New cracks have been found in one of the reactors at Hunterston B nuclear power station in North Ayrshire.
Two of about 3,000 graphite bricks in the core of reactor four are affected.
Plant operator, EDF Energy, said the cracking was predicted to occur as the station aged and it would not affect the safe operation of the reactor.
Deputy First Minister Nicola Sturgeon said the issue was “hugely concerning” to the public and that the Scottish government was seeking reassurances.
“The Scottish government is seeking the information we need to assure the public about safety, because safety is absolutely paramount and cannot be compromised,” she said.
A Scottish government spokesman added: “The Office for Nuclear Regulation have provided an assurance that there are no immediate safety implications affecting Hunterston B, and that it is safe to continue generating electricity.”
The spokesman said the government remained “opposed to the building of new nuclear capacity in Scotland” but the lifetime of existing nuclear plants could be extended to help maintain supply “subject to strict safety considerations” being in place.
Colin Weir, station director at Hunterston B, said: “Every time we take the reactor out of service for planned maintenance we inspect the graphite core, which is made up of around 6,000 bricks.
“During the current Hunterston outage we found two bricks with a new crack, which is what we predicted during Hunterston B’s lifetime as a result of extensive research and modelling.
“It will not affect the operation of this reactor and we also expect that a few additional cracks will occur during the next period of operation.”
Mr Cowell added: “They are safe.”
The reactor returned to service on Sunday evening, after the energy firm received approval from the Office for Nuclear Regulation.
It is, however, not supplying electricity to the grid as the start-up process takes time.
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