WNISR, 7 January 2022
Hunterston B-2, a 495 MWe Advanced Gas-cooled Reactor (AGR) was shut down for the last time around noon on 7 January 2022. The reactor, located on the Ayrshire coast in the west of Scotland and owned by Électricité de France (EDF) subsidiary EDF Energy, was connected to the grid on 31 March 1977, four years later than planned. Following the closure of Hunterston B-1 on 26 November 2021, it brings to an end nearly 58 years of commercial nuclear operations at the site, and leaves the United Kingdom (UK) with 11 operating reactors and two under construction (at Hinkley Point C).
Hunterston B-1 and B-2 Now Both Closed -
Photocredit: Jonathon Champton at English Wikipedia, CC BY-SA 3.0
The primary reason for the closure of both reactors, earlier than the 2023-2025 timeframe EDF had planned for, are major safety issues around the 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) specifications, the reactor core bricks are permitted to loose no more than a set percentage of their weight before they are considered having reached the end of their operational lifetime. As the bricks line the reactor’s core, they cannot be replaced, which means once the bricks have lost a certain percentage of their weight this should signal the end of the power station’s operation.
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. However, the Hunterston B reactors were permitted to continue operation and also had their operational lifetimes extended.
More than a decade later, in January 2017, it was disclosed that there were now thousands of cracks in the graphite bricks at Hunterston. EDF sought regulatory approval to increase permitted cracks from 10 to 20 percent of the bricks. The ONR in 2017 permitted both Hunterston B reactors to continue operating. In October 2018, Hunterston B-2 was shut down when graphite core inspection confirmed more rapid crack degradation. The ONR granted approval to operate for a limited period between August–December 2019. EDF decided to operate the reactor for two further periods of six months, one in 2020 and one in 2021. The ONR, on 13 April 2021, approved the last time operation for up to the equivalent of approximately six months of generation, which ended 7 January 2022.
As predicted by John Large 15 years ago, the risks from cracks extends across EDF’s remaining UK AGR fleet. In November 2020, EDF confirmed that due to cracks in the twin reactor units at Hinkley Point B in the southwest of England they would be closed no later than July 2022. The utility’s twin Torness AGR’s in the east of Scotland, are predicted to have cracks in 2022, and closure has already been brought forward from 2030 to 2028, but may be unavoidable as early as 2024/25. EDF’s four AGR’s at Heysham A1 and Hartlepool are planned for closure by March 2024.