[en] Fuel elements produced from spent fuel are porous media with spatially varying characteristics. A hierarchical discrete structure for the numerical modeling of heat-transfer processes in media with an anisotropic geometry that is characterized by both the microscopic voids and macroscopic changes in the parameters is proposed. The basic unit of the structure at its lower level is a cell that represents the local properties of the medium. The cells have a standard interface that allows one to form three-dimensional networks of such cells. Different types of cells in the network represent macroscopic changes. The potential for parallel processing is analyzed

[en] Application of the first collision probability (FCP) method to the system with boundary conditions of given currents or white reflection is considered. The equations of the method are generalized for the anisotropic case by introducing several angular modes in each zone.

[en] The first collision probability (FCP) method allows generalization by introducing several angular modes in each zone. Algorithms are proposed for calculating the components of the corresponding first collision tensor. These algorithms do not increase the computational complexity beyond that of the matrix calculation algorithm in the traditional FCP method and allow calculating a system described in terms of combinatorial geometry. Fourfold integrals are numerically taken in calculation of a three-dimensional system and twofold integrals are taken for two-dimensional geometry. The geometrical capabilities are ensured by using the standard geometrical module of the Monte Carlo method.

[en] The design of the ASTRA facility and critical assemblies that simulate physics features of modular high-temperature reactors (HTHR-Ms) with a graphite moderator and reflectors loaded with fuel particles having multilayer ceramic coatings is described in detail. Geometrical dimensions of the main elements and regions of the critical assemblies, composition of the materials used, and experimental results for various configurations of the critical assemblies are presented. A detailed computational benchmark model allowing for the structural and compositional features of the critical assembly configurations in question is developed on the basis of all the above data. The results are to be used for verification of the neutronics codes used for calculations of high-temperature helium-cooled reactors.