A COLLISION PROBABILITY ANALYSIS OF THE DOUBLE-HETEROGENEITY PROBLEM

A practical collision probability model is presented for the descripti on of geometries with many levels of heterogeneity. Regular regions of the macrogeometry are assumed to contain a stochastic mixture of sphe rical grains or cylindrical tubes. Simple expressions for the collisio n probabilities in the global geometry are obtained as a function of t he collision probabilities in the macro- and microgeometries. This mod el was successfully implemented in the collision probability kernel of the APOLLO-1, APOLLO-2, and DRAGON lattice codes for the description of a broad range of reactor physics problems. Resonance self-shielding and depletion calculations in the microgeometries are possible becaus e each microregion is explicitly represented.