Feasibility of Direct Disposal of Dual-Purpose Canisters-Options for Assuring Criticality Control

The concept of direct disposal of dual-purpose canisters (DPCs) has not been previously considered
for the Yucca Mountain geologic repository because of concerns, among other reasons,
about degradation of the reactivity-control material over the relatively long period of the repository
analyses. Aluminum-based neutron absorber materials, typically used in DPCs, are not
expected to have sufficient corrosion resistance necessary to retain their integrity over a 10,000+
year period. Using burnup credit, canister-specific criticality safety calculations were performed
without the criticality worth of aluminum-based neutron absorber materials to conservatively
assess the feasibility of the concept from a criticality safety perspective. Two Holtec MPC-32
DPCs located at the Sequoyah Nuclear Plant were arbitrarily selected to demonstrate the feasibility
of this approach. Calculations for the isotopic content of each of the 32 spent fuel assemblies
stored in each DPC were performed, and their reactivity with the actual isotopics and
loading positions, but without the neutron absorber materials, was determined. For already
loaded DPCs, the results show that criticality safety can not always be unequivocally demonstrated
through using burnup credit alone, except by taking into account a sufficiently large
number of neutron absorbing fission products and reasonable values for biases and uncertainties.
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.
It is more likely that a DPC would be partly flooded, rather than fully flooded as it is for typical
repository calculations. Calculations showed that the DPC is insensitive to the ingress of small
volumes of water, reducing the overall risk of criticality in the disposal environment. Calculations
also show that the repository temperature at time periods when water ingress becomes
possible has little effect upon the reactivity.