An Overview of the Crystal Chemistry, Durability, and Radiation Damage Effects of Natural Pyrochlore
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An Overview of the Crystal Chemistry, Durability, and Radiation Damage Effects of Natural Pyrochlore G.R. Lumpkin,1 R.C. Ewing,2 C.T. Williams,3 and A.N. Mariano4 1 Materials Division, ANSTO, Private Mail Bag 1, Menai, NSW 2234, Australia 2 Dept. of Nuclear Engineering, University of Michigan, Ann Arbor, MI 48109-2104, USA 3 Dept. of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK 4 48 Page Brook Road, Carlisle, MS 01741, USA ABSTRACT Numerous studies of pyrochlore group minerals have been completed over the previous 15 years, providing researchers in the field of nuclear waste disposal with a large body of data relevant to the behavior of these minerals in natural systems. The information obtained from studies of natural pyrochlore is applicable to the formulation of diverse waste form compositions and provides data for the assessment of the long-term behavior. Although resistant to dissolution, pyrochlore is subject to chemical alteration by ion exchange with hydrothermal fluids and low temperature ground water; however, Th and U are generally immobile and are retained in the structure. X-ray diffraction, TEM, EXAFS-XANES, and other techniques have been employed in studies of radiation damage. These studies reveal the classic sequence of damage microstructures with increasing dose and provide details about the structure of the amorphous state. Furthermore, the radiation damage studies are now complemented by data relating to the thermal histories of some of the host rocks. INTRODUCTION Pyrochlore, an oxide having ideal A2B2O7 stoichiometry, is the major phase in the baseline formulation designed for the encapsulation of fissile elements such as 235U and 239Pu from the decommissioning of nuclear weapons [1, 2]. The pyrochlore phase is designed to incorporate U, Pu, Gd, and Hf on the A-site, together with Ca. Gd and Hf are included in the formulation as neutron absorbers. To date, most of the research and development work has focussed on the processing, phase relations, and incorporation of elemental impurities. In the area of aqueous durability, only a limited amount of work has been published [3, 4]. Although a combined study of radiation damage and leaching using samples doped with 238Pu is under way at Pacific Northwest National Laboratory, the results are as yet inconclusive (although they report a high increase in release) [5]. In comparison, a large database now exists for minerals of the pyrochlore group in a variety of natural environments. These data can be used to evaluate the crystal chemistry of pyrochlore and to address some of the regulatory issues; e.g., the long term behavior with regard to radiation damage and aqueous durability [6, 7]. CRYSTAL CHEMISTRY As described elsewhere [8, 9], the structure of pyrochlore is considered to be an anion deficient derivative of the fluorite structure type. Minerals of the pyrochlore group conform to
the general formula A2-mB2X6-wY1-n·pH2O, where A represents cations in eight fold coordination, B represents cations in six fold coordina
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