COAL Experiments Investigating the Reflooding of a 7 × 7 Rod Bundle During a Loss-of-Coolant Accident: Effect of a Partially Blocked Area with Ballooned Rods

During a loss-of-coolant accident in a pressurized water reactor, the drying of the fuel assemblies leads to an increase in the fuel temperature and deformation of the fuel rod claddings. In addition to the restriction of the flow area, the relocation of the fragmented irradiated fuel within the ballooned area leads to an increase in the local residual power. The COAL (COolability of a fuel Assembly during Loca) experiments focused on the coolability issue of a partially deformed fuel assembly during water injection, with the safety systems using a 7 × 7 bundle of electrically heated rods. These experiments are part of the PERFROI (PERte de reFROIdissement) project launched by IRSN (Institut de Radioprotection et de Sureté Nucléaire) with the support of the French Agence Nationale pour la Recherche, Electricité de France, and the U.S. Nuclear Regulatory Commission.

The effects of the flow blockage [intact geometry up to long ballooning (100 to 300 mm) with different blockage ratios of 80% to 90%] were evaluated for various powers, inlet water mass flow rates, and different pressures representative of large-break loss-of-coolant accident (at 0.3 MPa) and medium-break loss-of-coolant accident (from 0.5 to 3 MPa) configurations. The relocation of fragmented fuel in the balloons was taken into account by a local increase in the power by a factor of 1.5.

This paper presents the thermal-hydraulic parameters and the main results of the experiments performed in a Canadian facility of 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 pressure. The presence of the balloons significantly increased the peak cladding temperature according to the flow rate, the pressure, and the power. These results can be used to improve and validate the heat exchange models of thermal-hydraulic codes dealing with the complex reflooding processes in such a configuration.