[en] Abstract only

[en] Increasing the reactivity of the major component, TiO₂, of oxide route Synroc precursor, led to a significant reduction in the hot pressing temperature required to achieve near-theoretical density. The combination of Ba and Ca hydroxides and a more reactive Al₂O₃ resulted in leach rates for Cs which approach those of alkoxide-route precursors. This improvement is believed to have been brought about by the dissolution of Al(OH)₃ in the highly alkaline slurries created when Ca and Ba hydroxides are used. This increased availability of Al³⁺ allows for the formation of more hollandite, into which the Cs can be incorporated, thus reducing the Cs leach rates. The use of a static mixer for waste addition improved the metallic phase distribution but did not affect the leach results. (author) 1 fig., 4 tabs., 10 refs

[en] Substitution of K for Na in certain nuclear fuel reprocessing cycles may allow an increase of high level radioactive waste loading in Synroc, because K can be incorporated in the barium hollandite phase more easily than Na. The use of rare-earth additions to stabilize Na in the perovskite phase may also have merit. (author)

[en] Sintering is the last stage of ceramic processing, where the final shape and microstructure are fixed. The driving force for sintering is the reduction in surface energy of the particles. While the sintering operation itself is very simple the actual mechanisms are complex. A brief introduction to the theory from a historical perspective is given. Various aspects of sintering such as liquid phase sintering, hot pressing, hot isostatic pressing and new developments in sintering are discussed with examples from the literature. Examples of Australian research and its contribution to the understanding of various sintering processes are given. Copyright (1996) Institute of Metals and Materials Australasia Ltd

[en] The immobilization of 13.5 wt% PuO₂ in a zirconolite-rich Synroc has been demonstrated at the 50 g PuO₂ scale in cans produced by hot isostatic pressing (HIPing) at 1280 C. The wasteform also contains more than an equimolar amount of neutron poisons such as Gd, Sm and Hf. The scale-up technology for production of Pu-containing cans of Synroc by HIPing has been demonstrated at the 10 kg scale using CeO₂ as an analog for PuO₂. The characterization of the products has shown that Ce is a good analog for Pu in zirconolite-rich Synroc produced by HIPing. Post-production thermal tests simulating the effects of a HLW glass pour in a surrounding canister have shown that the Synroc is stable and there is no exsolution of PuO₂ or CeO₂

[en] Synroc is a titanate ceramic developed for the incorporation of high level waste from nuclear fuel reprocessing. Nitrates present in the waste must be removed by calcination under reducing conditions to prevent the formation of undesirable phases. To date the bulk of Synroc calcinations have been in non-flammable 3.5% H₂ in Ar or N₂. It has shown that the use of pure H₂ will reduce the costs of a conceptual plant by: eliminating Ar and reducing the size of the off-gas system; increasing calciner throughput; and lowering gas velocity through the calciners (decreasing dust carry-over). A series of systematic experiments were designed to compare calcination in 100% and 3.5% H₂. The effect of calcination soak temperature and time were examined. In addition, the gas flow and heating rates were varied to give either excess or deficient hydrogen conditions during nitrate decomposition. The calcination conditions significantly affected the morphology of the calcined powder. However, provided the final Synroc was well consolidated there was little difference in the chemical durability, especially if titanium metal is added to the calcine before hot-pressing. 16 refs., 4 tabs., 2 figs

[en] This paper describes a small scoping study examining potential multiphase ceramic waste forms for wastes deriving from U-Mo research reactor fuel reprocessing. These fuels are being developed as replacements for silicide and aluminium fuels. The aim was to identify plausible phases that can be used in combination to achieve waste form monoliths with high waste loadings. These waste streams have unique challenges primarily because they have high Na, P and Mo contents. The approach taken was to utilize the Na and P and Mo to form phases that have been previously studied and are known to be durable. The Mo presents challenges because it is multivalent. In air it exists in the hexavalent state, but it can also be partially reduced to the tetravalent state and will substitute for Ti⁴⁺ in Synroc phases such as perovskite, rutile and pyrochlore. Under extremely reducing conditions it will be reduced to the metallic state. Five compositions were tested. The waste loadings ranged from 40 to ∼77 wt%. Powellite (nominally, CaMoO₄) was one of the main phases formed in all compositions when Ca was added. Powellite was also formed by the coupled substitution of Na and Gd for Ca. Ba and Sr were also incorporated in the powellite. NZP (NaZr₂P₃O₁₂) and NTP (NaTi₂P₃O₁₂) were also found as major phases in some of the compositions tested. Attempts to incorporate Na as a Na-Gd-titanate perovskite did not work, and instead the Na tended to react with the Gd and Mo to form powellite. Left over Gd reacted with P to form monazite. Pyrochlore was formed in one sample in which it was a target phase, with Mo in the tetravalent state. This pyrochlore appears to be a Gd-Mo-Ti pyrochlore with Na and some Al incorporated, plus traces of other waste elements. The XRD pattern suggests pyrochlore although the composition as measured suggests that it is a defect pyrochlore with vacancies in the A-site. Ca phosphate phases were also detected in some compositions. The initial results of this scoping study are promising with the results indicating that waste loadings of ∼50 wt% or maybe higher are feasible. The composition needs to be refined to eliminate the possibility of forming less durable secondary phases. Powellite is a major phase that forms, however we do not have durability data on this material and some testing would be needed to confirm its durability, particularly the (Ca,Na,Gd)MoO₄ composition. (authors)

[en] Lithium fluoride tiles of >95% theoretical density have been fabricated using conventional ceramic processing techniques, for use as thermal neutron shielding. In this paper, the authors discuss the problems associated with producing high density polycrystalline LiF. The quality is very dependent upon the characteristics of the raw materials and the processing procedures used. Changes in these parameters can result in the extremes of powders from those that sinter too rapidly, resulting in severely bloated tiles, to those which show little or no sintering at all

[en] Full text: The highest cost component the nuclear waste clean up challenge centres on high-level waste (HLW) and consequently the greatest opportunity for cost and schedule savings lies with optimising the approach to HLW cleanup. The waste form is the key component of the immobilisation process. To achieve maximum cost savings and optimum performance the selection of the waste form should be driven by the characteristics of the specific nuclear waste to be immobilised, rather than adopting a single baseline approach. This is particularly true for problematic nuclear wastes that are often not amenable to a single baseline approach. The use of tailored, high-performance, alternative waste forms that include ceramics and glass-ceramics, coupled with mature process technologies offer significant performance improvements and efficiency savings for a nuclear waste cleanup program. It is the waste form that determines how well the waste is locked up (chemical durability), and the number of repository disposal canisters required (waste loading efficiency). The use of alternative waste forms for problematic wastes also lowers the overall risk by providing high performance HLW treatment alternatives. The benefits tailored alternative waste forms bring to the HLW cleanup program will be briefly reviewed with reference to work carried out on the following: The HLW calcines at the Idaho National Laboratory; SYNROC ANSTO has developed a process utilising a glass-ceramic combined with mature hot-isostatic pressing (HIP) technology and has demonstrated this at a waste loading of 80 % and at a 30 kg HIP scale. The use of this technology has recently been estimated to result in a 70 % reduction in waste canisters, compared to the baseline borosilicate glass technology; Actinide-rich waste streams, particularly the work being done by SYNROC ANSTO with Nexia Solutions on the Plutonium-residues wastes at Sellafield in the UK, which if implemented is forecast to result in substantial cost savings to the UK Government; Immobilisation of surplus weapons plutonium. Alternatives for niche wastes containing technetium, caesium and strontium, particularly the elimination of volatility concerns during consolidation and the maximisation of the waste loading

[en] Current ANSTO scientific research on wasteform development for mainly high-level radioactive waste is directed towards practical applications. Titanate wasteform products we have developed or are developing are aimed at immobilization of: (a) tank wastes and sludges; (b) U-rich wastes from radioisotope production from reactor irradiation of UO2 targets; (c) Al-rich wastes arising from reprocessing of Al-clad fuels; (d) 99Tc; (e) high- Mo wastes arising from reprocessing of U-Mo fuels and (f) partitioned Cs-rich wastes. Other wasteforms include encapsulated zeolites or silica/alumina beads for immobilization of 129I. Wasteform production techniques cover hot isostatic and uniaxial pressing, sintering, and cold-crucible melting. In addition, building on previous work on speciation and leach resistance of Cs in cementitious products, we are studying geopolymers. Although we have a strong focus on candidate wasteforms for actual wastes, we have a considerable program directed at basic understanding of the wasteforms in regard to crystal chemistry, their dissolution behavior in aqueous media, radiation damage effects and processing techniques