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Fix caching issue when using NCrystal materials #3538

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Merged
merged 3 commits into from
Oct 2, 2025
Merged

Fix caching issue when using NCrystal materials #3538

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3039c28
Fix caching issue when using NCrystal materials
tkittel Aug 20, 2025
b3d4dea
Update plotter.py to the new API
marquezj Aug 20, 2025
fc1590f
Rename last_ncrystal_xs -> ncrystal_xs
paulromano Oct 2, 2025
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7 changes: 4 additions & 3 deletions include/openmc/particle_data.h
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Original file line number Diff line number Diff line change
Expand Up @@ -154,9 +154,10 @@ struct NuclideMicroXS {

// Energy and temperature last used to evaluate these cross sections. If
// these values have changed, then the cross sections must be re-evaluated.
double last_E {0.0}; //!< Last evaluated energy
double last_sqrtkT {0.0}; //!< Last temperature in sqrt(Boltzmann constant
//!< * temperature (eV))
double last_E {0.0}; //!< Last evaluated energy
double last_sqrtkT {0.0}; //!< Last temperature in sqrt(Boltzmann constant
//!< * temperature (eV))
double ncrystal_xs {-1.0}; //!< NCrystal cross section
};

//==============================================================================
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2 changes: 1 addition & 1 deletion openmc/plotter.py
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Expand Up @@ -500,7 +500,7 @@ def _calculate_cexs_nuclide(this, types, temperature=294., sab_name=None,
elif ncrystal_cfg:
import NCrystal
nc_scatter = NCrystal.createScatter(ncrystal_cfg)
nc_func = nc_scatter.crossSectionNonOriented
nc_func = nc_scatter.xsect
nc_emax = 5 # eV # this should be obtained from NCRYSTAL_MAX_ENERGY
energy_grid = np.union1d(np.geomspace(min(energy_grid),
1.1*nc_emax,
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4 changes: 3 additions & 1 deletion src/particle.cpp
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I think the update of the NCrystal XS could be taken out of the if condition. Then the ACE XS is not recalculated if only the NCrystal XS changes, but the XS is updated anyway:

void Particle::update_neutron_xs(
  int i_nuclide, int i_grid, int i_sab, double sab_frac, double ncrystal_xs)
{
  // Get microscopic cross section cache
  auto& micro = this->neutron_xs(i_nuclide);

  // If the cache doesn't match, recalculate micro xs
  bool recalculate_xs = ( this->E() != micro.last_E || 
                          this->sqrtkT() != micro.last_sqrtkT ||
                          i_sab != micro.index_sab || 
                          sab_frac != micro.sab_frac );
  if ( recalculate_xs ) {
    data::nuclides[i_nuclide]->calculate_xs(i_sab, i_grid, sab_frac, *this);
  }
  // If NCrystal is being used, update micro cross section cache
  if ((recalculate_xs || ncrystal_xs != micro.last_ncrystal_xs ) 
       && (ncrystal_xs >= 0.0) ) {
    ncrystal_update_micro(ncrystal_xs, micro);
    micro.last_ncrystal_xs = ncrystal_xs;
  }
}

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That is getting a bit too complicated for my liking, I can't really wrap my head around whether or not that will always end up correct.

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Ok, let's keep it your way.

Original file line number Diff line number Diff line change
Expand Up @@ -849,10 +849,12 @@ void Particle::update_neutron_xs(

// If the cache doesn't match, recalculate micro xs
if (this->E() != micro.last_E || this->sqrtkT() != micro.last_sqrtkT ||
i_sab != micro.index_sab || sab_frac != micro.sab_frac) {
i_sab != micro.index_sab || sab_frac != micro.sab_frac ||
ncrystal_xs != micro.ncrystal_xs) {
data::nuclides[i_nuclide]->calculate_xs(i_sab, i_grid, sab_frac, *this);

// If NCrystal is being used, update micro cross section cache
micro.ncrystal_xs = ncrystal_xs;
if (ncrystal_xs >= 0.0) {
data::nuclides[i_nuclide]->calculate_elastic_xs(*this);
ncrystal_update_micro(ncrystal_xs, micro);
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