CRACKS in one of Scotland’s nuclear reactors may prevent it shutting down in an emergency, according to a report by the industry watchdog.

Papers obtained by the BBC under freedom of information show that the Office for Nuclear Regulation (ONR) raised concerns about fractures at the core of Hunterston B power station in Ayrshire, and potentially similar problems affecting Hinkley B in Somerset.

The ONR documents referred to cracks in reactor three of Hunterston B, built in the 1970s.

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John Large, who helped design Britain’s early nuclear reactors, believes that if the cracks get any worse it could jeopardise the reactor’s stability in the event of a disaster and make it impossible to lower control rods to shut the reactor down.

He described how the structure would become “a very loose stack of bricks”.

He said: “These keyways are beginning to fracture... that means the locking together – the way that force can be transferred from one brick to another – is lost, so it becomes a very loose stack of bricks.”

Allan Jeffery, of campaign group Stop Hinkley, said he was concerned that the graphite core – which cannot be repaired – has become less dense because of the effects of radiation.

He said: “This... could end up distorting the channels the fuel and the boron control rods use.

“In cases of emergency there are sudden changes in temperature and pressure which could all end up starting to deform these channels.

“If you can’t get the control rod down then you can’t control the temperature inside the reactor and you’re heading for accidents – possibly even meltdowns.”

However, the regulator has agreed the stations can continue to operate after the reactor shutdown process was modified.

EDF Energy’s Brian Cowell said the level of cracking is “reasonable” and “far below anything which would affect the reactor’s safe operation”.

He added it was accepted that cracks will occur in some of the bricks “as part of the normal ageing process”.

Hunterston B was originally scheduled to begin decommissioning in 2016, but had its lifespan extended to 2023.

Mr Cowell said: “Observations from our comprehensive inspection programme were anticipated and are in line with our understanding, so our view of the best estimate lifetime planning date of 2023 for Hinkley Point B and Hunterston B has not changed.”