Hunterston B-1, a 490 MWe Advanced Gas-cooled Reactor (AGR), located on the Ayrshire coast in the west of Scotland, shut down for the last time on 26 November 2021. The 490 MWe reactor, owned by EDF Energy, was connected to the grid on 6 February 1976. The remaining reactor at the site, the 44-year-old Hunterston B-2, is scheduled to be closed in early January 2022, ending nearly 58 years of commercial nuclear operations at the site. The closures will reduce availability for electricity exports to France, struggling with multiple planned and forced reactor outages.
The primary reason for the closure of both reactors and the earlier than 2023-2025 timeframe EdF had planned for are major safety issues around cracks in the graphite bricks that make up the AGR cores. As a result of the effects of neutron bombardment the graphite bricks gradually lose weight and, under UK Office of Nuclear Regulation (ONR), the reactor core bricks are permitted to lose no more than a set percentage of their weight before they are classed as having reached the end of their lifetime. As the bricks line the reactor’s core, they cannot be replaced.
The late John Large, a well-known independent nuclear engineer, first assessed the cracking in AGR cores in 2006, and concluded that “…significant uncertainties over the structural integrity and residual strength of the moderator cores in …AGR plants … in view of the increased risk presented by continued operation of these nuclear plants, the reactors should be immediately shut down and remain so until a robust nuclear safety case free of such uncertainties has been established.” The entire AGR fleet of reactors was considered at risk of experiencing significant cracking.
The Hunterston B reactors were permitted to continue operation and also had their operational lives extended.
More than a decade later, in January 2017, it was disclosed that there were now thousands of cracks in the graphite bricks being confirmed, EDF Energy was seeking regulatory approval to increase permitted cracks from 10 percent to 20 percent of the bricks.
In 2017, John Large warned that “the integrity of the graphite bricks was vital to nuclear safety. If they failed, they could block the channels that enable control rods to be inserted to close down reactors and prevent them from overheating. Ageing problems like this serious cracking of the graphite bricks at the heart of each reactor are deeply worrying, so much so that these nuclear plants should now be permanently shut down.” However, the ONR in 2017 permitted both Hunterston B reactors to continue operating.
After shutting down again in March 2018, when graphite core inspection confirmed that more rapid crack degradation was discovered, the reactors remained offline until 2020. ONR on 27 August 2020 issued approval for Unit 1 to operate for six months. Unit 1 was granted one more operational period from April to November 2021.
As predicted 15 years ago, the risks from cracks extends across EDF Energy’s remaining AGR fleet. In November 2020, EDF Energy confirmed that due to cracks in the twin reactor units at Hinkley B in the southwest of England they would be closed no later than July 2022. The utility’s twin Torness AGRs in the east of Scotland, are predicted to develop cracks by 2022, and closure has already been brought forward from 2030 to 2028, but may be implemented as early as 2024/25. EDF Energy’s four AGRs at Heysham A 1 and Hartlepool are planned for closure by March 2024.