab.fraction;

          // If particle energy is greater than the highest energy for the
          // S(a,b) table, then don't use the S(a,b) table
          if (union_energy > data::thermal_scatt[i_sab]->energy_max_) {
            i_sab = C_NONE;
          }

          // Increment position in thermal_tables_
          ++mat_sab_table_idx;

          // Don't check for S(a,b) tables if there are no more left
          if (mat_sab_table_idx == mat.thermal_tables_.size()) {
            check_sab = false;
          }
        }
      }

      // ======================================================================
      // Compute the maximum smooth total cross section. This is either the
      // free gas cross section at energies larger than the Bragg edge, or
      // the bound cross section in the thermal scattering region.
      double micro_smooth_tot_xs = 0.0;
      if (i_sab >= 0) {
        // Thermal scattering cross sections using S(a,b) tables.
        micro_smooth_tot_xs = calculate_max_sab_tot_xs(
          mat.nuclide_[i], i_sab, sab_frac, union_energy);
      } else {
        // Free gas smooth cross section
        micro_smooth_tot_xs =
          calculate_max_smooth_xs(mat.nuclide_[i], union_energy);
      }

      // ======================================================================
      // Compute the URR cross section. This shouldn't intersect with the
      // S(a,b) cross section.
      double micro_urr_xs = calculate_max_urr_xs(
        mat.nuclide_[i], union_energy, micro_smooth_tot_xs);

      // ======================================================================
      // Accumulate the macroscopic cross section.
      mat_maj[i_energy] += std::max(micro_smooth_tot_xs, micro_urr_xs) *
                           mat.atom_density(i, max_density_mult);
    }
  }
}

double NeutronMajorant::calculate_max_smooth_xs(
  int i_nuclide, double energy) const
{
  const auto& nuc = *data::nuclides[i_nuclide];

  double max_smooth_tot_xs = 0.0;
  for (int i_temp = 0; i_temp < nuc.kTs_.size(); ++i_temp) {
    const auto& nuc_grid = nuc.grid_[i_temp];
    int i_grid = get_i_grid(energy, nuc_grid);
    auto total = nuc.xs_[i_temp].slice(openmc::tensor::all, 0);
    double xs = interpolate_lin_lin(nuc_grid.energy[i_grid],
      nuc_grid.energy[i_grid + 1], total[i_grid], total[i_grid + 1], energy);
    max_smooth_tot_xs = std::max(max_smooth_tot_xs, xs);
  }

  return max_smooth_tot_xs;
}

double NeutronMajorant::calculate_max_urr_xs(
  int i_nuclide, double energy, double smooth_xs) const
{
  // A tolerance on the URR check to make sure we include the URR energy grid
  // bounds.
  constexpr double URR_FUZZY_CHECK = 1e-6;

  const auto& nuc = *data::nuclides[i_nuclide];
  if (!nuc.urr_present_) {
    return 0.0;
  }

  double max_urr_xs = 0.0;
  for (const auto& urr : nuc.urr_data_) {
    if (!(urr.energy_in_bounds(energy - URR_FUZZY_CHECK) ||
          urr.energy_in_bounds(energy + URR_FUZZY_CHECK))) {
      continue;
    }

    int i_energy;
    if (energy <= urr.energy_.front()) {
      i_energy = 0;
    } else if (energy >= urr.energy_.back()) {
      i_energy = urr.energy_.size() - 2;
    } else {
      i_energy =
        lower_bound_index(&urr.energy_.front(), &urr.energy_.back(), energy);
    }

    // Find the maximum URR cross sections for the two bounding energy points.
    double max_urr_xs_E0 = 0.0;
    double max_urr_xs_E1 = 0.0;
    for (int i_cdf = 0; i_cdf < urr.n_cdf(); ++i_cdf) {
      max_urr_xs_E0 =
        std::max(max_urr_xs_E0, urr.xs_values_(i_energy, i_cdf).total);
      max_urr_xs_E1 =
        std::max(max_urr_xs_E1, urr.xs_values_(i_energy + 1, i_cdf).total);
    }
    // Handle the rare case where the points could be negative.
    max_urr_xs_E0 = std::max(max_urr_xs_E0, 0.0);
    max_urr_xs_E1 = std::max(max_urr_xs_E1, 0.0);

    // Interpolate the bounding energy points.
    double interp_urr_xs = 0.0;
    if (urr.interp_ == Interpolation::lin_lin) {
      interp_urr_xs = interpolate_lin_lin(urr.energy_[i_energy],
        urr.energy_[i_energy + 1], max_urr_xs_E0, max_urr_xs_E1, energy);
    } else if (urr.interp_ == Interpolation::log_log) {
      interp_urr_xs = interpolate_log_log(urr.energy_[i_energy],
        urr.energy_[i_energy + 1], max_urr_xs_E0, max_urr_xs_E1, energy);
    }

    // Multiply by the smooth cross section (after interpolation) if required.
    if (urr.multiply_smooth_) {
      interp_urr_xs *= smooth_xs;
    }

    max_urr_xs = std::max({max_urr_xs, interp_urr_xs, smooth_xs});
  }

  return max_urr_xs;
}

double NeutronMajorant::calculate_max_sab_tot_xs(
  int i_nuclide, int i_sab, double sab_frac, double energy) const
{
  const auto& nuc = *data::nuclides[i_nuclide];
  const auto& thermal = *data::thermal_scatt[i_sab];

  // Loop over the nuclide's temperature grid to ensure we're consistent.
  double max_sab_total = 0.0;
  for (int i_nuc_temp = 0; i_nuc_temp < nuc.kTs_.size(); ++i_nuc_temp) {
    double nuc_kT = nuc.kTs_[i_nuc_temp] * nuc.kTs_[i_nuc_temp];

    // Compute the elastic and inelastic scattering cross sections. The S(a,b)
    // cross sections are interpolated to match the nuclide temperature point.
    double thermal_elastic;
    double thermal_inelastic;
    const auto& tkTs = thermal.kTs_;
    if (tkTs.size() > 1) {
      if (nuc_kT < tkTs.front()) {
        thermal.data_.front().calculate_xs(
          energy, &thermal_elastic, &thermal_inelastic);
      } else if (nuc_kT > tkTs.back()) {
        thermal.data_.back().calculate_xs(
          energy, &thermal_elastic, &thermal_inelastic);
      } else {
        // Find temperatures that bound the actual temperature
        int i_sab_temp = 0;
        while (
          tkTs[i_sab_temp + 1] < nuc_kT && i_sab_temp + 1 < tkTs.size() - 1) {
          ++i_sab_temp;
        }
        // Interpolate the scattering cross sections to the nuclide temperature
        // grid point.
        double T0_elastic, T1_elastic, T0_inelastic, T1_inelastic;
        thermal.data_[i_sab_temp].calculate_xs(
          energy, &T0_elastic, &T0_inelastic);
        thermal.data_[i_sab_temp + 1].calculate_xs(
          energy, &T1_elastic, &T1_inelastic);
        thermal_elastic = interpolate_lin_lin(tkTs[i_sab_temp],
          tkTs[i_sab_temp + 1], T0_elastic, T1_elastic, nuc_kT);
        thermal_inelastic = interpolate_lin_lin(tkTs[i_sab_temp],
          tkTs[i_sab_temp + 1], T0_inelastic, T1_inelastic, nuc_kT);
      }
    } else {
      thermal.data_[0].calculate_xs(
        energy, &thermal_elastic, &thermal_inelastic);
    }

    // Compute the free gas total and elastic cross sections interpolated on the
    // majorant grid.
    const auto& nuc_grid = nuc.grid_[i_nuc_temp];
    int i_grid = get_i_grid(energy, nuc_grid);
    const auto& free_tot = nuc.xs_[i_nuc_temp].slice(openmc::tensor::all, 0);
    const auto& free_ela = nuc.reactions_[0]->xs_[i_nuc_temp].value;
    double tot_xs =
      interpolate_lin_lin(nuc_grid.energy[i_grid], nuc_grid.energy[i_grid + 1],
        free_tot[i_grid], free_tot[i_grid + 1], energy);
    double ela_xs =
      interpolate_lin_lin(nuc_grid.energy[i_grid], nuc_grid.energy[i_grid + 1],
        free_ela[i_grid], free_ela[i_grid + 1], energy);

    double thermal_xs = sab_frac * (thermal_elastic + thermal_inelastic);
    double sab_corrected_total = tot_xs + thermal_xs - sab_frac * ela_xs;
    max_sab_total = std::max(sab_corrected_total, max_sab_total);
  }

  return max_sab_total;
}

int NeutronMajorant::get_i_grid(
  double energy, const Nuclide::EnergyGrid& grid) const
{
  // Find energy index on energy grid
  int i_log_union =
    std::log(energy / data::energy_min[i_neutron_]) / simulation::log_spacing;

  int i_grid;
  if (energy <= grid.energy.front()) {
    i_grid = 0;
  } else if (energy >= grid.energy.back()) {
    i_grid = grid.energy.size() - 2;
  } else {
    // Determine bounding indices based on which equal log-spaced
    // interval the energy is in
    int i_low = grid.grid_index[i_log_union];
    int i_high = grid.grid_index[i_log_union + 1] + 1;

    // This catches the very rare case where floating point comparisons fail.
    if (i_low >= grid.energy.size() || i_high >= grid.energy.size()) {
      i_grid = grid.energy.size() - 2;
    } else {
      // Perform binary search over reduced range
      i_grid = i_low + lower_bound_index(
                         &grid.energy[i_low], &grid.energy[i_high], energy);
    }
  }

  // check for rare case where two energy points are the same
  if (grid.energy[i_grid] == grid.energy[i_grid + 1])
    ++i_grid;

  return i_grid;
}

//==============================================================================
// PhotonMajorant implementation
//==============================================================================

PhotonMajorant::PhotonMajorant(int i_universe) : Majorant(i_universe) {}

void PhotonMajorant::compute_unionized_grid()
{
  // This function generates a unionized cross section grid for all elements.
  std::unordered_set<int> processed_elements;
  for (int i_mat : contained_materials_) {
    const auto& mat = model::materials[i_mat];
    for (int i = 0; i < mat->nuclide_.size(); ++i) {
      // Only unionize elements we haven't checked yet.
      if (processed_elements.count(mat->element_[i]) > 0) {
        continue;
      }

      const auto& element = data::elements[mat->element_[i]];
      grid_.energy.insert(
        grid_.energy.end(), element->energy_.begin(), element->energy_.end());

      processed_elements.insert(mat->element_[i]);
    }
  }

  // Post-process the energy grid now that all points from photon interactions
  // are included. This sorts the energy points, removes duplicates, and removes
  // all energies exceeding photon transport bounds.
  post_process_grid(grid_);
}

double PhotonMajorant::calculate_photon_xs(double energy) const
{
  double log_energy = std::log(energy);
  int i_grid = get_i_grid<std::vector<double>>(log_energy, grid_.energy);

  // calculate interpolation factor
  double f = (log_energy - grid_.energy[i_grid]) /
             (grid_.energy[i_grid + 1] - grid_.energy[i_grid]);

  // interpolate the total cross section
  return std::exp(xs_[i_grid] + f * (xs_[i_grid + 1] - xs_[i_grid]));
}

void PhotonMajorant::fill_material_maj_xs(int i_material,
  double max_density_mult, const std::vector<double>& to_grid,
  std::vector<double>& mat_maj) const
{
  const auto& mat = *model::materials[i_material];

  mat_maj.resize(to_grid.size());

#pragma omp parallel for
  for (int i_energy = 0; i_energy < to_grid.size(); ++i_energy) {
    mat_maj[i_energy] = 0.0;
    const double union_log_energy = to_grid[i_energy];

    for (int i = 0; i < mat.nuclide_.size(); ++i) {
      const int i_element = mat.element_[i];

      mat_maj[i_energy] += calculate_elem_tot_xs(i_element, union_log_energy) *
                           mat.atom_density(i, max_density_mult);
    }
    mat_maj[i_energy] = std::log(mat_maj[i_energy]);
  }
}

double PhotonMajorant::calculate_elem_tot_xs(
  int i_element, double log_energy) const
{
  const auto& elem = *data::elements[i_element];
  int i_grid = get_i_grid<tensor::Tensor<double>>(log_energy, elem.energy_);

  // calculate interpolation factor
  double f = (log_energy - elem.energy_(i_grid)) /
             (elem.energy_(i_grid + 1) - elem.energy_(i_grid));

  // Calculate microscopic coherent cross section
  double coherent =
    std::exp(elem.coherent_(i_grid) +
             f * (elem.coherent_(i_grid + 1) - elem.coherent_(i_grid)));

  // Calculate microscopic incoherent cross section
  double incoherent =
    std::exp(elem.incoherent_(i_grid) +
             f * (elem.incoherent_(i_grid + 1) - elem.incoherent_(i_grid)));

  // Calculate microscopic photoelectric cross section
  double photoelectric = 0.0;
  tensor::View<const double> xs_lower = elem.cross_sections_.slice(i_grid);
  tensor::View<const double> xs_upper = elem.cross_sections_.slice(i_grid + 1);

  for (int i = 0; i < xs_upper.size(); ++i)
    if (xs_lower(i) != 0)
      photoelectric += std::exp(xs_lower(i) + f * (xs_upper(i) - xs_lower(i)));

  // Calculate microscopic pair production cross section
  double pair_production =
    std::exp(elem.pair_production_total_(i_grid) +
             f * (elem.pair_production_total_(i_grid + 1) -
                   elem.pair_production_total_(i_grid)));

  // Calculate microscopic total cross section
  return coherent + incoherent + photoelectric + pair_production;
}

//! Create a majorant cross section for photons or neutrons.
void create_majorants()
{
  simulation::time_build_majorant.start();

  write_message("Constructing a neutron majorant cross section");
  data::n_majorant = std::make_unique<NeutronMajorant>(model::root_universe);
  data::n_majorant->compute_unionized_grid();
  data::n_majorant->compute_majorant();

  if (settings::photon_transport) {
    write_message("Constructing a photon majorant cross section");
    data::p_majorant = std::make_unique<PhotonMajorant>(model::root_universe);
    data::p_majorant->compute_unionized_grid();
    data::p_majorant->compute_majorant();
  }

  simulation::time_build_majorant.stop();
}

//! Reset the photon and neutron majorant cross sections.
void reset_majorants()
{
  openmc::data::n_majorant.reset(nullptr);
  openmc::data::p_majorant.reset(nullptr);
}
} // namespace openmc

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