Accident in the Japanese NPP Fukushima: Large emissions of Cesium-137 and Iodine131/Spread of Radioactivity/ weather currently favourable/low level radioactivity
observed in Iceland (Update: 24 March 2011 16:00)

Weather in the crisis region
Currently, weak winds dominate in the crisis region. During the day, they turn from westerly into northerly directions. Air from theFukushima plant is predominately transported to the Pacific.
Tomorrow, the weather situation will change. A disturbance crosses the region with heavy rain. The wind turns for a short period tonortherly and easterly directions. Air from the reactors can thus be blown inland.
The day after tomorrow, north-westerly winds predominate. Air from Fukushima will thus be transported to the Pacific.

Emission estimates based on CTBTO data
Due to the numerous highly accurate radionuclide data (24-hour air samples) available to us from the world-wide network of theComprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) in Vienna, we can now estimate the source strengths of Iodine131 and Cesium-137 for the first few days of the accident. The measurement accuracy of the network amounts to about 10 µBqm -3for both isotopes.
During the first two days, March 12 and 13, the Fukushima emissions were mostly transported to the Pacific, eventually hitting theCTBTO station in Sacramento/California on March 17 (see Figure below). The maximum measurement there was 14000 µBqm -3 ofIodine-131 on March 18. Based on our plume dispersion model simulations, we estimated the source strengths of Caesium andIodine needed to explain these readings. They amounted to 1.3 1017 Bq and 5 1015 Bq per day, respectively.
On March 14, the wind changed its direction. Consequently, emissions were transported inland, hitting the CTBTO station inTakasaki, Japan (see Figure below). The measurements on March 15 at this station showed Iodine-131 activity concentrations of1.5 107 µBqm-3 (24 hours), exceeding the readings from California by a factor of 1000. Based on our plume model simulations, wenow estimated the source strengths of Caesium and Iodine needed to explain the readings at Takasaki. They amounted to 1.2 10 17
Bq and 4 1016 Bq per day, respectively. So, while the emission estimates for Iodine are very consistent, those for Cesium-137
showed some day-to-day variability.
On March 15 and 16, most of the material from Fukushima was transported to the Pacific, where it faded away before causingmajor detections at CTBTO stations. Therefore, we cannot make reliable source estimates for those two days without in-depth
analysis.
During the reactor accident in Chernobyl in 1986, the total releases of Iodine-131 and Cesium-137 (whole accident scenario) were1.76 1018 Bq and 8.5 1016 Bq, respectively. The three day emissions from Fukushima of Iodine-131 would be about 20% of the totalChernobyl emissions, while those of Cesium-137 would be between 20 and 60% of the total Chernobyl emissions, depending
whether one believes in the different Iodine to Caesium ratio measured in Japan.
ZAMG also estimated the effective dose rates that would follow from its emission estimates. In the highest contaminated grid cellof the model, namely the grid cell where the power plant is situated, the dose rates would be between 1 and 5 Micro Sievert perhour. This is well in line with data made available by the International Atomic Energy Agency (IAEA) in Vienna.

Figure: Iodine-131 from the Reactor in Fukushima reached the U.S. West Coast on March 17.

Figure: On March 15, the radioactive cloud from Fukushima reactor was transported inland, hitting the CTBTO station inTakasaki.

Dispersion Modeling
The results of the dispersion model show that radioactivity is transported mostly to the Pacific today. Tomorrow, the cloud would
be transported more inland. The day after tomorrow, the cloud is transported to the Pacific Ocean.
The colour scale shows a total of 5 colours. The area marked „E“shows an area with estimated current equivalent dose rate of 10mSv/h (in a 25x25 km2 square). The violet colour on the outer edge of contaminated areas (Area A) represents 0,3 µSv/h, whichcorresponds to the amount of the natural background radiation dose.

Figure: Spread of Radioactivity over Eastern Asia today and tomorrow 12:00 UTC

Figure: Spread of Radioactivity over Eastern Asia the day aftertomorrow 12:00 UTCRadiation data/CTBTO: What can we measure with modern technology?
The current CTBTO radioactivity data (status today, data from March 21) sow extremely low concentrations of Iodine-131 overIceland. Other isotopes were not measured there. The Iodine-131 activity concentrations were about 10 µBqm-3, which translates
into an effective dose rate of about 10 -6 (one in a million) Nano-Sievert per hour. This is hundred million times less the naturalradioactivity exposure in Austria. In other words, one needs to be exposed about 10000 years to this radioactivity level to get thesame dose as the natural exposition of one single hour.
There are currently no CTBTO data showing Iodine and/or Caesium in Central Europe. The measurement of these tiny traces ofradioactivity with the usually applied means, for example the total dose measurement devices, is impossible.

Dr. Gerhard Wotawa
Division for Data, Methods and Modelling
Central Institute for Meteorology and Geodynamics
Hohe Warte 38, 1190 Wien
gerhard.wotawa@zamg.ac.at

ZAMG will not answer any questions related to travel in Japan or in other parts of the world, since this is the responsibility of nationalradiation protection authorities. Travel advisories and warnings are available from your foreign ministry. In Austria, such advisories areavailable on www.bmeia.gv.at.

This information is updated daily, and whenever the development of the situation requires it.

Videos:Plume spread from Fukushima/Permanent Release/Iodine-131