Structural recovery of self-irradiated natural and 238Pu-doped zircon in an acidic solution at 175°C (original) (raw)
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Journal of Physics: Condensed Matter, 2003
Zircon has been proposed as a nuclear waste form to safely encapsulate weapons-grade plutonium. In order to study the impact of self-irradiation damage in zircon on its aqueous durability, we performed a hydrothermal experiment (2 M CaCl 2 solution, 600 • C, 100 MPa) with several variably radiation-damaged, i.e. amorphized, zircon samples. We found an anomalous increase in the alteration rate at two critical concentrations of amorphous domains. The first dramatic increase sets in when the amorphous domains form interconnected clusters in the structure. The second increase is related to the percolation of fast diffusion pathways consisting of nano-sized regions of depleted matter that are formed during strongly overlapping α-recoil events, as seen by molecular-dynamics simulations and small-angle x-ray scattering measurements. The two percolation thresholds provide model benchmarks for the safety performance of a zircon waste form.
Synthesis of Zircon for Immobilization of Actinides
MRS Proceedings, 1995
A new method of synthesis for actinide-doped zircon is presented based on studies of zircons formed by crystallization from the reactor core melt generated in the course of the accident at the Chernobyl Nuclear Power Plant. These zircons have compositions in the range (Zr0. 94 ,U0. 06 )Si0 4 to (Zro. 9 ,Uo.1)SiO 4 . Hot-pressing of oxides was studied to make Zr-based waste forms. The results demonstrate the efficacy of using metallic zirconium in synthesizing high-actinide zircons. In the event of deviations from zircon's ideal stoichiometry, ZrO2 forms, which is also an effective host phase for actinide elements. Waste streams high in zirconium and actinides could be converted into Zr-based waste forms. The adaptation and modification of the mixed-oxide reactor fuel (MOX) production process is proposed as a process for the production of (Zr,Pu)SiO4, a durable waste form for excess weapons plutonium.
On the breakdown of zircon upon “dry” thermal annealing
Mineralogy and Petrology, 2009
Zircon samples without and with secondary chemical alteration from diverse sources were subjected to heat treatment at 1400 °C for 96 hours. Resulting new phases and textures suggest that decomposition of zircon into component oxides occurred in all experiments to various degrees. The crucible material was found to have a strong influence on the extent of breakdown, especially in the case of altered starting materials. In this study the progressive stages of the breakdown of zircon grains are described. The factors that may govern the decomposition are discussed, including radiation damage, secondary alteration and external conditions (sample container, atmosphere). Based on the results, it is highly suggested to prefer platinum over alumina crucibles to avoid breakdown when dry annealing is done on zircon prior to U/Th-Pb isotopic analyses. * Calculated doses, given in 10 18 alpha events per gram. Values extracted from the references quoted. # Materials: Pt -platinum, Al 2 O 3 -sintered alumina.
Behavior of 238 Pu-Doped Ceramics Based on Cubic Zirconia and Pyrochlore under Radiation Damage
Journal of Nuclear Science and Technology, 2002
Crystalline ceramics based on durable actinide host phases such as cubic zirconia and titanate pyrochlore have been suggested for the immobilization of weapons grade plutonium and actinide wastes. Samples of crystalline ceramic based on the gadolinia-stabilized cubic zirconia, (Zr,Gd,Pu)02, structure doped with 9.9 wt.% 238pU were synthesized and characterized in comparison with samples of pyrochlore-based ceramic, (Ca,Gd,Hf,U,Pu)2Ti 2 0 7 , doped with 8.7 wt.% 238PU. It was found that a resistance of cubic zirconia to self-irradiation is much higher than that of pyrochlore. At the cumulative dose 1.lx10 25 alpha decays/m 3 , cubic zirconia retained its crystalline structure. No swelling or cracking were observed in the ceramic matrix. At the same cumulative dose the titanate pyrochlore became nearly amorphous and the density decreased by approximately 10 % in comparison with the initial, unaltered sample. Under self-irradiation, both ceramics demonstrated an increase of normalized Pu mass loss in deionized water depending on cumulative doses, but this increase is significantly greater for the pyrochlore-based ceramic.
The hydrothermal stability of zircon: Preliminary experimental and isotopic studies
Geochimica et Cosmochimica Acta, 1992
Experimental investigations of the stability of the U-Pb isotopic system in nonmetamict zircons show that appreciable losses of Pb and U can be induced at amphibolite-grade conditions (400°C to 6OO"C, 4 to 6 kb) in 2 M NaCl and 2% HNO3 solutions. The severity of U loss and, to a lesser extent Pb loss, varies with solution composition: in this case the 2 M NaCl solution induced more Pb and U loss than the 2% HNOs solution at the same P-T conditions. Scanning electron microscopy of the run products also revealed a range of corrosion-related surface features, which suggests that some of the observed trends in Pb and U loss must be attributed to zircon dissolution. Backscattered electron (BSE) imaging of the run products further suggests that partial homogenization of chemical zoning patterns occurred during the experiments. Microprobe analyses of treated and untreated grains show that both populations have a similar range of Hf contents. Thus, the apparent loss of sharp, well-defined zoning features is most likely due to small-scale "smearing out" of formerly sharp chemical gradients and is perhaps related to the annealing of lattice defects caused by alpha-recoil damage. Thus, experimentally induced U-Pb isotovic discordance in zircon is a complex function of zircon stability and annealing effects.
Journal of Nuclear Materials, 2003
Hydrothermal experiments were carried out with powder from an almost fully amorphous, natural zircon under various P-T-t conditions mainly in a 0.1 N HCl solution. Powder infrared spectroscopic measurements on the experimental products reveal that first structural changes occurred at a fluid temperature as low as 75°C. Significant recrystallization started at 200°C, as indicated by an increase in the absorption intensity of the zircon fundamental IR bands and the formation of sharp OH stretching bands at 3385 and 3420 cm À1. Although the powder has fully reacted at 400°C, the zircon fundamental absorption bands are not fully recovered, indicating the occurrence of significant amounts of amorphous remnants. The experimental results in neutral to acidic solutions are consistent with the idea that water (H þ and possibly H 2 O) diffuses into the amorphous network where it ÔcatalysesÕ solid state recrystallization. During this process, Zr and Si were leached from the amorphous network.
Hydrous species in ceramics for the encapsulation of nuclear waste: OH in zircon
Journal of Physics: Condensed Matter, 2006
The partition of hydrous species in radiation-damaged zircon was investigated using infrared spectra from a large range of samples with different degrees of radiation damage and a variety of geological conditions. The results of the present study showed uniformly a partition coefficient of K = 0.3. This indicates that OH is enriched in the damaged crystalline phase and not in the amorphous phase. The uniformity of the results for all samples indicates equilibrium rather than a frozen-in kinetic behaviour. It is possible that the enrichment of OH in the damaged crystalline phase of zircon indicates a catalytic effect of hydrogen during recrystallization events.
Journal of Physics: Condensed Matter, 2002
Radiation damage and recrystallization in natural zircons have been studied by analysing Si-O stretching overtones/combinations, hydrous species, and U-ion spectra in the frequency region between 1200 and 11 000 cm −1 . The effects of radiation are characterized by a dramatic variation of intensity, a decrease in frequencies of multi-phonon bands (e.g., Si-O stretching overtones), a change of spectral profile of OH species, a formation of new OH species, and new signals related to U ions. The formation of new anisotropic OH species in the crystalline regions of metamict zircon is observed and this could account for the different thermal behaviour of OH species between metamict zircon and titanite during high-temperature annealing. The results imply systematic modifications of the local environments of the OH and U ions in the damage process. Both U 4+ and U 5+ spectra show dramatic variations during metamictization. We observe, for the first time, that as a result of radiation damage, the U 5+ signals near 6668 and 9030 cm −1 become undetectable at a dose of around 1.5 × 10 18 αevents g −1 while extra lines near 6650 and 8969 cm −1 appear. These variations are interpreted as radiation-induced local modifications in crystalline regions. The general shape of the U-ion spectrum of the crystalline zircon is somehow still preserved in highly damaged zircon. A decomposed zircon, consisting of ZrO 2 , SiO 2 , and ZrSiO 4 , shows spectral features different from those of metamict zircon samples. Thermal annealing of a highly damaged zircon leads to recovery of the structure of zircon, indicated by spectral changes of multiphonon bands and U ions, accompanied with the appearance of new OH species. The results confirm that the recrystallization process in heavily damaged zircon involves the decomposition of metamict ZrSiO 4 into SiO 2 and ZrO 2 near 1100 K and the significant crystal growth of ZrSiO 4 near 1400 K as indicated by the recovery of Si-O stretching overtones and U 4+ and U 5+ bands.