Zirconolite stability under alpha self-irradiation: investigation of natural analogs rich in thorium and uranium

Full text of publication follows: Zirconolite, with the generic formula CaZrTi2O7, is capable of immobilizing up to 15-20 wt% tri and tetravalent actinides at atomic-scale in its crystal structure. Under alpha self-irradiation (>1019 alpha disintegrations per gram of zirconolite), its structure is disorganized and becomes metamict. The containment matrix thus gradually becomes amorphous over the long term. Do microscopic changes in the initial crystalline structure affect its macroscopic properties, for example chemical durability? This issue was investigated through static leach testing at 90 deg. C of natural zirconolite containing Th/U some 2 billion years old from Phalabowra (South Africa),. The same experimental protocol was also implemented on synthetic zirconolite ceramic pellets loaded with neodymium, an inactive actinide surrogate. Leach tests were carried out on natural zirconolite after separation and purification of zirconolite crystals incorporated in a mineral assemblage of apatite, phlogopite, rutile and ilmenite. The results indicate that the behavior of fully metamict natural zirconolite in water is no different than that of the same zirconolite previously recrystallized by heat treatment at 1000 deg. C, as verified by analysis of the calcium, titanium and zirconium released in solution. Steady-state dissolution conditions are quickly reached (after 1 day of alteration), as systematically revealed with neodymium-loaded synthetic zirconolite ceramic samples crystallized or damaged by heavy ion bombardment simulating alpha irradiation damage. The available data on synthetic zirconolite containing Nd and Pu confirm that equivalent steady-state conditions are found in natural zirconolite 2 billion years old. (authors)