[en] Simulated high level radioactive waste (HLW) similar to these generated from nuclear fuel reprocessing has been incorporated in a very stable, inert, crystalline, durable solid waste form. This solid form is an assemblage of three mineral phases, Hollandite (BaAl₂Ti₆O₁₆), Perovskite (CaTiO₃) and Zirconolite (CaZrTi₂O₇). these phases are known for their high thermodynamic stability, their compatibility with each other, and their ability to incorporate a wide range of HLW ions in their crystal lattice structures. A subsolidus sintering technique has been developed in this research as an alternative method for processing which has sown its advantages over a previously proposed hot pressing technique particularly for remote operation in high gamma radiation fields. The sintered SYNROC-B ceramic produced by this technique has fine-grained micro-structure (approx. 1 to 3 μm grain diameter) with uniform distribution for HLW ions in a locally heterogeneous three-phase system. Phase identification has been carried out by x-ray diffraction techniques

[en] Leachability studies for sintered Modified SYNROC-B waste forms containing up to 20 wt % simulated high-level radioactive waste have been investigated. Significant decreases in the leach rates of Ba, Ca, and Cs are noticed with the increase in waste loadings. The values of leach rates for Cs in deionized H²O at 25₀ Care 0.007 and 0.001 g-m₂-d^-aexclamation for 10 and 20 wt % simulated radioactive waste respectively. Barium and calcium showed similar leaching behavior. The results are presented and discussed

[en] The chemical alteration formed by ground water on the solidified radioactive waste during deep geologic disposal represents the most likely mechanism by which dangerous radioactive species could be reintroduced into the biosphere. Knowing the geologic history of the repository, the chemistry of the ground water and the mechanisms involved in the corrosion of the radioactive solids can provide help to predict the long-term stability of these materials. The factors that must be considered in order to assess the safety and the risk associated with such a disposal strategy are presented. The leaching behavior of a solidified radioactive waste form called SYNROC-B (SYNthetic ROCks) is discussed. Different simulated ground water brines similar to those of the repository sites were prepared and used as the leaching media in leaching experiments

[en] Simulated High Level Radioactive Waste has been incorporated in sintered SYNROC-B Ceramic matrices prepared by subsolidus sintering techniques. Phase and compositional analyses have been carried out by x-ray diffraction techniques and scanning electron microscope equipped with energy dispersive analysis of x-ray (SEM/EDAX) analyses on fracture surfaces. Modified SNYROC-B compositions have been formulated based on computer-calculated ion substitutions relative to the stoichiometries of the major phases, [Hollandite (H), Perovskite (P), Zirconolite (Z)] and their respective abilities to retain HLW ions in their lattice structures, either substitutionally or interstitially. When prepared and sintered, the Modified SYNROC-B compositions yield clean H,P,Z assemblages, free of minor phases which might be questionable with respect to compatibility with major phases and HLW ions, and/or chemical and/or radiation stability under long-term disposal conditions. Chemical stability of the Modified SYNROC-B loaded with 10 and 15 wt% simulated HLW has been studied by leaching in pure distilled water at approx. 100⁰C and in simulated brines similar to those found in geologic disposal sites at their boiling temperatures (approx. 110⁰C) at atmospheric pressure. High pressure, high temperature leaching in simulated brine has also been investigated at approx. 50 atm and 270⁰C for the Modified SYNROC-B Ceramic waste form loaded with 15 wt% simulated PW-4b radwaste. In all instances, the observed leach rates have been near or below (mostly below) present analytical detection limits

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[en] The leaching behavior of a sintered Modified SYNROC-B ceramic waste form containing 10 wt % simulated high level radioactive waste, under the influence of ν-irradiation, has been investigated. Similar leaching experiments have been carried out in the absence of ν-irradiation, and the leach rates of Ba and Cs are presented for comparison studies. Acid/Base formation has been monitored through pH measurements of the leachants. A decrease in pH is found with ν-irradiation due to radiolytic nitric acid generation. Higher initial leach rates with a more pronounced subsequent drop in the case of Ba and Cs were observed with ν-irradiation than without irradiation. The results of the study are presented and discussed. 1 figure, 1 table

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[en] The effects of gamma irradiation on the chemical stability and particularly the leaching behavior of Modified SYNROC-B ceramic waste form for high level radioactive waste immobilization are presented. Experimentally obtained leach rates for uranium, iron and cerium from γ-irradiated and unirradiated SYNROC-B are given. The results of the study are discussed. A decrease in pH is found with γ-irradiation due to radiolytic nitric acid generation. Higher initial leach rates with a more pronounced subsequent drop in the case of iron and cerium was observed with γ-irradiation than without irradiation. The effect of a TiO₂ film and uranium leach rate behavior are also discussed. (author)