Remote Welding at Nuclear Reactor Sites

Safety at nuclear power stations is of paramount importance and operators need to specify the best fail-safe repair systems in order to guarantee minimum risk. An essential part of EDF Energy's comprehensive safety system in their UK nuclear fleet is the high quality welding systems provided by Arc Machines Inc (AMI) for secure repairs to damaged re-heater tubes.

Following a three-year development process, from concept to design and application testing, AMI has recently supplied a custom Model 20 weld head with full remote deployment to plug and seal any leaking tubes on the re-heaters at EDF Energy's Advanced Gas-cooled Reactors (AGR). The new weld head, paired with a Model 415 Power Supply, will be used in a repair procedure to stop leaking inlet/outlet tubes by plugging them during reactor shutdown.

Bespoke adaptations to the weld system include the facility to remotely drive a specially designed plug to the weld area and hold it in position during the application of a two pass weld sequence with the addition of filler wire. The weld head includes a high precision vision system that can quickly locate the pipe, monitor weld progress and enable a post-weld visual check, using a high definition camera, to determine weld acceptance.

Ensuring a safe method of sealing off a damaged pipe during a reactor shut down is an essential part of EDF Energy's safety case. With the integrity of the weld being so critical and the difficulty of pipe access within the re-heater structure, an automated system was the only solution and AMI's stringent manufacturing standards, workmanship and quality of materials made it the first choice for a reliable solution.

The M20 weld system is AMI's latest venture in a long term relationship with EDF Energy, providing both standard and bespoke weld heads and power sources. The modified internal diameter (ID) weld head carries a 48mm diameter plug between 8 and 12 metres from the pipe entry point, depending on the length and location of the pipe, to complete a precise locate and seal operation using AMI's remote welding video vision system.

This patented vision system incorporates direct-view optics and colour video cameras to show both the leading and trailing sides of the weld puddle. The miniature video camera assemblies are combined with remote-controlled dual wire manipulators to allow welding in both directions and provide a clear view of weld progress as well as the facility to run non-destructive evaluation (NDE) of weld and pipe integrity once the weld is complete.

EDF Energy already uses a modified AMI weld system at the Heysham 1 and Hartlepool sites and was happy to specify further AMI equipment to the engineering contractor, Doosan Babcock. With an anticipated working life of up to 20 years the M20 weld system will be deployed across the AGR sites at Hinkley Point B, Torness, Heysham 2 and Hunterston B, to replace a welding system in operation since the reactors were first commissioned around 30 years ago.

Andy Purvis, regional boiler programme manager at EDF Energy, stresses that safety was the company's first priority: "Repairs to the re-heater pipes are very infrequent but EDF Energy has a responsibility to safely maintain these stations.

"Each re-heater has around 250 stainless steel pipes and if one of these is damaged the whole reactor needs to be shut down. The AMI system provides us with a safe, methodical welding system that can get us back up as quickly as possible, which is vital to our commercial operation, and it should see out the rest of the stations' operational lives.

AMI fully commissioned the welding system, provided operation and safety training and, from its new site at Daventry, maintains a comprehensive backup and customer support service.

Michael Allman, AMI's regional director, sees great opportunity for the company within the nuclear sector: "Arc Machines has been at the forefront of orbital welding for the nuclear industry for a long time and we are determined to maintain that position. Our welders are rugged, flexible, functional and, above all, reliable."

Most nuclear reactors have restricted physical access to pipe work so a remote automated weld head is far safer and more accurate than a manual system. Additionally, in nuclear applications, weld integrity and the ability to adapt to exotic materials are critical. The accuracy and consistently high quality welds achieved by the AMI weld heads, power sources and control systems are now essential to an industry that, until recently, relied almost entirely on manual welding."

AMI has been setting the standards for automated orbital welding technology since it was first established in 1976 and its welding systems now provide a range of applications for the nuclear industry from secure seals on 3013 high level waste containers to precision welding for reactor tubesheets, turbine shafts, reactor piping, vessel maintenance and repair and superheated steam generator piping.