Directional diffusion coefficients and leakage-corrected discontinuity factors: Implementation in Serpent and tests

A recently proposed method has been extended to treat the generation of radial directional diffusion coefficients with the Monte Carlo code Serpent. In addition to a previous implementation for the axial direction, the new method was applied to the study of a sodium-cooled fast reactor by combined Monte Carlo and diffusion theory. The performance of anisotropic diffusion coefficients was found to be superior to that one of standard diffusion coefficients already available in Serpent. A diffusion solver was also implemented in Serpent in order to compute leakage-corrected discontinuity factors in hexagonal-Z geometry for multiplicative and non-multiplicative multi-group problems. The use of face discontinuity factors improved the agreement among results achieved by diffusion theory and by full-core Monte Carlo calculations. The performance of anisotropic diffusion and discontinuity factors is compared in terms of sensitivity to the micro-energy group structure used in Serpent, core eigenvalue, power distribution, sodium void reactivity worth and control rod worth. Anisotropic diffusion coefficients furnished a better overall agreement. Means of improving the performance of both modeling approaches are proposed.