EDF Energy said on 8 March it had extended outages at the two-unit Dungeness B nuclear plant in the UK by about six months to complete inspections on steam pipes and corrosion repairs.

Units B-21 and B-22, both 535MWe advanced gas-cooled reactors (AGRs), have been offline for planned outages since late summer 2018 after an inspection revealed that several systems providing a safety function were  “corroded to an unacceptable condition”.  The corrosion has been rated level 2 on the International Nuclear and Radiological Events Scale (INES).  

The two units at Dungeness B began operation in 1983 and 1985 and have been upgraded for continued operation until 2018. Unit B-21, which was closed for planned maintenance in September and was scheduled to restart in April, will now remain closed until 30 September 2019. Unit B-22 closed in August and was also scheduled to restart in April, but will now remain offline until 15 October.

The two-unit Hunterston B plant in Scotland has also been offline since last year due to cracks developing faster than expected in the graphite bricks in the reactor cores. On 8 March EDF Energy released video footage of inspections showing the cracks as a prelude to seeking regulatory approval to restart the units.

The two AGR units were taken offline after significant cracks were discovered in March and October 2018. In November 2015, EDF Energy had said cracks found in three of the graphite bricks in reactor 3 and two bricks in reactor 4 had no safety implications. However, in May 2018 EDF admitted the cracks were expanding more quickly than anticipated and in December extended the outages by several weeks, for further work. EDF Energy said reactor 3 is now expected to return to service on 30 April and reactor 4 on 31 March.   

The two units at Hunterston B started operating in 1976 and 1977. They were originally scheduled to shut in 2011, but their operating life was extended to 2016. After a technical and economic evaluation of the plant, EDF later confirmed it could operate until 2023. The graphite core of each AGR reactor includes around 6000 graphite bricks, 3000 of which are the graphite bricks containing fuel channels. The core structure is designed to contain many redundant bricks meaning a "very large number of bricks" would have to crack before there were any significant safety concerns, EDF Energy said in 2015.

In a letter to site stakeholder groups dated 5 March, EDF Energy’s acting station director for Hunterston, Roddy Angus, said the videos are representative examples of the kind of “keyway root cracking” that has been discovered during inspections. “Cracks are around 1mm when first observed, and they are opening very slowly. The average crack size is currently approx. 2mm and we have demonstrated in our safety cases that cracks up to a width of 10mm on the inside of the fuel channel (i.e. the bore) and 18mm on the outside of the fuel channel are acceptable.”

Photo: Hunterston B (Photo: EDF Energy)