6. Sorption Values for Thorium, Uranium, Plutonium, Neptunium, and Protactinium

The actinide elements—thorium, uranium, plutonium, neptunium, and protactinium—are important constituents of radioactive waste from nuclear fuel. Uranium also occurs in a variety of wastes, e.g. from mining. These elements all hydrolyse extensively in aqueous solutions and correspondingly strong sorption onto concrete, hydrated cement paste and individual CSH phases is observed. Sorption is also more or less constant over all states of cement evolution. The sorption mechanisms are, however, not well understood to date. In case of Th, relatively fast kinetics of uptake and reversibility are observed, indicating that uptake occurs through surface processes and probably does not involve incorporation into the structures of hydrated cement phases. In contrast to Th, which exists only in the tetravalent oxidation state in aqueous environments, uranium, plutonium, neptunium, and protactinium can exist also in lower and/or higher oxidation states. In analogy to their aqueous chemistry, very similar sorption behaviour is expected for all tetravalent forms. Sorption of U(VI) on cementitious materials is similar to Th(IV)/U(IV) in magnitude, but appears to involve different mechanisms (probably solid-solution formation). In lack of specific information, the same is assumed for Pu(VI). Relatively little is known for the pentavalent forms. Strong sorption is observed for Pa(V), but the underlying mechanism is not known. For Np(V), relatively weak sorption is assumed in lack of specific data.