[en] Comparative evaluations of research sodium cooled fast reactors fueled with plutonium or uranium carbides have been made. The core represents a homogeneous mixture of major constituents - fuel, sodium and steel. The physical calculations have been fulfilled in terms of the 26-group P₁-approximation in the spherical geometry within the wide range of core volumes (20-1000 l). The plutonium fuel is concluded to be superior to the uranium one from the standpoint of the reactor ''quality'' (ratio of the maximum neutron flux to the reactor power), fuel concentration in the critical volume of the core and the neutron spectrum hardness