Neptunium Association with Selected Uranyl Phases Anticipated in the Yucca Mountain Repository

摘要

Spent nuclear fuel (SNF) in the United States is planned to be permanently stored in a geological respository located at Yucca Mountain. Under the anticipated moist, oxidizing environment, a suite of secondary uranium solids is expected to form, which will likely impact the release and potential mobility of other radionuclides contained in the SNF. This chapter provides a summary of selected studies of neptunium (Np) with three phases suggested likely to form under the anticipated respository conditions: metaschoepite ([(UO2)8O2(OH)i2]-10H2O), a-uranophane (Ca[(UO2) (SiO3OH)]2-5H2O), and studtite (UO44H20). Results suggest that uranophane and studtite have a higher capacity for neptunium association than metaschoepite, although uptake of Np by metaschoepite increases proportionally to the quantity of Na included in the synthesis. The release of Np into solution from batch dissolution studies exceeded congruent release with uranium for the uranophane and studtite solid phases. Ultimately, the mechanism of Np association with these phases is inconclusive and further studies will need to be conducted to answer this important question.