Monte Carlo current-based diffusion coefficients: Application to few-group constants generation in Serpent

• Implementing a new Monte Carlo method for axial diffusion coefficients in Serpent.
• Fast reactor diffusion calculations via Monte Carlo XS data generated by Serpent.
• Comparison between the use of isotropic or directional diffusion coefficients.
• Comparison between two formalisms to account for neutron leakage at cell level.
• Quantification of the effect of group condensation onto a coarse 7-group structure.

Criticality eigenvalue and power distributions of a medium-sized sodium-cooled fast reactor core were studied by combined diffusion theory and Monte Carlo methods. Few-group XS data sets generated by the Monte Carlo code Serpent for both normal and sodium-voided cells were used in the multi-group diffusion code TRIZ. Two different approaches were adopted in order to account for neutron leakage at fuel assembly level: a radially-reflected and axially-heterogeneous model with vacuum boundary conditions at the bottom and the top; and a more typical infinite cell calculation, followed by criticality spectrum corrections. In addition to the standard diffusion coefficients calculated by Serpent, a novel method for the calculation of directional diffusion coefficients was implemented and tested, yielding satisfactory results for normal and sodium-voided conditions, using Monte Carlo results as a reference. The feasibility of using the B1B1 criticality spectrum as a weighting function for these diffusion coefficients was also tested, and slightly better estimates of k-eff   were obtained when compared to the direct use of diffusion coefficients arising from the solution of the B1B1 equations.