Iodine removal from the vessel off gas in Tokai reprocessing plant

Through the active operation of Tokai reprocessing plant started on Sep. 1977, the measurements of iodine (129I) in the liquid and gaseous streams have been carried out. From the result of measurements, it was confirmed that more than 99% of the calculated amount of 129I in the spent fuels was released into the off gas circuit during the dissolution. Most of the released iodine was desorbed to liquid streams and then transferred to the waste disposal facility, where some amounts of iodine were released into the vessel off gas line. Through the investigation of iodine distribution in the plant, it was understood that the removal of iodine in the vessel off gas led to decreasing the iodine discharged to the atmosphere, so that the charcoal and the silver-exchanged zeolite (AgX) filters were installed in the vessel ventilation systems at the waste disposal facility. Then the removal tests have been carried out through the active operation. The decontamination factor (DF) of the charcoal filter for iodine (129I) achieved about 103 at the beginning but the breakthrough occurred at 1,100 hours loading. On the other hand, DF of AgX filter has been kept in the range of 50 to 102 through 18,000 hours loading (about two years since the start of operation in Nov. 1979). Consequently, it was confirmed that AgX adsorbent was effective for the removal of airborne iodine (129I) in the vessel off gas. At present, by means of the installation of AgX filters on the vessel ventilation systems, the iodine (129I) discharged to the atmosphere has been controlled at low level in the Plant. (author)