Performance of radio-iodine discharge control methods of nuclear reprocessing plants

Highlights: • Systematic review of performance of iodine retention methods in reprocessing plants. • Relates plant capacity, type of spent fuel and burn-up data. • Helps in selecting suitable optimized process and operating conditions. • Long-term exposure of radio-iodine may increase the risk of thyroid cancer. • Iodine discharge control can reduce radiation dose to public significantly. It is imperative to control radio-iodine discharges to atmosphere from nuclear reprocessing plants. Inhalation and ingestion of radio-iodine cause its concentration in the thyroid gland leading to risk of thyroid cancer in humans. Two isotopes of iodine viz. iodine-131 (¹³¹I) and iodine-129 (¹²⁹I) are generated in considerable quantities in the nuclear fuel as fission products in the nuclear reactors. From nuclear reactors, no iodine is released to the atmosphere during normal operations, whereas from spent fuel reprocessing plants, during normal operation, iodine is discharged to the atmosphere, mainly through gaseous discharges. Shortly after the initial periods of reprocessing in 1944, iodine emission control methods were incorporated in the design of reprocessing plants. At the time of spent fuel discharge from reactor, quantity of ¹³¹I is high and can contribute radiation dose to humans during reprocessing operations. A delay or cooling period of spent fuel, before reprocessing for a definite number of days can reduce the quantities to below the permissible limits of discharge due to its short half-life of 8 days. ¹²⁹I has a very long half-life, and is only significant for reprocessing plants of large throughput and high fuel burn-ups. Minimum required de-contamination factor (DF) for iodine for a reprocessing plant can be estimated from the limits of discharge of iodine stipulated by regulatory authority of each country. Though many processes were developed and demonstrated extensively in lab and pilot scale, only a few of these processes were found to be suitable for commercial deployment. This paper reviews systematically the operation experiences and performance characteristics of iodine control methods implemented so far. The review also focus on the effect of integrating various iodine control methods on the main reprocessing operations and thereby facilitate selection of the optimum iodine control method.