Yes
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".
In the worst-case scenario taken into account by the ILL, i.e. a core meltdown in air, a fraction of the radioactive fission products is directly released into the containment.
Levels of radioactivity in the containment are continuously monitored by three radiation detectors in the containment itself and by three others in the ventilation system. If readings from any two of three identical detectors exceed a pre-set threshold, the containment is automatically and completely isolated.
However, the total isolation of the containment involves shutting down all the ventilation systems, including the air conditioning, which keeps the air in the containment cool. The pressure in the containment will therefore rise due to the heat given off by various sources inside the containment, in particular the residual power of the fuel. The evaporation of liquid nitrogen and helium used for ILL's research experiments will also lead to a slight increase in the pressure inside the containment.
In the worst possible situation, i.e. on a very hot summer day, the pressure in the containment would rise within a few hours to stabilise at about 0.1 bar.
This explains why the ILL reactor containment is double-walled. It comprises an inner wall made of 40 cm-thick concrete and an outer shell of 11 mm steel. Between the two walls an overpressure of 0.135 bar is constantly maintained, using clean air from the outside. Given the dimensions of the building (60 m wide and 35 m high), there are inevitably some minors leaks. The double-walled containment guarantees that these leaks are from the outside to the inside, and not the other way round. It is clean air from the outside that enters the containment rather than air potentially polluted by radioactive materials that leaves the building.
It is nevertheless preferable to reduce the pressure inside the concrete containment to slightly below atmospheric pressure. This guarantees, this time with absolute certainty, that there are no unmonitored leaks, even if the systems that maintain the overpressure in the annular space between the inner and outer containment walls fail. To achieve the underpressure inside the inner concrete containment, the air to be released is analysed for radioactivity and then directed through the 45m exhaust stack, across very high efficiency filters and iodine traps. The radioactivity released is, of course, measured and monitored before it enters the stack.