Repair and maintenance

Sizeable problem solved

10 August 2006

A distorted flange on a reactor access hatch at an EdF nuclear power plant was repaired using a purpose-developed machine – on a scale unprecedented in Europe – to allow the issue to be addressed in situ.

The materials access hatch flange on the reactor at EdF’s Chooz B1 had become distorted over years of service, primarily because of subsidence of the concrete internal enclosure, and flange face machining was required to restore the flatness. Given the hatch’s colossal weight of over 70t and 8m internal diameter, an in situ repair solution was clearly the preferred option but the scale exceeded the onsite machining capabilities available. Moreover, accuracy of the finish to within high-level tolerances was required.

In discussion with the plant operator, Furmanite developed a plan to construct the largest diameter milling and turning machine in Europe to enable the necessary remedial work to be undertaken in situ, within the high precision tolerances required.

In situ flange face machining of the materials access hatch flange using Furmanite's orbital facing machine

Designed for both single point turning and milling operations, the Furmanite 4000-8500 modular orbital facing machine can handle work faces up to 8.5m diameter and can be used in a horizontal or vertical plane.

With the modular orbital facing machine designed, built, and onsite, the internal hatch cover was removed so that the collar face, which was set in concrete, could be accessed. Up to 16mm was removed from the face in the most distorted areas and a sealing groove was re-machined in. The mating face on the hatch cover was also machined to ensure a airtight seal.

In addition to the scale, the fact that the machining was performed in the vertical plane made it particularly challenging. Measures also had to be applied to undertake the work in a nuclear environment, including specific training for the technicians, monitoring of radiation exposure, and special measures for decontamination of tools.

The capabilities of the machine include being able to deliver 0.4mm flatness and 0.1mm roundness at the maximum diameter (8.5m), with a surface finish of 3.2 microns Ra under all cutting, milling and turning operations.

“This equipment now has the potential for similar applications across other industries where large scale in situ flange facing to high tolerances is required,” said Furmanite European managing director Tony Nicholls. He added that substantial time and cost savings could be achieved “both by avoiding the need to remove and transport such large structures to be worked on offsite, and by minimising downtime while the work is carried out in situ.”

Following the completion of the work on Chooz B1, EdF worked together with Furmanite and the other suppliers on the second part of the contract for Chooz B2, which was successfully completed earlier this year.

Author Info:

Article supplied by Furmanite International Ltd - UK, Furman House, Shap Road, Kendal, Cumbria LA9 6RU, UK

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