Iodine-131 is a fission product having a half-life of 8 days, and is important in view of its radiological risk to children by accumulation in the thyroid, which was a major issue at Chernobyl. The estimated total amount of iodine-131 released into the atmosphere lies in the range from 120 to 500 PBq, which means that there is still considerable uncertainty associated with the calculated estimate. This amount corresponds to about one fifth or one tenth of the release from Chernobyl. The latest estimate by TEPCO is about 500 PBq, while those by JAEA, NISA, and the Japan Nuclear Safety Commission (NSC) are in the range from 120 to 150 PBq. According to the study on the analysis of gaseous sample by JAEA, the release rate of iodine — 131 was initially 1015 Bq/h, and continued to be of the order of 1014Bq/h in the period from March 15 to 24. It is probable that the majority of the iodine release occurred in these ten days. Analysis of iodine deposition has been performed at 2,200 locations, and with this a map created of the radioactive contamination. This map showed that iodine-131 spread northwest of the plant, just like cesium-137 as was indicated on an earlier map. Iodine-131 was also found south of the plant at relatively high levels, even higher than those of cesium-137 in coastal areas south of the plant. According to the Ministry, clouds moving southwards apparently acquired large amounts of iodine-131 that were emitted at the time.

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