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.
In such a case, the fuel meltdown would result in:
- the loss of the fuel cladding, which serves as the first barrier to contain the radioactive fission products;
- the release of some of the fission products normally stored in the fuel element matrix.
The radioactive fission products would therefore be released into the reactor's primary cooling system.
The worst-case scenario included in the design basis of the ILL reactor involves the extremely pessimistic assumption that the primary cooling system, which is the second barrier to contain radioactive substances, is itself damaged. In this case, the radioactive materials are released directly into the reactor containment - the third and final barrier - and not into the water of the pool surrounding the reactor vessel. This is a particularly unfavourable assumption, as the water in the pool is an extremely efficient filter for retaining a very large proportion of the radioactive fission products.
This worst-case scenario was included in the initial design of the reactor and is taken into account for the improvements made continuously to the safety of the installation. Preventive measures are in place to reduce the probability of its occurrence to as low as is reasonably achievable; damage limitation measures are also in place to reduce to the lowest level reasonably possible the severity of the accident, should it nevertheless occur.
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. It is this scenario which, despite its improbability, was used to calculate the "300 m danger zone" (evacuation of personnel of companies on the site) and the "500 m danger zone" (“take cover” order for the few hundred people living in the Bastille district of Fontaine).