SAFE DISPOSAL OF HIGH-LEVEL NUCLEAR WASTE - A GEOCHEMICAL PERSPECTIVE. IF NOT YUCCA MOUNTAIN, THEN WHERE?

The choice of Yucca Mountain as a proposed repository for high-level nuclear waste was based, in part, on the following technical factors: 1) dry climate, 2) thick sequence of strata above the repository horizon, 3) unsaturated zone above a deep water table, 4) potential adsorption of radionuclides by zeolites, and 5) competent hostrock. Geochemically, the advantages of Yucca Mountain are diminished by one disadvantage – the presence of oxygen in the proposed repository. Uranium oxide, which is the main component of spent nuclear fuel, is soluble in water under oxidizing conditions. The dissolution of spent UO₂ fuel pellets in infiltrating meteoric water would release uranium and associated byproducts developed during fission and irradiation. Because of the oxidizing environment, Yucca Mountain is unique among the several proposed high-level repositories in other countries. The scientists and engineers charged with preventing release and migration of the radionuclides undertook extraordinary measures to design canisters and protective drip shields to prevent oxidation, corrosion, and release of radionuclides for at least 10,000 years, with all of the uncertainties involved in modeling and extrapolation over an enormous time. Much of the research effort could have been avoided simply by choosing a reducing subsurface environment. Other factors, such as the flow of groundwater, become irrelevant if the canisters and the spent fuel cannot dissolve. So, if not Yucca Mountain, then where? One technically attractive location is in the Precambrian Nonesuch Shale in the subsurface of the Upper Peninsula of Michigan. At that location metallic copper occurs in abundance and has been mined extensively. The fresh metallic copper is at least hundreds of millions of years old, demonstrating that spent nuclear fuel could be encapsulated in copper containers and safely stored for hundreds of millions of years. The main technical issue would be the effect of the thermal pulse on the interactions among the copper cannisters, the minerals and organic matter in the Nonesuch Shale, and the saline brines in the deep subsurface of the Upper Peninsula. The White Pine Mine, which closed in 1995, encompasses more than 25 square miles in the Nonesuch Shale and could be utilized as a pre-excavated underground laboratory.