Colin Weir, station director at Hunterston B in North Ayrshire, said the reduction in output came after the “right and appropriate” decision was taken to take one of the reactors offline after a “slight increase” in the number of defects.
A planned inspection of the graphite bricks at the core of reactor 3 in March found new “keyway root cracks”.
These were found to be developing at a slightly higher rate than anticipated, operator EDF Energy said. The reactor had been expected to restart a few weeks after it was taken offline, but that has been delayed.
It comes as anti-nuclear campaigners warned the problem represented “new territory” for the industry, given the materials involved, while Ross Greer, the Scottish Greens MSP, said the local community in Ayrshire should have a say in Hunterston B’s future.
Officials at the site, which opened in 1976, now expect the reactor to return to service before the end of the year.
Mr Weir told the BBC that “because of the slight increase” in cracks found in reactor 3, “it is right for us to take the time to review these findings and do some more assessments and potentially do some more inspections of the core”. He added: “Obviously this year we will be reduced in output – it will be around a 40 per cent reduction in our planned output for this year –taking this decision, the right and appropriate safe decision to have the unit off while we do this assessment.”
But he insisted that the plant, scheduled to remain in operation until 2023, is still safe.
“Our safety margins are way beyond where we are just now,” he added. “This is a far distance from any safety margins.”
Peter Roach, editor of the No to Nuclear Power website, said: “I have my doubts over whether the reactor will ever reopen. It’s not up EDF, it’s up to the Office for Nuclear Regulation, and they can’t inspect all the graphite bricks.
“It’s a gamble to keep things going as they are. We’re in new territory here, as we’ve never had graphite reactors dissolve before.
“Economics will also come into it. If control room staff are being paid to oversee the one reactor, at one point does it become unsustainable to keep it going? Maybe they should cut their losses.”
Mr Greer said: “This is obviously of major safety and economic concern to the local community.”