News of LHC incident.
*************************************************************
Source: CERN
Content: Press Release
Date Issued: 16 October 2008
*************************************************************
CERN releases analysis of LHC incident
Geneva, 16 October 2008. Investigations at CERN* following a large helium
leak into sector 3-4 of the Large Hadron Collider (LHC) tunnel have
confirmed that cause of the incident was 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.
Proper safety procedures were in force, the safety systems performed as
expected, and no one was put at risk. Sufficient spare components are in
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."
The summary report follows:
SUMMARY OF THE ANALYSIS
OF THE 19 SEPTEMBER 2008 INCIDENT AT THE LHC
On 19 September 2008, during powering tests of the main dipole circuit in
Sector 3-4 of the LHC, 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.
After a period during which the temperature of the magnets in question was
allowed to rise close to room temperature, inspections started and a
number of clear findings have now been established. Investigations are
continuing and the complete findings will be reported at a later date.
A - In the following summary, a brief description is given of the
chain of events which occurred around mid-day on 19th September. A more
detailed technical report is available at
https://edms.cern.ch/file/973073 ... dent_at_LHC__2_.pdf
1. 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. The power supply, unable
to maintain the current ramp, 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. Prior to this
fast discharge, it is certain that a magnet quench can be excluded as the
cause of the initial event. During the discharge, many magnet quenches
were triggered automatically in the arc and the helium from their cold
masses was recovered through the self actuated relief valves.
2. 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 neighbouring subsectors*.
3. The spring-loaded relief discs on the vacuum enclosure opened when
the pressure exceeded atmospheric, thus releasing helium into the tunnel,
but they 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.
B - 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 lsocation 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 the electric and fluid
connections pulling the dipole cold masses in the subsector from the cold
internal supports inside their undisplaced cryostats. The displacement of
the quadrupoles cryostats damaged "jumper" connections to the cryogenic
distribution line, but without rupturing its insulation vacuum.
C - Pending further inspection of the inside of the dipole cryostats,
it has been established that the number of magnets to be repaired is at
most 5 quadrupoles and 24 dipoles from the three subsectors 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. The
plan for removal/reinstallation, transport and repair of magnets in Sector
3-4 is being established and integrated with the maintenance and
consolidation work to be performed during the winter shutdown across the
whole CERN facility. The corresponding manpower resources have been
secured.
D - Once all possible inspections are completed, an analysis of the
events will lead to recommendations for future actions to prevent the
reoccurrence of this type of initial event, and to mitigate its
consequences should it accidentally reoccur. Although the cause of the
initial growth of connection resistance has not yet been established, and
knowing that a similar event has not occurred in the test of all other
sectors and of their thousands of connections, it has nonetheless been
decided that additional measurements to generate early warnings and
interlocks, improvements in pressure relief devices and in external
anchoring of the quadrupole cryostats with vacuum barrier will be
implemented before any further powering of the LHC circuits at high
current.
Technical appendix: LHC design
* The arcs of the LHC, extending over most of the length of each 3.3 km
long sector, are composed of a periodic lattice, the elementary cell of
which (107 m long) is composed of a horizontally focusing quadrupole,
three dipoles, a vertically focusing quadrupole and another three dipoles.
In each family, the magnets are electrically powered in series throughout
the sector. The magnets, equipped with their helium vessel and end covers,
constitute the "cold masses", which, in normal operation, contain
superfluid helium at 1.9 K and 0.13 MPa, and are thermally insulated from
the vacuum enclosure. The neighbouring cold masses are electrically and
hydraulically interconnected. The weight of the cold mass is transmitted
to the vacuum enclosure via cold support posts and is further transmitted
to the tunnel floor by adjustable support jacks, anchored in the concrete.
The lattice cell corresponds to the extent of the local cooling loops of
the cryogenic system, fed from the cryogenic distribution line through a
"jumper" connection every 107 m at the location of a quadrupole. Two
subsequent cells constitute a vacuum subsector sharing a common insulation
vacuum; the insulation vacuum enclosures of neighbouring subsectors are
separated by "vacuum barriers". The two beam pipes constitute two other
separate vacuum systems, extending over the whole length of the continuous
cryostat, and segmented at the arc ends.
