[en] The main function of sacrificing layer (SL) of external catcher is decreasing of influence on a refractory layer. This function can be overlapped with the capability of SL to immobilize radionuclide if SL composition is identical to the Synroc composition for example. For this purpose a part of substances can be used from corium of a reactor, the other part can be complemented with substances from SL. There were studied different compositions of SL, their behavior under high temperatures and crystallization and also interaction with refractory layer. The particular attention was paid to the action of iron oxides from corium on the properties of melt, eutectic temperature, viscosity, damping, infiltration into the refractory layer from zirconium dioxide stabilized by calcium oxide bricks. (orig.)