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Accident: Helium leak halts CERN facility tests; root cause

09 December 2008

At CERN, during commissioning testing, an arc flash punctures a cryogenic tank, damages steering and focusing magnets, and contaminates beamline. Could this CERN accident have been prevented?

Superconducting quadrupole electromagnets direct protons beams to four intersection points, where interactions between protons take place. Source: Wikipedia; see http://en.wikipedia.org/wiki/Hadron_collider.
Superconducting quadrupole electromagnets direct protons beams to four intersection points, where interactions between protons take place. Source: Wikipedia; see http://en.wikipedia.org/wiki/Hadron_collider.

What were the details and cause of the fall accident at CERN? Might it have been prevented? On September 19, 2008, only nine days after proton beams were first circulated at the Large Hadron Collider (LHC) at the European Organisation 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 = 10^12 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 27km at a depth ranging from 50m to 175m 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.

The incident occurred approximately two months before the facility’s planned winter shutdown, scheduled for November 2008, leaving insufficient time to repair the damage and put the facility back in operation. Repairs will be carried through the shutdown period and the facility will be operational in time to resume operations in spring 2009.

In their report, CERN investigators say that proper safety procedures were in force, the safety systems performed as expected, and no one was put at risk. Furthermore, they say sufficient spare components are on hand to ensure that the LHC is able to restart in 2009, and measures to prevent a similar incident in the future are being put in place.

‘This incident was unforeseen,’ said CERN director general Robert Aymar, ‘but I am now confident that we can make the necessary repairs, ensure that a similar incident can not happen in the future, and move forward to achieving our research objectives.’

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.

During the ramping-up of current in the main dipole circuit at the nominal rate of 10 A/s, a resistive zone developed, leading in less than one second 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 three and four seconds, the beam vacuum also degraded in beam pipes 2 two and one, respectively. Then the insulation vacuum started to degrade in the two neighbouring 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 neighbouring subsectors.
After restoring power and services in the tunnel and ensuring mechanical stability of the magnets, the investigation teams proceeded to open up the cryostat sleeves in the interconnections between magnets, starting from the central subsector. This confirmed the location of the electrical arc, showed absence of electrical and mechanical damage in neighbouring interconnections, but revealed contamination by soot-like dust, which propagated over some distance in the beam pipes. It also showed damage to the multilayer insulation blankets of the cryostats.

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.

Pending further inspection of the inside of the dipole cryostats, investigators estimate that at most five quadrupoles and twenty-four dipoles from the three subsectors were involved, but it is possible that more magnets will have to be removed from the tunnel for cleaning and exchange of multilayer insulation.

Spare magnets and spare components appear to be available in adequate types and sufficient quantities to allow replacement of the damaged ones during the forthcoming shutdown.

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.

— C.G. Masi, senior editor
Control Engineering News Desk


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