The disposal of low-level radioactive waste (LLW) containing special nuclear material (SNM) presents some unusual challenges for LLW disposal site operators and regulators. Radiological concerns associated with the radioactive decay of the SNM are combined with concerns associated with the avoidance of a nuclear criticality both during handling and after disposal of the waste. Currently, there are three operating LLW disposal facilities: Envirocare, Barnwell, and Richland. All these facilities are located in U.S. Nuclear Regulatory Commission (NRC) Agreement States and are regulated by their respective state: Utah, South Carolina, and Washington. As such, the amount of SNM that can be possessed by each of these facilities is limited to the 10 CFR Part 150 limits (i.e., 350 g of uranium-235, 200 g of uranium-233, and 200 g of Pu, with the sum-of-fractions rule applying), unless an exemption is issued. NRC has applied these SNM possession limits to above-ground possession. The purpose of this report is to provide data which could demonstrate that SNM waste at emplacement will not cause a nuclear criticality accident. Five different SNM isotopic compositions were studied: 100 wt% enriched uranium, 10 wt% enriched uranium, uranium-233, plutonium-239, and an isotopic mixture of plutonium (76 wt% plutonium-239, 12 wt% plutonium-240, and 12 wt% plutonium-241). Three different graded-approach methods are presented. The first graded-approach method is the most conservative and may be applicable to facilities that dispose of very low areal densities of SNM, or dispose of material with a low average enrichment. It relies on the calculation of average areal density or on the average enrichment of SNM. The area over which averaging may be performed is also specified, but the emplacement depth is not constrained. The second graded-approach method relies on limiting the average concentration by weight of SNM in the waste, and on limiting the depth of the emplacement. This method may apply to facilities that emplace somewhat higher areal densities of SNM but do not use vaults or segmentation in the disposal emplacement. The third graded-approach method relies on limiting the average concentration by weight of SNM in the waste, and on the presence of segmenting barriers, such as vaults, which will mitigate interaction between units of SNM. This method may apply to facilities that use concrete vaults in their disposal areas, and allows even higher areal density of SNM in the disposal site.
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