Results of computational studies reactor fuel elements BREST-OD-300 and BR-1200 with gas and liquid metal sublayers

To increase the burnup in fast reactors fuel elements with mixed nitride fuel it is necessary to attenuate the factors, influencing on the mechanical loading of the fuel element cladding: such as the swelling fuel, fuel gas release and increasing the time until contact between the fuel and the fuel element cladding occurs. Obvious way to do this is the temperature decrease during irradiation. The thermal conductivity of nitride fuel is quite high, so that its temperature during irradiation is largely determined by the thermal resistance of the gap. In case of gas gap the important factor is the positive feedback, associated with the fuel gas release, which reduces the thermal conductivity of the gap, increases fuel temperature and with it further increases fuel gas release. The role of this factor can be significantly reduced by using fuel elements with a liquid metal sublayer, in which the gap is filled with one or another molten metal. For lead-cooled reactor fuel elements BREST-OD-300 and BR-1200 with a helium and liquid metal sublayer options for filling the gap with lead and various alloying elements are currently being actively studied. The paper presents the results of computational studies of the performance of fuel elements with gas and liquid metal sublayers of different compositions in order to select the geometric dimensions of mixed nitride fuel pellets. The results obtained are necessary for experimental confirmation of the fuel elements performance with cladding made of EP823-Sh steel at maximum burnup 12.5 % of heavy atoms and maximum damaging dose 169 FIMA (Fraction of initial metal atom).