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The Waste Isolation Pilot Plant (WIPP), located at a depth of 650 m in bedded salt at a site approximately 40 km east of Carlsbad, New Mexico, was constructed by the US Department of Energy for the disposal of transuranic wastes arising from defense-related activities. The disposal site is regulated by the US Environmental Protection Agency (EPA). During the process leading to certification of the site for initial emplacement of waste, EEG and their contractors reviewed the DOE Compliance Certification Application (CCA) and raised a number of issues. This report reviews the issues related to the chemistry of plutonium as it will affect the potential for release of radioactivity under WIPP conditions. Emphasis is placed on conditions appropriate for the Human Intrusion scenario(s), since human intrusion has the largest potential for releasing radioactivity to the environment under WIPP conditions. The most significant issues that need to be addressed in relation to plutonium chemistry under WIPP conditions are (1) the effects of heterogeneity in the repository on Pu concentrations in brines introduced under the human intrusion scenario, (2) the redox state of Pu in solution and potential for plutonium in solid phases to have a different redox state from that inmore » the solution phase, (3) the effect of organic ligands on the solubility of Pu in WIPP-relevant brines, and (4) the effects of TRU waste characteristics in determining the solubility of Pu. These issues are reviewed with respect to the treatment they received in the DOE CCA, DOE’s response to EEG’s comments on the CCA, and EPA’s response to those comments as reflected in the final EPA rule that led to the opening of the WIPP. Experimental results obtained in DOE’s Actinide Source-Term Test Program (STTP) during the last two years are reviewed and interpreted in the light of other developments in the field of Pu solution chemistry. This analysis is used as the basis for a conceptual model for Pu behavior under WIPP conditions. This report identifies three issues that can be addressed as part of the five-year re-certification cycles mandated by the WIPP Land Withdrawal Act (U.S. Congress, 1996, *8.(f)(1)) and implemented in the EPA Final Rule (U.S. EPA, 1998a). First, the impact of organic ligands on the complexation of a system with multiple cations could be delineated by some simple experiments with Th (IV), citrate,EDTA, Ca2+ and one or more transition metals. An experiment with Pu (IV) could be designed to determine the lability of Pu oxidation states in the presence of organic ligands. Second, the issue of using analogs for Pu could be addressed with appropriate experiments using U and Pu at redox conditions in the range anticipated at the WIPP. Attention to the published experimental results of U and Pu could make a considerable contribution to this effort. Third, the issue of uncertainty in calculated actinide solubility can be resolved with experimental data for Pu (IV) compounds of appropriate compositions. The report also recommends starting by constructing a data base for U and Pu using the published, peer-reviewed OECD/NEA U and Pu data bases.« less

Plutonium chemistry under conditions relevant for WIPP performance assessment. Review of experimental results and recommendations for future work