Cracks found in the graphite bricks of a nuclear reactor at a power station in Scotland.

As a result the unit at Hunterston B in North Ayrshire has been shut for 12 months.

Although “keyway root cracks” were expected to appear in the graphite bricks that make up the core of reactor three, EDF Energy said they had been growing faster than expected.

They have now grown to an average of 2mm wide, the firm said.

After the problem was discovered last march during a planned inspection, the reactor was shut down.

In pictures: Hunterston B nuclear power station will operate until 2023 Show all 7 1 /7 In pictures: Hunterston B nuclear power station will operate until 2023 In pictures: Hunterston B nuclear power station will operate until 2023 power-st-2.jpg A worker at Hunterston B power station monitors the control room Getty Images In pictures: Hunterston B nuclear power station will operate until 2023 power-st-1.jpg Workers at Hunterston B power station work at the charge machine Getty Images In pictures: Hunterston B nuclear power station will operate until 2023 power-st-6.jpg Hunterston B power station is operated by EDF Energy, employing over 700 hundred people it generates enough electricity to power almost half the homes in Scotland Getty Images In pictures: Hunterston B nuclear power station will operate until 2023 power-st-3.jpg A worker at Hunterston B power station walks over the reactor 3 pile- cap Getty Images In pictures: Hunterston B nuclear power station will operate until 2023 power-st-4.jpg Workers at Hunterston B power station attend to some maintenance in the gas circulation Getty Images In pictures: Hunterston B nuclear power station will operate until 2023 power-st-5.jpg A worker at Hunterston B power station looks over the cartridge cooling pond Getty Images In pictures: Hunterston B nuclear power station will operate until 2023 power-st-7.jpg The plant, near Largs, was due to be decommissioned in 2016 but following a technical and economic evaluation last year it will now operate until 2023 Getty Images

EDF has now released footage of the cracks, which was taken in 2017 and 2018.

“Nuclear safety is our overriding priority and reactor three has been off for the year so that we can do further inspections," the plant’s director Colin Weir told BBC Scotland. “We’ve carried out one of our biggest ever inspection campaigns on reactor three, we’ve renewed our modelling, we’ve done experiments and tests and we've analysed all the data from this to produce our safety case that we will submit to the Office for Nuclear Regulation.

Exterior of Hunterston B power plan in Ayrshire, Scotland. (EDF)

“We have to demonstrate that the reactor will always shut down and that it will shut down in an extreme seismic event.”

About 370 hairline fractures have been discovered, which means there are cracks in about one in every 10 bricks in the reactor core.

The operational limit for the latest period of operation was 350 cracks but EDF plans to ask the regulator for a new operational limit of up to 700 cracks.

Hunterston B is expected to continue producing electricity until 2023 – but it could be forced into decommissioning before then because of the cracks.