MCNP Variance Reduction Developments in the 21st Century

Improvements incorporated into MCNP variance reduction methodology and code releases since 2000 are discussed. Some of the improvements are modifications or generalizations of older techniques, and some are entirely new. In particular, pulse-height-tally variance reduction is now possible in MCNP, and the dxtran technique has been generalized to allow an arbitrary nesting of dxtran spheres. A new precollision, next-event estimator is discussed along with flux-at-a-point image tallies. Additionally, the event log analyzer is a tool designed to help the user understand what causes the variance in the user's particular MCNP calculation.