Cracks have been discovered in bricks which make up the core of one of two nuclear reactors at the Hunterston B power station in Ayrshire.
Operator EDF Energy said the cracks in three graphite bricks were found during planned maintenance on Reactor Three.
The firm insisted there were no safety implications and the finding had no impact on the operation of the reactor.
A similar issue – known as “keyway root cracking” was identified in Hunterston’s other reactor last year.
EDF Energy said it was publicising the latest findings “as part of its commitment to openness and transparency”.
Station director Colin Weir said: “Nuclear safety drives everything we do. This means we work within very large safety margins. This applies to graphite bricks too.
“The level of cracking which is considered reasonable is far below anything which would affect the reactor’s safe operation.
“It is accepted by our regulators and materials experts that cracks will occur in some of the bricks and that the core will lose some of its mass as part of the normal ageing process.”
Mr Weir added: “The observations were anticipated and are in line with our understanding, so our view of the best estimate lifetime planning date of 2023 has not changed.”
Hunterston B has two advanced gas-cooled reactors, similar to those found at nuclear plants around the UK.
The core of the reactors is made up of thousands of graphite bricks.
The station began operating in 1976 and its working life has already been extended to 2023 – well beyond its planned closure date.
WWF Scotland director Lang Banks said the issue with cracks in both reactors emphasised the need “to embrace the clean energy transition”.
He said: “Despite the assurances given by the nuclear industry, with cracks now found in both reactors it’s clear the problem is spreading and that we can expect this facility to become even more unreliable in the future.
“News of more cracks in the country’s ageing fleet of nuclear power stations underscores why we’re right to be taking steps to harness cleaner, safer forms of energy.”