Upgrade on risk analysis following the 080919 incident in the LHC sector 3-4

On 19th September 2008, during powering tests of the main dipole circuit in sector 3-4 of the LHC, an electrical fault occurred producing an electrical arc and resulting in mechanical and electrical damage, release of about 6 tons of helium from the magnet cold mass to the insulation vacuum enclosure and consequently to the tunnel, via the spring-loaded relief discs on the vacuum enclosure. The helium discharge from the cold mass to the vacuum enclosure exceeded by an order of magnitude, the maximum credible incident (MCI) flow described in the preliminary risk analysis performed in 1998. Based on the experience gained from the 19th September 2008 incident, a new MCI has been formulated and the cryogenic risk analysis has been revised and updated. The recommendations concerning the safety relief system protecting the vacuum vessels and the mechanical properties of the doors installed in the tunnel have been formulated.