A Comparison of Wasteforms and Processes for the Immobilisation of Iodine-129
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A Comparison of Wasteforms and Processes for the Immobilisation of Iodine-129 E.R. Maddrell, P.K. Abraitis British Nuclear Fuels plc, Sellafield, Seascale, Cumbria, United Kingdom, CA20 1PG ABSTRACT A range of potential wasteforms for the immobilisation of iodine-129 have been synthesised and compared. The work used both iodosodalite and iodovanadinite, as identified in previous studies, as baseline iodine host phases. Syntheses of related compounds using alternative precursors had varying degrees of success. In addition it was shown that iodine could potentially be isolated in a simple composite wasteform of silver iodide particles in a rutile or anatase matrix. Preliminary leaching studies suggest that iodosodalite dissolves congruently whilst iodovanadinite shows preferential release of iodine at a rate approximately ten times that of matrix alteration.
INTRODUCTION Fission product iodine arising from reprocessing of spent nuclear fuel is currently discharged to sea because a dilution and dispersal approach results in a low dose to the general population and is therefore seen as the most environmentally benign option. With the continuing pressure to reduce discharges to the marine environment, however, a package of work has been initiated to evaluate methods by which iodine can be conditioned into a durable wasteform suitable for geological disposal. Previous work has shown that iodide analogues of natural halide containing minerals have potential as iodine hosts. Two prominent examples of these are iodovanadinite (Pb5(VO4)3I), [1] with an apatite structure, and iodosodalite (Na4Al3Si3O12I), [2,3] however, no direct comparison of the durability of these two phases could be found. In addition to producing wasteforms in which iodine is chemically bound into a lattice, it may be sufficient simply to incorporate silver iodide into a passive matrix of glass or ceramic and this approach has also been investigated. Choice of Iodine Containing Precursor Our first consideration necessarily involved capturing iodine from current process streams, and the most practical means for doing this was viewed as precipitation as AgI in the fuel dissolver off-gas system. To minimise processing requirements it would therefore be desirable to use AgI as a wasteform precursor. If this is not possible, reduction of the AgI could lead to a more suitable aqueous iodide stream through the reaction: AgI(s) + OH(aq) + [H] → Ag(s) + H2O(l) + I(aq) in which [H] indicates a generic reducing agent as opposed to nascent hydrogen. This reaction was viewed as being compatible with barium and sodium hydroxides as reagents leading to their iodides as wasteform precursors. Reprecipitation as PbI2 would be necessary for fabrication of pure iodovanadinite but would be an undesirable additional processing stage.
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&KRLFHRI,QWHQGHG:DVWHIRUPV The majority of wasteform formulations prepared were intended to form apatite structured phases with a variety of cation permutations on the A site chosen from Pb, Ba, Sr, Ag, Na and La. Lanthanum was used on
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