The IRSN (Institute of Radiationprotection and Nuclear Safety) doesn't have the data to measure directly the composition and amounts of ejected radioactive materials, but rather it collects technical information about accidents at nuclear installations.
The interpretation of such information has permitted the IRSN to devise probable scenarios for the degradation of the three reactors since 12 March upon assuring their coherence with the the dose rates measured on the site. The IRSN has equally retained the hypothesis that the radioactive releases continued through the 20th of March.
The readioactive elements emitted in the course of different episodes of ejection consist of rare gasses (chemically non-reactive radioactive elements resting in the atmosphere and not falling to the ground) and volatile elements, principally radioactive iodide, i.e., iodine 131, with a halflife of 8 days, and radioactive cesium, i.e., cesium 137.
The IRSN has simulated the atmospheric dispersion of the estimated amounts of material ejected between 12 and 22 March with the aid of a computer model applicable to long distances (on a scale of several hundreds of kilometers) by using the observations and meteorological forecasts furnished by Météo France.
This simulation has been applied to cesium 137 as the tracer of the radioactive plume in the course of this period. The results of this modelling are expressed in becquerels of cesium 137 per cubic meter of air (Bq/m3).
Play this simulation here.
This modelling effecting the scale of Japan shows that the plume is headed in different directions at different times: first toward the northeast just until 14 March, next toward the south and the southwest, in the direction of Tokyo on 15 March, then toward the east in the direction of the Pacific Ocean.
The IRSN has compared the results of this simulation with the measures contamination of the air effected in Tokyo, finding the results of the simulation to be of the same order of magnitude as those measured in the city, as illustrated in the graphics below for iodide 131 and cesium 137.
The IRSN has estimated the doses likely to be received by a person exposed the the radioactive plume, assuming that this person remains in the same area without protection (outside) from 12 to 22 March. In order to caluculate the dose, the IRSN has considered a one-year-old baby who is the the most suseptible to iodine 131 (dose to the thyroid).
The next simulations will show the evolution of doses in the course of time.
In cases of accidents, the maximum whole body doses without special protection recommended are 10 mSv (milliSivert) to seek shelter and 50 mSv for evacuation. Below 10 mSv the risk to health is judged sufficiently slight as to not make necessary protective actions. By comparison, the average annual dose received in France due to natural and medical radiation exposures is 3.7 mSv.
Doses to the thyroid likelt to be received by a one-year-old baby in the absence of protection
4 - Modelling the dispersion of radioactive ejecta in the atmosphere on the global scale
Based on the ejecta estimated by the IRSN, Météo France has simulated the the long distance dispersion of radionuclides over global distances, projected to 26 March 2011.
View the animated simulation here.
For comparison, the values measures in the days following the Chernobyl accident were higher than 100,000 Bq/m3 in the first few kilometers surrounding the center; they were of the order of 100 to 1000 Bq/m3 in the countries most affected by the radioactive plume (Ukraine, Belarus); in eastern France the measured values were of the order 1 to 10 Bq/m3 (May Day 1986). Today, a very week activity of cerium 137 is still in the air, of the order 0.000001 Bq/m3.