Electricity generation from the Hunterston B nuclear power station could fall by 40% this year after cracks were discovered in one of the reactors.
Colin Weir, station director at the North Ayrshire power plant, said the reduction in output came after the "right and appropriate" decision was taken to put one of the reactors offline after a "slight increase" in the number of defects.
A planned inspection of the graphite bricks that make up the core of Reactor 3 in March uncovered new "keyway root cracks".
These were found to be developing at a slightly higher rate than anticipated, operator EDF Energy revealed.
The reactor had been expected to restart a few weeks after it was taken offline, but that has now been delayed.
Bosses at the site, which opened in 1976, now expect the reactor to return to service before the end of the year.
Mr Weir told BBC Radio Scotland's Good Morning Scotland programme 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% 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, which is scheduled to remain in operation until 2023, is still safe.
"Our safety margins are way beyond where we are just now," Mr Weir said.
"This is a far distance from any safety margins."
Why are you making commenting on The Herald only available to subscribers?
It should have been a safe space for informed debate, somewhere for readers to discuss issues around the biggest stories of the day, but all too often the below the line comments on most websites have become bogged down by off-topic discussions and abuse.
heraldscotland.com is tackling this problem by allowing only subscribers to comment.
We are doing this to improve the experience for our loyal readers and we believe it will reduce the ability of trolls and troublemakers, who occasionally find their way onto our site, to abuse our journalists and readers. We also hope it will help the comments section fulfil its promise as a part of Scotland's conversation with itself.
We are lucky at The Herald. We are read by an informed, educated readership who can add their knowledge and insights to our stories.
That is invaluable.
We are making the subscriber-only change to support our valued readers, who tell us they don't want the site cluttered up with irrelevant comments, untruths and abuse.
In the past, the journalist’s job was to collect and distribute information to the audience. Technology means that readers can shape a discussion. We look forward to hearing from you on heraldscotland.com
Comments & Moderation
Readers’ comments: You are personally liable for the content of any comments you upload to this website, so please act responsibly. We do not pre-moderate or monitor readers’ comments appearing on our websites, but we do post-moderate in response to complaints we receive or otherwise when a potential problem comes to our attention. You can make a complaint by using the ‘report this post’ link . We may then apply our discretion under the user terms to amend or delete comments.
Post moderation is undertaken full-time 9am-6pm on weekdays, and on a part-time basis outwith those hours.
Read the rules here