Solubility of Nd 3+ and UO 22+ in WIPP brine as oxidation-state invariant analogs for plutonium

Abstract

A general status of the Waste Isolation Pilot Plant (WIPP), as it relates to plutonium chemistry and inventory, is given. We also report on the results of two redox-invariant studies, Nd 3+ and UO 2 2+, that are being investigated as analogs for Pu(III) and Pu(VI) chemistry under WIPP-relevant conditions. The WIPP repository is the only working repository for TRU waste in the world and continues to perform above expectations. Recertification of the WIPP was recently received in April 2006 and plutonium research to support these ongoing recertification activities continues. As oxidation-state analogs for plutonium, the solubility of uranium(VI) and neodymium(III) were established in long-term experiments as a function of pC H+ for two WIPP simulated brines that bracket the range in brine composition expected. The solubility of uranium(VI) measured at high pC H+ in the absence of carbonate was ∼2 × 10 −7 M in ERDA-6 brine. This is almost 100 times lower than the uranium solubility published in the literature for carbon dioxide-free systems and is almost four orders of magnitude lower than current assumptions in the WIPP Performance Assessment (PA). The Nd 3+ solubility determined was ∼1 × 10 −7 M. There was essentially no significant effect of carbonate on the solubility measured for Nd 3+. Taking into consideration experimental errors in our data, the neodymium solubilities determined in the present work are in good agreement with the solubilities calculated by WIPP PA.