Impact of corium thermophysical properties on fuel-coolant interaction

Abstract

During a severe accident in a nuclear reactor, the molten core may relocate in the lower head of the reactor vessel or in the reactor pit in case of vessel failure. In both cases, if water is present, it may lead to the so-called “fuel-coolant interaction”. When the melt jet enters into a water pool, it undergoes fragmentation and simultaneously initiates the rapid vapor generation due to high heat transfer. In the worst case, it could lead to steam explosion that may threat the integrity of the vessel or of the containment.The phenomenology of FCI phenomena depends on the nature of jet interaction with water: fragmentation/breakup, droplet deformation, vapor film destabilization. Many correlations have been developed to model these phenomena demonstrating the need for corium physical properties data. Nevertheless, experimental data about corium thermophysical properties are scarce, due to the difficulty to manage high temperature measurement.In order to assess the sensitivity of FCI scenarii to the corium thermophysical properties, French 900 MW PWR calculations have been performed using the MC3D code. Ten thermophysical properties (density, solidus-line temperature, liquidus-line temperature, solid heat capacity, liquid heat capacity, surface tension, emissivity, thermal conductivity, melting specific heat) were varied following the Morris statistic methodology. The sensitivity of the main calculation outputs (pressure peak, mass of fragment, steam production) has been quantified.The aim of the paper is to assess the impact of corium thermophysical properties on fuel-coolant interaction. In the first part, the phenomenology of FCI, together with the main correlations introducing the corium thermophysical properties, are presented. The main assumption of the calculation scenario is also given. The second part is devoted to the presentation of the Morris methodology and the definition of property variation range according to the literature or to model prediction. The results of MC3D calculations are presented in the fourth part. It exhibits the important role played by the corium surface tension on the progression of the FCI.