Ball joints against earthquakes

Upon exiting the cryoplant's termination cold box, which acts as a dispatcher for the different cooling fluids, two sets of cryolines leave the facility over a massive industrial bridge that starts by spanning the 75-metre distance between the cryoplant and the Assembly Hall, and then takes a 90-degree turn left to run along the building for another 50 metres before reaching the Tokamak Building at L3 level.

Along with the steel bellows placed at regular intervals along both the cryolines' inner pipes and outer jackets to compensate for metal contraction due to the flow of intensely cold fluids, the decoupling spools are dimensioned to withstand the displacements that an SL-2 earthquake would cause—an event predicted to occur once every 10,000 years. "In case of a seismic event, even of a lower intensity than SL-2, the Tokamak Building, the Assembly Hall (to which the cryobridge is attached), and the cryobridge itself would each respond in a different manner," explains Lahcene Benkheira, the ITER cryogenic systems responsible officer. "Without a decoupling device, the cryoline network could rupture which—in a nuclear installation—would amount to a loss of confinement." There will be a total of six such decoupling spools in the cryobridge; three of them have already been installed.

The difference in the response to a seismic event stems from structural properties: the Tokamak Building is part of a 400,000-tonne reinforced concrete nuclear building (the Tokamak Complex) that sits on anti-seismic system that allows an important amplitude in lateral displacement; the Assembly Hall is a steel structure with a small degree of flexibility, as is the cryobridge attached to it. In order to decouple and absorb the specific response of these different structures, the decoupling spools are equipped with lateral and transversal gimbles that act as ball joints, a bit like the hip joint in the human body.

Another major element in ensuring the integrity of the cryoline network is the quality of the welding for the 4 or 5 process pipes within every spool. One of the first challenges lies in aligning these massive components that measure between 6 and 9 metres in length and weigh between 2.2 and 9 tonnes. Once the inevitable offsets are compensated (sometimes by workers pushing and pulling) and the protruding process pipes at each end of the spools are almost in contact, the welders, duly qualified, can proceed. The task is difficult, the space constricted, the body contortions sometimes painful but the work gets done, millimetre after millimetre, for a total length of several kilometres.

The installation of the cryoline spools along the cryobridge leg parallel to the Assembly Hall is expected to be completed by the end of the summer, while the activities along the perpendicular leg leading to the cryoplant should be finalized by the end of the year. All in all, the cryoline network installation is now approximately 70 percent completed.