An easy to implement global variance reduction procedure for MCNP

In this article, an easy to implement global variance reduction procedure is demonstrated. The procedure is capable of flattening the relative error distribution in a global mesh tally covering the geometry of a complex system. The method is fully based on the I/O capabilities of the MCNP Monte Carlo radiation transport code family and requires little additional support code and almost no user judgment to work properly. Increases, with respect to a standard simulation, in a global figure of merit ranging up to a factor 276 for a PWR core configuration have been observed. Also, the relative error distribution was flattened substantially in all studied systems by the application of the procedure and the number of empty mesh tally cells was reduced.

► An easy to implement global variance reduction procedure for the MCNP codes is explained and demonstrated.
► Objectives and measures are defined to test the efficacy of the procedure.
► Procedure tested on four different systems, including full scale nuclear reactor core and fixed source problem.
► Increases in a global Figure of Merit ranging up to a factor 276 for a PWR configuration have been observed.
► Procedure is simple, works well (in some cases extremely well) and is capable of meeting all stated objectives.