Capture of Iodine from Nuclear-Fuel-Reprocessing Off-Gas: Influence of Aging on a Reduced Silver Mordenite Adsorbent after Exposure to NO/NO₂

Abstract

Iodine radioisotopes released during nuclear fuel reprocessing must be removed from the off-gas stream before discharge. One promising material for iodine capture is reduced silver mordenite (Ag 0 Z). Nevertheless, the adsorbent's capacity will degrade, or age, over time when the material is exposed to other off-gas constituents. Though the overall impact of aging is known, the underlying physical and chemical processes are not. To examine these processes, Ag 0 Z samples were prepared and aged in 2% NO 2 in dry air and in 1% NO in N 2 gas streams at 150 °C for up to six months. Aged samples were then characterized using scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and X-ray absorption spectroscopy. These techniques show that aging involves two overarching processes: (i) oxidation of the silver nanoparticles present in Ag 0 Z and (ii) migration of oxidized silver into the mordenite's inner network. Silver on the nanoparticle's surface is oxidized through adsorption of O 2 , NO, and NO 2 . Raman spectroscopy and X-ray absorption spectroscopy indicate that nitrates are the primary products of this adsorption. Most of these nitrates migrate into the interior of the mordenite and exchange at framework binding sites, returning silver to its unreduced state (AgZ). The remaining nitrates exist at a persistent concentration without aggregating into bulk-phase AgNO 3 . X-ray absorption spectroscopy results further indicate that iodine adsorption occurs on not just Ag 0 Z but also on AgZ and a portion of the nitrates in the system. AgZ adsorbs a sizable quantity of iodine early in the aging process, but its capacity drops rapidly over time. For well-aged samples, nitrates are responsible for up to 95% of mordenite's iodine capacity. These results have enhanced our understanding of the aging process in silver mordenite and are expected to guide the development of superior adsorbents for the capture of radioactive iodine from reprocessing off-gas.

Silver Sulfide and Silver Sulfate as Aging Byproducts and Adsorbents for Gaseous Iodine Capture in Spent Nuclear Fuel Reprocessing

“…One major obstacle to the use of Ag adsorbents is, however, their tendency to steadily lose capacity (i.e., age) when they are exposed to other SNF reprocessing off-gas components (e.g., O 2 , H 2 O, NO x ) at elevated temperatures for prolonged periods of time. Our previous studies have focused on the processes governing aging in two prototypical porous Ag adsorbents, Ag 0 Z and reduced Ag functionalized silica aerogel (Ag 0 -aerogel), in gas streams containing the aforementioned off-gas components. − Ag 0 -aerogel is a relatively new material developed at Pacific Northwest National Laboratory (PNNL), and its high Ag content and adequate specific surface area aid in iodine capture, making it an appealing alternative to zeolites that have traditionally been the primary focus in this area. , By exposing Ag 0 Z and Ag 0 -aerogel to gas streams containing the aforementioned off-gas components at 150 °C, we found that NO 2 significantly reduced the I 2 capacity for both materials (∼90% reduction after 1 month), , while O 2 , H 2 O, and NO were less impactful (30%–80% reduction for Ag 0 Z and ∼20% reduction for Ag 0 -aerogel after 1 month). , Though the existence of the aging phenomenon is well established, there are still, nevertheless, some discrepancies that arise when attempting to understand its underlying processes.…”

“…The formation of new Ag species after aging is, for example, a key feature in our previous studies though these aging byproducts may or may not directly reduce the I 2 capacity of the adsorbent. For instance, Wiechert et al reported that molecular Ag nitrate (AgNO 3 ) was generated as a byproduct of aging in Ag 0 Z samples exposed to NO x -containing (NO and NO 2 ) gas streams . AgNO 3 is a well-documented I 2 -favored species and contributes to a portion of I 2 adsorption at the early stages of Ag 0 Z aging.…”

“…AgNO 3 is a well-documented I 2 -favored species and contributes to a portion of I 2 adsorption at the early stages of Ag 0 Z aging. It was postulated that AgNO 3 initially forms on the mordenite surface and then, over time, migrates into the interior channels of the mordenite, where it is held in an immobilized form that is unavailable for I 2 adsorption . Thus, while the formation of AgNO 3 is an important component of the aging process, it does not directly diminish the capacity of the mordenite.…”

Radioiodine sorbent selection criteria

“…An initial step in assessing oxidation potential is to conduct aging experiments, whereby candidate sorbents are subjected to relevant gas streams expected in the process stream for extended time periods and the sorbent performance is evaluated after these aging experiments. Aging experiments in 2% NO 2 in dry air and 1% NO in N 2 streams at 150 °C (for up to 6 months) were noted to drastically reduce the iodine capacity of reduced silver mordenite (Ag 0 Z) in a study by Wiechert et al (2020) , whereby Ag 0 is oxidized to form molecular nitrates that are believed to be on the surfaces of the mordenite crystals. In their study, X-ray absorption spectroscopy was used to verify that a majority of the Ag present in the aged samples was in the oxidized (Ag + ) state.…”

Evaluating the Physisorption and Chemisorption of Iodine on Bismuth-Functionalized Carbon Foams

“…A main challenge that remains to be resolved is the effective control of the radioactive product emissions that potential reprocessing technologies produce. These complex off-gas streams include the long-lived radioisotope 129 I, which readily volatilizes and partitions into reprocessing off-gas streams …”