Analysis of experiments on the corium-to-water thermal interaction during a severe accident at a VVER/PWR-based NPP

Abstract

The basic aim of the FARO experimental program is to obtain data on the mechanisms governing thermal interaction between a large mass of high-temperature corium with water through simulating the in-vessel stage of a severe accident at an NPP equipped with VVER/PWRtype reactors. The study is devoted to validating the thermal interaction model used in the SOCRAT/VAPEX-M code. The article presents the system of model equations, including those describing the corium jet dynamics and the corresponding closing relations describing the energy and force interaction between the corium jet and steam--water medium. Special attention is paid to the corium fragmentation models, including the initial corium jet fragmentation model and the so-called model for secondary fragmentation of the produced fragments. The FARO L-14 and L-19 experiments aimed at investigating the corium-to-water thermal interaction were numerically simulated using the SOCRAT/VAPEX-M code. The specific feature of these experiments was that they were carried out at a high initial pressure of around 5 MPa. The experiments differed from each other in the initial water level inside the vessel. Thus, the effect the initial water level has on the thermal interaction process was investigated. The time dependences for the pressure in the vessel, corium cooling rate, and distribution of the produced fragments calculated using the SOCRAT/VAPEX-M code were found to be in good agreement with the experimental data. Qualitative and quantitative agreement between the experimental data and calculation results has been shown, which confirms the adequacy of the thermophysical and hydrodynamic models laid down in the SOCRAT/VAPEX-M code.