Extending the Direct Statistical Approach to Include Particle Bifurcation Between the Splitting Surfaces

A general volumetric particle bifurcation capability has been inserted into the Direct Statistical Approach (DSA) surface parameter and cell models. The resulting extended DSA describes the second moment and time functions in terms of phase-space surface splitting/Russian roulette parameters (surface parameter model) or phase-space cell importances (cell model) in the presence of volumetric particle bifurcations including both natural events [such as (n,xn) or gamma production from neutron collisions] and artificial events (such as DXTRAN). At the same time, other limitations in the DSA models (concerning tally scores direct from the source and tracks surviving an event at which a tally score occurs) are removed. Given the second moment and time functions, the foregoing surface or cell parameters may then be optimized.