Reflooding model for quasi-intact rod configuration: Quench front tracking and heat transfer closure laws

Abstract

Reflooding is the main accident management measure in order to stop the progression of a severe accident in a pressurised water reactor (PWR). However, it remains difficult to predict the effects of reflooding in a core at very high temperature, where the core might have been significantly damaged. Some difficulties come from the uncomplete knowledge of the possible enhancement of Zircaloy oxidation caused by the strong steam production during reflooding. But other difficulties come from the uncertainties in the basic understanding and modeling of the flow and heat transfers across the fuel assemblies, damaged or not. Most of the codes used for severe accident calculations, in particular for PSA studies, must use rather large meshes (tens of cm) in order to keep the computation time reasonable. Therefore, they cannot benefit from models developed recently, taking into account phenomena occurring at a very small scale like the axial heat conduction in the wall. The present paper introduces a new model that takes advantage of recent experimental observations of the structure of the two-phase flow in the near the quench front. The basic idea of the model is to calculate an integrated heat flux over the mesh where the quench front is located, instead of calculating a heat transfer coefficient which is not the relevant parameter in such situation. In order to be consistent, the model requires an accurate tracking of the quench front position, which is done thanks to a method similar to the enthalpy method used to solve Stefan’s problem on a fixed grid (e.g. for solidification). The new model is assessed by comparing the predicted results with various sets of experimental data obtained in the large scale tests PERICLES (CEA, France) and RBHT (PSU, USA). The quench front progression appears to be well predicted. The time evolution of the cladding temperature during reflooding is also well reproduced. The model appears suitable for calculations of reflooding under various conditions (pressure, inlet velocity) and may be adapted to any code using large meshes, as it is the case for codes used to simulate severe accidents.