Numerical Investigation of the Possibility for “Dry” Recriticality of the Molten Corium during Late In-Vessel Phase of Severe Accident at LWR – D. Popov, O. Runeval

During a hypothetical severe accident with LWR, a molten corium pool would be formed in RPV bottom head containing large amount of fissile material as tons of Uranium and hundred kilograms of Plutonium for EOC. Extensive part of these are oxidized, but some part is transformed into pure heavy metals, due to high temperature chemical interactions between molten core components and with the wall of RPV, as shown in some experimental investigations during the last decades in Russia and France. The question arises whether critical mass could be formed in this case. In the open literature, there was no quantification of the “dry” (i.e. without water overflow) recriticality of the molten corium in the bottom of reactor head. This recriticality would be mainly on fast neutrons and several kilograms of Plutonium are enough for instance to form a critical mass. To answer to this, a numerical investigation of keff of the molten corium pool has been performed by a special methodology, which consists in a consecutive usage of several numerical codes – ORIGEN and MCNP for neutronphysics calculations for EOC, ASTEC for system calculations of severe accident phenomena, THERMOCALC for thermochemical calculations of the system U-Pu-Zr-O and again MCNP for keff calculation of molten corium. VVER-1000 core was used as referent. The results show that “dry” recriticality is very unlike to happen, because keff obtained is less than 1.