The impact on people located in the vicinity of a nuclear facility where an accident has occurred is always evaluated in terms of the radiation dose received.
In the ILL’s internal emergency plan (PUI) (which is the responsibility of the nuclear operator) and the off-site emergency response plan (PPI) (which the responsibility of the public authorities and, in particular, the Prefect), there are two danger zones defined around the facility:
- An inner circle corresponding to the zone that must be evacuated. The reference dose value for defining the radius of this zone is 50 mSv. For the worst-case accident for the ILL reactor, this circle has a radius of 300 m and only concerns employees working for companies in the immediate vicinity of the ILL: the ESRF, EMBL, PSB, LPSC, ST and IBS.
- An outer circle corresponding to the zone in which “take cover” procedures apply. The reference dose value used to define the radius of this zone is 10 mSv. For the worst-case accident for the ILL reactor, this circle has a radius of 300m. It concerns a small number of the staff at the neighbouring CNRS and CEA sites. The only local people concerned are the 300 residents of the Bastille district of Fontaine on the other side of the river Drac opposite the ILL.
The values used by the ILL to define the danger zones are based on WHO recommendations and have been adopted in most countries.
The dose beyond these zones is not nil, of course (the cloud does not stop at the border), but it decreases with distance. The doses received after one week are given below. They are calculated for a person located in the radioactive plume with no protection (a person outdoors, breathing the air of the plume for a whole week).
- 3 mSv at a distance of 1 km;
- 0.9 mSv at a distance of 2 km;
- 0.15 mSv at a distance of 5 km.
For comparison:
- The statutory annual dose limit for the general public, excluding natural sources and medical procedures, is 1 mSv;
- The natural radiation dose received by those living in the Grenoble basin is 2.4 mSv per year;
- The natural radiation dose received in certain highly populated areas of India or Brazil is 30 mSv per year;
- The average annual dose received in France for medical purposes is 1.3 mSv, although there are large disparities: an abdominal scan, for example, results in a radiation dose of over 10 mSv.
Basically there are two types of exposure to radiation:
- External exposure: the radiation source is located outside the body. The radioactive substances which are the source of the radiation decay and emit particles, mainly gammas, which enter into contact with the body. It is relatively easy in this situation to protect oneself, by reducing to a minimum the time spent near the source (TIME), by moving away from the source (DISTANCE), and by placing screens (SHIELDING) between oneself and the source. A house wall, for example, will reduce the dose of radiation received by a factor of 10. For external exposure, the dose rate received per unit of time (expressed in mSv /h) is the relevant value.
- Internal exposure: the source of radiation is inside the body. This is the case if we inhale contaminated air (by breathing when inside the radioactive plume or “cloud”) or ingest contaminated foodstuffs (drinking water, milk, vegetables, meat, etc ...). The radioactive substances enter the body and are then absorbed by various body organs, depending on their physical and chemical characteristics. The calculations take into account all radionuclides released (see fission products). The organs are irradiated by the particles emitted during decay, only in this case, because the radiation is emitted directly inside the body, the beta and, in particular, alpha particles if any, contribute substantially to the dose received. Obviously, if the source of radiation is inside the body, the simple techniques described above to protect against external exposure can no longer be used. For internal exposure, the amount of radioactivity incorporated (expressed in Becquerel (Bq)) is the relevant value.
In an accident at the ILL reactor, the radiation doses received by those situated within the 300 m zone would primarily be due to external exposure; the source of the radiation would be the reactor building itself. The thickness of the reactor containment would reduce this radiation at its source by a factor of at least 100.
The maximum dose rate would be less than 1 mSv/h at 170 m from the containment, on the edge of the ILL site, and about 0.1 mSv/h at 300 m. These low dose rates would give staff time to take all the necessary action calmly and without haste. This explains why, for example, in such a scenario ILL staff would evacuate the site on foot. This is also the most efficient solution, as it solves the problem of possible traffic jams.
For those outside the 500 m zone, the dose received would mainly be due to releases of gas from the exhaust stack. With a southerly wind, for example, as is usually the case in the morning, the inhabitants of Grenoble would not receive any dose at all. In the afternoon with a northerly wind, they might receive a fraction of the low doses mentioned above.