Modelling Reflooding Of Intact Core Geometries In Astec V2.1: Improvements And Validation On Pericles Experiments

The injection of water into core (core reflooding) is a key Accident Management strategy to prevent or mitigate core degradation in water-cooled nuclear reactors. Among the flow regimes implicated during reflooding, the most significant one in terms of core heat transfer removal is the transition boiling, which takes place over a short distance near the quench front. In the ASTEC V2.1 integral severe accident code, the user can select among two concurrent two-phase thermalhydraulics modelling (five- or six-equation scheme) and for each one, two alternative reflooding models to describe the transition boiling region (TBR). On one hand, the DROPLET model calculates a large heat transfer coefficient over a user-defined distance above the quench front. On the other hand, the EXPCHF model calculates the heat flux downstream by imposing the Critical Heat Flux at the quench front and prescribing an exponentially decreasing heat flux along a flow-dependent length. This article aims at selecting the most appropriate reflooding model for the evaluation of the heat transfer associated with transition boiling in ASTEC V2.1 both for the five and the six-equation hydrodynamic model. For that purpose, experimental data from the PERICLES facility dealing with the reflooding of intact core geometries have been used. After an exhaustive analysis of the different reflooding and hydrodynamic models, this study has found that the DROPLET model should be selected when using the five-equation scheme, since it provides enough rod precooling above the quench front. However, the EXPCHF model should be selected when using the six-equation scheme, since it benefits from the larger amount of liquid available near the quench front. This last combination gives the best results on the simulated PERICLES tests and is thus recommended for reactor applications.