01 Overview
Inspections at the Hunterston B Advanced Gas-cooled Reactor (AGR) revealed the graphite moderator bricks forming the reactor core had cracked far more extensively than safety models predicted. By 2021, inspectors had counted over 2,000 cracks across the bricks of both reactors, significantly above the limit regulators had set for continued operation.
02 Cause
Graphite moderator bricks in AGR reactors crack over time due to radiation damage and oxidation. The rate of cracking at Hunterston B exceeded predictions because the bricks were older and more irradiated than safety models had accounted for. Extended operation beyond original design life accelerated the degradation.
03 Impact
The cracked bricks raised concerns that channel geometry could distort in an accident, potentially jamming control rods and preventing reactor shutdown. Both reactors were taken offline for extended safety assessments. The economic and regulatory burden of continued operation was deemed too high.
04 Response
EDF Energy worked with the Office for Nuclear Regulation (ONR) to conduct extensive inspections. Regulators required detailed safety cases demonstrating bricks would not impede control rod insertion before permitting any restart. Short test operations were permitted, but the plant spent most of 2020โ2022 offline.
05 Legacy
Hunterston B was permanently closed in January 2022. The incident highlighted the challenges of operating nuclear plants beyond their original design life. Several other UK AGR plants face similar graphite cracking issues, raising broader questions about Britain's existing nuclear fleet.