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

Long-term spent fuel (SF) storage is one option included in nuclear fuel cycle strategies considered in Japan. In facilities for this, criticality safety should be confirmed. Generally, to reduce the neutron multiplication factor, wide spaces are provided between spent fuel assemblies. However in the case where it is difficult to increase the distance between assemblies, we can alternatively separate the assemblies from each other in the axial direction to reduce neutron interaction efficiently between regions where contains much fissile materials. In this study, the possibility of reduction of neutron multiplication factor by this alternative axial separation of spent nuclear fuel is considered. Several criticality calculations were conducted in order to evaluate the effect of this alternative axial assembly separation (A3S) method, which separates the assemblies not in a horizontal direction rather in the vertical direction. For both PWR and BWR fuels, A3S results in smaller neutron multiplication factors than a normal configuration. Also A3S reduces the end effect which is important in determining the criticality of spent nuclear fuels. This study shows several design configurations optimize criticality safety of spent fuel storage facilities in order to reduce the neutron multiplication factor and the end effect, for effective operation of such facilities.