Geologic disposal of HEU aluminum-based DOE fuels

This paper describes the disposal criticality analysis for canisters containing highly enriched uranium (HEU) in the form of aluminum-based Department of Energy spent nuclear fuel (DOE-SNF). The analysis is based on Massachusetts Institute of Technology (MIT) U-Al fuel with 93.5% enriched uranium which has a relatively heavy linear uranium loading. A canister design with 64 MIT assemblies (16/layer, 4 layers) totaling 35.2 kg of uranium was developed for this fuel type. A radial cross-sectional view of the MIT SNF canister is shown in a figure. Gadolinium phosphate (GdPO{sub 4}) distributed on, or in, carbon steel plates was incorporated into a carbon steel internal basket structure within a 439 mm OD, 15 mm thick XM-19 canister shell. The DOE-SNF canister is placed in a Codisposal waste package (CWP) centered in a pentagonal array of five HLW canisters. The disposal criticality analysis methodology was applied to develop the following scenarios unique to the CWP in general, and to the Al-based fuel in particular: (1) aqueous corrosion of stainless steel HLW and fuel containers; (2) degradation of HLW glass to clay; (3) degradation of Al-based fuel concurrent with or after the HLW glass; and (4) degradation of fuel canister basket materials including criticality control material, coupled with flushing soluble compounds from the CWP. The EQ3/6 program was used to analyze the chemistry/geochemistry of the system. Parametric analyses were run on a range of possible distributions of fuel within the CWP using MCNP to identify the most reactive configurations and determine the minimum amount of neutron absorber required to be distributed with the fuel in its degraded configuration