Major safety issues around cracks in the graphite bricks that make up the AGR cores

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. 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.