March 19, 2011

Simulation of atmospheric dispersion of the radioactive plume formed byreleases from the Fukushima Daiichi nuclear power plant since 12 March
1 – What is known about the radioactive releases emitted since 12 March 2011?
IRSN does not have any direct information regarding the composition and scale of the radioactive
releases. From the interpretation of dose rates measured at the site and probable degradation
scenarios for the 3 reactors since 12 March, together with the assumption that these releases willcontinue until 20 March, we draw the conclusion that the radioactive elements released during thevarious release episodes are noble gases in combination with volatile elements.
The noble gases are radioactive elements with a very low chemical reactivity, and they remain in
the air without being posed on the ground. The volatile elements consist of mainly radioactive
iodine, including iodine-131 which has a radioactive half-life of 8 days, and radioactive caesium,including caesium-137 with a half- life of 30 years. These volatile elements constitute aerosols,
which due to their weight will end up falling on the ground.
The proportions of these different radioactive elements are estimated on the basis of the generalknow-how and experience feedback in relation with nuclear reactors.
2 – Dispersion of radioactive releases in the air
IRSN has simulated the atmospheric dispersion of estimated releases emitted between 12 and 20
March using its long-range numerical model (a scale of several hundred kilometres), and based on
the meteorological forecasts supplied by Météo France.

This simulation was applied to caesium-137, used as a tracer in the radioactive plume during thisperiod. From the first day on (12 March) this simulation has been continuously carried out every
other hour and the results of this simulation are given in becquerels of caesium-137 per cubic metreof air (Bq/m3). In addition the results are compared to the values measured in the vicinity of theChernobyl plant, just after the accident that occurred on 26 April 1986. The values exceeded
100,000 Bq/m and they were around 100 to 1000 Bq/m3 in countries seriously affected by theradioactive plume (Ukraine and Belarus) in France, the values measured in the east were around 1
to 10 Bq/m3 (on 1 May 1986). A very low level of radioactive caesium-137 still remains in the air,around 0.000001 Bq/m3.
The same model applied in the case of Japan shows that the plume travelled in various directions
over the course of time: first, up to the northeast until 14 March, then down to the south andsouthwest, towards Tokyo, on 15 March, and then to the east and towards the Pacific Ocean.
IRSN compared the results of the simulation with the results of air contamination measurements
taken in Tokyo. For iodine-131 and caesium-137I the results are of the same order of magnitude as
the values measured in Tokyo, shown in the graphs below.
In view of this comparison, it is possible to consider that the atmospheric dispersion modelling
carried out by IRSN provides satisfactory results for the agglomeration of Tokyo and that the doses
calculated by IRSN based on this model are representative of the doses likely to have been received
by the population exposed to the radioactive plume.
3 – Estimating the doses likely to have been received by people exposed to the radioactive
plume
IRSN has estimated the doses likely to have been received by a person exposed to the radioactive
plume, assuming that this person remained in the same place and with no protection (i.e. outside)throughout the entire release period (from 12 to 20 March). For these dose calculations, IRSN

studied the case of a one-year old infant, the most sensitive to iodine-131 (dose to thethyroid). In other words, these are the most cautious hypotheses.
The simulations below show how the doses evolve over time, throughout the simulation period. If
there are further releases in the future, these doses may increase further if the most exposed
people are unprotected.
•Whole body doses likely to be received by a one-year old infant with no protection during
the releases

In the event of an accident, the recommended minimum dose values used to launch protective
actions are 10 mSv for taking shelter and 50 mSv for evacuation. Below 10 mSv, the health risk isdeemed to be sufficiently low and therefore such protective actions are considered unnecessary. To
compare, the average annual dose received in France due to natural radioactivity and medical
exposure is 3.7 mSv.

•Doses to the thyroid likely to be received by a one-year old infant with no protection during
the releases

In the event of an accident, the recommended minimum dose values for prescribing the ingestion ofstable iodine are 100 mSv in Japan.

4- Modelling the dispersion of radioactive releases into the atmosphere on a global scale
Using the release values estimated by IRSN, Météo France simulated the dispersion of radioactive
releases over a very great distance, with forecasts up to 26 March.

According to this simulation, the radioactive plume will now have reached north-eastern Siberia,
the United States and the western Atlantic. It should reach France from 23 or 24 March on.

According to this simulation, the concentrations that can eventually be expected may bearound 0.001 Bq/m3 in the mainland of France and in the French Overseas Territories in thenorthern hemisphere. As expected, the southern hemisphere is not significantly affected by thislarge-scale dispersion.