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Could the fuel in the ILL's reactor melt?

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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.

The reactor vessel and its associated cooling systems are designed in such a way that there must always be at least two independent failures for the water inventory of the vessel to be lost and the core to be exposed. The frequency of such an occurrence is therefore extremely low, around 10-5 to 10-6 per year (once every hundred thousand to a million years, on average).
This scenario was nevertheless taken into account in the most recent safety review of the installation. Following this review, an emergency water makeup system was installed. This allows water to be reinjected directly into the reactor vessel in this sort of scenario to ensure that there is always enough water in the vessel to cool the core by natural convection. Recovery pumps have been installed in the ‘crypt’ of the reactor to reinject the water that has leaked from the reactor vessel and thus re-establish a “closed” cooling system.
Finally, as part of the continuing process of safety improvement, ILL has recently proposed a new emergency core cooling system that will allow a communication passage to be opened up if necessary between the reactor vessel and the reactor pool and adjacent storage pool, providing a total volume of 1000m3 of water. Water from the reactor pool will then fill the reactor vessel by simple gravity, i.e. without the need for an external power source, allowing core cooling to continue. This new system is currently being considered by the relevant safety authorities (ASN and IRSN). It is expected to be in place by early 2012.
With these new emergency systems the probability of a core meltdown will then be extremely low, approaching a frequency of 10-7 per year, the value at which the risk is considered to be “residual” and no longer needs to be taken into account in the installation’s design basis.

In the Borax-type accident automatically included in the design basis, the core also undergoes partial meltdown but remains covered by the water in the pool. Most of the radioactivity released during the meltdown is therefore retained in the pool, which acts as a filter. A small amount of the radioactivity, predominantly the noble fission gases krypton and xenon, is released immediately into the reactor containment.