Study of Pyrochlore and Garnet-based Matrices for Actinide Wastes Produced by a Self-propagating High-temperature Synthesis

Abstract

Actinide-containing wastes are among the most dangerous for the environment. Such waste streams originate from reprocessing operations with irradiated nuclear fuel and conversion of weapons-grade plutonium metal into dioxide. The long-term toxicity of actinides derives from the presence of isotopes with half-life varying from hundreds of years (Am²⁴¹) to tens of thousands (Pu²³⁹) or even millions of years (Np²³⁷). Therefore, these waste fractions need to be incorporated into durable crystalline host phases. The matrices have to incorporate substantial amounts of actinides, and possess chemical durability and resistance to radiation damage. Complex oxides with fluorite-derived and garnet lattices meet these requirements. Self-propagating high-temperature synthesis (SHS) based on exothermic oxidizing-reduction reactions may be used for production of these waste forms. This technology has the following advantages: absence of extrinsic heating sources, low energy requirements for equipment, high reaction velocity, simplicity of design of processing equipment, feasibility of remote-handling the processes, and lack of considerable amounts of facility decommission wastes. The basic features of the SHS technology are as follows: duration of initiation is 0.05–5.0 sec, temperature in a combustion wave is within 1500–3000 °K, and the velocity of advance of the combustion wave is 1–150 mm/s. Two sets of samples composed of pyrochlore and garnet-type phases were produced with SHS. The first of them corresponds to nominal pyrochlore formulation Y₂Ti₂O₇ doped with various amounts of actinides: 10–30 wt.% UO₂, 10 wt % PuO₂, 10 wt % NpO₂, or 9.5 wt % UO₂ + 0.5 wt % Am₂O₃. The precursor was prepared from oxides of the base phases (TiO₂, Y₂O₃, AnO₂), an oxidizer (MoO₃), and Ti. In the second set of runs, the target phase was garnet (Y_2.8Gd_0.2)(Al_4.7Ga_0.3)O₁₂, where Gd₃₊ was used as a surrogate for Am₃₊. The initial batches were composed of MoO₃, Y₂O₃, Gd₂O₃, Al₂O₃, Ga₂O₃, and metallic Al. Phase compositions of the samples have been determined by XRD and SEM/EDS. Samples of the first series are composed of major pyrochlore with minor metallic Mo. The samples with 10 wt % actinides do not contain any separate actinide oxide phase. In the samples with 20 and 30 % UO₂ a separate uranium oxide phase was observed. SEM/EDS data allows determination of the limit of solid solution of the pyrochlore phase with respect to tetravalent actinide (U) as 12–14 wt. %. Principle phases in the second series were garnet:

(Y_2.82–2.88Gd_0.13–0.14)(Al_4.69–4.74Ga_0.17–0.22Mo_0.05–0.16)O₁₂ and Mo-Al-Ga alloy. Small amounts of perovskite - (Y_0.86Gd_0.12)(Al_0.93Ga_0.05Mo_0.03)O₃, Mo, and Al oxides were also observed. Gd and Ga mainly entered in the garnet; small amounts of the elements were incorporated into perovskite (Gd), a metallic alloy, and perovskite (Ga).