Influence of corium composition on ex-vessel steam explosion

Abstract

The worst event that could happen in a nuclear power plant is a severe accident. If during such accident the corium melt comes into contact with the coolant water, a steam explosion may occur. At some stage of the accident progression, the primary system and the containment integrity of the nuclear power plant are jeopardized.In the article, the material influence of the oxide and metal corium on the ex-vessel steam explosion is investigated. The main purpose of the research work was a comparative analysis of different scenarios of steam explosion in the flooded reactor cavity. The purpose was to determine the influence of the material properties of the melt (comparison of oxide and metallic melt) and the oxidation of the metallic melt on the steam explosion strength. A PWR ex-vessel steam explosion study was carried out with the MC3D computer code. A premixing simulation and an explosion simulation were performed for each type of the corium. The explosion was triggered at the time of melt bottom contact. The global jet breakup model was used to simulate the premixing simulations.In the first part comparative calculations with oxide corium and oxide corium with prescribed thermal properties of metal corium are presented to find the isolated influence of thermal properties. The analysis of PWR ex-vessel steam explosion with oxide corium was already performed in the past (Leskovar and Uršič, 2014), but in this paper the comparative calculations with oxide corium and oxide corium with prescribed thermal properties of metal corium are presented for the first time. The purpose was to determine the differences in premixing and the steam explosion strength at simulations considering solely the different thermal properties of the oxide and metal corium.In the second part of the paper the influence of the metal corium oxidation on the ex-vessel steam explosion is presented. Due to the focusing effect the reactor vessel would most likely fail at the side and the outflow of metal corium would occur. First of all it was necessary to collect material properties of metal corium. The ex-vessel steam explosion was investigated and discussed by varying the metal corium oxidation rate. Additionally, based on experimental findings, the hydrogen film hypothesis for oxidation is presented. In this paper the calculations with properties of metal corium are presented for the first time. With the comparison of the simulation results the influence of the corium composition on the strength of the steam explosion is analysed. The simulation results revealed that with metal corium the strength of the steam explosion is in some cases higher than with oxide corium.