Modelling of fuel fragmentation, relocation and dispersal during Loss-of-Coolant Accident in Light Water Reactor

Abstract

Recent LOCA tests with high burnup fuel at the OECD Halden Reactor Project and at Studsvik demonstrated the susceptibility of the fuel to fragment to small pieces, to relocate and possibly cause a hot-spot effect and to be dispersed in the event of cladding rupture. However, the LOCA safety criteria defined by the US NRC are still based on fuel tests with fresh and low burnup fuel and therefore require revision for high burnup fuel. In this context a PhD project with the goal of developing new models for high burnup fuel fragmentation, relocation and dispersal during Loss of Coolant Accident in Light Water Reactors was launched at Paul Scherrer Institute in June 2013 with a financial support from swissnuclear. The work continued with base irradiation simulation with a closer look at the fission gas release measurements of high burnup BWR fuel rods.The data showed significant scatter for fuel rods at the same average burnup, originating from the same fuel assembly and having almost the same enrichment. The scatter was explained with a BWR-specific mechanism for fission gas trapping. It was motivated with Scanning Electron Microscopy images at the pellet-cladding interface that showed very strong fuel-cladding bonding layer on one side of the fuel rod. Base irradiation with the EPRI's FALCON code coupled with an in-house advanced fission gas release and gaseous swelling model GRSW-A was done for selected high burnup BWR fuel rods. The calculated fission gas release was overestimated in all cases. This was expected, because at the time the modelling did not account for fission gas trapping. From the available modelling and experimental data, a model for fission gas trapping was proposed. After calibration, the agreement between calculated and measured fission gas release was significantly improved. Fuel fragmentation modelling was focused on fuel pulverization. This is a high burnup fuel-specific phenomenon, in which the fuel pellet periphery may fragment to sizes less than 100 IŒm. Such fragments are very mobile and can be easily relocated and dispersed in the event of cladding rupture regardless of the rupture opening size. Therefore, the fuel fragmentation model addresses the potentially most important mode of fragmentation - fuel pulverization. Fuel relocation inside fuel rod is simulated during cladding ballooning and until cladding failure. The model takes as input the time-dependent cladding deformation supplied by FALCON and the fragment size distribution either provided directly by experimental data or by the fuel fragmentation model. Fuel relocation was modelled under the specific assumption that outermost fragments (e.g. pulverized fuel) relocate first, resulting in a large packing factor and local cladding temperature increase at the balloon (i.e. hot-spot effect) and, as a consequence, in enhanced cladding oxidation at the balloon. Fuel dispersal is modelled by solving the mass, energy and momentum conservation equations for two phases - the gas inside the fuel rod and the fraction of fuel which is considered moveable. Although the model uses simplified geometrical representation of the fuel rod and some other simplifying assumptions, the underlying reason for fuel dispersal (namely the interfacial friction between the gas outflow and the solid) is explicitly simulated. The model for fuel dispersal is calibrated using Halden and Studsvik LOCA tests.