A new uranyl oxide hydrate phase derived from spent fuel alteration

An alteration phase that formed during the corrosion of commercial oxide spent nuclear fuel has been characterized with analytical transmission electron microscopy (AEM). The phase is a Cs-Ba uranyl molybdate oxide hydrate that has an orthorhombic structure related to the alkaline earth uranyl oxide hydrates of the protasite-group minerals. On the basis of the compositional analysis and a proposed model of the structure, 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 present 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 actual radioactive waste forms is important. This study suggests that the natural U-Mo deposits at Shelby, WY, and Bates Mountain Tuff, NV, may be good analogues for the long-term behavior of U-Mo phases formed due to spent fuel corrosion. (C) 1997 Elsevier Science B.V.