Development and Durability Testing of a Low‐Temperature Sintering Bi–Si–Zn Oxide Glass Composite Material (GCM) ¹²⁹I Waste Form

Abstract

The capture and safe storage of radiological iodine ( 129 I) from nuclear fuel reprocessing is of concern due to its long half-life and potential mobility in the environment. The development of durable waste forms in which to store captured iodine requires materials that are both compatible with the iodine capture phases and durable to repository environments. To that end, Sandia is developing a low-temperature sintering Bi-Si-Zn oxide glass composite material (GCM) waste form and herein presents results of durability testing. Furthermore, durability studies were extended to both the occluded iodine capture material Ag-Zeolite (Mordenite, MOR) as well as the GCM, synthesized with compositional variations including: amount of Ag flake added, AgI-MOR particle size in the GCM, and mass loading of I within the AgI-MOR. Product consistency test (PCT-B), chemical durability (MCC-1) tests, and single-pass flow-through (SPFT) tests, were performed on both the individual components of the GCM and the completed GCMs. Durability tests indicate low GCM dissolution rates (<10 -3 g/m 2 Ád) across wide variable ranges including: pH, AgI-MOR loading, I loading, and AgI-MOR particle size. Results indicate that the Bi-Si-Zn oxide glass matrix sharply limits the release of iodine from the otherwise relatively fast degrading AgI-MOR getter material. Furthermore, the formation of an amorphous AgI phase during sintering of the GCM results in the limitation of iodine release during waste form degradation. Durability of GCM and release rates approximate those of established nuclear waste glasses, or analogues such as basaltic glass. This suggests that the Bi-Si-Zn GCM is a viable candidate as a repository iodine waste form.

Determination of Desorbed Species During Heating of AgI-Mordenite Provided by ORNL

“…We believe that incorporation of the ORNL AgI-MOR in the GCM, with silver flake added to capture volatilized iodine to form AgI, will result in a durable waste form on par with that recorded with AgI-MOR incorporation into the GCMs. 8 Furthermore, past studies of ours shown that the iodine is better entrapped in the Ag-MOR when a number of other variables are introduced to the zeolite Mordentie for iodine capture. These include: (1) use of a nonnatural/geologic MOR (which is much more highly crystalline and does not contain many…”

“…and density = 5.675 g/cm 3 ) 3 has a low solubility in water, 3 x 10 -6 g/L at 20°C 3 [4][5][6][7] They are easier and less expensive to prepare than conventional ceramics but also offer high durability, as shown in our recent work. 8 In our previous reports 5-8 on I 2 -loaded Ag-MOR, we identified a commercially available low-temperature sintering glass powder based on silicon and bismuth oxides that sinters to full density at 550C and has excellent durability. To prevent surface/bulk chemisorbed I 2 vapor from escaping from the zeolite during GCM sintering, additional Ag flake was added to the GCM mixture to capture the desorbing I 2 vapor.…”

Candidate Low-Temperature Glass Waste Forms for Technetium-99 Recovered from Hanford Effluent Management Facility Evaporator Concentrate

“…This feature is especially useful for addressing the low-temperature processing objective for a technetium secondary waste stream. Waste loadings of 20-25% are typical in research publications on iodide encapsulation, which are described below [Mowry 2015, Garino 2011]. There are a large number of glasses in Appendix B with reported sintering temperatures below 700 °C, and an even greater number that have softening points in the 400-500 °C range, a good indicator of their potential to be used in GCMs for technetium.…”

“…Specifically, for an AgI-mordenite monolith containing 5 wt% iodide exposed to near-neutral water, the authors reported an average of 19.5 ppm of silicon in the effluent in the MCC-1 test (pH 8.2-8.3) and 53.1 ppm in the PCT-B test (pH 7.5-7.6). Using the density and surface area estimates provided in the Experimental section of the paper, the average leach rates for silicon were 0.24 g Si/m 2 /day (MCC-1) and 0.081 g Si/m 2 /day (PCT-B) [Mowry 2015].…”

“…The peak firing temperatures, a typical temperature to melt the powder into a glass, are lower than 600 o C (Appendix B).Bismuthates have a good track record in bench-scale testing to date. For example, researchers at Sandia National Laboratories have shown that GCM waste forms are easily synthesized and yield durable monoliths that retain iodine during sintering and during leach testing[Garino 2011;Mowry 2015]. For…”