COAL reflooding experiments during a loss of coolant Accident: Effect of the water flow rate, the pressure and the rod power with ballooned rods

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).

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, the effect of the pressure and the effect of the rod power as a function of the moment of availability of the safety pumps after the reactor scram. We provide experiments data on the coolability limits for different rod powers, which is given by the minimum of water flow to consider that the reflooding may be not impaired (PCT below the LOCA criterium of 1204 °C). The needed flow is ranging from 7 7 kg/s/m² (with intact rods geometry) at low power up 35 kg/s/m² (with at the high power that remaining in the core 1 min after the reactor scram) with a strong effect of the presence a partially local area due to rod ballooning during the large break LOCA accident. We outlined also the effect of the system pressure with a strong effect on the reflooding process above 10 bar up to 30 (for medium break LOCA).

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.