Impact of self-irradiation damage on the aqueous durability of zircon (ZrSiO 4 ): implications for its suitability as a nuclear waste form (original) (raw)
Related papers
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.
Leaching of a zirconolite ceramic waste-form under proton and He2+ irradiation
Radiochimica Acta, 2008
In the hypothesis of a nuclear waste geological disposal, zirconolite is a candidate host material for minor triand tetra-valent actinides arising from enhanced nuclear spent fuel reprocessing and partitioning. Its chemical durability has been studied here under charged particle-induced radiolysis (He 2+ and proton external beams) to identify possible effects on dissolution rates and mechanisms in pure water. Two experimental geometries have been used to evaluate the influence of the following parameters: solid irradiation and total deposited energy. Results on the evolution of the elemental releases due to the enhanced dissolution of the zirconolite surface during charged particle-induced irradiation of water are presented. Under radiolysis, elemental releases are first kinetically controlled. When the titanium and the zirconium releases reach (or exceed) their corresponding hydroxide solubility limits, the zirconolite dissolution becomes thermodynamically controlled.
Radiation damage effects in candidate titanates for Pu disposition: Zirconolite
Journal of Nuclear Materials, 2008
Results from studies of radiation-induced damage from the alpha decay of 238 Pu on the density and crystal structure of a nominally phase-pure zirconolite and two other zirconolite-bearing ceramics are discussed. Macro and micro swelling were found to be temperature independent, whereas the density determined with He gas pycnometry was temperature dependent. Approximately 2.6 · 10 18 a/g were needed to render the specimens X-ray amorphous-more to saturate the swelling. Unlike pyrochlore-based ceramics, we did not observe any phase changes associated with storage temperature and damage ingrowth. The forward dissolution rate at a pH value of 2 for material containing essentially all zirconolite is 1.7(4) · 10 À3 g/(m 2 d) with very little pH dependence and no dependence on the amount of radiation-induced damage. Even after the radiation-induced swelling saturated, the specimens remained physically intact with no evidence for microcracking. Thus, the material remains physically a viable material for the disposition of surplus weapons-grade Pu.
Revision 1 1 Quantification of-particle radiation damage in zircon 1 2
2014
Analysis of radiation damage in natural mineral analogues such as zircon is important for the evaluation of the long-term behavior of nuclear waste forms and for geochronology. Here we present results of experiments to determine the partitioning of radiation damage due to the heavy nuclear recoil of uranium and thorium daughters and the-particles ejected in an-decay event in zircon. Synthetic polycrystalline zircon ceramics were doped with 10 B and irradiated in a slow neutron flux for 1, 10, and 28 days to achieve the reaction 10 B + n 7 Li + (+2.79 MeV), creating an alpha event without a heavy nuclear recoil. The 7 Li atoms produced in the nuclear-c-dose applied to each sample. The amount of damage (number fraction and volume fraction) created by each-event (one-event being a 7 Li +-particle) has been quantified using radiological nuclear magnetic resonance and X-ray diffraction data. The number of permanently displaced atoms in the amorphous fraction was determined by 29 Si NMR to be 252 ± 24 atoms for the 10 B(n,) event when the heavy recoil is absent, which is broadly in agreement with This is a preprint, the final version is subject to change, of the American Mineralogist (MSA) Cite as Authors (Year) Title. American Mineralogist, in press. (DOI will not work until issue is live.
Structural recovery of self-irradiated natural and 238Pu-doped zircon in an acidic solution at 175°C
Journal of Nuclear Materials, 2005
We have investigated the aqueous stability of self-irradiated natural and synthetic 238 Pu-doped zircon (4.7 wt% of 238 Pu) in an acidic solution at 175°C. Both zircon samples have suffered a similar degree of self-irradiation damage, as given by their degree of amorphization. X-ray diffraction measurements revealed that during the hydrothermal treatment only the disordered crystalline remnants recovered in the natural zircon, whereas in the 238 Pu-doped zircon the amorphous phase strongly recrystallized. Such a different alteration behavior of natural and Pu-doped zircon is discussed in terms of two fundamentally different alteration mechanisms. Our results demonstrate that further experimental studies with Pu-doped zircon are required before any reliable prediction about the long-term aqueous stability of an actinide waste form based on zircon can be made.
Radiation damage in zircon and monazite
Geochimica et Cosmochimica Acta, 1998
Monazite and zircon respond differently to ion irradiation and to thermal and irradiation-enhanced annealing. Monazite cannot be amorphized by 800 keV Kr ϩ ions at temperatures greater than 175°C; whereas, zircon can be amorphized at temperatures up to 740°C. The damage process (i.e., elastic interactions leading to amorphization) in radioactive minerals (metamictization) is basically the same as for the ion-beamirradiated samples with the exception of the dose rate which is much lower in the case of natural samples. The crystalline-to-metamict transition in natural samples with different degrees of damage, from almost fully crystalline to completely metamict, is compared to the sequence of microstructures observed for ion-beamirradiated monazite and zircon. The damage accumulation process, representing the competing effects of radiation-induced structural disorder and subsequent annealing mechanisms (irradiation-enhanced and thermal) occurs at much higher temperatures for zircon than for monazite. The amorphization dose, expressed as displacements per atom, is considerably higher in the natural samples, and the atomic-scale process leading to metamictization appears to develop differently.
Characterization of a Plutonium-Bearing Zirconolite-Rich Synroc
MRS Proceedings, 1996
ABSTRACTA titanate-based ceramic waste form, rich in phases structurally related to zirconolite (CaZrTi2O7), is being developed as a possible method for immobilizing excess plutonium from dismantled nuclear weapons. As part of this program, Lawrence Livermore National Laboratory (LLNL) produced several ceramics that were then characterized at Argonne National Laboratory (ANL). The plutonium-loaded ceramic was found to contain a Pu-Gd zirconolite phase but also contained plutonium titanates, Gd-polymignyte, and a series of other phases. In addition, much of the Pu was remained as PuO2-x. The Pu oxidation state in the zirconolite was determined to be mainly Pu4+, although some Pu3+ was believed to be present.
Actinide-doping experiments using short-lived 238 Pu and 244 Cm have demonstrated that pyrochlore and zirconolite become fully amorphous at a dose of 0.2-0.5 x 10 16 α/mg at ambient temperature and exhibit bulk swelling of 5-7%. Detailed studies of natural samples have included determination of the critical amorphization dose, long-term annealing rate, microstructural changes as a function of dose, and the thermal histories of the host rocks. Together, the laboratory based work and studies of natural samples indicate that the critical amorphization dose will increase by about a factor of 2-4 for samples stored at temperatures of 100-200 °C for up to 10 million years. These studies of alpha-decay damage have been complemented by heavy ion irradiation studies over the last ten years. Most of the irradiation work has concerned the critical amorphization dose as a function of temperature in thin films; however, some work has been carried out on bulk samples. The irradiation work indicates that most pyrochlore and zirconolite compositions will have similar critical amorphization doses at low temperatures (e.g., below 300-400 °C). Pyrochlores with Zr as the major B-site cation transform to a defect fluorite structure with increasing ion irradiation dose, but do not become amorphous.