Experiments on MCCI with oxide and steel

Recently performed experimental programmes at the French VULCANO and the German MOCKA and SICOPS facilities aimed at the further elucidation of various phenomena of molten core-concrete interaction (MCCI). Questions on these phenomena arose during the scientific discussion of MCCI in the last years. The large-scale MOCKA (KIT, Karlsruhe) experiments study the interaction of a simulant oxide (Al2O3, ZrO2, CaO) and metal melt (Fe) with concrete. To allow for a long-term interaction, internal heating was provided by alternating additions of alumino-thermite and Zr metal to the upper oxide layer of the stratified melt. Since the heat generated by the thermite reaction and the exothermal oxidation reaction of Zr is mainly deposited in the oxide phase, prototypic heating of both melt phases is achieved. Recent tests in the MOCKA (KIT, Germany) program are focused on assessing the influence of a typical 6wt.% reinforcement in the concrete on the erosion behaviour. The experiments were performed in siliceous concrete crucibles with an inner diameter of 25cm and a height of 1.3m. In these experiments, the overall downward erosion by the metal melt was of the same order as the sideward one. In addition, the lateral erosion in the overlaid oxide melt region was about the same as in the metal melt region. Experiments with prototypic UO2-containing melts have been conducted in parallel in the VULCANO (CEA, Cadarache) and SICOPS (AREVA, Erlangen) facilities. In VULCANO a plasma arc furnace melts the oxide corium while three 1-L steel induction furnaces melt the steel. All these melts are poured in a concrete cavity where the decay heat is applied to the oxide phase via 40-kHz induction coils. VULCANO VBS tests with oxide and metal have shown an increased ablation in front of the metallic masses. More unexpected results concern a very high steel oxidation (especially in VBS-U1 with limestone-rich concrete in which almost all the steel was oxidised) and the fact that post test examinations did not show the expected horizontal steel layer. Induction heating was also used for decay heat simulation in the laboratory-scale SICOPS tests, using a “cold crucible” technique and a higher frequency of about 1.3MHz. Tests were performed to study the influence of an additional metal phase on the 1D erosion behaviour. For these mixed melt tests, concrete specimen of 10cm diameter, made of siliceous concrete with grain aggregates, were used. Oxide melts were either simulant or prototypic melts. Metallic melt was form by addition of steel (steel 37.1) pellets after oxidic melt has been produced. It was found that the impact of the metal phase depends on heating power and gas release rate. The analysis suggests that the mechanism of concrete attack by metal melt can be understood as a thermal destruction mechanism.