Cumulative migration method for computing rigorous diffusion coefficients and transport cross sections from Monte Carlo

The cumulative migration method (CMM) for computing lattice-homogenized multi-group neutron diffusion coefficients and transport cross sections from Monte Carlo is proposed in this paper. CMM is demonstrated to be both rigorous and computationally efficient, while eliminating inaccuracies inherent in commonly-applied transport approximations. In the limit of a homogeneous hydrogen slab, the new method is shown to be equivalent to the long-used, and only-recently-published CASMO transport correction method employed for production LWR analysis. Results demonstrate that CMM can produce rigorous few-group assembly-homogenized diffusion coefficients directly from heterogeneous Monte Carlo lattice tallies-without requiring the intermediate step of tallying of fine-group cross section data commonly required for P1 or B1 calculations of diffusion coefficients. Comparisons with several common diffusion coefficient approximations are made for both simple homogeneous media and fully heterogeneous lattices. CMM is demonstrated to produce 2-group diffusion data for the BEAVRS PWR lattices, as well as 11-group directional-dependent diffusion coefficients for the TREAT graphite/fuel lattices. Core flux distributions and eigenvalues computed using CMM diffusion coefficients are demonstrated to be more accurate than those obtained with traditional methods for approximating diffusion coefficients.