The Waste Isolation Pilot Plant (WIPP) is located at a depth of 655 m in bedded salt at a site about 40 km east of Carlsbad, New Mexico.The 1996 U. S. Department of Energy (DOE) application for certification by the U. S. Environmental Protection Agency (EPA) included results of a performance assessment (PA) for the planned repository.After the EPA certified the facility in May, 1998, emplacement of contact handled transuranic waste (CHTRU) began in March, 1999.The WIPP facility must undergo a recertification by EPA every 5 years to demonstrate compliance with disposal regulations.Performance assessment is expected to be a key part of the recertification process.The PA will include probabilistic calculations to predict the release of actinides to the accessible environment over a 10,000 year period using a variety of plausible scenarios.The 1996 PA used a model for Pu(IV) solubility based on Th data and the assumption of analogous behavior for all actinides in the (IV) oxidation state.That model did not allow for mobility between the various possible oxidation states of Pu.The possible effects of increases in solubility of Pu through complexation with organic ligands was also not included in the PA because it was argued by DOE that such complexation would be insignificant. Subsequent data from tests using actual TRU wastes show strong evidence that the importance of organic ligands and the potential for multiple oxidation states must be carefully considered in estimating the expected solution concentrations of Pu in WIPP brines.This paper reviews the present state of knowledge of the behavior of Pu-containing wastes in contact with brines similar to those expected to be important for the WIPP repository, recent advances in understanding of the nature of Pu-oxide phases, and analytical methods that have high sensitivity for Pu speciation. A conceptual model for Pu in TRU wastes is presented and a series of steps - including experiments and calculations - that could led to an improved basis for the PA calculations to be done during the WIPP recertification process is outlined.
