Performance of the improved version of Monte Carlo Code A3 MCNP for cask shielding design

A3 MCNP (Automatic Adjoint Accelerated MCNP) is a revised version of the MCNP Monte Carlo code, that automatically prepares variance reduction parameters for the CADIS (Consistent Adjoint Driven Importance Sampling) methodology. Using a deterministic “importance” (or adjoint) function, CADIS performs source and transport biasing within the weight-window technique. The current version of A3 MCNP uses the 3-D Sn transport TORT code to determine a 3-D importance function distribution. Based on simulation of several real-life problems, it is demonstrated that A3 MCNP provides precise calculation results with a remarkably short computation time by using the proper and objective variance reduction parameters. However, since the first version of A3 MCNP provided only a point source configuration option for large-scale shielding problems, such as spent-fuel transport casks, a large amount of memory may be necessary to store enough points to properly represent the source. Hence, we have developed an improved version of A3 MCNP (referred to as A3 MCNPV) which has a volumetric source configuration option. This paper describes the successful use of A3 MCNPV for cask neutron and gamma-ray shielding problem.