Iodosodalite Waste Forms from Low-Temperature Aqueous Process
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.225
Iodosodalite Waste Forms from Low-Temperature Aqueous Process Junghune Nam,1 Saehwa Chong,2 Brian J. Riley,3 John S. McCloy1,2,3 1
School of Mechanical & Materials Engineering, Washington State University, Pullman, WA 99164, USA
2
Materials Science and Engineering Program, Washington State University, Pullman, WA 99164, USA
3
Pacific Northwest National Laboratory, Richland, WA 99352, USA
ABSTRACT Nuclear energy is one option to meet rising electricity demands, although one concern of this technology is the proper capture and storage of radioisotopes produced during fission processes. One of the more difficult radioisotopes is 129I due to its volatility and poor solubility in traditional waste forms such as borosilicate glass. Iodosodalite has been previously proposed as a viable candidate to immobilize iodine due to high iodine loading and good chemical durability. Iodosodalite was traditionally synthesized using solid state and hydrothermal techniques, but this paper discusses an aqueous synthesis approach to optimize and maximize the iodosodalite yield. Products were pressed into pellets and fired with glass binders. Chemical durability and iodine retention results are included. INTRODUCTION Iodine-129 (129I) is a highly volatile radionuclide that is difficult to immobilize in a nuclear waste form using conventional vitrification processes [1]. It is highly water soluble and thus poses a serious concern for storage in geological repositories. It also has a very long halflife of about 16 million years. Attempts to make waste forms have been made using vitrification processes, but iodine typically has low solubility in typical borosilicate glass chemistries [2], and it is highly volatile at normal glass processing temperatures, between 1000-1150°C. Iodinecontaining mineral structures have received some attention as iodine waste form options, the more commonly studied being sodalite [3] and apatite [4]. Iodosodalite [Na8(AlSiO4)6I2] can be synthesized at temperatures below 200°C, and is the focus of this study. The structure of iodosodalite is shown in Figure 1. It consists of SiO4 and AlO4 tetrahedra arranged in rings, with a central “cage” containing a Na4I tetrahedron. It is this so-called β-cage, containing the iodide anion, which makes sodalite attractive for immobilizing 129 I, because iodine is tightly bound within the cage thus reducing the tendency for release into the environment.
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Figure 1. (a) Structure of the β-cage and (b) its arrangement in the sodalite unit cell. Oxygen atoms are not shown for better visualization.
Background on sodalite synthesis Iodosodalite was first artificially synthesized using a solid state method in 1968 [5], where nepheline and NaI were mixed, sealed insi
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