The multipurpose fast research reactor (MBIR) is intended for research and experimental work using reactor radiation. The problems and questions arising in the course of neutron-physical calculations are examined and ways to resolve them and alternative variants are described. As a result of the computational and analytical studies and development work, a reactor core for MBIR meeting nuclear safety requirements and possessing high research potential took shape.The multipurpose fast research reactor (MBIR) now under development is intended for experimental research using reactor radiation [1]. According to the concept of a reactor plant, a large part of the experimental facilities must be placed within the confines of the core, and the maximum neutron flux density must reach at least 5·10 15 sec -1 ·cm -2 . In accordance with the problems posed, the reactor plant includes three auxiliary loops, autonomous instrumented experimental channels, more than 10 materials science and isotopic assemblies as well as vertical and horizontal experimental channels located behind the reactor vessel.The versatility of MBIR and the diversity of experimental facilities and different restrictions which must be taken into account in the neutron-physical calculations dictate the specific features of the core and side screen.The following conditions were taken as initial data for the neutron-physical calculations of the core: thermal power of the reactor 150 MW, mixed uranium-plutonium oxide fuel, maximum lineal thermal power density of the fuel-elements 500 W/cm, interval between refuelings at least 100 days, reactor service life limited by the damaging dose on the structural elements, number of control rods in the control and protection system (CPS) and absorber based on enriched boron carbide or natural isotopic composition.The use of two types of fuel elements with the dimensions 6.9 × 0.4 and 6 × 0.3 mm are studied in the project [2]. The neutron-physical characteristics of the core were calculated for the equilibrium operating regime, fuel assemblies in the core were rearranged before extraction and fuel assemblies accumulated fluence not exceeding the maximum admissible level. The arrangement of the core with 6.9 mm in diameter fuel elements is characterized by larger size and more fuel assemblies, which makes it possible to place in it more cells for materials science and isotopic assemblies with better quality in regards to some neutron physical aspects (location). Nonetheless, the decisive factors in comparing two variants were the volume power density and, correspondingly, higher neutron flux density (about 20%) using 6 mm in diameter fuel elements with maximum lineal thermal power 500 W/cm. Thus, a hexahedral fuel assembly with span size 72.2 mm and 91 fuel elements with diameter 6 mm were adopted for modeling.The core arrangement was determined by the problems facing the facility or the particulars of the design. For example, the conditions of the planned experiments in an auxiliary channel with sodium coolant required in...