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

Summary 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 (UO4·2H2O), while the corrosion present on the surface of the fuel itself was determined to be studtite [(UO2)(O2)(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 90Sr, 137Cs, and 99Tc, relative to the fuel inventory, had concentrated at the air–water interface along with the aggregates of metastudtite. Concentrations of 241Am were at least two orders of magnitude lower than expected in these solids, and retention of 237Np and 239Pu 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.