Remote welding

One of the more significant design improvements of the UK’s Advanced Gas-cooled Reactors (AGR)—some built more than 30 years ago—was the inclusion of a reheater system to re-energise exhaust steam from the high pressure turbines in order to drive intermediate or lower pressure turbines further down the line.

However, the introduction of any new feature to a nuclear power generation system creates further levels of complexity and additional safety issues. These issues must be assessed and integrated into a planned monitoring and maintenance programme.

Each reheater has about 250 high-grade stainless steel pipes, and if one of these is damaged the whole reactor needs to be shut down. Failures are infrequent but could be critical to the safe operation of the station. These can be caused by design deficiencies, material defects, fabrication defects or service-related deterioration (such as thermal fatigue) or a combination of these factors. It is therefore essential that conditions with the potential to result in failure are identified, monitored and addressed.

When damaged pipes are identified, either through failure or as part of a planned outage inspection, operators have to repair them safely and reliably but also within the planned duration of the outage (for commercial reasons). Due to the reheater design, the pipes are hard to access.

“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,” says Michael Allman, regional director of Arc Machines Inc (AMI), a specialist supplier of automated welding systems.

“Additionally, in nuclear applications, weld integrity and the ability to adapt to exotic materials are critical.”

Many nuclear site operators are turning to automated orbital welding systems for these reasons. EDF Energy has recently invested in a bespoke weld system from AMI to replace the original 25-year old automatic system, now obsolete.

System design

EDF Energy commissioned AMI to supply a custom-built Model 20 weld head with full remote deployment. The modified internal diameter weld head, with an M415 power supply, will be used to stop leaks from inlet/outlet tubes by plugging a damaged tube during a reactor shutdown, or by repairing an existing plug. Development of the system, from concept to design and application testing, took three years. It was delivered in early 2011.

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. A pneumatic collet holds a pin on the plug during its transportation along the header pipe.

The weld head also includes a high precision vision system that can quickly locate the pipe, monitor weld progress and enable a post-weld visual check to determine weld acceptance. This system incorporates direct-view optics and three colour video cameras, with lenses specially formulated to filter out ultra-violet and infra-red light and give a clearer view of weld progress. Miniature video camera assemblies show both the leading and trailing sides of the weld puddle and are combined with remote-controlled dual-wire manipulators to allow welding in both directions.

The AMI weld head is integrated with a delivery vehicle developed in partnership with James Fisher Nuclear Services specifically for the project. The same vehicle is used for the abrasion cleaning of any oxidation from the header port prior to welding. It has also been designed to accommodate other technologies that may be required as non-destructive testing and examination (NDT and NDE) systems continue to evolve.

The delivery system and weld head need to work within different sizes of header pipe to allow for design variations between the sites operated by EDF Energy.

The delivery vehicle carries the weld head between 8 and 12 metres from the entry point, depending on the length and location of the pipe. To avoid any conflict, controls for the delivery vehicle and the weld head cannot operate synchronously. Once at the repair point, the delivery vehicle is clamped securely to the inside of the tube, locked out and remote operation switched to the weld head system. Small-scale precision controls then locate the weld head axially and radially around the header to effect a precise weld seal operation using AMI’s remote welding video vision system.

EDF Energy already used a modified 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 and Hunterston B. It will replace a welding system in operation since the reactors were first commissioned over two decades ago. The weld system is expected to see out the rest of the stations’ operational lives.