Decreasing in neutron multiplication factor in spent fuel storage facilities by changing fuel assembly position in axial direction

Suyama, Kenya; Murazaki, Minoru; Okuda, Yasuhisa*

Long-term spent fuel (SF) storage becomes one of the options in nuclear fuel cycle strategies in Japan. In such facilities, criticality safety should be confirmed. Generally, to reduce the neutron multiplication factor, taking wide spaces between spent fuel assemblies is usually considered. However, from the view point that reducing neutron interaction between regions where contain much fissile materials, we are able to shift the assemblies in axial direction alternatively. In this study, the possibility of reduction of neutron multiplication factor by the alternative axial shift of spent nuclear fuel is presented. Several criticality calculations were conducted in order to evaluate the effect of alternative axial assembly shift (A3S) method, which shift the assembly not to horizontal but to vertical directions. For both PWR and BWR fuels, we have smaller neutron multiplication factors if we adopt A3S method than normal configuration. Concerning the end effect which is important for criticality of spent nuclear fuels, it was confirmed that A3S method reduces the end effect. This study shows that several configurations can be considered to optimize criticality safety design of spent fuel storage facilities to reduce the neutron multiplication factor and the end effect for effective operation of such facilities.