Criticality analysis of canned MOX fuel rods in the TN 7-2 cask

To assess the feasibility of using the TN 7-2 cask for the transport of MOX fuel rods from a prototype fast reactor, a criticality analysis has been conducted with Monte Carlo simulation calculations. The fuel rods are placed loosely within steel cans, which poses a unique challenge with regards to criticality safety due to the wide variety of possible configurations.The basic design approach for the transport basket is a solid aluminum body with six separated drilled holes for loading the steel cans. A borated steel tube surrounds each position to decrease the neutronic coupling between the can inventories. The solid aluminum basket protects the inventory from the mechanical effects during all transport conditions.The fuel rods have been stored for more than a decade, so the radiation and thermal source strengths are not problematic, and the welded steel cans in combination with the fuel rod cladding and the TN 7-2 cask provide ample protection against any release of radioactive material. Accordingly, criticality safety is a crucial part of the overall safety analysis.A default approach to assess the criticality risk of loose fuel rods is to assume a regular hexagonal lattice and to determine the rod-to-rod pitch that defines the point of optimal moderation. In this case, however, this approach severely underestimates the potential of this inventory: by placing most of the fuel rods in a tight ring near the wall of the can and optimizing the lattice  distance of the remaining rods in the center of the can, the effective neutron multiplication factor keff can be increased by about 10%.To counteract this effect from extremely unfavorable fuel configurations, we capitalize on the fact that the cask is intended solely for transport and not for storage. This allows us to use moderator material like polyethylene to increase the neutron absorption in the borated steel walls and thereby decouple the seven cask positions from each other. The presentation will show the special design features of the basket which ensure safe subcriticality