Potential incorporation of transuranics into uranyl phases: Implications for the immobilization of radionuclides in the alteration phases of spent nuclear fuel.

摘要

The UO₂ in spent nuclear fuel is unstable under the moist oxidizing conditions expected in the proposed repository at Yucca Mountain, Nevada and will be altered to secondary uranyl (UO₂2+) phases. The transuranics released during the corrosion of spent fuel may also be incorporated into the structures of these phases. The incorporation of radionuclides into alteration products will limit their mobility.; A series of precipitation tests were conducted to determine the potential incorporation of Ce4+ and Nd3+ (surrogates for Pu4+ and Am3+, respectively) into uranyl phases. The uranium alteration phases examined in this study include ianthinite ([U₂4+(UO₂)₄O₆(OH) ₄(H₂O)]₄](H₂O)₅), becquerelite (Ca(UO₂)₆O₄(OH)₆(H₂O) ₈), Na-compreignacite (Na₂(UO₂)₆O ₄(OH)₆(H₂O)₇), boltwoodite (K(UO ₂)(SiO₃OH)(H₂O)_1.5), and other unidentified uranyl compounds. The results of these experiments demonstrate that significant quantities of both Ce4+ (up to 800 ppm) and Nd3+ (up to 24,800 ppm) maybe incorporated within the uranium alteration phases. By analogy, it is likely that Pu4+ and Am3+ will behave in a similar fashion. Incorporation of cerium and neodymium by the uranyl phases can be accomplished by co-precipitation of the elements within the crystalline structure, by adsorption of the elements onto surfaces of the solid phases, or by inclusion of the components into the host solids in which they ultimately become absorbed by a process of surface overgrowth. The incorporation of Ce4+ and Nd3+ into or onto the uranium phases appeared to be dependent on the solution pH. The pH dependence for enhancing tetra- and trivalent element incorporation varies with various incorporation reactions and host phases.