Polished tiles (7×7×2 mm3) of Nd-bearing zirconolite were fabricated and then some were irradiated on both large faces with 3 MeV or 2 MeV Au2+ ions (total fluence of ≥ 1 × 1015 ions/cm2) in order to render the zirconolite amorphous and so simulate displacement damage caused by alpha decay. Both the irradiated and non-irradiated tiles were then subjected to static dissolution tests in 0.01M nitric solution (pH2) at 90 C, for periods of 0–1, 1–7, 7–14 and 14–28 days. It was found that radiation damage did not affect the dissolution rate of zirconolite as indicated by the elemental leach rates of Nd, Ti, Ca and Al. The results of solution analyses are consistent with those obtained from X-ray Photoelectron Spectroscopy (XPS) in that the Ca, Nd, Ti and Al concentrations in the top surface layer (< 5 nm) all decreased with respect to that of Zr after dissolution testing, and the leached surface composition of the non-irradiated zirconolite is very similar to that of the two irradiated specimens. The implications of these results are discussed in the context of previous work.

Synroc containing mixed fission products and four actinides (237Np,239Pu,241Am and244Cm) was fabricated by hot pressing on a 75 g-scale in separate hot-cell and glove-box facilities. Samples were physically characterised by density, which ranged from 98.2 to 99.1% of theoretical, by scanning electron microscopy (SEM) and by the cc-track etch technique to determine the distribution of actinide elements. The chemical durability of specimens prepared from the hot-pressed Synroc was determined in 460 leach tests carried out under static conditions at 70°C. Release of137Cs dominated the fission-product source term while237Np was the most leachable actinide element. Short-term release is dominated by the initial spilce from Cs and other readily soluble species at grain boundaries and in non-equilibrium phases. Long-term release is probably controlled by matrix solubility. A phenomenological model is developed which accounts for both short- and long-term release.

The chemical durabilities of two Pu-doped pyrochlore samples were studied by Single-Pass-Flow-Through (SPFT) tests at 70°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 TiO 2 (brookite and probably anatase) have been identified on the surface of the leached sample and the principal secondary phase is an unknown polymorph of TiO 2 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% PuO 2 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.

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 behaviour 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 rutile 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.

Hydrothermal veins, rich in Ti, Zr, rare earth elements (REE’s), and actinides, occur in the pure dolomitic marbles of the Adamello contact metamorphic aureole. A selective phase extraction using 9M HC1 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 REE’s 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 analysed 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 HC1, whereas spinel, phlogopite, titanite, chal copy rite, 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 REE’s 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 HC1 treatment. The sample from the forsterite zone contained substantial amounts of REE’s 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.

Chemical extraction techniques and scanning electron microscopy were used to study the distribution and behavior of actinides and rare earth elements (REE) in hydrothermal veins at Adamello, (Italy). The six samples discussed in this paper were from the phlogopite zone, which is one of the major vein zones. The samples were similar in their bulk chemical composition, mineralogy, and leaching behavior of major elements (determined by extraction with 9M HCl). However, there were major differences in the extractability of REE and actinides. The most significant influence on the leaching characteristics appears to be the amounts of U, Th and REE incorporated in resistant host phases. Uranium and Th are very highly enriched in zirconolite grains. Actinides were more readily leached from samples with a higher content of U and Th, relative to the amount of zirconolite. The results show that REE and actinides present in chemically resistant minerals can be retained under aggressive leaching conditions.

Abstract Uranium mining and milling produces significant quantities of solid and liquid wastes. Solid wastes include waste rock arising from the mining activity and tailings produced by processing of the ore through the millin circuits. Of these, tailings have the most pronounced impact on the environment as they represent almost the entire mass of material processed in the milling circuits and also contain over 85% of the original radioactivity present in the ore. Modern rehabilitation schemes for tailings impoundments include a number of layers designed to prevent infiltration of water, oxidation of minerals, capillary rise of salts, and release of Rn, thereby minimizing the impact of the disposal of tailings on the environment. Although the effectiveness of these engineered barriers may degrade with time, it is possible, using evidence from natural systems and archaeological constructions, to provide evidence of the long-term isolation of the contaminants contained in the uranium tailings from the environment.

Confidence in the ability of repository systems to isolate high level wastes from the environment can be strengthened by placing greater reliance on robust designs for the repository system, and by using multiple lines of evidence to demonstrate that parameters, models and predictions developed for the repository are relevant. A particularly useful approach is to demonstrate that models and predictions incorporate processes that have been shown to be important in existing natural systems.

Ceramics rich in pyrochlore-structured titanate and fluorite-structured zirconate phases designed for surplus Pu immobilisation, with and without process impurities, have been leach tested at 90°C in deionised water. The zirconates consist mainly of a defect-fluorite with secondary impurity-containing phase-powellite/scheelite (when sintered in Ar but not when sintered in air), a spinel or magnetoplumbite type phase, a glass forming silicate and a secondary U-rich phase (when sintered in air with added impurities). The pyrochlore-rich baseline titanate ceramic consists of pyrochlore, brannerite and Hf-rutile. When impurities are added zirconolite and a silicate glass are also present. The pyrochlore-rich titanate with 5 wt% of impurities sintered at 1300°C is highly durable. A well-densified zirconate version without impurities has comparable elemental releases to those of the titanate ceramic but a zirconate with 5 wt% of impurities sintered at 1400°C in air or Ar shows much higher U and Ca releases than the titanate ceramic. Sintering atmospheres, changing from Ar to air, can influence Pu and U release rates up to an order of magnitude. High Ga releases from zirconates with impurities show that the secondary phase containing Ga is not durable. The higher processing temperature and the apparent inability to incorporate many impurity elements suggest that zirconates are not as flexible as titanates in respect of processing conditions and aqueous durability.

The dissolution of synthetic brannerite is incongruent and pH dependent with a minimum in the dissolution rate at near pH 8. Preferential release of U leaves TiO2 on the surface with different morphologies; smooth uniform layers in acidic media and nano-spherule agglomerations in alkaline media. The measured apparent activation energies at pH 5.6 to 9.8 suggest surface reaction-controlled dissolution mechanisms. A natural brannerite sample is amorphous and has shown very little alteration over geological time; under laboratory test conditions the U release is less than an order of magnitude more than in synthetic brannerite.

Aqueous alteration of five Ti-Zr-oxide-based ceramics containing elements simulating long-lived radionuclides was studied experimentally by leaching at 90 degreesC in deionized water for more than one year under conditions of high solid/liquid ratios. Four of these ceramics were synthesized by cold-crucible melting (two Synroc-like materials, one zirconia, and one aluminotitanate) and the fifth ceramic was a hot-pressed Synroc. Melted Synroc-like ceramics have the same major constitutive phases as hot-pressed Synroc, but crystal sizes are very different, millimetric as opposed to micrometric, respectively.After the first seven days of leaching, the alteration appeared to cease as solution concentrations for all of the constituent elements attained constant values. The altered mass percentages determined from the release of Ca and Mo were less than 0.2% of the initial mass. Thermodynamic equilibrium calculations using data or estimations for pure phases, or using a model of ideal solid solutions, showed that the cessation of the alteration cannot be explained by the solubility limit of the primary phases of these ceramics. But, the data could be interpreted by the development of a passivating layer of secondary phases, e.g., hydroxides. Examination of the altered surfaces was carried out using SEM, XRD and XPS; however, the thickness of the alteration layer, estimated as 3-5 nm, was below the resolution limit of these techniques. Finally, despite the differences in the crystal size and therefore the amount of grain boundaries, and in the synthesis redox conditions, the leaching behaviors of melted and hot-pressed Synroc are the same for the present experiments.

This paper presents experimental studies on the kinetics of U release from near single-phase zirconolite, pyrochlore, brannerite and pyrochlore-rich titanate ceramic materials. The dissolution tests were conducted at 20–75°C with initial pHs from 2 to 12, and flow rates from 10 to 80 ml d−1 in the open atmosphere. The U releases from these titanate materials are controlled by initial fast process and then followed by linear kinetics. The close-to-stoichiometric U release from zirconolite and pyrochlore-rich materials and preferential U release from brannerite are consistent with the alterations observed for the natural samples. The rate constants for U releases were determined and the effects of pH and temperature were examined. For each material, the U release vs. pH exhibits a V-shape with a minimum near pH 8. The measured activation energies suggest surface reaction controlled dissolution mechanism. Pyrochlore-rich materials and zirconolite demonstrated higher chemical durability and more resistance to aqueous attack than brannerite. However, impurities and minor brannerite inclusions do not appear to have a detrimental effect on U releases from pyrochlore-rich multi-phase ceramics.

A multi-phase pyrochlore-rich titanate ceramic has been chosen by the US Department of Energy for the immobilisation of surplus military plutonium. The pyrochlore-rich wasteform was designed to have a high capacity for plutonium, uranium and neutron absorbers such as hafnium and gadolinium as well as tolerance for high impurity loadings from the plutonium feed stocks. More than 99 wt% of the plutonium is distributed among the pyrochlore, zirconolite and brannerite titanate phases that make up the multi-phase wasteform. This paper presents the results of our current studies on the preparation and characterisation of single phase Pu or U containing specimens of the three major titanates prevalent in the pyrochlore-rich ceramic wasteform. The results cover aqueous durability, radiation damage sensitivity and natural mineral analogues of the above titanates.

Brannerite, as a minor phase, exists in the pyrochlore-rich titanate ceramic formulations designed for immobilization of surplus weapons Pu. The dissolution of synthetic brannerite was studied at 90°C using static tests in pH 4 solution, deionized water and Finnsjön synthetic groundwater. After 140 days the normalized U release rates into a pH 4 solution and deionized water reach similar values, ~10−2 g m−2 d−1, and are about 2 orders of magnitude higher than those in Finnsjön synthetic groundwater. The normalized Ti release rate into Finnsjön synthetic groundwater is about an order of magnitude higher than those in pH 4 solution and deionized water. The dissolution of brannerite is incongruent in the pH 4 solution and deionized water (preferential release of U over Ti), and nearly congruent in Finnsjön synthetic groundwater. SEM observations of the samples after 140 days in pH 4 solution and deionized water revealed minor surface alteration, in the form of a thin surface layer, probably TiO2, as a result of preferential releases of U in both cases.

A multi-phase pyrochlore-rich titanate ceramic has been chosen by the US Department of Energy for the immobilisation of surplus military plutonium. The pyrochlore-rich wasteform was designed to have a high capacity for plutonium, uranium and neutron absorbers such as hafnium and gadolinium as well as tolerance for high impurity loadings from the plutonium feed stocks. More than 99 wt% of the plutonium is distributed among the pyrochlore, zirconolite and brannerite titanate phases that make up the multi-phase wasteform. This paper presents the results of our current studies on the preparation and characterisation of single phase Pu or U containing specimens of the three major titanates prevalent in the pyrochlore-rich ceramic wasteform. The results cover aqueous durability, radiation damage sensitivity and natural mineral analogues of the above titanates.

Synroc specimens doped individually with ˜ 1 wt% of Np or Pu have been studied, after leaching for >2,500 days, using α-spectroscopy and SEM. In the last leaching period, the leachant was replaced on either a daily or monthly basis. When the leachant was replaced on a daily basis the release rate of Np was similar to that measured when the samples were first leached, whereas for the monthly replacement the release rates of Np or Pu were about a factor of 20 to 30 lower than that for daily replacement. These findings agree generally with the results obtained from surface examination of the samples which showed that the thickness of actinidedepleted surface layer of anatase was 0.3 μm for monthly replacement but only 0.1 μm when the leachant was replaced frequently. Overall, the results suggest that release of actinides from Synroc are controlled by solubility limiting effects at the surface area to volume ratios employed in MCC-1 tests.

From elastic recoil detection analysis (ERDA) of 2 MeV He ions and secondary ion mass spectroscopy (SIMS), exposure of Synroc-C to D 2 O at 150°C for ~ 30 days produced surface deuteration products of a few nm in thickness, with surface roughness after polishing down to 0.25 μm diamond finish not being of critical importance in the thickness determination. Reaction at 250°C produced more extensive deuteration and general surface alteration, over depths of about a micron. SIMS did not show any surface enhancement of rare earths or Zr on Synroc-C surfaces reacted at 90°C for up to 336 days. Pu-doped SynrocC exposed to deionised water at 70°C showed surface depletion of Pu by alpha-spectroscopy. Zirconolite-rich Synroc showed less surface deuteration than Synroc-C after reaction for 3 weeks at 150°C in D 2 O. Admixtures of 0.001 M of fluoride ions to dilute HCI (pH = 2) produced deposits of anatase, ~ 20 μm thick, on perovskite after a few weeks at 90°C; these deposits were much thicker than those produced by the dilute HCI without the fluoride ions being present.