Coupled Monte Carlo - Discrete ordinates computational scheme for three-dimensional shielding calculations of large and complex nuclear facilities

Abstract

Shielding calculations of advanced nuclear facilities such as accelerator based neutron sources or fusion devices of the tokamak type are complicated due to their complex geometries and their large dimensions, including bulk shields of several meters thickness. While the complexity of the geometry in the shielding calculation can be hardly handled by the discrete ordinates method, the deep penetration of radiation through bulk shields is a severe challenge for the Monte Carlo particle transport simulation technique. This work proposes a dedicated computational approach for coupled Monte Carlo - deterministic transport calculations to handle this kind of shielding problems. The Monte Carlo technique is used to simulate the particle generation and transport in the target region with both complex geometry and reaction physics, and the discrete ordinates method is used to treat the deep penetration problem in the bulk shield. To enable the coupling of these two different computational methods, a mapping approach has been developed for calculating the discrete ordinates angular flux distribution from the scored data of the Monte Carlo particle tracks crossing a specified surface. The approach has been implemented in an interface program and validated by means of test calculations using a simplified three-dimensional geometric model. Satisfactory agreement was obtained for the angular fluxes calculated by the mapping approach using the MCNP code for the Monte Carlo calculations and direct three-dimensional discrete ordinates calculations using the TORT code. In the next step, a complete program system has been developed for coupled three-dimensional Monte Carlo- deterministic transport calculations by integrating the Monte Carlo transport code MCNP, the three-dimensional discrete ordinates code TORT and the mapping interface program. Test calculations with two simple models have been performed to validate the program system by means of comparison calculations using the Monte Carlo technique directly. The good agreement of the results obtained demonstrates that the program system is suitable to treat three-dimensional shielding problems with satisfactory accuracy. Finally the program system has been applied to the shielding analysis of the accelerator based IFMIF (International Fusion Materials Irradiation Facility) neutron source facility. In this application, the IFMIF-dedicated Monte Carlo code McDeLicious was used for the neutron generation and transport simulation in the target and the test cell region using a detailed geometrical model. The neutron/photon fluxes, spectra and dose rates across the back wall and in the access/maintenance room were calculated and are discussed. The comparison to the results of shielding analyses conducted previously for IFMIF on the basis of approximate methods and models shows severe discrepancies. These are due to the poor geometry representation and the approximate treatment of the neutron source simulation employed in those calculations. The successful application to IFMIF thus demonstrates the suitability of the program system for the analysis of shielding problems of large and complex nuclear facilities based on coupled Monte Carlo - discrete ordinates calculations.