Influence of accessory phases and surrogate type on accelerated leaching of zirconolite wasteforms

Abstract

Abstract A fraction of the UK Pu inventory may be immobilised in a zirconolite ceramic matrix prior to disposal. Two zirconolite compositions, targeting CaZr 0.80 Ce 0.20 Ti 2 O 7 and CaZr 0.80 U 0.20 Ti 2 O 7 , were fabricated by hot isostatic pressing, alongside a reformulated composition, nominally Ca 0.80 Zr 0.90 Ce 0.30 Ti 1.60 Al 0.40 O 7 , with an excess of Ti and Zr added to preclude the formation of an accessory perovskite phase. Materials were subjected to accelerated leaching in a variety of acidic and alkaline media at 90 °C, over a cumulative period of 14 d. The greatest Ce release was measured from CaZr 0.80 Ce 0.20 Ti 2.00 O 7 exposed to 1 M H 2 SO 4 , for which 14.7 ± 0.2% of the original Ce inventory was released from the wasteform into solution. The extent of Ce leaching into the solution was correlated with the quantity of perovskite present in the wasteform, and associated with the incorporation and preferential dissolution of Ce 3+ . CaZr 0.80 U 0.20 Ti 2.00 O 7 exhibited improved leach resistance relative to CaZr 0.80 Ce 0.20 Ti 2.00 O 7 , attributed to the decreased proportion of accessory perovskite, with 7.1 ± 0.1% U released to in 8 M HNO 3 after 7 d. The Ca 0.80 Zr 0.90 Ce 0.30 Ti 1.60 Al 0.40 O 7 composition, with no accessory perovskite phase, presented significantly improved leaching characteristics, with < 0.4%Ce released in both 8 M HNO 3 and 1 M H 2 SO 4 . These data demonstrate the need for careful compositional design for zirconolite wasteforms with regard to accessory phase formation and surrogate choice.