Fuel-coolant interaction models based on corium experimental data under release of corium into water

Abstract

In the event of a severe accident with corium meltdown, if heat from the molten corium could not be removed, the failure of the integrity of the reactor vessel may lead to an accident where the corium leaks out of the vessel. If the molten corium discharged to the ex-vessel, the containment building is the only barrier against radioactive materials from leaking into the environment. Therefore, various types of core catchers and flooding strategies have been studied to prevent damage to concrete in containment buildings, and this thesis focused on the fuel-coolant interaction (FCI) in the pre-flooded cavity: coolability of the molten core and steam explosion. If high-temperature corium pours into the pre-flooded cavity, the steam explosion might occur due to its rapid reaction with the coolant, which might cause early containment failure. In order to analyze the conversion ratio of melt thermal energy to vapor mechanical energy during steam explosion, we developed a model based on two thermodynamic processes: thermal equilibrium and constant-volume process during the heating period, and the isentropic and adiabatic volume expansion process of coolant and fuel during the expansion period. The conventional thermodynamic models for the conversion ratio were modified introducing three parameters

fractions of corium and water participating in steam explosion, and the diameter of molten drops after the fine fragmentation process. We developed the model for the estimation of the fraction of water participating in steam explosion considering the contribution of conduction and convection to the heating process during the molten drop-water contact period. By analyzing 62 spontaneous vapor explosion corium experimental data, it was confirmed that spontaneous vapor explosion occurred in some cases only when the corium with UO2: ZrO2 component ratio of 70:30 was ejected into a water tank of 1.2 m or less. Based on these results, assuming that 100% of spontaneous steam explosions occurred under the conditions, the probability of occurrence of spontaneous steam explosions was estimated to be 0.28% for 92,099 randomly sampled cases. Even when 200 tons of the corium is ejected, the maximum energy conversion ratio is calculated to be 0.82%, so the possibility that ex-vessel steam explosion will cause early containment failure is considered negligible. We developed a COrium COolability Analysis tool named COCOA. COCOA is a transient calculation tool for the evaluation of the phenomena from the molten corium release to debris formation through the corium falling stage of FCI producing the transient results of pressure, released energy, average water temperature, fragmented particle size distribution, and particulate debris fraction. The jet breakup length prediction model experimentally optimizes the existing Epstein model to improve the prediction performance by 50% of RMSE to about 24%. The fragmented particle size distribution model was proposed using the critical Weber number criteria and the upper-limit log-normal distribution and validated by FARO and TROI experiments. COCOA was validated against FARO. Comparing to COMETA, which is a 2D severe accident analysis code, COCOA showed better accuracies on the prediction of the particulate debris fraction during the corium falling stage of the FCI phase. COCOA and COMETA predict the particulate debris fraction with 15.89% and 15.95~47.88% (with IKEJET model and with original model) of RMSE, respectively. Since COCOA is a semi-empirical model-based simple 1-D tool, only small computational effort is required for the calculation.