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FAQ reactor safety

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FAQ on the ILL reactor safety

What are the technical specifications of the ILL’s reactor?

The ILL’s high-flux reactor is devoted exclusively to research. It operates continuously during 50-day cycles.

Its core comprises a single highly enriched uranium fuel element (10 kg) that is cooled by heavy water. The reactor produces the most intense continuous neutron flux in the world, namely 1.5 x 1015 neutrons per second and per cm2. Its thermal power of 58 MW is not reused and is removed by a secondary cooling system supplied with water from the river Drac.

The heavy water vessel that contains the core is situated in a pool filled with demineralised water which provides shielding from the neutron and gamma radiation produced by the core. The reactor is controlled by means of a neutron-absorbing rod, which is gradually withdrawn from the core as the uranium is burned up. It also has 5 safety rods, which are also neutron-absorbing devices and are used to shut down the reactor in the event of an emergency. [Find out more]

Is the ILL’s high-flux reactor an “old” reactor?


It is true that the ILL was founded in 1967 and that the high-flux reactor went critical for the first time in 1971.
However, the lifetime of a reactor is linked to the aging of the components “bombarded” by the neutrons (the flux), in particular the reactor vessel. In power plants, this vessel cannot be replaced.

On the high-flux reactor, however, these components are all replaced regularly. The reactor vessel was replaced in its entirety at the beginning of the 1990s, and the “new” installation then restarted in 1995, the first and only time this operation has ever been performed on a reactor. This vessel has currently been in use for the equivalent of just 8 years of operation at full power.


Similarly, between 2004 and 2007, 30 million euro were invested in the seismic reinforcement of the reactor building. As a result, the building is fully compliant with the most recent seismic design regulations in France (known as the Règle Fondamentale de Sûreté or Basic Safety Rule).
All other components are maintained, upgraded and replaced in accordance with standard practice.

What magnitude of earthquake is the ILL’s reactor designed to withstand?

An earthquake with a magnitude of 5.7 occurring at a depth of 7 km right under the reactor.

When it was built in 1970, the reactor was designed to withstand an earthquake corresponding to the criteria defined in the seismic regulations in force at the time (intensity VIII, as per French Seismic Code PS 67). Since then, our knowledge of seismic hazards has evolved, as have the regulations, which have become even more stringent.

In 2004, studies were resumed to define the earthquake characteristics to be taken into consideration and to verify the behaviour of the ILL’s installations under seismic conditions. These studies led to the carrying out of major seismic reinforcement work, which was completed in 2006 at a cost of around 30 million euro. The ILL’s reactor is now designed to withstand an earthquake with a magnitude of 5.7 occurring at a depth of 7 km right under the reactor building.

[Find out more]

Is a more powerful earthquake possible?

The studies carried out were based on the rate of seismicity of the Alps, which is considered to be moderate on a world scale.

Scientists estimate that in south-east France:
•    the frequency of earthquakes with a magnitude greater than 4 is one every 3 years,
•    the frequency of earthquakes with a magnitude greater than 5 is one every 30 years
•    the frequency of earthquakes with a magnitude greater than 6 is one every 300 years

The strongest historic earthquake occurred in Lambesc in the south of France in 1909, with a magnitude estimated at 6. It is therefore highly unlikely that an earthquake with a magnitude greater that 5.7 would ever occur directly beneath the installation.

As far as the region around Grenoble is concerned, seismologists consider that an earthquake with a magnitude of between 5.5 and 6 could occur on the Belledonne Border Fault located 15 km from the ILL. The acceleration levels at ILL would not exceed the limit values of the SSE. It is therefore extremely unlikely that the reactor will ever be subjected to more powerful seismic movements than those taken into account in its design.

Could a dam break cause a tsunami-like wave?

Yes and no.

The event that would cause the greatest rise in water levels in the Grenoble basin is a breach of the Monteynard dam.

It would not create a wave travelling at high speed as in the case of a tsunami. On flowing into the valley, the water from the dam would form a water surge whose speed would slow down the further it travelled. As a result, the water would reach the ILL approximately one hour after the dam burst and would be travelling at a speed of 10 km/h; the water level would rise by 4 m in 20 minutes.

Is the ILL’s reactor designed to withstand a dam break?


The reactor building is designed to mechanically withstand the pressure of the water. Even in a situation of this kind, it would remain watertight.
The rise in water levels caused by the breach of the Monteynard dam would not therefore affect the building’s structure. On the other hand, it would lead to a total loss of electrical power, as the two 20 kV substations and the two emergency diesel generators would be under water.

The loss of these power supplies would automatically trigger the shutdown of the reactor by safety rod drop. Cooling would be adequately provided by simple natural convection using the water in the reactor pool.

Is human intervention required to shut down the reactor in the event of an earthquake?


The reactor has three sensors (3-axis accelerometers) which permanently monitor ground movements. If two of these three sensors detect an acceleration greater than 0.01 g, the safety rods automatically fall, shutting down the reactor. This acceleration corresponds to a weak earthquake with a magnitude of less than 3 close to the reactor.

Of course, this automatic shutdown takes place under the supervision and control of the reactor operator teams.

[find out more]

Is a power supply needed to cool the reactor after its shutdown?


After the reactor shuts down, the core can be cooled by simple natural convention, i.e. without any source of electrical energy.

[find out more]

Is an external source of cooling water necessary after the reactor is shut down?


The reactor core is situated at the centre of a heavy water vessel with a volume of around 15 m3, which itself is located at the bottom of a pool containing 450 m3 of light water. In the event of the total loss of all power supplies, both external and emergency backup supplies, the core is cooled by natural convection in the reactor vessel, during which process the water in the vessel stabilises at a temperature of around 60°C. The reactor vessel itself is cooled, again by natural convection, by the water in the reactor pool, the temperature of which stabilises at below 60°C.

Could the ILL’s reactor explode?


An explosion such as the one that occurred in the n° 4 reactor of the Chernobyl power plant due to a runaway fission reaction cannot happen with the ILL’s reactor.

The equivalent scenario for research reactors like ours, known as a BORAX accident, does not produce an explosion capable of damaging all the reactor’s structures, including its containment. The energy stored in the reactor core and released in the “explosion” is much too low to do such damage. Obviously, this is due to the fact that the core of the ILL’s reactor is very small (10 kg of uranium compared with the 190 tonnes contained in the core of Chernobyl's RBMK-type reactor).

[find out more]

Could the fuel in the ILL's reactor melt?


We have already seen that as soon as the reactor shuts down the reactor core is adequately cooled by simple natural convection. Of course, for this to be possible there must be sufficient water in the reactor vessel. Accidents liable to result in the loss of the water in the reactor vessel can therefore lead to the meltdown of the fuel, as natural convection is no longer possible.

[find out more]

What is the most serious accident that could happen at the ILL?

The most serious accident would be a core meltdown following the loss of the water inventory.

It is on the basis of this extremely pessimistic scenario that the Institute's internal emergency plan (PUI) and the off-site emergency response plan (PPI) have been established.

[Find out more.]

Would the ILL have to release radioactive gases into the atmosphere if this accident occurred?


In the event of a core meltdown, the reactor containment is immediately isolated. In the hours following the accident, the pressure inside the containment may increase slightly. It stabilises at a maximum of around 0.1 bar, so there is no risk of damage to the containment. However, to guarantee that there are absolutely no unfiltered and unmonitored radioactive releases into the atmosphere, it is preferable to maintain the pressure inside the containment at slightly less than atmospheric pressure. This is done by regularly releasing small quantities of air from the containment via the 45m exhaust stack, through very high efficiency aerosol filters and iodine traps. As these releases are calculated, monitored and controlled, they are known as "planned releases".

[Find out more]

What would be the impact of such an accident on Grenoble and the surrounding area?

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.

[Find out more]

How would this accident evolve in kinetic terms?

In an accident involving a core meltdown due to the loss of the water inventory, the kinetics are relatively slow: the number of backup systems available ensures that water levels remain sufficient for the core to continue to be properly cooled by natural convection.

The Basic Safety Rules (RFS - Règles Fondamentales de Sûreté) are based on the assumption that certain emergency backup systems fail after 24 hours of use. It is this additional failure that, in this scenario, would result in the exposure of the core and its fusion in air.
The operator and the authorities would therefore have sufficient time to implement their respective emergency response plans (PUI and PPI) before the radiation accident itself (i.e. the core meltdown) occurs. [find out more]

Is it necessary to take iodine tablets if there is an accident at the ILL’s reactor?

Yes and no.

Taking into account the nominal efficiency of the iodine traps, calculations made for the worst-case accident scenario (i.e. a core meltdown) show that the equivalent dose in the thyroid gland is about 10mSv for children (the most vulnerable group) within the 500 m zone.
In France the decree of 20 November 2009 fixes the thyroid equivalent dose above which stable iodine must be administered at 50 mSv. The administration of iodine tablets would therefore not be compulsory in our case.
The Prefect may, however, order tablets to be distributed to people in the 300 m and 500 m zones as an added precaution.

[find out more]

Is the spent fuel in the storage pool as well protected as the reactor core?


The pool used to store spent fuel awaiting reprocessing at La Hague is situated, like the reactor pool itself, inside the reactor containment; its design constraints (resistance to various stresses and possible damage) are exactly the same as those for the reactor pool.

[Find out more]

What are the dangers of ionising radiation?

Ionising radiation has two main types of effect:

  • Deterministic effects: these are effects that occur at high doses, typically 1000 mSv and above, and are observed in anyone who receives such a dose.
  • Stochastic effects: these are effects that occur randomly, irrespective of the dose received, to some of the individuals in a population exposed equally to the same dose of radiation.

[Find more]