[en] Actinide-doped Synroc samples were leached in deionized water and buffered solutions (pH 2 to 10) under MCC-1 conditions at 70deg C for 28 days. Leachates were filtered through 0.45 μm and colloid filters to determine the α-activity associated with colloids and particulates. The EQ 3/6 thermodynamic package was used to calculate whether actinide solubility limits were affecting Synroc dissolution kinetics. For Pu, the solubility in all leachants, except the pH 2 buffer, appeared to control the dissolution kinetics, but for Np and Am, the dissolved actinides were below solubility limits in each case. To avoid Pu saturation effects in MCC-1 tests on Synroc, shorter leaching times will have to be used. (orig.)

[en] The Australian Nuclear Science and Technology Organisation (ANSTO) has established a project to undertake research relevant to the safety case for the proposed Australian radioactive waste facility. This facility will comprise a store for intermediate level radioactive waste, and either a store or a near-surface repository for low-level waste. In order to identify the research priorities for this project, a structured analysis of the features, events and processes (FEPs) relevant to the performance of the facility was undertaken. This analysis was based on the list of 137 FEPs developed by the IAEA project on 'Safety Assessment Methodologies for Near Surface Disposal Facilities' (ISAM). A number of key research issues were identified, and some factors which differ in significance for the store, compared to the repository concept, were highlighted. For example, FEPs related to long-term groundwater transport of radionuclides are considered to be of less significance for a store than a repository. On the other hand, structural damage from severe weather, accident or human interference is more likely for a store. The FEPs analysis has enabled the scientific research skills required for the inter-disciplinary project team to be specified. The outcomes of the research will eventually be utilised in developing the design, and assessing the performance, of the future facility. It is anticipated that a more detailed application of the FEPs methodology will be undertaken to develop the safety case for the proposed radioactive waste management facility. (authors)

[en] Betafite and zirconolite occur in Ti-rich hydrothermal veins emplaced within dolomite marble in the contact aureole of the Adamello batholith, northern Italy. Zirconolite contains up to 18 wt% ThO₂ and 24 wt% UO₂, and exhibits strong compositional zoning. Some zirconolite grains were corroded by the hydrothermal fluid. Betafite, the Ti-rich member of the pyrochlore group, often occurs as overgrowths on zirconolite. The betafite is weakly zoned and contains 29--34 wt% UO₂. In terms of end-members, betafite contains approximately 50 mole percent CaUTi₂O₇ and is the closest known natural composition to the pyrochlore phase proposed for use in titanate waste forms. Amorphization and volume expansion of the betafite caused cracks to form in the enclosing silicate mineral grains. Backscattered electron images reveal that betafite was subsequently altered along crystal rims, particularly near the cracks. EPMA data reveal little difference in composition between altered and unaltered areas, except for lower totals, suggesting that alteration is primarily due to hydration. The available evidence demonstrates that both betafite and zirconolite retained actinides for approximately 40 million years after the final stage of vein formation. During this time, betafite and zirconolite accumulated a total alpha-decay dose of 3--4 x 10¹⁶ and 0.2--2 x 10¹⁶ α/mg, respectively

[en] The capacity for zirconolite to incorporate actinides, combined with a very high chemical durability, makes zirconolite-rich ceramics potential candidates for the containment of minor actinides from reprocessing of nuclear spent fuel. Our studies examine the chemical durability of sintered Nd-bearing zirconolite under simulated geological repository conditions, demonstrating the effect of citric acid (at varying concentrations), as an organic complexing agent, on the leaching behaviour of zirconolite under a low flow regime. The complexation limit of zirconolite at pH=5 in citrate media appears to be reached by 0.001 M citric acid concentration. Secondary phase development (titania and possibly titania-rich hydrolyzed species) was evident on the zirconolite leached in 0.0001 M citric acid, similar to that on zirconolite leached in water. (authors)

[en] Perovskite and zirconolite are two of the major phases of the Synroc titanate mineral assemblage. Their aqueous durability under a range of pH conditions at 90 C has been examined. Solution analysis, electron microscopy and X-ray diffraction have been used to investigate the dissolution behavior of these phases, and a perovskite phase doped with Nd, Sr and Al, using buffered solutions at pH levels of 2.1, 3.7, 6.1, 7.9 and 12.9. After 43 days of leaching, Ca and Ti extractions from perovskite and zirconolite show only a weak pH-dependence. SEM investigation of the samples leached at pH 2.1, 6.1 and 12.9 showed that a titanaceous surface layer formed on the perovskite specimens. XRD analysis of the perovskite samples showed that anatase formed on the leached surface at acidic and neutral pHs, but not under alkaline conditions, and that minor amounts of rutile also formed. In the leached perovskite specimens doped with Nd, Sr and Al, no rule was found by XRD and anatase was only detected in the sample leached at pH 2.1. There were no detectable changes in the leached zirconolite samples examined by SEM and XRD

[en] Hydrothermal veins, rich in Ti, Zr, rare earth elements (REEs), and actinides, occur in the pure dolomitic marbles of the Adamello contact metamorphic aureole. A selective phase extraction using 9M HCl was applied to samples from within and near these veins to chemically separate acid-soluble phases from residual phases, and to study the associations of U, Th, and REEs with these phases. The samples were from the phlogopite, titanian clinohumite, and forsterite vein zones, and from the country rock. The effects of the extraction were studied by SEM/EDS and by chemical analysis. Isotopes of U and Th were analyzed by alpha-spectrometry. The chemical data and SEM/EDS results indicated that dolomite, calcite, apatite, and much of the pyrrhotite were dissolved by the 9M HCl, whereas spinel, phlogopite, titanite, chalcopyrite, and zirconolite were among the acid-resistant phases. In all vein samples, the REE-patterns of acid-soluble phases were consistent with the dissolution of REE-rich apatite. In samples from the phlogopite zone, the majority of U, Th, and REEs were in residual phases, and the REE pattern of the residue was similar to that of REE-rich titanite. In the titanian clinohumite zone, a substantial proportion of these elements were in acid-soluble phases, and the REE pattern of the residual phases resembled that of zirconolite. Clinohumite was partially dissolved by the HCl treatment. The sample from the forsterite zone contained substantial amounts of REEs in acid-soluble phases, whereas Th and U were mostly in residual phases. The dolomite (country rock) samples contained small amounts of an acid-resistant, uranium-rich phase which, while only comprising about 0.2% of the mass, accounted for 40--60% of the uranium present

[en] Full text of publication follows: Performance assessment for nuclear waste repositories requires the ability to model source term data for different radionuclides and waste-forms over long periods of time commensurate with geological disposal. For an analysis of the entire repository system, it is necessary to consider interactions involving different system components, for example solid state and water-mediated interactions involving the waste-forms and materials comprising the Engineered Barrier System. Our experimental studies on the chemical interactions between a candidate wasteform (zirconate-based ceramic-glass bearing simulated fission products (Cs, Mo and Sr) and an actinide simulant (Gd)), 'can' material, backfill clay material and ingressing groundwater, and the effect of these interactions on waste-form performance, are guided by assessing the outcomes of previous work compiled through an extensive literature review. In our experiments the wasteform fabrication technology centres around the application of hot isostatic pressing (HIP) to consolidate precursor powders contained in a stainless steel (SS) 304 can. The can effectively forms the most intimate of the engineered barriers in the waste package system. The intention is to optimise the performance of the system (for example, by minimising corrosion) by matching other appropriate engineered barrier materials and controlling the local environmental conditions. This is particularly important as the waste package will very likely be required to provide effective containment of the waste for more than 10,000 years. The starting point of our experimental work has been to examine solid-state reaction of ceramic-glass waste-forms with the surfaces of the HIP can material during fabrication. The results of microstructural characterisation show that the reaction between the glass-ceramics and the stainless steel HIP can resulted in the formation of an outer diffusion layer, subsequently changing the chemical composition of the glass matrix and the ceramic phases present in the wasteform. A key issue for aqueous durability testing of the system is whether the diffusion layer produced by the reaction of the SS 304 with the glass-ceramic affects the wasteform leach rates, and whether any plating out of elements occurs on the surface of the HIP can. We also seek to describe the likely leaching mechanisms operating with respect to the ceramic-glass waste-form in the presence of corrosion products. Leach testing has been carried out on both the diffusion layer and the main body of the wasteform to assess the effect of the presence of Fe, Cr and Ni, in particular, on the durability of the waste-form. Results on the material exclusive of the diffusion layer have shown that chemical durabilities significantly exceed that for EA glass even at waste loadings of up to 90%. Leach testing of material from within the diffusion layer, and of the HIP can material alone, is in progress. (authors)

[en] The chemical durabilities of two Pu-doped pyrochlore samples were studied by Single-Pass-Flow-Through (SPFT) tests at 70 deg C. The dissolution of pyrochlore is incongruent with preferential releases of Ca and Gd over Ti, close to stoichiometric releases of U and Ti, and lower releases of Hf and Pu than Ti. Altered pyrochlore and polymorphs of TiO₂ (brookite and probably anatase) have been identified on the surface of the leached sample and the principal secondary phase is an unknown polymorph of TiO₂ containing Hf and varying amounts of Gd and Pu. These surface alteration phases are consistent with reported studies of natural samples. The releases of U, Gd, Ca and Ti into solution follow linear kinetics, whereas the releases of Pu and Hf exhibit non-linear behavior. The presence of ∼ 5% PuO₂ and trace amounts of glass does not appear to have an effect on the overall durability of the material. Further, the low Pu release rate and the similar kinetics for Pu and Hf releases limit the possibility of nuclear criticality under repository conditions. Overall, this study provides useful information on the lower bounds of durabilities of the materials. Copyright (2001) Material Research Society

[en] Zirconolite-glass and sphene -glass specimens, doped with REE as simulants for trivalent actinides, were leached under two conditions. The first was a solubility test using a powdered sample in deionized water at an SA/V ratio of 200 cm^-1, to examine the long-term leaching behavior of the composite materials. The other test was carried out in the presence of moist clay, to assess the degree of surface alteration of the composites in the presence of potential geological repository materials. Both tests were carried out at 90 deg C. The specimens leached in clay showed signs of preferential attack on the glassy matrix along zirconolite and sphene grain boundaries. EDS results showed no gross changes in composition of the constituent phases as a result of leaching. For the solubility tests, steady state conditions of elemental release were attained within 7 days of leaching, suggesting development of a surface passivation layer hindering movement of reactive species between the surface of the material and the leachant. Calcium, Si and Al releases were similar between composites. Titanium and Ce releases were also similar between composites, and were two orders of magnitude lower than those for Ca, Si and Al. The actinide simulants partitioned into the glass phase and into the crystalline component of the composite materials in approximately similar proportions. Although the surrogates were contained in the less durable glassy phase at these levels, this was not reflected in the release of Ce, for example, which was similar to that for a single-phase zirconolite. Copyright (2001) Material Research Society

[en] The heat treatment conditions are a key factor in fabricating zirconolite ceramics and glass-ceramics following high-temperature melting. An oxide mixture melted at 1450 deg C and subsequently heat-treated at 1200 deg C yielded a glass-ceramic containing crystallized zirconolite-2M. The silica-enriched residual glass represented about 60-70 vol% of the total; the actinide surrogates (Nd, Ce) were equally distributed between the residual glass and the zirconolite crystals. Zirconolite ceramics obtained after melting an oxide mixture at 1600-1700 deg C consisted of zirconolite, perovskite and rutile. Rapid cooling rates (> 100 deg C-min^-1) were obtained by pouring the melt into ingot molds; the resulting zirconolite ceramics were characterized by crystals of zirconolite-2M ranging from 1 to no more than 20 μm. Slow cooling (< 25 deg C.min^-1) produced ceramics with crystals several hundred micrometers long. Despite the microstructural differences, the chemical durability of the zirconolite ceramics was identical. The initial alteration rates r₀ were about two orders of magnitude lower than those measured for the residual aluminosilicate glass of the zirconolite glass-ceramics. Moreover, during long-term leach tests at high S/V ratios to obtain advanced degrees of reaction progress, the alteration rates of all the materials diminished by over 3 to 4 orders of magnitude below r₀. Copyright (2001) Material Research Society