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 (Y2.8Gd0.2)(Al4.7Ga0.3)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:(Y2.82–2.88Gd0.13–0.14)(Al4.69–4.74Ga0.17–0.22Mo0.05–0.16)O12 and Mo-Al-Ga alloy. Small amounts of perovskite - (Y0.86Gd0.12)(Al0.93Ga0.05Mo0.03)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).

Self-propagating high-temperature synthesis of ZrO2 incorporated Gd2Ti2O7 pyrochlore

“…Compared with conventional solid state reaction process, SHS leads to high temperature, low cost, simplified equipment, and convenient handling. According to Yudintsev et al [27], the duration of initiation is 0.05-5 s and the velocity of combustion wave is 1-150 mm/s. In addition, the industrial scale synthesis of actinidecontaining pyrochlore will require handling relatively large volumes of materials.…”

Self-Propagating High-Temperature Synthesis of Simulated ⁹⁰SrO-Contained Radioactive Graphite in N₂ Atmosphere

“…Generally, the technologies for the treatment and disposal of radioactive graphite could be categorized into following: incineration, steam pyrolysis method and immobilization. As for the studies on the treatment of radioactive graphite with self-propagation technologies, they have been focused upon the preparation of man-made rock with self-propagation high-temperature synthesis (SHS) to immobilize the high-level wastes (HLW) [3][4][5][6][7][8][9][10]. Compared with the conventional high-temperature immobilization technologies for the nuclear waste treatment, SHS technology is characterized in multiple aspects, including: higher reaction rate (reaction time duration: about 0.05-5.0s), simplified procedure, shortened flow-process, lower energy consumption (no additional energy requirement after ignition), lower requirement for equipment to facilitate the remote control, and no requirement for the additional decommission for the SHS facilities.…”

“…Compared with the conventional high-temperature immobilization technologies for the nuclear waste treatment, SHS technology is characterized in multiple aspects, including: higher reaction rate (reaction time duration: about 0.05-5.0s), simplified procedure, shortened flow-process, lower energy consumption (no additional energy requirement after ignition), lower requirement for equipment to facilitate the remote control, and no requirement for the additional decommission for the SHS facilities. Due to all of the above-mentioned aspects, self-propagation technologies demonstrate their exclusive advantages in high temperature grid immobilization and stabilization for the treatment of nuclear wastes [5]. However, there has been no report found in dealing with the treatment of the radioactive graphite with SHS technologies both in China and abroad.…”