Lost particles in small boxes with reflective/periodic BCs

[ChasingNeutrons]

On doing simulations with very small boxes (width ~1um) we obtain a very low k, despite having reflective/periodic boundary conditions. This is due to particles erroneously leaking from the geometry due to the absolute surface tolerance. This should probably be relative to the size of the surface.

[Mikolaj-A-Kowalski]

Can I be annoying and ask for 'Minimal Reproducible Example"? 😉

[ChasingNeutrons]

Sure thing.

type eigenPhysicsPackage;

pop 20000;
active 200;
inactive 1;
XSdata ceData;
dataType ce;
seed 3;
outputFile example;

collisionOperator { neutronCE {type neutronCEstd;}

transportOperator { type transportOperatorDT;
}

inactiveTally {
}

activeTally {
}

geometry {
type geometryStd;
boundary (2 2 2 2 2 2); graph {type shrunk;}
surfaces
{
squareBound { id 1; type box; origin ( 0.0 0.0 0.0); halfwidth (1.40725456E-6 1.40725456E-6 1.40725456E-6); }
sphere { id 2; type sphere; origin (0.0 0.0 0.0); radius 1.0E-6;}
}
cells
{
fuel {id 1; type simpleCell; surfaces (-2); filltype mat; material fuel;}
matrix {id 2; type simpleCell; surfaces (2); filltype mat; material matrix;}
}
universes
{
root { id 1; type rootUniverse; border 1; fill u<2>; }
cellUni {id 2; type cellUniverse; cells (1 2);}
}
}

viz {
bmp {
type bmp;
output xy;
what material;
centre (0.0 0.0 0.0);
axis z;
res (500 500);
}
}

nuclearData {

handles {
ceData { type aceNeutronDatabase; aceLibrary $SCONE_ACE;}
}

materials {

fuel {
  temp       273;
  composition {
    92235.03  0.002;
  }
}

ZrH {
  temp       273;
  composition {
    6012.03  0.05;
  }
}

}

}