CERN accident findings released
On Sept. 19, 2008, only nine days after proton beams were first circulated at the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN), a fault occurred resulting in mechanical damage and release of helium. A recently released investigators’ report confirmed the incident was caused by a faulty electrical connection between two of the accelerator’s magnets. This resulted in mechanical damage and release of helium from the magnet cold mass into the tunnel.
LHC is a dual-beam synchrotron designed to accelerate protons to a kinetic energy of 7 TeV (1 TeV = 1012 electron volts). When the beams intersect, protons collide with relative energies of 14 TeV. Superconducting magnets immersed in vacuum-insulated liquid helium tanks (Dewars) at an operating temperature of 1.9 K turn the beams to follow a circular tunnel with a circumference of 27 kilometers (17 mi) at a depth ranging from 50 to 175 meters underground, and to keep the beams focused. Two types of magnets are used: 1,232 dipole magnets keep the beams on their circular path, while an additional 392 quadrupole magnets keep the beams focused.
During power-up tests of the main dipole circuit, a fault occurred in the electrical bus connection in the region between a dipole and a quadrupole, resulting in mechanical damage and release of helium from the magnet cold mass into the tunnel. Proper safety procedures were in force, the safety systems performed as expected, and no-one was put at risk, investigators report.
While the ramp-up of current in the main dipole circuit was occurring at the nominal rate of 10 A/s, a resistive zone developed, leading to a resistive voltage of 1 V at 9 kA. Unable to maintain the current ramp, the power supply tripped off and the energy discharge-switch opened, inserting dump resistors into the circuit to produce a fast current decrease. In this sequence of events, the quench detection, power converter, and energy discharge systems behaved as expected.
Within one second, an electrical arc developed, puncturing the helium enclosure and leading to a release of helium into the insulation vacuum of the cryostat. After 3 and 4 seconds, the beam vacuum also degraded in beam pipes 2 and 1, respectively. Then the insulation vacuum started to degrade in the two neighboring subsectors.
Spring-loaded relief discs on the vacuum enclosure opened when the pressure exceeded atmospheric, releasing helium into the tunnel. The relief valves were unable to contain the pressure rise below the nominal 0.15 MPa in the vacuum enclosure of the central subsector, thus resulting in large pressure forces acting on the vacuum barriers separating the central subsector from the neighboring subsectors.
The forces on the vacuum barriers attached to the quadrupoles at the subsector ends were such that the cryostats housing these quadrupoles broke their anchors in the concrete floor of the tunnel and were moved away from their original positions, with electric and fluid connections pulling the dipole cold masses in the subsector from their internal supports inside their undisplaced cryostats. The displacement of the quadrupoles’ cryostats damaged “jumper” connections to the cryogenic distribution line, but did not rupture its insulation vacuum.
The extent of contamination to the beam vacuum pipes is not yet fully mapped, but is known to be limited; in situ cleaning is being considered to keep the number of magnets to be removed to a minimum. Removal/reinstallation, transport and repair of magnets will be integrated with the maintenance and consolidation work to be performed during the winter shutdown across the CERN facility.