The formation of hydrated zirconium molybdate in simulated spent nuclear fuel reprocessing solutions

Hydrated zirconium molybdate (ZM_h) is known to precipitate from solutions of dissolved spent nuclear fuel, particularly from the waste fission product solution after the uranium and plutonium have been extracted during reprocessing. Its precipitation can cause major problems during waste treatment, and therefore a complete understanding of its chemical behaviour, especially with regard to its role in the nuclear fuel cycle, is desirable. We have used a number of complementary analytical techniques to elucidate the hitherto incompletely understood chemistry of formation of ZM_h in synthetic fuel reprocessing solutions. We have demonstrated that ZM_h formation was governed by multi-step surface reactions and does not involve the formation of colloids or particulates in solution. The first step in the deposition of ZM_h onto surfaces is the formation of an amorphous film with a Zr ∶ Mo ratio close to unity. It is followed by the formation, growth and nucleation of ZM_h particles of varying degrees of crystallinity with a Zr ∶ Mo ratio close to 0.5. The X-ray diffraction pattern of deposited ZM_h particles is in agreement with the reported crystallographic data. The structural features of the film and ZM_h were also examined at the nanometer scale.