A NEW URANYL OXIDE HYDRATE PHASE DERIVED FROM SPENT FUEL ALTERATION

An alteration phase that formed during the corrosion of commercial oxi de spent nuclear fuel has been characterized with analytical transmiss ion electron microscopy (AEM). The phase is a Cs-Ba uranyl molybdate o xide hydrate that has an orthorhombic structure related to the alkalin e earth uranyl oxide hydrates of the protasite-group minerals. On the basis of the compositional analysis and a proposed model of the struct ure, the ideal structural formula is (Cs0.8Ba0.6)(UO2)(5)(MoO2)O-4(OH) (6) . nH(2)O (where n is around 6). Low levels of strontium are also p resent in the phase. The estimated unit cell parameters are a = 0.754 nm, b = 0.654 nm, and c = 3.008 nm. Although many of the phases formed during corrosion of spent oxide fuel are similar to those observed in natural uraninite deposits, such as Pena Blanca in Mexico, there are important differences owing to the presence of fission products in the spent fuel. Thus, accurate determination of corrosion processes in ac tual radioactive waste forms is important. This study suggests that th e natural U-Mo deposits at Shelby, WY, and Bates Mountain Tuff, NV, ma y be good analogues for the long-term behavior of U-Mo phases formed d ue to spent fuel corrosion. (C) 1997 Elsevier Science B.V.