Criticality control for direct disposal of dual purpose canisters

Dual Purpose Canister-specific burnup credit criticality calculations were performed, omitting the criticality worth of relatively short-lived aluminum-based neutron absorber materials, in order to conservatively assess the feasibility of direct disposal of DPCs without repackaging into disposal canisters. Calculations using the actual isotopic content and loading position of each of the 32 spent fuel assemblies stored in each of two Holtec MPC-32 DPCs were performed, but without the neutron absorber materials. For already loaded DPCs, the results show that criticality safety can not always be unequivocally demonstrated through using burnup credit alone. Criticality control, however, may be enhanced by the inclusion of moderator displacement by burnable poison rod assemblies (BPRAs) stored in the assemblies. For as-yet to be loaded DPCs, reactivity management during loading through emplacement of the least reactive spent assemblies in the center of the canister is also possible. Other options include placement of surrogate control rods, containing fresh, durable neutron absorber material, into center locations of the DPC; such an approach would permit any DPC to satisfy criticality safety for direct disposal. Repository calculations with partially flooded canisters showed much lower reactivities and the effect of cool time was also evaluated. (authors)