Cross section of pressure channel of light water reactor core: a) pressure channel 10-14-3М of icebreaker OK-900A reactor core У (Vatulin et al. 2005); b) pressure channel of UNITERM reactor core (IAEA-TECDOC-1536); c) model of fuel cluster of ship propulsion nuclear reactors. 1 – brace of spacer grid; 2 – fuel rod; 3 – operational neutron source; 4 – burnable absorber rod; 5 – tabular displacer; 6 – casing; 7 – coolant; 8 – fuel layers; 9 – interchannel water.