CMS Bulletin

SAFETY

Fire Safety – Essential for a particle detector

The CMS detector is a marvel of high technology, one of the most precise particle measurement devices we have built until now. Of course it has to be protected from external and internal incidents like the ones that can occur from fires. Due to the fire load, the permanent availability of oxygen and the presence of various ignition sources mostly based on electricity this has to be addressed.

Starting from the beam pipe towards the magnet coil, the detector is protected by flooding it with pure gaseous nitrogen during operation. The outer shell of CMS, namely the yoke and the muon chambers are then covered by an emergency inertion system also based on nitrogen. To ensure maximum fire safety, all materials used comply with the CERN regulations IS 23 and IS 41 with only a few exceptions. Every piece of the 30-tonne polyethylene shielding is high-density material, borated, boxed within steel and coated with intumescent (a paint that creates a thick cover if thermally exposed). The same level of detail was considered while analysing all other materials chosen.

Both sides of the detector galleries support additional installations (mostly racks and cryogenic equipment etc.). Most of them are covered by small carbon-dioxide extinguishing systems automatically triggered by melt cables and smoke detectors. Of course, manual intervention is also possible.

Below the detector, a region called “X0” takes up most of the cables leading towards the underground service cavern. Due to the density of the cable trays and their load, a medium-pressure water-mist extinguishing system was installed in situ. In case of activation it will limit the extent of a propagating fire and due to its nature the water will bind and condense smoke and its ingredients like soot, aerosols etc.

In addition an active fire detection system is installed everywhere inside the cavern. Here, the term “active” describes a system that is constantly extracting air from a volume and analysing it with two redundant units. Only if both are triggered a confirmed fire detection is then forwarded to the fire brigade by the detector safety system. Such systems are far more sensitive than the standard smoke detectors.

Last but not the least, a high-expansion foam system could be manually triggered to flood the whole volume of the experimental cavern up to one meter above the detector. This powerful last means of defence is currently undergoing a revision during LS1 to ensure its readiness and operation.

Even if the technical fire safety is state of the art, all users should never stop looking out for unusual incidents. A machine could not replace a vigilant human.