[en] The use of tight pitch lattice and MOX fuel is one of the ways to improve fuel utilization in WWERs. The presented report is devoted to an analysis of main neutron-physical and thermohydraulic problems connected with such core design. In order to achieve the needed accuracy of tight pitch lattices burnup calculations it is necessary to provide a high accuracy of reaction rates calculations in the resonance energy range and therefore the cross sections of the used Pu, Am, Cm isotopes and fission products should be reliable. For such calculations the UNIRASOS-2 and SAPFIR codes are used at the Kurchatov Institute. The ''second'' equivalence theorem with specially selected parameters and generalized subgroup approach are used for resonance treatment in these codes. MCU code package calculations have been used for their validation. This code package is based on the Monte-Carlo method with a detailed description of cross section energy dependence in the energy range of resolved resonances. A comparison of MCU, UNIRASOS and SAPFIR calculations with the results of precision calculations, results of benchmark problems on tight lattices burnup solutions and measured data obtained at the PROTEUS critical assembly, has shown that the obtained accuracy of these codes is satisfactory for practical purposes. However, in order to estimate the reliability of void reactivity coefficient calculations under low moderator density, it is necessary to perform special investigations. For two dimensional pin power distribution calculations, the 4-group code PERMAK, in which both diffusion and nodal type balance equations are realized, has been used. (author). 30 refs, 4 figs, 6 tabs