Corrosion of commercial spent nuclear fuel. 2. Radiochemical analyses of metastudtite and leachates

Immersing commercial spent nuclear fuel (CSNF) in deionized water produced two corrosion products after a 2-year contact period. Suspensions of aggregates were observed to form at the air-water interface for each of five samples. These suspended aggregates were characterized by X-ray diffraction (XRD) to be metastudtite (UO4 center dot 2H(2)O), while the corrosion present on the surface of the fuel itself was determined to be studtite [(UO2)(O-2)(H2O)(2)](H2O)(2)]. The presence of unreacted UO2 matrix was below the limits of detection by XRD for the three samples examined. The result prompted a radiochemical analysis of the solids collected from the sample air-water interface. The analysis indicated that high concentrations of Sr-90, Cs-137, and Tc-99, relative to the fuel inventory, had concentrated at the air-water interface along with the aggregates of metastudtite. Concentrations of Am-241 were at least two orders of magnitude lower than expected in these solids, and retention of Np-237 and Pu-219 into the corrosion product was observed. The combined radiochemical analyses of the air-water interface aggregates and leachate samples are a rare example of radionuclide partitioning to an alteration phase and may provide preliminary evidence for mechanisms that give rise to such noticeable departures from fuel-inventory values. The leachate radiochemical data are compared to existing data from hydration of the same CSNF.