Durability of Zirconolite in Hydrothermal Fluids: Implications for Nuclear Waste Disposal
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Durability of Zirconolite in Hydrothermal Fluids: Implications for Nuclear Waste Disposal R. Gieré1, J. Malmström2, E. Reusser2, G.R. Lumpkin3, M. Düggelin4, D. Mathys4, R. Guggenheim4 and D. Günther5 1 Earth & Atmospheric Sciences, Purdue University, West Lafayette, IN 47907-1397, USA; Email: [email protected] 2 Institute of Mineralogy and Petrography, ETH-Zentrum, 8092 Zürich, Switzerland 3 Australian Nuclear Science & Technology Organisation, Menai, NSW 2234, Australia 4 SEM-Laboratory, University of Basel, Klingelbergstrasse 50, 4056 Basel, Switzerland 5 Institute of Inorganic Chemistry, ETH-Zentrum, 8092 Zürich, Switzerland ABSTRACT Synthetic zirconolite (CaZrTi2O7) doped with rare earth elements (REEs) and Hf has been subjected to corrosion tests in a closed system at elevated temperature and pressure in fluids with various compositions. Together with previous studies, the results indicate only a very weak corrosion below 250°C at a pressure of 50 MPa. Above that temperature and up to 500°C zirconolite suffers from relatively rapid corrosion and, depending on the fluid composition, it may be covered by various secondary phases. Above 500°C in Na-rich fluids, zirconolite is replaced by perovskite and calzirtite (nominally CaTiO3 and Ca2Zr5Ti2O16, respectively), but the REEs and Hf (acting as actinide analogues and/or neutron absorbers) are almost quantitatively incorporated into the secondary phases. The breakdown of zirconolite and its replacement by other phases in the laboratory tests are comparable to reactions observed in natural systems. Additional experiments with U-doped zirconolite revealed differences in the behavior of the used actinide analogues (Nd, Ce, Gd) and U during corrosion. The results of this study, together with observations on natural samples, strongly support the use of zirconolite-based ceramic waste forms for actinide-rich wastes. INTRODUCTION Zirconolite, ideally CaZrTi2O7, is a major constituent of Synroc-type nuclear waste forms. This synthetic rock is designed to immobilize liquid, high-level nuclear waste (HLW), and zirconolite acts as a principal host phase for actinides. One of the essential requirements for a HLW host phase is that it should be highly durable at the conditions expected at a final disposal site. A high resistance against corrosion has been observed for naturally occurring zirconolite and is also reported from experiments in deionized water at temperatures up to 150ºC [1, 2]. The present study had the aim to investigate the durability of zirconolite at higher temperatures, at elevated pressure and in various fluid compositions. Closed-system hydrothermal experiments were carried out in externally heated pressure vessels at temperatures between 150 and 700ºC, and at 50 and 200 MPa. In addition to deionized water, fluids containing various amounts of NaOH, HCl, H3PO4, SiO2, and CO2 were used to study the corrosion behavior of zirconolite. Starting material was a single-phase, polycrystalline zirconolite-2M, doped with REEs and Hf (as actinide analogues and/or neutro
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