COAL Reflooding experiments during a Loss Of Coolant Accident : effect of the water flow rate and the residual power with ballooned rods

Abstract

During a loss of coolant accident (LOCA) in a pressurized water reactor, the drying of the fuel assemblies leads to an increase in the fuel temperature and a deformation of the fuel rod cladding. The COAL experiments focused on the coolability issue of a partially deformed fuel assembly during water injection with the safety systems using a 7x7 bundle of electrically heated rods. The relocation of the fragmented fuel in the balloons is taken into account by a local increase in power by a factor of 1.5, and the effect of the flow area restriction is provided with various flow blockage {intact geometry up to moderate and long ballooning (100 and 300 mm) with different blockage ratios (80 and 90%)}. These experiments, in the frame of the PERFROI project, were launched by the “Institut de Radioprotection et de Sureté Nucléaire” (IRSN) with the support of the “Agence Nationale pour la Recherche” (ANR), Electricité de France (EDF) and the US-Nuclear Regulatory Commission. This paper presents the thermal hydraulics parameters and the main results of some experiments carried out in a facility of the STERN Laboratories. We studied the effect of the inlet water flow rate which is the consequence of the amount of water entering the reactor core after the break of the primary circuit and the effect of the residual power as a function of the moment of availability of the safety pumps after the reactor scram. These results are used to improve and validate the heat exchange models of thermal hydraulics codes dealing with the complex reflooding processes in such a configuration.