Radioactive Iodine-129 Capture in Mixed Cation Sodalites: ab initio Modelling
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.249
Radioactive Iodine-129 Capture in Mixed Cation Sodalites: ab initio Modelling Authors: E.Y. Kuo1, D.J. Gregg1, E.R. Vance1, E.R. Maddrell2, G.R. Lumpkin1 Affiliations: 1Nuclear Fuel Cycle Research, NSTLI, Australian Nuclear Science & Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia; 2National Nuclear Laboratory, Sellafield, Seascale, Cumbria CA20 1PG, UK
Abstract: Sodalites have been investigated experimentally for the capture and long-term containment of 129I, a significant and hazardous waste product of the nuclear fuel cycle. Sodalites are zeolite-type structures commonly occurring in nature in alkaline igneous rocks and having the prototype formula Na8(AlSiO4)6Cl2. The crystal structure is based around -cages consisting of corner-sharing SiO4 and AlO4 tetrahedra. In the centre of the -cage is an anion X. Iodine captured by sodalites sits in the centre of the -cages as iodide anions. Silver iodide (AgI) plays an important role in the capture and subsequent processing of 129I in the nuclear fuel cycle. Using ab initio density functional theory (DFT) modelling, we investigate the energetics and feasibility of iodine capture and containment in mixed cation sodalites Na8-xAgx(AlSiO4)6I2, and compare the results with experimental observations.
INTRODUCTION The fission product 129I is produced in the off-gas at used nuclear fuel reprocessing facilities and current best practice management for radioiodine at these facilities is by an approach known as ‘dilute and disperse’, with final discharge into the sea [1]. That is, dilution into a much larger reservoir of natural iodine and dispersal by marine currents. However, evolving social and political attitudes may mean that future used nuclear fuel reprocessing facilities will not have access to this discharge approach. The immobilisation of radioiodine is therefore a growing priority for nuclear wasteform research and development and in this regard a review of 129I immobilisation has recently been published [2,3]. The iodine isotope has a long half-life (t1/2 ~1.6 x 107 yr.) and high mobility in most geological environments and incorporating it into a durable wasteform is an attractive solution. One approach is to capture iodine via solid sorbents, such as silver-containing sorbents [4] and most recently silver-exchanged mordenite-type-zeolites [5] and this has led to the development of hot-isostatically pressed (HIP) silver iodosodalite, Ag4Al3Si3O12I, as a candidate wasteform [6]. Practically, this could involve impregnating a commercial zeolite of appropriate composition with silver, and then using these to sequester iodine from the gas phase as the iodine capture substrate. This material could then be converted to the iodosodalite via hot isostatic pressing (HIP). In this regard, although the end members of the iodosodalite (Na4(AlSiO4)3I and Ag4(AlSiO4)3I) are known, there has been little evidence for the mixed cation solid solution ((Na,Ag)4(AlSiO4)3I). In the p
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