Removal of iodine by dry adsorbents in filtered containment venting system after 10 years of Fukushima accident

Abstract

Radioactive iodine is a hazardous fission product and a major concern for public health. Special attention is paid to iodine out of 80 fission products because of its short half-life of 8.02 days, high activity, and potential health hazards like its irreversible accumulation in thyroid gland and ability to cause thyroid cancer locally. Radioactive iodine can get released in the form of aerosols (cesium iodide), elemental iodine, and organic iodide after a nuclear accident and can cause off-site and on-site contamination. Filtered containment venting system (FCVS) is a safety system whose main objective is mitigation of severe accidents via controlled venting and removal of different forms of iodine to ensure safety of people and environment. After nuclear accidents like Fukushima, extensive research has been done on the removal of iodine by using dry scrubbers. This review paper presents research status of iodine removal by dry adsorbents especially after 10 years of Fukushima to assess the progress, research gap, and challenges that require more attention. A good adsorbent should be cost-effective; it should have high selective adsorption towards iodine, high thermal and chemical stability, and good loading capacity; and its adsorption should remain unaffected by aging and the presence of inhibitors like CO, NO₂, CH₃Cl, H₂O, and Cl₂ and radiation. Research on different dry adsorbents was discussed, and their capability as a potential filter for FCVS was reviewed on the basis of all the above-mentioned features. Metal fiber filters have been widely used for removal of aerosols especially micro- and nanoscale aerosols. For designing a metal fiber filter, optimal size or combination of sizes of fibers, number of layers, and loading capacity of filter should be decided according to feasibility and requirement. Balance between flow resistance and removal efficiency is also very important. Sand bed filters were successful in retention of aerosols, but they showed low trapping of iodine and no trapping of methyl iodide at all. For iodine and methyl iodide removal, many adsorbents like activated carbon, zeolites, metal organic frameworks (MOFs), porous organic frameworks (POPs), silica, aerogels, titanosilicates, etc. have been used. Impregnated activated carbon showed good results but low auto-ignition temperature and decline in adsorption due to aging and inhibitors like NO_x made them less suitable. Silver zeolites have been very successful in methyl iodide and iodine removal, but they are expensive and affected by presence of CO. Titanosilicates, macroreticular resins, and chalcogels were also studied and they showed good adsorption capacities, but their thermal stability was low. Other adsorbents like silica, MOFs, aerogels, and POPs also showed promising results for iodine adsorption and good thermal stability, but very limited or no research is available on their performance in severe accident conditions. This review will be very helpful for researchers to understand the merits and demerits of different types of dry adsorbents, the important operating parameters that need optimization for designing an efficient scrubber, margin of research, and foreseeable challenges in removal of different forms of iodine.

Keywords: Dry adsorbents; Dry scrubbers; Environmental safety; Filtered containment venting system; Fukushima Accident; Iodine removal; Porous adsorbents; Removal of hazardous gases.