Reflooding of degraded cores in ASTEC V2.1: Modelling and validation on PEARL experiments

Abstract

The injection of water into the reactor vessel at a late stage of a severe accident (core reflooding) is one of the key Severe Accident Management measure to stop the progression of the accident in Light Water Reactors. Therefore, an appropriate modelling of the physical phenomena involved in this process is of paramount importance for the enhancement of safety of these nuclear power plants.This article presents the improvements performed in the ASTEC V2.1 integral severe accident code in order to obtain a comprehensive reflooding model, which is valid regardless the core damage state. The main development consists in replacing, in presence of debris, the single momentum conservation equation (plus the drift flux relation) by two momentum conservation equations (one for each phase) with specific porous friction terms. Moreover, the calculation of the quench front position and the heat transfers downstream of the quench front have been harmonized.The physical models are validated against experimental data from the PEARL facility where a large heated debris bed is quenched. A wide range of thermal–hydraulic (pressure, injection velocity) and geometrical parameters (bed particle diameter and bypass thickness) is investigated. Generally, ASTEC V2.1 provides a good agreement for tests involving either a one or two-dimensional quench front progression. However, conversion ratios of produced steam to injected water seem to be overestimated in two-dimensional tests with fine particle diameters, which raises questions about the modelling of capillarity in those situations.The validated version of the code is then used to gain further insights on degraded core reflooding: radial steam redistribution is mainly driven by the large pressure gradients generated within the bed, whereas water entrainment is driven by the interfacial drag between the redistributed steam and the liquid, both contributing to hinder the coolability of the debris bed.