EXTENDED EXPERIMENTS ON THE STABILITY OF NEUTRON ABSORBER COMPOSITE MATERIAL IN DUAL PURPOSE CANISTERS: MINERAL AND STRUCTURAL PHASE CHANGES AT ELEVATED REPOSITORY TEMPERATURES

The U.S. Department of Energy Spent Fuel and Waste Disposition Campaign is investigating the use of Dual Purpose Canisters as a potential spent nuclear fuel storage system. Such dry spent fuel canisters would be loaded with fuel bundles, and then transported and stored at a nuclear repository. Separating the fuel bundles is a composite thermal Neutron Absorber Composite Material (NACM) consisting of sintered aluminium and boron carbide (B₄C) powders laminated within aluminium metal sheets.

This egg crate structure within the canisters is a vital component to prevent criticality issues. Although various tests have been performed on NACM, this material has not been subjected to hydrothermal conditions such as when a canister is breached after emplacement. The original set of experiments reacted NACM coupons with DI water at 150, 225, and 300 °C at 150 bar for 2 weeks. Full results were published by Sauer et.al. (2020). The aluminium metal was converted primarily to boehmite as

2Al_(s) + 2H₂O = 2AlO(OH)_(s) +H_2(gas).

Al metal + water = boehmite + hydrogen gas

Over 1700 ml of H₂ gas were generated. The coupons doubled in volume and increased by ~63% in mass. The coupons remained structurally intact, but likely warped due to preferential crystal growth on the Al sheets. Reaction fluids were enriched in aluminium and boron.

To further our understanding of NACM stability we chose to study galvanic coupling. Galvanic coupling between steel and aluminum has been shown to cause preferential corrosion of aluminum, and, therefore, may be important to consider in models of waste package internal degradation.

The NACM plates was sandwiched between steel coupons and secured with platinum wire. Three experiments (two weeks each) were then completed at the same temperatures. Aqueous boron concentrations in the completed NACM-steel experiments are elevated in comparison to values in all of the NACM-only experiments. Gas generation of H₂ has doubled due to the galvanic coupling. The NACM coupons in the galvanic experiments increased in volume and mass at slightly higher values than the NACM-only experiments. In the 300C experiment the NACM coupons disintegrated. 304SS coupons included in the experiment did not experience a change in volume or mass. The results are significant for understanding material stability in a breached canister scenario.