The 100<font size="2"><sup>th</sup></font> service module is repaired
In Building 110, a small team is working round the clock to complete the repair of the service modules needed for the cryogenic line between Points 7 and 8. The team has just repaired the one-hundredth module.
The magic number of one-hundred is often a cause for celebration, and CERN is no exception in this. But some centuries are more special than others, as was the case for the repair of the one-hundredth service module in Building 110. This small hall at the far end of the Meyrin site is where the service modules for the cryogenic line, which has been completely dismantled between LHC Points 7 and 8, are being repaired.
The fourteen members of the TS/MME Group working in the building have been pulling out all the stops since last October, even postponing their holidays, in order to complete the repairs as quickly as possible to keep the Project on schedule. "The team is highly dedicated and is working quickly and with a high degree of accuracy", explains Jean-Paul Bacher, Head of the Group's AS Section, which is responsible for the repairs.
While the repairs on the 256 straightforward components of this part of the cryogenic line have already been completed, work is still in progress on the service modules, the most complex components of the installation. "There are 156 of these modules, which are located at intervals of around 100 metres in the tunnel and link the cryogenic line to the magnets", explains Said Atieh, the engineer who is supervising the work.
As in the case of the straightforward components, the work consists mainly of replacing the faulty sliding tables (see Bulletin No. 42/2004). The difficulty lies in the complexity of the modules, of which there are seven different types. They contain bends and many welding points and solder joints, etc., and have to be cut before the many layers of insulation can be removed.
The team is taking advantage of this work to carry out other repairs that have proven necessary. Certain connecting bends have had to be completely replaced, for example. On a few modules, traces of corrosion have been discovered around the soldering points between one of the helium tubes and the vacuum barrier. It was quickly established that the cause of the corrosion was insufficient neutralisation of the soldering flux, in other words insufficient cleaning. "We are now aware of this problem of corrosion under stress and are able to resolve it", explains Said Atieh. "It affects around thirty of the modules that were made right at the beginning, before CERN imposed a change in the technique". The damaged part of the tube is replaced and connected to the vacuum barrier by mechanical means.
The operators then have to weld all the components together again. This is a delicate process as the welds must not damage the insulation of the tubes, which cannot withstand excessive heat. Cooling devices and heat barriers therefore have to be brought in during the welding operation. The last stage of the work is to prepare the ends of the modules for connection in the tunnel. Finally, the modules are tested under vacuum: an ultra-high vacuum is produced in the tubes and argon is injected around them to check that they are leak-tight.
The work is subject to strict quality control procedures involving a whole series of inspection reports. The 25 operations carried out on each module are all meticulously recorded and checked. Various X-ray and geometric checks are also performed. Given the complexity involved, repairs are proceeding at the rate of three modules a week. The work is due to be completed in October.
