The Use of Th, U, Np, Am as Oxidation-State Analogs for Pu in the WIPP Actinide Chemistry Program

Nathalie A. Wall, Laurence H. Brush and Donald E. Wall

Sandia National Laboratories — Carlsbad Programs Group

4100 National Parks Highway - Carlsbad, NM 88220 - USA



The Waste Isolation Pilot Plant (WIPP), located in southeastern New Mexico and operational since March 1999, is the first deep geological disposal facility for transuranic (TRU) waste. In the 1996 Compliance Certification Application, the Department of Energy (DOE) demonstrated a reasonable expectation that repository performance will meet compliance standards for the 10,000 years regulatory period. Performance assessment calculations show that, in the absence of human intrusion, migration of radionuclides horizontally or vertically through the Salado Formation, the bedded-salt formation hosting the repository, or through the shaft seal system will be insignificant. However, borehole intrusions resulting from exploratory drilling may release radioactive materials to the surface and subsurface environments. Knowledge of Pu chemistry is important to model repository behavior in response to intrusion scenarios, but the factors influencing Pu solubility (e.g. Pu complexation with organic ligands) are difficult to measure due to the redox lability of this element. Pu oxidation-state analogs were used in complexation studies to eliminate the additional experimental difficulties inherent in oxidation state control and monitoring. Am(III), Th(IV), Np(V) and U(VI) were used as analogs for Pu(III), Pu(IV), Pu(V) and Pu(VI), respectively. However, WIPP reviewers have questioned the validity of such an analogy. This paper discusses the strengths and weaknesses of the oxidation state analogy in high ionic strength solutions, such as the increased experimental reliability in exchange for small, but quantifiable, deviations in chemical behavior. Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL8500.