Cesium-137 is the major nuclide that causes environmental and radiological affects lasting over a long period. The estimated total amount of cesium-137 released into the atmosphere is summarized in Table 24.2 and ranges from 8.2 to 15 PBq suggesting significant uncertainty. Estimates of the released amount have been performed by several foreign institutes and researchers, e. g. [13-16]. The estimation by the French Institut de Radioprotection et de Surete Nucleaire (IRSN), which provided 30 PBq as the sum of cesium-137 and 134, agrees quite well with those of Japanese authorities. These esti­mates correspond to about one eighth to one tenth of the release from the Chernobyl accident. It should be noted that the distance of the cesium deposition with higher contamination than 1,000 kBq/m2 is limited to within 80 km of the NPP in specific directions (Plate X, between pages 448 and 449), while in the case of the Chernobyl accident, more distant dispersion was observed. The release rate of cesium-137 reached 1015 Bq/h on March 15, but it gradually decreased to between 1012 and 1013 Bq/h, and finally to 1011-1012Bq/h after March 25.

The cesium-134/cesium-137 ratio observed in deposition samples over a wide area, about one month after the accident, is mostly in the range from 0.95 to 1.0, which is different from that of Chernobyl. This is expected to decrease gradually along with the decay of cesium-134 which has a half-life of 2-1 years.

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