[en] The RSG-GAS reactor was designed to work at nominal thermal power of 30 MW with average thermal neutron flux of 2.10^-14 cm^-2s^-1. Forty fuel assemblies and eight control assemblies are expected to form the typical working core (TWC) so that the pattern of the in-core fel management could be carried out properly. One of the requested safety conditions is that the reactor could still be shut down even if the control assemblies with highest negative reactivity were in failure work properly. It is not possible, with respect to the prerequested safety condition, to construct directly a typical working core. Utilizing the IAFUEL code program, a series of transition cores (the cores before reaching the TWC) were set up. The TWC is expected to be reached after eight transition cores. (author)