Iodine Pathways and Off-Gas Stream Characteristics for Aqueous Reprocessing Plants ? A Literature Survey and Assessment

Abstract

Used nuclear fuel is currently being reprocessed in only a few countries, notably France, England, Japan, and Russia. The need to control emissions of the gaseous radionuclides to the air during nuclear fuel reprocessing has already been reported for the entire plant. But since the gaseous radionuclides can partition to various different reprocessing off-gas streams, for example, from the head end, dissolver, vessel, cell, and melter, an understanding of each of these streams is critical. These off-gas streams have different flow rates and compositions and could have different gaseous radionuclide control requirements, depending on how the gaseous radionuclides partition. This report reviews the available literature to summarize specific engineering data on the flow rates, forms of the volatile radionuclides in off-gas streams, distributions of these radionuclides in these streams, and temperatures of these streams. This document contains an extensive bibliography of the information contained in the open literature.Iodine removal has always been primarily focused on the head-end off-gas operations. But, in light of the estimated iodine emissions control efficiencies of up to 99.9% or higher needed to meet US regulatory requirements, the control of iodine in the dissolver off-gas is insufficient. Operating experience and tests with actual nuclear fuel show that iodine evolution from the dissolver solution into the dissolver off-gas ranges between 95 and 99%. Iodine evolution from the dissolver solution can be improved to >95% with NO 2 sparging, addition of KIO 3 to the dissolver, and addition of iodate. Several studies have shown that much of the residual iodine in the dissolver solution is in the form of colloidal iodine (AgI and PdI 2 ); organic iodides formed in the dissolver also factor into the iodine retention. Even if the iodine in the dissolver off-gas is efficiently controlled, the iodine in other reprocessing facility off-gas streams may also need to be controlled.

Chemically Stable and Heteroatom Containing Porous Organic Polymers for Efficient Iodine Vapor Capture and its Storage

“…In the UNF reprocessing plant, the temperatures of the off-gas steams may often be cooler, such as those of ambient temperatures. 66 Therefore, the iodine capture experiments (using HPOPs as adsorbents) were also studied at ambient temperature, i.e., 25 °C. The experimental setup for the capture experiments was similar to the ones for the static iodine capture described earlier, excepting that the large closed glass chamber was not placed in a heated oven.…”

Technology Development Roadmap for Volatile Radionuclide Capture and Immobilization

“…Effective removal of 129 I from off-gas waste streams requires high DFs (Jubin et al 2013;Jubin et al 2011Jubin et al , 2012aJubin et al 2012c). Iodine chemisorption on AgZ is the current iodine capture baseline, although silver functionalized aerogel (Ag aerogel) can also capture iodine.…”

Effects of moisture and aging upon decomposition of methyl iodide by reduced silver mordenite

“…This process involved the dissolution of used nuclear fuel and chemical separation of the fuel components into various waste and product streams. While the majority of the material handled by an aqueous used fuel reprocessing facility is readily collected, radioiodine especially organic radioiodine, remains a material of concern [20,36]. This is due to its ability to bypass the current off-gas treatment methods and readily bioaccumulate within animal and human populations.…”

“…However, small quantities of I-129 would not have decayed and can interact with organic solvents to produce organic iodides, most abundantly methyl iodide. Methyl iodide must be abated even in concentrations as low as 18 ppbv to remain in compliance with United States federal regulations and is thus part of the subject of this study [5,20].…”