Higher estimate of Fukushima releases

Friday, 25 May 2012

The latest analysis has upped the figures for the amount of radioactivity released during the accident at Fukushima Daiichi, although the estimate for the main long-term contaminant, caesium-137, was actually reduced. 

In previous official estimates from Japanese government agencies only broad figures were given for gross releases of iodine-131 and caesium-137. Now Tepco has compiled a more detailed estimate based on data from radiation monitoring cars and validated by government measurements of materials deposited on the ground. The figures are divided between air and sea and raise the overall scale of the iodine-131 release by a factor of three, while marginally reducing the scale of caesium-137 release.

Figures for the releases can only be estimated but their effects on radiation dose rates have been accurately mapped using aircraft survey data

The potential health risk posed by the Fukushima accident was that released iodine-131 could accumulate in the thyroid glands of growing children and lead to cancer cases. Tepco put the amount of this radionuclide at 500 petabecquerels (PBq). The risk from iodine, however, passes quickly as it has a half-life of only eight days. The World Health Organisation said this week that in only one place did children receive a higher dose to their thyroids due to iodine-131, and estimated this at 100-200 millisieverts - well below the levels received by children in the vicinity of the Chernobyl accident which for some groups averaged 1400 millisieverts.

In terms of contamination the most serious radionuclide has been caesium-137, which has a half-life of about 30 years. Tepco said that approximately 10 PBq of this isotope was released to the air. Another 10 PBq of caesium-134 was also released to air, but it has lower contamination potential due to a shorter half life of only two years.

Noble gases amounting to 500 PBq were also released during the accident, but these do not represent a contamination risk or health issue away from the site because they are not biologically active, are quick to decay and disperse easily in the atmosphere.

Similarly, releases to sea have not resulted in contamination beyond the plant's immediate area because the mixing power of ocean currents has dispersed the substances beyond the limits of detection in seawater samples. Tepco estimated that 11 PBq of iodine-131, 3.6 PBq of caesium-137 and 3.5 PBq of caesium-134 were released.

Revisions

The first estimates made shortly after the accident put releases of iodine-131 at 130 PBq. This was raised in June 2011 to 160 PBq and the latest figures have it at 511 PBq. For caesium-137, however, the estimates grew from an initial 6.1 PBq to 15 PBq but have now been revised downwardly to 13.6 PBq.

Tepco warned that the latest figures may not be the final ones: "Further data still needs to be collected to review the validity of our evaluation result considering that the evaluation was done based on a small amount of data in a limited area [the power plant site]." 

Researched and written
by World Nuclear News