Modelling axial relocation of fragmented fuel pellets inside ballooned cladding tubes and its effects on LWR fuel rod failure behaviour during LOCA

Downward axial relocation of fuel pellet fragments may occur when overheated and internally overpressurized cladding tubes of light water reactor fuel rods distend due to creep during a loss-of-coolant accident (LOCA). The relocation is of safety concern, since it changes the axial distribution of heat load along the rod and also has the potential to increase the amount of fuel material dispersed into the reactor coolant, should the cladding fail. Here, we present a computational model that calculates the fuel relocation on the basis of estimated fuel fragment size distributions and the calculated cladding distension along the fuel rod. The model has been implemented and fully integrated with the FRAPTRAN-1.5 computer program, such that thermal feedback effects of fuel relocation on the axial redistribution of fuel mass, stored heat and power are accounted for in FRAPTRAN’s calculations of the fuel rod thermo-mechanical behaviour. The model has been validated against the IFA-650.4 integral LOCA test in the Halden reactor, Norway, which was done on a very high burnup UO2 fuel rodlet and resulted in extensive fuel pellet pulverization, axial relocation and dispersal into the coolant. Our simulations of this test suggest that thermal feedback effects from axial fuel relocation are strong enough to significantly affect the dynamics of cladding ballooning and rupture, in spite of the short duration of these processes. Moreover, for the considered LOCA test, the axial relocation has a strong effect on the calculated peak cladding temperature and oxidation after rupture