Impact of Pretreatment and Aging on the Iodine Capture Performance of Silver-Exchanged Mordenite - 12314
- Oak Ridge National Laboratory (United States)
Volatile gas emissions from a nuclear fuel recycle facility in the United States are governed by several key regulations, including 10 CFR 20, 40 CFR 61, and 40 CFR 190. Under 40 CFR 190, the total quantity of iodine that may be released to the environment from the entire fuel cycle is limited to 5 millicuries of I-129 per gigawatt-year of electrical energy produced by the fuel cycle. With a reasonable engineering margin, an iodine decontamination factor (DF) of approximately 1000 will be required for the complete fuel cycle. Off-gas treatment in a fuel reprocessing plant must address several gas streams containing iodine, among a number of volatile radionuclides. Past research and developmental activities identified silver-exchanged mordenite (AgZ) as a very promising sorbent based on its acid resistance, relatively high iodine and methyl iodide capacity, and high achievable DF. Recent studies at ORNL have focused on the impacts of long-term exposure to simulated off-gas streams (aging) and pretreatment on the iodine adsorption performance of hydrogen-reduced silver-exchanged mordenite (Ag{sup 0}Z). Experiments were conducted to determine the effects of long-term exposure to both dry and moist air on the iodine sorption capacity of Ag{sup 0}Z. The data indicates that aging reduces the capacity of Ag{sup 0}Z, which must be accounted for to prevent degradation of DF. Because of its high acid resistance, a AgZ sorbent has been selected specifically for application in treating off-gas streams containing iodine. While extensive tests have been conducted in the United States on a form of this sorbent, the specific material previously tested is no longer commercially available and similar materials are currently being evaluated. As part of this evaluation, tests were conducted to determine the iodine sorption properties of this replacement media and the effects of long-term (up to 6 months) exposure to simulated off-gas streams. The ultimate goal is to develop an understanding of the fundamental phenomena that controls aging for this material and other zeolites that could be considered for use in off-gas treatment in the future. The trends in the study results indicate that the amount of elemental silver observed by XRD increases from 0.3 wt% in vendor-supplied AgZ to approximately 5 wt% by reducing the material with hydrogen. The study also concluded that aging decreases the quantity of elemental silver in the material. After 2 months of aging, the Ag{sup 0} content of an experimental sample was reduced from 5 wt% to about 1.3 wt%. The form into which the elemental silver is converted during aging was not determined. Experimental tests have been initiated to study how aging of Ag{sup 0}Z impacts iodine loading on the zeolite. Loading tests with un-aged Ag{sup 0}Z resulted in an 81% silver utilization. The loading capacity of iodine on Ag{sup 0}Z was reduced with aging in dry air. Material aged for 6 months in dry air had a 40% reduction in iodine loading capacity. Under moist-air aging conditions, a significant decrease in the rate and total loading (∼45% of theoretical) of iodine uptake can be observed beginning with the shortest aging period (i.e., after 1 month) when compared with the loading curve using Ag{sup 0}Z with no aging. Increasing exposure time to the humid air used to age the Ag{sup 0}Z beyond 1 month resulted in a slight additional reduction in capacity to about 35% of theoretical at 2 months. Virtually identical capacity was observed with 4 months of aging. Compared to the non-aged material, the 1 month dry-air aged Ag{sup 0}Z shows about a 35% reduction (approximate) in iodine loading capacity and the 6 month dry-air aged Ag{sup 0}Z shows about a 50% reduction. These studies generated several questions that will be addressed in future tests. They include the following: Is there indeed degradation over time (in storage) in the iodine adsorption performance of Ag{sup 0}Z? Once reduced, how should the Ag{sup 0}Z be stored- under a hydrogen atmosphere, an inert atmosphere, a desiccant, or some other method or combination of methods? Does Ag{sup 0}Z have a 'shelf life' that must be considered after receipt from a vendor and before use in a plant? Also, how should a column of Ag{sup 0}Z be stored offline in a processing plant (parallel column) arrangement? Future tests will also include increasingly challenging aging environments such as acid vapor conditions. (authors)
- Research Organization:
- WM Symposia, 1628 E. Southern Avenue, Suite 9-332, Tempe, AZ 85282 (United States)
- OSTI ID:
- 22293586
- Report Number(s):
- INIS-US-14-WM-12314; TRN: US14V1242115110
- Resource Relation:
- Conference: WM2012: Waste Management 2012 conference on improving the future in waste management, Phoenix, AZ (United States), 26 Feb - 1 Mar 2012; Other Information: Country of input: France; 11 refs.
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
RADIOCHEMISTRY AND NUCLEAR CHEMISTRY
12 MANAGEMENT OF RADIOACTIVE WASTES, AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES
ADSORPTION
CAPACITY
CONTROL
EFFICIENCY
EMISSION
FUEL CYCLE
FUEL REPROCESSING PLANTS
HYDROGEN
IODINE
METHYL IODIDE
MORDENITE
NUCLEAR FUELS
ORNL
SILVER
SIMULATION
VOLATILITY
X-RAY DIFFRACTION