Zeolite - Salt Occlusion: A Potential Route for the Immobilisation of Iodine-129?
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=HROLWH±6DOW2FFOXVLRQ $3RWHQWLDO5RXWHIRUWKH,PPRELOLVDWLRQRI,RGLQH " Neil C. Hyatt,1* Joseph A. Hriljac,2* Alia Choudhry,2 Laura Malpass,2 Gareth P. Sheppard2 and Ewan R. Maddrell.3 1 Immobilisation Science Laboratory, Dept. of Engineering Materials, University of Sheffield, Mappin Street, Sheffield, S1 3JD. UK. 2 School of Chemical Sciences, The University of Birmingham, Birmingham, B15 2TT. UK. 3 BNFL Technology Centre, Sellafield, Seascale, Cumbria, CA20 1PG. UK. $%675$&7 Reactions of zeolite Na-A with AgI, and the sodium, copper and lead forms of zeolites A, LTA, X and Y with NaI, have been examined as possible starting routes to the long term immobilisation of iodine-129. Heating the salts in air, at 500oC, with the sodium forms of the zeolites leads to the formation of occlusion products, where the iodide salt migrates into the zeolite pores. Detailed studies of the Na-A / 5AgI complex indicate it has a uniform distribution of Na, Si, Al, Ag and I, and is thermally stable to ca. 750oC, where there is a substantial weight loss as iodine is released. ,QVLWX powder X-ray diffraction studies have been used to monitor the occlusion reaction at 400oC, and show that the occlusion product decomposes to produce a single crystalline phase at 800oC prior to further decomposition at 850oC to a mixture of nepheline and elemental silver. ,1752'8&7,21 The long term immobilisation of iodine-129 arising from nuclear fuel reprocessing operations is a complex challenge. Secure containment of iodine-129 (half life 15.7 million years) is difficult to assure, due to the volatility of elemental iodine, the corrosive nature of iodide salts and the propensity for the displacement of iodide-129 by other anionic species in ground waters. At present, therefore, the preferred route for disposal of iodine-129 is through discharge to the ocean where dilution ensures minimal exposure to mankind. However, future regulatory decisions to reduce discharges to the marine environment may require the development of an inexpensive process for the safe and effective long term immobilisation of iodine-129. We are exploring the occlusion of metal iodide salts into zeolite hosts as a potential route to the immobilisation of iodine-129. The occlusion of a metal (iodide) salt into a suitable zeolite host affords a nano-composite material in which the constituent cations and anions of the salt are dispersed within the microporous channel network of the host zeolite [1]. Effective sequestration of iodine-129 during reprocessing operations may be achieved by precipitation from the Dissolver Off-Gas liquor, using AgNO3 [2]. The AgI thus formed may be converted to NaI by washing with NaOH in the presence of a suitable reductant, thereby regenerating Ag, if required [2]. Occlusion of AgI or NaI into a suitable aluminosilicate zeolite affords a primary wasteform suitable for further elaboration. For example, calcination of an iodide salt occluded zeolite under ambient or isostatic pressure may yield iodo-sodalite, Na4Al3Si3O12I. This phase has b
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