Durability of zirconolite in hydrothermal fluids:: Implications for nuclear waste disposal

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 degreesC at a pressure of 50 MPa. Above that temperature and up to 500 degreesC zirconolite suffers from relatively rapid corrosion and, depending on the fluid composition, it may be covered by various secondary phases. Above 500 degreesC 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.