Development and Implementation in the KIR Code of the Method for Accounting for the Continuity of Material Density Distribution

A method for accounting for the inhomogeneous density of a material in neutron transport modeling using the Monte Carlo method [1], where the density inhomogeneity is specified in the initial data of the program on the basis of piecewise continuous analytical functions of spatial coordinates, has been considered. This approach is similar to the method of aligned cross sections and is implemented in the KIR calculation code [2, 3] as one of the possibilities. Despite some kinship with the delta-tracking method [4, 5], the presented improved method is a completely independent development. The resulting algorithm for accounting for the continuous material density has been used in conventional zones of the NCG geometric module [6] with tracking of the transitions of particles across the boundary. The algorithm has been tested by calculating test models of VVER reactor cells with a sharp change in coolant density, similar to systems with supercritical coolant parameters, for example, SCWR [7]. In this case, the option of specifying initial data with the continuously changing material density can be applied if it is necessary to any type of problem and systems with any neutron spectrum (thermal, intermediate, and fast).