Variance of Certain Flux Estimators Used in Monte Carlo Calculations

Calculations were set up to determine the effect of the grazing flux on the surface-crossing flux estimator and the pillbox track-length estimator used in Monte Carlo calculations. It was found that grazing particles make only higher order contributions to current or flux but significantly increase the variance. These effects are more pronounced at internal surfaces than at external boundaries. The use of nonstochastic methods to estimate contributions of grazing particles is justified.