EDF Energy has extended outages at its Hunterston nuclear plant in Scotland by several weeks, for further work at two reactors which were taken offline after cracks were discovered in March and October.

EDF Energy said on its website on 17 December that the Hunterston B  unit 3 is now expected to return to service on 30 April and unit 4 on 31 March. The cracks were found during routine inspections at the plant, which is more than 40 years old. The two 610MWe advanced gas reactors began operation in 1976.

EDF Energy announced in May that new cracks were being investigated in the graphite core of the reactors. Whilst cracks are expected as part of the ageing process of the graphite bricks, they have appeared more quickly than anticipated.

EDF noted that since May, additional inspections of the graphite core
have taken place, and that 100 keyway root cracks have been identified in reactor 3, following inspection of just over a quarter of the reactor.  Using modelling the number of cracks across the whole reactor is estimated at around 370. This takes the core over the operational limit of 350 contained in the existing safety case for that period of operation; although EDF Energy added that it always sets "conservative operational limits which are well below what would be considered safe."

Inspections at Hunterston reactor 4 were carried out on more than a tenth of the core, and around 30 keyway root cracks were observed. Modelling estimates the number of cracks in the core at around 200, which is within the operational limits of the existing safety case, EDF said.

EDF Energy submitted a safety case to the Office for Nuclear Regulation (ONR) for reactor 4 in early November and is preparing one for reactor 3. The safety cases will be assessed by the regulator to determine whether the reactors are safe to return to service. Neither reactor can restart without ONR's consent, which will be given only if it is safe to do so.

"Our current view remains that the best lifetime judgement for Hunterston B Reactor 3 (2023) continues to be supported, given the large nuclear safety margins," EDF noted.

Photo: Hunterston B (Credit: EDF Energy)