Crystalline plutonium hosts derived from high-level waste formulations

The Department of Energy has selected immobilization for disposal in a repository as one approach for disposing of excess plutonium (1). Materials for immobilizing weapons-grade plutonium for repository disposal must meet the ''spent fuel standard'' by providing a radiation field similar to spent fuel (2). Such a radiation field can be provided by incorporating fission products from high-level waste into the waste form. Experiments were performed to evaluate the feasibility of incorporating high-level waste (HLW) stored at the Idaho Chemical Processing Plant (ICPP) into plutonium dispositioning materials to meet the spent fuel standard. A variety of materials and preparation techniques were evaluated based on prior experience developing waste forms for immobilizing HLW. These included crystalline ceramic compositions prepared by conventional sintering and hot isostatic pressing (HIP), and glass formulations prepared by conventional melting. Because plutonium solubility in silicate melts is limited, glass formulations were intentionally devitrified to partition plutonium into crystalline host phases, thereby allowing increased overall plutonium loading. Samarium, added as a representative rare earth neutron absorber, also tended to partition into the plutonium host phases. Because the crystalline plutonium host phases are chemically more inert, the plutonium is more effectively isolated from the environment, and its attractiveness for proliferation is reduced. In the initial phase of evaluating each material and preparation method, cerium was used as a surrogate for plutonium. For promising materials, additional preparation experiments were performed using plutonium to verify the behavior of cerium as a surrogate. These experiments demonstrated that cerium performed well as a surrogate for plutonium. For the most part, cerium and plutonium partitioned onto the same crystalline phases, and no anomalous changes in oxidation state were observed. The only observed difference in behavior between cerium and plutonium was that plutonium partitioned more completely into the major host phases than cerium. Where cerium was sometimes observed at up to a few atom percent in crystalline or glassy phases, plutonium could not be detected in these phases. The crystalline plutonium host phases identified in this work included zirconolite, cubic zirconia, sphene, and an anorthite-like calcium aluminosilicate. Zirconia has been suggested as a possible material for immobilizing actinides (3), but this appears to be the first synthesis of such a material. Plutonium appears to stabilize the cubic (fluorite) structure through abroad solid solution range. Samarium can also be incorporated into this material, but is not necessary to stabilize the cubic structure. Plutonium leach rates, as measured by the Product Consistency Test (4), were on the order of 10-5 to 10-6 g/m2/day