Neptunium incorporation into uranyl compounds that form as alteration products of spent nuclear fuel: Implications for geologic repository performance

Abstract

Summary Alteration of spent nuclear fuel in a geological repository under oxidizing conditions is likely to result in abundant uranyl compounds. Incorporation of radionuclides into the uranyl alteration phases may significantly reduce their mobility, thereby impacting repository performance. The following compounds have been synthesized from solutions containing from ∼10 to 500ppm Np5+: meta-schoepite, UO3·2H2O; Na-compreignacite, Na2[(UO2)3O2(OH)3]2(H2O)7; uranophane, Ca(UO2)2(SiO3OH)2(H2O)5; and β-(UO2)(OH)2. The structures of each involve sheets of uranyl polyhedra; the interlayers contain cations and H2O groups in Na-compreignacite and uranophane, only H2O in meta-schoepite, and β-(UO2)(OH)2 does not contain interlayer constituents. Synthesized powders were characterized by X-ray powder diffractometry, and were analyzed by ICP-AES (U, Na, Ca) and ICP-MS (Np). Aliquots of the powders were washed in 0.5M acetic acid to remove sorbed Np prior to analysis. The powders of meta-schoepite and β-(UO2)(OH)2 contained at most a few ppm Np5+. In contrast, powders of Na-compreignacite and uranophane contained Np5+ in proportion to the Np5+ concentration in the mother solution, and powders of each containing more than 400ppm Np5+ (of total Np + U) were obtained. Incorporation of Np5+ into uranyl compounds that form in a geological repository appears possible, and may impact the mobility of Np.