NEAR-FIELD REDOX CONTROL: IMPACT ON ACTINIDE SOLUBILITY AND SPECIATION

A summary of progress made in establishing the source term for key actinides in a salt-based permanent geologic repository is provided. This source term is defined by the expected near-field conditions. The most important of these are pH, redox conditions, and the inorganic/organic complexants present in the site-specific groundwater. We have identified the reactivity of reduced iron, reduction/oxidation by organic complexants, and the bio-activity of halo-tolerant microorganisms as the key processes that define actinide oxidation state distribution under repository-relevant conditions.

The use of bedded salt deposits for the permanent disposal of nuclear waste continues to receive much attention in the United States and internationally. This is largely based on the highly successful Waste Isolation Pilot Plant (WIPP) transuranic waste repository that was opened in 1999 in Southeastern New Mexico. A bedded salt formation, such as the one in which the WIPP is located, has many advantages that make it an ideal geology for permanent disposal of nuclear waste. This includes well established mining techniques, self-sealing that lead to a naturally-induced geologic isolation, a relatively dry environment, and a favorable chemistry.

Redox control in the near-field is one of the more important aspects of this favorable chemistry. Establishing reducing conditions for the key multivalent actinides (e.g., U, Pu and Np) is important since these have low solubility in their lower oxidation states. The effects of reduced iron as Fe(0/II), anaerobic activity by indigenous microorganisms, and organic chelating agents that typify TRU waste were established for plutonium, uranium, and in some cases neptunium. Under a wide range of conditions investigated, the predominant oxidation states established are Pu(III) and Pu(IV) for plutonium, U(IV) for uranium, and Np(IV) for neptunium. Reduced iron is the most effective in reducing the multivalent actinides. Bioreduction was demonstrated for all three actinides but is often coupled with iron cycling and is less understood and certain. Organic chelating agents tend to slowly reduce or oxidize actinides to favor the IV oxidation state. Understanding the detailed mechanisms for all of these processes is the focus of ongoing research.