The formation of hydrated zirconium molybdate in simulated spent nuclear fuel reprocessing solutions Fre ´de ´ric J. Doucet,*y a David T. Goddard, b Carol M. Taylor, b Iain S. Denniss, c Sheila M. Hutchison c and Nicholas D. Bryan a a The Centre for Radiochemistry Research, Department of Chemistry, The University of Manchester, Oxford Road, Manchester, UK M13 9PL. E-mail: f.doucet@bham.ac.uk b British Nuclear Fuels plc, Springfields, Preston, UK PR4 0XJ c British Nuclear Fuels plc, Sellafield, Seascale, UK CA20 1PG Received 19th February 2002, Accepted 16th April 2002 First published as an Advance Article on the web 7th June 2002 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. 1. Introduction Hydrated zirconium molybdate, ZrMo 2 O 7 (OH) 2 2H 2 O (herein abbreviated ZM h ), is one of the most common insoluble resi- dues encountered during fuel reprocessing operations. 1–5 It is known to cause major problems in the clarification and/or extraction process in nuclear reprocessing plants. 6 For this rea- son, a major effort has been made to characterise ZM h 7–10 and to determine the chemical and physical factors that impinge on its precipitation. 10–12 For instance, it is now well established that the formation of ZM h follows a typical ‘‘ S-shaped ’’ pre- cipitation curve, where the onset of precipitation occurs slowly at first, followed by the fast formation of ZM h up to a plateau of precipitation where the rate slows or stops. 1 However, a sig- nificant number of variables have been identified as affecting the rate and yield of ZM h precipitation. Perhaps paramount amongst these are the acidity of the solution and the tempera- ture. Increasing the concentration of HNO 3 from 3 mol dm 3 to 6 mol dm 3 or decreasing the temperature from 100  C to 70  C was shown to significantly increase the initiation period and decrease the rate of precipitation of ZM h . 1,9,10,13 The pre- sence of iron in solution is another factor which might influ- ence ZM h formation. The amount of insoluble ZM h formed in iron-rich solutions was found to be much lower than in iron-free solutions. 14 The author speculated upon the role of a soluble iron–molybdenum complex in preventing ZM h preci- pitation. He also suggested that the composition of the experi- mental solutions may govern the size and shape of ZM h particles. For instance, the use of ammonium molybdate rather than molybdic acid as the molybdenum source promoted the formation of elongated particles. Moreover, the presence in solution of gadolinium as a nuclear poison promoted the for- mation of regularly shaped cubic particles, although gadoli- nium itself was not identified in the ZM h cubes. Although the physical nature of ZM h particles appeared to be influenced by solution composition, their XRD pattern suggested that its chemical structure (i.e. ZrMo 2 O 7 (OH) 2 2H 2 O) was unaffected by these physical changes. Despite the acknowledged importance of the precipitation of ZM h in nuclear fuel reprocessing solutions, no attempt has been made to elucidate the mechanism of formation under the conditions encountered during reprocessing operations. This is surprising since understanding their formation in acidic media will help to explain how and why they form under fuel reprocessing operations, and also to improve operating condi- tions in order to minimise its formation and prevent interfer- ence with reprocessing operations. Herein we have examined the different stages in ZM h forma- tion in synthetic spent nuclear fuel reprocessing solutions (also called short-simulant solutions). We have suggested a possible mechanism of formation of ZM h , and the new knowledge we have gained might now be used to investigate means of con- trolling or preventing their formation within the nuclear fuel cycle. 2. Materials and methods 2.1. Preparation of short-simulant solutions Considerable effort has been made to prepare aqueous solu- tions that closely resemble those found in nuclear fuel repro- cessing plants. The physical and chemical properties simulated in this study include time, acidity and nitrate con- y Present address: Division of Environmental Health and Risk Man- agement, School of Geography and Environmental Sciences, Univer- sity of Birmingham, Edgbaston, Birmingham, UK B15 2TT. DOI: 10.1039/b201792j Phys. Chem. Chem. Phys., 2002, 4, 3491–3499 3491 This journal is # The Owner Societies 2002 PCCP