Both 570MWe Wylfa reactors have been restarted after a 15-month outage. The outage lowered BNFL’s annual capacity by about 40% and was cited as a major contributor to BNFL’s loss of £210 million for the year ended 31 March 2001 (see NEI August 2001, p12).
The routine maintenance outage of unit 2 in April 2000 provided the first opportunity to use new ultrasound inspection equipment capable of accessing the lower part of steam pipe welds. Previously these welds had been assumed to be perfect, but the ultrasound images revealed tiny defects that had been present since manufacture.
The discovery meant that the other reactor had to be shut down, while BNFL had to prove to the Nuclear Installations Inspectorate that the strength of the welds had not been compromised by the defects. Several months of analysis followed, which included carrying out tests on a replica weld. Having proved the integrity of the welds, BNFL were required to design and fit restraints on all 64 steam pipes to hold them in place, in case a weld would fail.
Although the work cost BNFL several millions of pounds, the station produces over £100 million per year worth of electricity when in full operation. The 31-year-old plant has a design lifetime of 50 years.