Colloids from the aqueous corrosion of uranium nuclear fuel

Abstract Colloids may enhance the subsurface transport of radionuclides and potentially compromise the long-term safe operation of the proposed radioactive waste repository at Yucca Mountain. Little data is available on colloid formation for the many different waste forms expected to be buried in the repository. This work expands the sparse database on colloids formed during the corrosion of metallic uranium nuclear fuel. We characterized spherical UO2 and nickel-rich montmorilonite smectite-clay colloids formed during the corrosion of uranium metal fuel under bathtub conditions at 90 °C. Iron and chromium oxides and calcium carbonate colloids were present but were a minor population. The estimated upper concentration of the UO2 and clays was 4 × 1011 and 7 × 1011–3 × 1012 particles/L, respectively. However, oxygen eventually oxidized the UO2 colloids, forming long filaments of weeksite K2(UO2)2Si6O15 · 4H2O that settled from solution, reducing the UO2 colloid population and leaving predominantly clay colloids. The smectite colloids were not affected by oxygen. Plutonium was not directly observed within the UO2 colloids but partitioned completely to the colloid size fraction. The plutonium concentration in the colloidal fraction was slightly higher than the value used in the viability assessment model, and does not change in concentration with exposure to oxygen. This paper provides conclusive evidence for single-phase radioactive colloids composed of UO2. However, its impact on repository safety is probably small since oxygen and silica availability will oxidize and effectively precipitate the UO2 colloids from concentrated solutions.