Improved MCNP Memory Locality by Neutron Grouping

This research presents new code for Monte Carlo N-Particle (MCNP) to achieve an improved time during criticality calculations. Modifications implementing the grouping and sorting of neutrons takes advantage of memory locality by processing all neutrons in a group to achieve the temporal reuse of cross section data. This prevents unnecessary data lookups. Various groupings and their results are compared.;The modified code utilizing neutron energy groups provided the best result of a 16.7% +/- 0.5% speedup for a criticality determination of a two slab tank experiment. This is a savings of 2 ½ hours for a system that normally takes approximately 15 ½ hours to execute. The code implemented was chosen to require minimal modifications to the MCNP program thus avoiding the need to rewrite a new version. Verification and validation is still needed in order to show that a speedup using neutron groups can be achieved in all cases.