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But according to independent nuclear engineer John Large, the new cracks signal the “death knell” for Hunterston reactor three. “This means that reactor four is doomed to the same fate, followed by similar plants at Hinkley Point and Hartlepool, thereafter progressively followed by other advanced gas-cooled reactors,” he said.

EDF announced last week that it was extending the shutdown of reactor three at Hunterston by six months because it had discovered more cracks than expected in its graphite core. Instead of reopening early in May, it is now scheduled to reopen on November 17.

The company said it had found a total of 39 “keyway root cracks” in the reactor and they were “happening at a slightly higher rate than modelled”. The discovery of new cracks was first revealed by the Sunday Herald on April 22.

The integrity of the thousands of graphite blocks that make up the reactor core is vital to nuclear safety. They ensure that the reactor can be cooled and safely shut down in an emergency.

But bombardment by intense radiation over decades causes the blocks to start cracking. If they fail, experts say, nuclear fuel could overheat, melt down and leak radioactivity in a major accident.

Large argued that EDF’s decision to keep reactor three closed until the end of the year was prompted by the UK Government’s safety watchdog, the Office for Nuclear Regulation (ONR). “ONR’s doubts about the reactor safety have not been satisfied by this most recent inspection,” he said.

Hunterston reactor three, which started generating power in 1976, is the oldest in EDF’s fleet. The other working reactor on the site, number four, is 18 months behind, has five cracks and is likely to develop more.

The six other advanced gas-cooled reactor plants in the UK are also likely to crack, including the only other working nuclear power station in Scotland at Torness in East Lothian. The others are: Hinkley Point B in Somerset; Hartlepool in County Durham; Heysham 1 and 2 near Lancaster; and Dungeness B in Kent.

Large also highlighted the uncertainties in tracking cracks, which are mostly modelled rather than measured. “There is little that EDF can do to physically resolve this problem,” he said.

Meanwhile, Edinburgh-based nuclear critic and consultant Pete Roche pointed out that Hunterston is now 42 years old.

“This must surely be the end for reactor three,” he said. “We are gambling with public safety by extending the lives of old reactors.”

He expects Hinkley Point B to close “very soon”, followed by other nuclear stations in England. “Even Torness has passed the 30-year threshold, so may not make it to its expected 2030 closure date,” Roche said.

Paul Mummery, a professor of nuclear materials from the University of Manchester, pointed out that the ONR would not allow EDF to restart reactors until it can be demonstrated that they are safe. “This is quite a task for EDF and not something that can be rushed,” he said.

“EDF is undertaking extensive modelling and experimental programmes to demonstrate the safety of the reactors but it will take time to generate and interpret the results. Time is against them as the reactors will continue to age during service.”

The ONR has welcomed EDF’s decision to keep Hunterston reactor three closed as “responsible, conservative, and made in the best interest of public safety”. It confirmed that the reactor could not be restarted without its permission.

One ONR advisor, professor Paul Bowen from the University of Birmingham, argued that the process showed how the regulatory system was working.

“In my technical opinion and noting that I am not at all influential in any decisions, a return to service for Hunterston reactor three will be justified,” he said.

The director of Hunterston B, Colin Weir, told BBC Radio Scotland last week that he was “100 per cent confident” that the reactor would restart.

A Scottish Government spokesperson said: “We expect ONR will exercise its duties diligently to ensure the nuclear industry controls its hazards effectively and maintains the highest nuclear safety and security standards.”