Numerical Investigation on the Effect of Fuel Pulvers on Axial Fuel Relocation

Burnup extension above the current regulatory burnup limit of 62 GWd/tU is an effective way economically increase cycle lengths of pressurized water reactors to 24 months. However, high burnup (>62 GWd/tU) fuel has been observed to severely fragment and even pulverize when subjected to temperature transient conditions such as loss-of-coolant accidents. After the cladding ballooning under loss-of-coolant conditions, the fuel fragments and fuel pulvers can axially relocate into the balloon region. The purpose of this work is to perform the three-dimension simulations of fuel relocation based on the validated simulation framework with coupled FEM-DEM approach to investigate the effect of fuel pulvers on axial fuel relocation. Parametric investigations are conducted on the relative increase of cladding volume and volume fraction of fuel pulvers. Based on the quantitative analysis, it is found that fuel pulvers can increase the filling ratio after fuel relocation, which means more fuel will accumulate in the ballooned region, probably resulting in higher local temperature during the loss-of-coolant transients. A series of fitting correlations of fuel stack reduction, mass fraction and filling ratio are proposed with respect to volume fraction of fuel pulvers. The proposed correlations can support the interpretation of experimental data, and can enhance the evaluation accuracy of fuel performance during loss-of-coolant accidents by the more realistic prediction of the critical parameters influenced by fuel relocation.