Effect of Extended Exposure to Simulated LMFBR Fuel Reprocessing Off-Gas on Radioiodine Trapping Performance of Sorbents.

Abstract

For LMFBR fuel reprocessing plants, a required radioiodine retention factor of ^ 1()8 is projected. Also projected is a cumulative retention factor of ^ 10f or iodine fixation achieved upstream of the filter-adsorber assembly used for final off-gas treatment. Thus, this assembly will need to provide a decontamination factor (DF) of *v 102, but considerable reserve capability is desired. Certain silver-containing sorbents in a 2-in. depth at 100°C and/or 200°C have, in shortterm laboratory tests, exhibited DF T s of > 103 for iodine as methyl iodide or elemental iodine under conditions including 3 vol % water vapor and iodine loadings estimated equivalent to that for at least one year of operation. Present objectives are to obtain information on the effective service life of such sorbeuts and to ascertain the desirability of locating a catalytic oxidation bed upstream of the sorbents to minimize poisoning by organics and to convert organic iodides to l£. Accordingly, a laboratory facility for exposing sorbents to simulated fuel reprocessing off-gas is being operated. The main air stream is humidified to around 85% saturation (referred to room temperature). Elemental iodine (60-day 125l-labeled), dodecane, and nitric oxide are continuously injected at their anticipated concentrations. One group of sorbents is exposed to untreated off-gas while another group is exposed to off-gas which has traversed a Hopcalite bed at 350°C. Methyl iodide (12.3-hr 130i-labeled) is injected periodically to furnish short-term results, whereas long-term cumulative results are obtained with the I 2 .Sorbents investigated include three varieties of silver zeolite (designated AgX with the percent Ag + for Na + exchange denoted), GX135 (silver salt-treated alumina-silica), and a type of iodized charcoal. Sorbent depths are 2 in. Excellent performance (indicated 12 and CH3I DF's £ 10-?) has been observed thus far for the following test situations: 26% AgX, 1/16-in. pellets, at 200°C, both with and without Hopcalite upstream, 196 days exposure; 99% AgX, 1/16-in. pellets, 100°C, both with and without Hopcalite, 105 days; GX135 (8 x 16 mesh, U.S.), 200°C, with Hopcalite, 196 days; and GX135, 100°C, without Hopcalite, 119 days. (Data are not yet available for GX135, 100°C, with. Hopcalite.) Performances observed for other test situations ranged from poor to good. The importance of sorbent particle size was demonstrated by the less favorable performance of 88% AgX, 1/8-in. pellets, 100 and 200°C, relative to that for the 26% AgX, 1/16-in. pellets (200°C). The use of Hopcalite was observed highly beneficial in trapping iodine injected as CH3I; in trapping iodine injected as Io» its use was beneficial for 26% AgX, 99% AgX, and GX135 and deleterious for 88% AgX (l/8~in. pellets) and iodized charcoal (the latter at 66°C). This investigation is being continued.

Evaluation of Getter Metals in Na–Al–Si–O Aerogels and Xerogels for the Capture of Iodine Gas

“…However, some sorbents have different binding mechanisms from physisorption to molecular sieving (e.g., graphene, MOFs). Several studies have focused on assessing metal-exchanged zeolites including the metals Cd, Cu, Hg, Mn, Pb, Pd, and Tl. − The approach of using a porous scaffold provides a means of allowing a gaseous stream containing I 2(g) to flow through the sorbent and have a higher probability of getter–I 2(g) interactions. Thus, the higher the specific surface area (SSA) present within the sorbent, the greater the opportunity for these interactions. …”

Silver-Loaded Aluminosilicate Aerogels As Iodine Sorbents

“…The main drawbacks to using Ag as a getter are that (a) it is a precious metal (it is expensive), (b) it is sensitive to oxidation and thus Ag-containing sorbents have a shelf life before requiring regeneration, and (c) it is one of eight environmentally toxic elements imposed by the Resource Conservation and Recovery Act that controls the release of toxic metals into the environment. Several other metals have been evaluated as iodine sorbents such as Ag, Bi, Cd, Cu, Hg, Mn, Pb, Pd, Sb, Sn, and Tl, ,,,− but Ag has proven the most effective. It is likely that other 1+ (e.g., Cu + , Hg + ), 2+ (e.g., Cu 2+ , Pb 2+ ), 3+ (e.g., Bi 3+ , Sb 3+ ), or 4+ (e.g., Sn 4+ ) getters could be introduced into the Na–Al–Si–O gel network using a similar approach as was taken here (i.e., with AgNO 3 ) through an ion-exchange process.…”

“…The capture of radioiodine (e.g., 131 I, 129 I) has been a topic of focus for many decades by the community studying the capture and immobilization of potential nuclear fuel reprocessing wastes and legacy nuclear weapons production wastes. − Iodine capture technologies discussed in the literature include various types of solution-based processes like Mercurex, , Iodox, electrolytic scrubbing, , and caustic scrubbing as well as solid sorbents including metal-exchanged ceramics such as mordenite (AgZ) or faujasite (AgX), chalcogen-based aerogels, − metal–organic frameworks, , granular activated carbon, graphene powders/aerogels, copper metal, Bi-compounds, Ag 0 -functionalized silica aerogels, , Ag-impregnated Al 2 O 3 , and Ag-impregnated SiO 2 . Many different metals have been evaluated with solid sorbents as getters for I 2 (g) including Ag, Bi, Cd, Cu, Hg, Mn, Pb, Pd, Sb, Sn, and Tl, ,,,− although Ag is one of the most effective getters tested to date due to strong chemisorption with iodine. In the current work, a new form of iodine sorbent is evaluated, which is an Ag 0 -functionalized aluminosilicate (Al–Si–O) aerogel.…”

Impact of the Si/Al ratio on the selective capture of iodine compounds in silver-mordenite: a periodic DFT study

“…Due to the ionic exchange character of zeolites, the nature of the compensating cation plays a prominent role and exhibits strong adsorption properties suitable for iodine trapping. Different metals have been tested in faujasite (Na, Cu, Ag, Pb, Cd, Tl) [14][15][16] and all these experimental works found that silver is the most efficient cation, the other cations trap I 2 and ICH 3 to a lesser extent. Indeed, Nenoff and co-workers have successively used silver-exchanged mordenite in nuclear facilities.…”