PNNL

Abstract

The quantity of high-level radioactive waste in the United States requiring permanent disposal continues to increase at a steady rate. The Yucca Mountain site in southwestern Nevada is currently the subject of a license application expected to be submitted by the U.S. Department of Energy in 2005 to host the nation’s first repository for high-level radioactive waste. Approximately 70,000 metric tons of spent nuclear fuel (SNF) from commercial power plants (as UO2) is planned to be emplaced in the repository at Yucca Mountain. Of interest is the capability of the repository to retain the vast array of radionuclides with different chemical properties for the lifetime of the repository, currently debated to be between 10,000 to greater than 270,000 years. Under the conditions anticipated at Yucca Mountain, the interaction of SNF with the local environment is expected to form a suite of secondary U solids. Natural analog studies as well as work with SNF have shown that UO2 undergoes oxidative corrosion in the presence of oxidizing water to form hexavalent U phases that reflect the composition of the water. One issue under consideration is the fate of long-lived isotopes present in the waste, particularly their interaction with the predominant hexavalent U solids expected to form at the repository. Neptunium-237 (237Np) is present in SNF at 400–600 ppm and will be one of the major contributors to dose after 50,000 years because of its long half-life of 2.14 × 106 years. Under the conditions expected at Yucca Mountain, Np will likely exist in the pentavalent oxidation state as the neptunyl ion, NpO2+, resulting in a relatively high mobility. Burns et al. suggested some time ago that radionuclides such as Np may be incorporated into the predominant uranium (U) solids present in the repository, forming solid solutions and thereby reducing their potential mobility in the environment. This chapter summarizes the results of work done to investigate the interaction of dissolved Np with three uranyl solids, each of differing structure type and deemed likely to form in a repository for SNF at Yucca Mountain.