When designing an underground repository for spent nuclear fuel it is important to know if and to what extent Np and Pu are reduced. At the moment only Np has been studied, but further investigations with Pu are planned.
Np in the pentavalent state and Th in the tetravalent state (234Th, about 10−9 M) have been used to study the sorption onto TiO2 (no reduction is expected) and onto UO2 with different atmospheres (air, nitrogen and nitrogen and hydrogen at 5 MPa pressure), and with different Np- concentrations (about 10−12M, 239Np and 10−6 M,237Np).
Th is sorbed to a high extent at pH>3 on both TiO2 and UO2. The sorption increases 3 orders of magnitude over one pH-unit when log Ka (the distribution coefficient with respect to surface area) is plotted against pH.
The Np sorption on TiO2 is independent of the Np concentration and shows a slope of one when pH is plotted against log Ka. The higher sorption of Np onto UO2 compared with TiO2 indicates that Np is partially reduced even when it is in contact with air. No large differences can be observed for Np between the nitrogen and hydrogen-atmosphere, indicating that the absence of oxygen is the important factor for the reduction. The different Np concentrations had no great impact on the reduction/sorption on UO2.