[en] An annular cross-section of the 80% enriched fuel elements enriched fuel elements in the thermal core of the coupled fast thermal system HERBE designed at the RB reactor in the VINCA, Beograd, ( Institute permits possibility of generation of heavy water steam in fuel cooling channels during an hypothetical accident. Influence of this local void effect at the reactor reactivity is examined using computer codes DENEB, AVERY and GALER developed at the NET Laboratory and verified in the experiments. Independent influence of steam generation inside and outside hollow fuel elements on reactor reactivity are calculated. For that purpose, the thermal reactor core is radially separated in two zones (TC1 with 16, and TC2 with 28 fuel elements). Steam voids are modeled by air contents into heavy water of the cooling zones of the fuel elements. The extreme conditions - total void (i.e. air gaps) in TC1 and TC2 cooling channels are verified in especially designed experiments at HERBE system confirming validity of the void model applied in the calculation