Cross-section of fuel assembly of cassette-type core of SNPP reactors: а) cassette of ABV-6 reactor (IAEA-TECDOC-1536); b) Cassette of KLT-40S reactor (Alekseev et al. 2016); в) Model of cylindrical reactor cell; 1 – fuel element; 2 – absorber element; 3 – burnable absorber rod; 4 – internal tube; 5 – external tube; 6 – burnable absorber rods (BAR) or operational neutron sources (ONS) with external diameter equal to 6.2 mm; 7 – BAR or ONS with external diameter equal to 4.6 mm; 8 – “heavy” fuel element; 9 – “light” fuel element; 10 – burnable absorber; 11 – fuel; 12 – fuel cladding; 13– coolant (water); 14 – structural materials.