Gadolinium Credit Application for Transport and Storage Casks loaded with BWR UO2 Spent Fuel Assemblies

Transport and storage casks for BWR UO2 spent fuel assemblies are usually designed taking into consideration the criticality safety analysis with the assumption of fresh fuel and with an isotopic composition corresponding to the most penalizing fresh fuel without gadolinium. The effects of the burnup and the presence of burnable neutron poison fuel rods in the BWR fuel design, which can lead to an important gain in reactivity, were consequently not considered. In the last few years, AREVA TN has studied a conservative method by taking into account the gadolinium (Gd2O3) contained in the burnable poison fuel rods in order to increase the performance of the casks or to limit the amount of neutron absorbers (boron content or thickness of the absorber plates in new basket designs). This method is based on the definition of the operating conditions of the core and the fuel assembly parameters ensuring the conservatism of the calculations (spent fuel inventory and criticality assessments). Therefore, several calculations were performed to evaluate the sensitivity of different parameters. Indeed, current BWR UO2 fuel designs are highly heterogeneous both radially and axially, with the presence of partial-length fuel rods and positions of these rods in the fuel assemblies, each assembly with its specific initial fissile enrichment distribution and gadolinium fuel rod configuration. Moreover, depletion calculations of BWR UO2 fuel assemblies depend on the local core conditions: mainly the coolant void fraction, the presence of control rods during the operating cycle, the distribution of fissile content in the fuel assemblies, the number of burnable neutron poison fuel rods per fuel assembly, and positions of these rods in the fuel assemblies. This paper presents the results of the sensitivity calculations, the conservatism of the method developed for both depletion and criticality calculations, and the reactivity gained compared to the fresh fuel assumption.