Radiation damage effects in candidate titanates for Pu disposition: Zirconolite (original) (raw)
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Radiation Damage Effects in Candidate Ceramics for Plutonium Immobilization: Final Report
2004
In this document, we summarize our study of the effects of radiation induced damage to the titanate ceramics that were to be the immobilization form for surplus weapons-grade Pu. In this study, we made five ceramic materials: pure-phase pyrochlore, pure-phase zirconolite, pyrochlore-rich baseline, zirconolite-rich baseline, and impurity baseline. Two-hundred specimens were made of which 130 contained approximately 10 mass% 238 Pu and 70 contained 10 mass% 239 Pu. The specimens containing 239 Pu served as materials against which the behavior of the 238 Pu-bearing specimens could be compared. In our studies, we measured the "true" density (density exclusive of surface connected porosity), bulk density, crystalline-phase composition with X-ray diffraction (XRD), and dissolution rates as radiation induced damage accumulated in the 238 Pu-bearing specimens. We routinely took photographs of the specimens during each characterization period.
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.
Journal of Nuclear Materials, 2014
Zirconolite and monazite matrices are potential ceramics for the containment of actinides (Np, Cm, Am, Pu) which are produced over the reprocessing of spent nuclear fuel. Actinides decay mainly through the emission of alpha particles, which in turn causes most ceramics to undergo structural and textural changes (amorphization and/or swelling). In order to study the effects of alpha decays on the above mentioned ceramics two parallel approaches were set up. The first involved the use of an external irradiation source, Au, which allowed the deposited recoil energy to be simulated. The second was based on shortlived actinide doping with 238 Pu, (i.e. an internal source), via the incorporation of plutonium oxide into both the monazite and zirconolite structures during synthesis.
Titanate Ceramic Phases for Surplus Plutonium Immobilisation
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.
Irradiation Effects in Ceramics for Plutonium Disposition
Fox/Advances, 2010
Understanding the effect of radiation damage and noble gas accommodation in potential ceramic hosts for plutonium disposition is important for determining long-term behaviour during geological disposal. Polycrystalline samples of zirconolite and Nd-doped zirconolite were irradiated ex-situ with 2 MeV Kr + ions to simulate plutonium nuclei recoil which occurs during alpha decay. The microstructural and chemical modifications induced by the irradiation were investigated by Transmission Electron Microscopy (TEM) on thin sections prepared by Focused Ion Beam (FIB). During this study, the feasibility of TEM section preparation from pristine and irradiated ceramics by FIB was demonstrated. After Kr + irradiation, the samples were observed to be amorphous at the surface and an interface between the pristine and irradiated parts of the specimen was identified. No significant chemical modification was observed.
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.
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.
Ion irradiation-induced amorphization of six zirconolite compositions
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2000
Zirconolite is an important phase in Synroc, a polyphase titanate ceramic, designed for the immobilization of high level waste from nuclear fuel reprocessing. Zirconolite is a principal host phase for actinides in Synroc. We have studied radiation eects of six zirconolite compositions: CaZrTi 2 O 7 , Ca 0X8 Ce 0X2 ZrTi 1X8 Al 0X2 O 7 , Ca 0X85 Ce 0X5 Zr 0X65 Ti 2 O 7 , Ca 0X5 Nd 0X5 ZrTi 1X5 Al 0X5 O 7 , CaZrNb 0X85 Fe 0X85 Ti 0X3 O 7 and CaZrNbFeO 7 . The samples have been irradiated by 1.0 MeV Kr in a temperature range from 25 to 973 K and observed by in situ transmission electron microscopy. The radiationinduced crystalline-to-amorphous transformation was studied by electron diraction and high-resolution electron microscopy. Concurrent with amorphization, monoclinic zirconolite has been found to transform to a¯uorite sublattice through cation disordering during irradiation. All the zirconolites amorphized after a similar dose $ 2 Â 10 15 À 3X9 Â 10 15 ionsacm 2 at room temperature. The amorphization dose increased at elevated temperatures with varying rates of increase for each phase. The critical temperatures for amorphization were 550 K for CaZrNb-FeO 7 , 590 K for CaZrNb 0X85 Fe 0X85 Ti 0X3 O 7 , 640 K for CaZrTi 2 O 7 , 900 K for Ca 0X8 Ce 0X2 ZrTi 1X8 Al 0X2 O 7 , 1000 K for Ca 0X85 Ce 0X5 Zr 0X65 Ti 2 O 7 , and 1020 K for and Ca 0X5 Nd 0X5 ZrTi 1X5 Al 0X5 O 7 . The trend of the critical temperatures indicates that decreasing calcium content increases susceptibility to amorphization. The role of calcium in the radiation-induced amorphous structure is considered to be that of a network-modi®er of the aperiodic structure formed by the polyhedra of high-valence cations. Ó
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.