The maximum solubilities of several different cations in zirconolite (CaZrTi2O7) were investigated using X-ray diffraction and electron probe microanalysis. A parameter termed the relative radius ratio, defined as the ratio of the average radius of all atoms substituting for Ca and Zr to the average radius of all atoms substituting for Ti, was calculated for each chemical system, using coordination-dependent ionic radii obtained from the literature. It was found that with the possible exception of systems containing A13+, regardless of the chemical system studied, and for additional chemical systems described in the literature, the relative radius ratio for zirconolite is 1.59, with a less than one percent standard deviation, at its solubility limit. Similar strong correlations were found between relative radius ratio and phase stability for crystallographically related rhombohedral and pyrochlore structures that also appear in these systems. These results suggest that the phase stability of zirconolite is generally governed by geometry and that chemical effects are of secondary importance

Garnet solid solutions, Y3A15O12-Gd3A15O12-Gd3Ga5O12 (YAG-GAG-GGG), are being considered as prospective durable host phases for the immobilization of actinide-containing waste with complex chemical compositions. Garnet samples with the suggested simplified formula: (Gd,Ce…)3(Al,Ga,Pu,…)5O12 containing from 3.4 to 5.3 wt.% 239Pu and 3.6-5.5 wt.% Ce have been synthesized through melting of oxide starting materials in air using a hydrogen torch. Calcium and Sn were added to increase the Pu incorporation into the garnet lattice through ion charge and size compensation for Pu4+. Polycrystalline materials obtained in the experiments consist of garnet, perovskite and other phases and were studied by scanning electron microscopy (SEM) and powder X-ray diffraction (XRD). Our results confirmed that the use of compensating elements such as Ca and Sn allow for significant incorporation of Pu and Ce (not less than a few wt.%) into the garnet structure. The preliminary conclusions thus so far indicate that garnet solid solution compositions may incorporate simultaneously trivalent and tetravalent actinides in significant quantities because they occupy different positions in the garnet structure

This paper presents microscopic characterization of four samples of a ceramic waste form (CWF) developed for disposal of actinide-containing electrorefiner salts. The four samples were prepared to investigate the influence of water content and the Pu:U ratio on CWF microstructure and performance. While the overall phase content is not strongly influenced by either variable, the presence of water in the initial zeolite has a detectable effect on CWF microstructure. It is found to influence the distribution of the major actinide host phase, a (U, Pu)O2 mixed oxide

Solid solubility limits of U, Pu, and the neutron absorbers Hf and Gd have been measured for zircon (ZrSiO4), monazite (CePO4), titanite (CaTiSiO5), perovskite (CaTiO3), apatite (Ca10(PO4)6O), in almost all cases where these limits were not known beforehand. The method used was to oversaturate the host phase with the dopant, using a nominated substitutional scheme, and then establish the dopant content of the host phase by microanalysis/scanning electron microscopy. Tetravalent U has limited solid solubilities in titanite, perovskite and apatite. X-ray absorption near-edge and diffuse reflectance spectroscopies were used to show that U was tetravalent in U-doped perovskite prepared in both argon and hydrogen-nitrogen atmospheres, with different charge compensating schemes. Tetravalent Pu has solubilities of 0.13 and 0.02 formula units (f.u.) in perovskite and titanite respectively. Trivalent Pu has a solubility of 0.05 f.u. in titanite. Pu3+ dominates tetravalent Pu in monazite fired in air at 1400°C. At least 0.5 and < 0.1 f.u. of Hf are soluble in titanite and monazite respectively.Hf solubility in apatite is estimated as < 0.1 f.u. Approximately 0.3 and < 0.1 f.u. of Gd are soluble in titanite and zircon respectively

Ceramic materials based on garnet, (Y,Gd,An,…)3(Al,Ga,An,…)5O12 and perovskite (Y,Gd,An,…)(Al,Ga,An,…)O3 structures have been proposed for the immobilization of weapons-grade actinide-containing waste materials with complex chemical compositions. Cathodoluminescence (CL) images and emission spectra of synthetic garnet and perovskite crystals containing Ce, U and Pu were studied. It was determined that Pu3+ incorporated into the garnet matrix has characteristic CL emission bands at 1.9 and 1.6 eV. The loading capacity of the garnet lattice for Pu4+ incorporation is significantly higher than for Pu3+ and similar to U4+. The maximum amount of Pu3+ that may be incorporated into the garnet structure experimentally is 0.3 wt.% in comparison with 5.3 wt.% for Pu4+. The CL emission spectra of Ce3+ in different materials is a characteristic property and can be used for identification of garnet and perovskite phases into the multiphase ceramic matrices simultaneously with microprobe analysis

We studied the phase stability and melting temperature of alumina-doped alkaline earth and rare earth -Ti-O system with the composition variation. Melting of the mixture of Nd2O3, CeO2, SrO, TiO2 and A12O3 at 1673 K in 1 hour produced RE2Ti3O9 phase, and the chemical formula of the oxide was assumed to be (Ce,Nd,Sr)2(Ti,Al)3O9. A Differential Scanning Calorimetry (DSC) measurement showed that the melting temperature of this compound was 1646 K. Al and Sr was considered to contribute to the reduced melting temperature of the oxides in the system. The hardness comparing with ceramic wastes and the leach rate superior to glass waste forms were confirmed

Calcined simulated high level liquid waste (HLLW) with a desired amount of TiN and AIN mixture was heat-treated at 1673-1873 K. It was revealed that the mixture of TiN and AIN (the atomic ratio of Al to Ti is 1:9) caused the melting of the specimen at 1673 K and the separation of the elements into two groups: alloy phase and oxide phase. The analysis of the oxide phase showed that the compounds in it could be divided into four phases, and that all fission product elements formed the complex oxides with Ti and Al. It is considered that Al and Zr dissolution in each phase contribute to the melting of the oxide phase at 1673K. A 30-days Soxhlet leach test showed that the chemical durability of the oxide phase as a waste form was superior to that of glass waste form[l]

Phase relations in the system: CaO-TiO2-SiO2-(Na2O,Al2O3,Gd2O3,UO2) were studied. This system is of interest due to the formation of sphene, perovskite,. and other phases potentially suitable for immobilization of high level waste (HLW) elements. Along with sphene, other phases found in the samples were ru-tile, chevkinite, anorthite, crystobalite, and pyrochlore-structured phases. Sphene is able to incorporate up to 21.5 wt.% Gd2O3 and 9.3 wt.% UO2 or, in formula units: 0.25 Gd3+and 0.07 U4+

Ce-pyrochlore, CaCeTi2O7 is a chemical analogue for CaPuTi2O7, which is a proposed ceramic endmember waste form for the disposition of excess weapon-usable plutonium in geological repositories. Ce-pyrochlore was synthesized by firing and annealing in air a mixture of CeO2, TiO2, and CaCO3 with a stoichiometry of CaCeTi2O7. The annealed products contain Cepyrochlore, Ce-bearing perovskite, CeO2, and minor CaO. The mixture annealed at a temperature of 1140 °C contains more pyrochlore phase than that annealed at a higher temperature (1300 °C), indicating that a low temperature condition favors the formation of the Ce-pyrochlore. The Ca/Ce ratio of the pyrochlore is slightly lower than the ideal ratio (one). Electron energy-loss spectroscopy results show that there is a small fraction of Ce3+ present in the pyrochlore. Ce present in perovskite is dominated by Ce3. High-resolution TEM images show that the boundary between pyrochlore and perovskite is semi-coherent. No glassy phases were observed at the grain boundary between pyrochlore and perovskite, nor between CeO2and pyrochlore. It is postulated, based on the presence of trivalent Ce in the Ce-pyrochlore, that neutron poisons such as trivalent cation Gd would be incorporated into the CaPuTi2O7 phase

Interim results are presented on the alpha-decay damage study of a 238pu-loaded ceramic waste form (CWF). The waste form was developed to immobilize fission products and transuranic species accumulated from the electrometallurgical treatment of spent nuclear fuel. To evaluate the effects of α-decay damage on the waste form, the 238Pu-loaded material was investigated by electron microscopy for microstructure characterization, x-ray diffraction for bulk phase properties, bulk density measurement, and the product consistency test (PCT) for waste form durability. While the predominate phase of plutonium in the CWF, PuO2, shows the expected unit cell expansion due to a-decay damage, currently no significant change has occurred to the macro- or microstructure of the material. The major phase of the waste form is sodalite and contains very little Pu, although the exact amount is unknown. Interestingly, masurement of the sodalite phase unit cell is also showing very slight expansion; again, presumably from α-decay damage.

Synthetic zirconolites doped with rare earth elements (REEs) are corroded in a closed system at elevated temperature and pressure for various fluid compositions. Together with previous studies, the results indicate only a weak corrosion below 250°C at 50 MPa. Above that temperature and up to 500°C zirconolite (CaZrT2O7) displays more rapid rates of corrosion and may be covered by various secondary phases, depending on the fluid composition. Above 500°C in Na-rich fluids, zirconolite is replaced by perovskite (CaTiO3) and calzirtite (Ca2Zr5Ti2O16), but the REEs and Hf (as actinide analogues and/or neutron absorbers) are incorporated into the secondary phases. Perovskite and calzirtite exhibit an unusual crystal chemistry as determined by transmission electron microscopy and microanalysis.

Irradiation effects which occur in nuclear waste glasses are often reproduced by external irradiations. Interesting questions arise about the application of observed phenomena to real waste glasses. For a better understanding of the role of the surface, we have simulated a surface on a 7 oxides glass using Molecular Dynamics. Modifications of the glassy structure, limited to the first 4 Å, have been observed. We also report in this paper on the influence of surface on the behaviour of displacement cascades, characterised by an increased number of displaced atoms, and also by a larger displacements of sodium atoms in the surface region.

The critical dose for amorphisation, Dc, of two end-member zirconolites (CaZrTi2O7) with different stacking fault densities, was measured as a function of irradiation temperature from 20 K to 623 K using the HVEM-Tandem Facility at Argonne National Laboratory (ANL). Below 473 K, the Dc, values of both samples are identical within experimental error, showing only a small increase in Dc, from (2.5 to 4.6) × 1018 ions m−2 between 20 K and 473 K. At temperatures above 473 K, the data for the zirconolite containing many stacking faults is bracketed by two data sets from almost crystallographically perfect end-member zirconolites: one collected in this study and one collected in a previous study. The raw Dc versus temperature data from the zirconolites in this and a previous study suggest that the critical temperature above which samples cannot be amorphised and/or recrystallisation is complete, Tc, is between 600 and 1000 K. The data sets collected in this study are discussed in relation to a current model.

Zeolite-NaY is susceptible to both irradiation- and thermally-induced amorphization. Amorphized zeolite-NaY loses approximately 95 percent of its ion exchange capacity for cesium due to the loss of exchangeable cation species and /or the blockage of access to exchangeable cation sites. A secondary phase was formed during the ion exchange reaction with cesium. The Cs-exchanged zeolite-NaY phase has a slightly higher thermal stability than the unexchanged zeolite-NaY. A desorption study indicated that the amorphization of cesium-loaded zeolite-NaY enhances the retention capacity of Cs due to the closure of structural channels.

Radiation effects on the composition of NaCl (70 mole %) and PbC12 (30 mole %) salts irradiated by a high energy accelerator proton beam were analyzed. In the temperature interval from ∼50 to ∼1200 °C this composition can be considered as radiation-resistant material, that can be recommended for use as target material for accelerator-driven transmutation systems.

Hydrogen gas evolution from water dispersing nanoparticles under 60Co γ-ray irradiation was investigated under various conditions. Dispersion of TiO2, A12O3, ZnO, ZrO2 and CeO2 nanoparticles with average sizes of 6 nm - 11 µm showed hydrogen yields much higher than that obtained by pure water radiolysis, and the yields seemed to depend on their particle size and shape rather than their chemical species. Hydrogen yield increased with increasing γ-dose but not linearly, which implies the occurrence of a kind of reversing reaction. Some of the nanoparticles supported with Pt, Au and Pd were also examined. The yields were strongly affected by these noble metals, which indicates an important role of the particle surface in the total mechanism of the present hydrogen evolution.

Recent advances in the synthesis of diamond films led to the fabrication of new devices that enable the direct measurement of the alpha activity of a corrosive liquid source. Such devices can be directly immersed in radioactive liquid solutions for the in-situ and real time measurement of their alpha activity. We report here on the fabrication of diamond from the chemical vapour deposition (CVD) technique and its use for the monitoring of the alpha activity of a plutonium 239 radioactive source diluted in concentrated nitric acid. Tests have demonstrated the excellent reliability of the CVD diamond detectors, that would give great benefit to the monitoring of the nuclear fuel assembly re-processing, and in particular for corrosive wastes identification and treatment.

Uranium-rich microlite, a pyrochlore-group mineral, occurs in 440 Ma old lithium pegmatites of the Mozambique Belt in East Africa. Microlite exhibits a pronounced growth zoning, with a U-free core surrounded by a U-rich rim (UO2 up to 17 wt.%). The core exhibits conjugate sets of straight cracks (cleavage planes) which provided pathways for a late-stage U-enriched pegmatitic fluid which interacted with the U-free microlite to produce a distinct U enrichment along the cracks and led to the formation of the U-rich rim. Following the stage of U incorporation into microlite, a second generation of hydrothermal fluids deposited mica along the cleavage planes. Subsequent to these two hydrothermal stages, the host rock was uplifted and subjected to intense low-temperature alteration during which Na, Ca and F were leached from the microlite crystals. This alteration also led to a hydration of microlite, but there is no evidence of U loss. These low-temperature alteration effects were only observed in the U-rich rim which is characterized by a large number of irregular cracks which are most probably the result of metamictization, as indicated by electron diffraction images and powder X-ray patterns.

Various REE-Sr-(Pb)-crandallites, uraninite, and coffinite in the near-field of the 2 Ga old super-gene-altered Bangombé U-deposit and its natural fission reactor (RZB) have been examined. The crandallite minerals may have formed during syncriticality host-rock alteration, continous alteration of phosphates, episodic Pb-loss and/or supergene weathering. Coffinitization with P2O5 and SO4-substitution has occurred immediately below RZB and resulted in extensive loss of U (≤ 46%) and enrichment of Ce (≤ 190%) and Nd (≤ 780%). Additional loss of U during coffinitization also may have occurred due to dissolution. Current alteration under oxidizing conditions has resulted in partial dissolution of uraninite and coffinite and the formation of uranyl phases. Despite supergene alteration, the hydrogeochemistry (3.09 ppt U [235U/238U = 0.7012 to 0.7019%], 4.96 ppt Ce, and 1.92 ppt Nd) suggests a remarkable retardation of lanthanides and depleted uranium by REE-Sr-(Pb)-crandallites, uraninite, coffinite, and uranyl phases at RZB.

The use of natural analog studies in performance assessments has been widely discussed and debated, but its accomplishment has been limited. Given recognized uncertainties and challenges, scientific contributions to performance assessments and support for the validity of performance assessment models are valuable from all possible sources, including natural analog studies. The conceptual basis for geologic disposal of nuclear waste and for performance assessments relies on scientific expertise based largely on studies of natural systems analogous to possible repository systems, i.e., natural analogs. Natural analog studies offer contributions to model validation based both on inductive and deductive reasoning. The utility of analog studies as a deductive tool in performance assessment is enhanced by specificity of the analog system to the repository system. As geologic sites are selected and repository designs detailed, the use of analog data in supporting deductive performance assessments should increase. Consideration of Yucca Mountain for the proposed US high level nuclear waste repository affords site specificity conducive to applications of natural analog data in performance assessment. The primary use of Peñia Blanca natural analog data in recent Yucca Mountain performance assessments stems from observations of mineral products formed by alteration of natural uraninite, an analog of spent fuel. Alternate performance assessment source term models based on the Peñia Blanca oxidation rate model and the schoepite solubility model yield lower, yet comparable estimated doses than the base case model in the NRC performance assessment for Yucca Mountain.