The Office for Nuclear Regulation (ONR) will allow the 40-year-old generator in North Ayrshire to run for four months, 17 months after it was shut down to investigate 350 hairline cracks in its graphite bricks.
The reactor is expected to be re-connected to the grid at the end of this month. EDF is seeking permission to reopen the second reactor, while two are being decommissioned.
Donald Urquhart, the ONR deputy chief inspector, said there had been a “long and detailed assessment of the safety case.”
He said: “Nuclear safety remains our utmost priority and we would only allow a reactor to restart with clear evidence that it remains safe for workers and the public.
“Our assessment included extensive scrutiny of the underpinning evidence provided by the licensee and concluded that an adequate safety case has been provided to allow a further period of operation.”
The energy company said it invested £125 million and over “1,000 person years” into research to demonstrate that “even in the most extreme conditions our reactors operate within large safety margins”.
It said: “Since the decision was made to take both reactors offline last year, we have completed the most extensive investigation of the reactor core that has ever been undertaken.”
The reactor has been subject to “extensive scrutiny and testing” by its own internal Independent Nuclear Assurance, the Nuclear Safety Committee – a body of independent industry experts – and by the ONR.
“This has provided us with valuable knowledge about condition of the graphite core. As part of this programme we have worked with the University of Bristol to confirm our understanding of how the core would react in the event of a major earthquake, of a magnitude never experienced in the UK.
“Along with Bristol University, we have been working with leading consultancies and expert academics at other universities across the UK including Strathclyde, Glasgow, Manchester, Oxford, Sussex, Nottingham and Durham as well as with leading UK companies such as Fraser-Nash, Wood and SNC-Lavalin.”
EDF, owned by the French government, operates all of Britain’s nuclear plants which provide a fifth of the country’s electricity. It hopes to build new nuclear plants at the Hinkley Point C and Sizewell B nuclear sites.
It is extending the operation of its Heysham 1 and Hartlepool nuclear plants, which were due to close this year but will now run until 2024. Heysham 2 and Torness nuclear plants will remain in operation until 2030.
EDF Energy hopes to run the Hunterston nuclear plant until 2023.
