Immobilizing Iodine with Silver-Functionalized Silica Aerogel
Advanced material developed by researchers at PNNL captures and sequesters radioiodine
Spent nuclear fuel reprocessing plants face many environmental challenges. Preventing the release of radioactive isotope iodine-129 into the environment is one of the most significant challenges. At PNNL, a team of researchers led by Josef Matyáš are developing silver-functionalized silica aerogel (Ag0-aerogel) for the removal and immobilization of iodine compounds from the off-gas of nuclear fuel reprocessing plants. This research is being conducted for the Sigma Off-Gas Team for the DOE Office of Nuclear Energy Fuel Cycle Research & Development Program.
The iodine-129 is released during dissolution of the fuel in nitric acid and is a particular concern for many reasons, including:
- it tends to concentrate in the human thyroid gland, causing cancer
- it has an extremely long half-life of 15.7 × 106 years, and therefore can persist in the environment for millions of years
- it is a highly mobile and chemically reactive element that is difficult to control. This presents a significant challenge for meeting regulatory control requirements for radioactive discharges. Other radioisotopes of iodine—namely iodine-131 and iodine-135—produced during reprocessing have relatively short half-lives (e.g., 8 days and 6.5 hours, respectively) and will naturally decay over a short time.
Efficient Capture and Sequestration of Iodine-129
In laboratory tests with simulated dissolver off-gas streams, the Ag0-aerogel exhibited excellent sorption properties for iodine, with capacities up to 480 mg/g demonstrated along with decontamination factors over 10,000 (removal of more than 99.99 percent of iodine). In addition, Ag0-aerogel retained high selectivity and sorption capacity for iodine even after a long-term exposure to dry/humid air containing either 2 percent nitrogen dioxide or 1 percent nitric oxide at 150°C. This is important considering that in off-gas treatment systems, sorbents will be exposed to gas streams containing water and nitrogen oxide at elevated temperature for extended periods during their lifetime.
The results from consolidation tests clearly showed that rapid consolidation of iodine-loaded Ag0-aerogel with hot uniaxial pressing, hot isostatic pressing, and spark plasma sintering yielded a final waste form consisting of AgI particles encapsulated in a silica-based matrix. This fully densified waste form had a bulk density of 3.3 × 103 kg/m3 (a density similar to AlN 3.26 × 103 kg/m3) and contained ~39 mass% of iodine.
PNNL Research Team: Josef Matyáš, Sarah Burton, Nathan Canfield, Eugene Ilton, Libor Kovarik, Xiaohong (Shari) Li, Timothy Roosendaal, and Sulaiman Sannoh