The Waste Isolation Pilot Plant (WIPP) is located in the northern portion of the Delaware Basin in Southeastern New Mexico east of Carlsbad. It was certified by the Environmental Protection Agency (EPA) in May 1998 as a repository for transuranic (TRU) waste and is operated by the Department of Energy, Carlsbad Field Office (DOE CBFO). The WIPP is currently in the process of recertifying its EPA license.

The potential for the release of actinides from the WIPP was an important, if not the key, consideration in WIPP PA. Models developed by the WIPP were oxidation-state specific and relied on the predicted distribution of oxidation states in the subsurface to establish the source term concentration and therefore predict the rate of multivalent actinide release (e.g. plutonium, americium, neptunium, and uranium). In the expected iron-rich anoxic reducing environment, it was predicted that lower oxidation states, with their lower overall solubilities, would predominate should brine inundation occur.

Herein we report the results of both past WIPP supported research and recently initiated confirmatory studies to establish the prevalent oxidation states of multivalent actinides for the expected WIPP environment. These studies are centered on the reduction pathways of plutonium (V/VI) and uranium (VI) under anoxic conditions in the presence of iron but have general applicability to all multivalent actinides present in the WIPP. Parameters investigated include the form of the iron (powder, metallic and aqueous), the pH, the composition of the brine, and the presence of carbonate. Total actinide concentration and, when possible, the speciation of the actinide is established using a combination of absorption spectrometry, ICP-MS and radio-analytical methods. The data from the ongoing studies are expected to confirm the prevalence of plutonium (IV) and uranium (IV) species in the WIPP, as was predominantly observed in past research.