Dissolution of plutonium hydroxide under slightly reducing conditions

摘要

In Sweden, the proposed method of disposal of spent nuclear fuel is to deposit it 500 m down in granitic bedrock. Due to lack of oxygen at these depths it is important to study the behaviour of elements present in the spent fuel at reducing conditions. The dissolution of plutonium (Ⅲ, Ⅳ) hydroxide has been studied in near neutral conditions in 0.1 M NaCl for almost two years. For the study two solids were prepared using precipitation with deoxygenated NaOH followed by filtration and washing of the precipitates with deionised water. Prior to the precipitation of the first solid the plutonium solution was passed through a lead column reductor and then filtered to remove any residues from the column. The resulting solution was verified to be more than 95% Pu(Ⅲ) using optical spectrometry. The precipitate was initially blue but became khaki after drying. The second precipitate was made directly on the plutonium stock consisting mainly of Pu (Ⅳ). This precipitate was bright green. Microgram amounts of the two precipitates were put in 0.1 M NaCl solutions adjusted to pH 5.6, 6.7 and 7.0 (Exp. 1-3) for the Pu(Ⅲ) hydroxide and pH 5.5 (Exp. 4) for the Pu(Ⅳ) precipitate. For the concentration measurements samples were withdrawn and centrifuged at 41000 rpm (288 000 g_(max)). The pH and on occasion Eh was measured directly in the centrifuge tubes after some supernatant had been withdrawn for liquid scintillation counting (LKB Wallac, RackBeta). All handling was performed in an inert (N_2) glove box with active removal of oxygen down to < 10 ppm.