Workers at Hunterston B (Credit: EDF Energy)The UK Office for Nuclear Regulation (ONR) on 24 September provided EDF Energy Nuclear Generation Ltd with permission for Reactor 4 at Hunterston B power station to return to service for a limited period of operation.

The permission is for operation up to a total of 16.25 terawatt days, or approximately six months operation. It follows ONR’s assessment on whether cracking observed in the graphite bricks that form the reactor core could compromise its fundamental nuclear safety requirements.

ONR said it satisfied that Reactor 4 is able to operate safely and can be safely shutdown (including in a significant seismic event) as required.

Donald Urquhart, ONR deputy chief Inspector, said: “In reaching this decision, ONR’s team of independent expert inspectors are committed to ensuring the safety of site workers, local residents and the wider public. We have scrutinised the safety justification and supporting evidence for this next period of operation in great detail, and I am satisfied that the high standards of nuclear safety that we expect have been demonstrated. We will only allow nuclear facilities to operate if we are satisfied that they are safe to do so.”

According to ONR, the fundamental nuclear safety requirements of the graphite core of an advanced gas cooled reactor are to:

  • Allow unimpeded movement of control rods and fuel.
  • Direct gas flows to ensure adequate cooling of the fuel and core.
  • Provide neutron moderation and thermal inertia.

ONR said it has long been understood that irradiation of the fuel channel graphite bricks will eventually lead to shrinkage and cracking of these bricks late in reactor lifetime; a phenomenon know as keyway root cracking. This has the potential to challenge the nuclear safety requirements and therefore it needs to be demonstrated that these fundamental requirements continue to be met in normal operation, fault conditions and after a design basis seismic event.

Keyway root cracking was first observed in Hunterston B Reactor 4 in August 2014, although this was in one of a small number of bricks with a high shrinkage, known to be more susceptible to cracking. The first observation in the main population of graphite fuel bricks was at Hunterston B Reactor 3 in October 2015, and then in September 2017 in Reactor 4. In order to monitor the core condition and the number of cracks, the reactor cores have been regularly inspected.

In August, EDF Energy announced that it will begin decommissioning Hunterston B by January 2022 at the latest. The plant, which began operation in 1976, has two AGR reactors.

On 27 August 2020, Reactor 3 at Hunterston B was granted permission to return to service for a run of approximately six months.

Hunterston B station director, Paul Forrest welcomed ONR's decision to approve restart of Hunterston B's second reactor "Having both reactors back online will mean that Hunterston B can continue the important job of making low carbon electricity and help reduce reliance on gas-powered generation until the site moves into defuelling by January 2022."

Photo: Workers at Hunterston B (Credit: EDF Energy)