The ONR said the permission is for operation up to a total of 16.25 terawatt days, or about six months operation.
The ONR’s assessment focussed on whether cracking observed in the graphite bricks that form the reactor core of Hunterston B-2 could compromise its fundamental nuclear safety requirements.
Those requirements are:
* Unimpeded insertion of control rods and unimpeded movement of fuel;
* Ensuring that gas flow will be maintained to ensure cooling of the fuel and core; and
* Ensuring that appropriate moderation (slowing neutrons to sustain the nuclear chain reaction) and thermal inertia (reducing the speed of temperature changes) are maintained.
ONR inspectors concluded that all issues had been addressed and an adequate safety justification for a limited period of operation had been provided by EDF.
“ONR is satisfied that [the reactor] is able to operate safely and can be safely shutdown (including in a significant seismic event) as required,” a statement said.
Hunterston B has two advanced gas-cooled reactor units. Hunterston B-1 is a 490-MW GCR that began commercial operation in February 1976 and Hunterston B-2 is a 495-MW GCR that began commercial operation in march 1977.
Together the two units, owned and operated by EDF subsidiary EDF Energy, generate enough electricity to power around 1.7 million homes.
EDF Energy had regulatory approval to operate the units until March 2023, but both were taken offline for inspections in 2018 after cracks were found in the graphite bricks that form the reactor core.
In August 2019 the ONR granted permission for Hunterston B-2 to be returned to service for four months. The ONR has now granted permission for a run of about six months.
In August, EDF was given permission to restart Hunterston B-1 “for a limited period of operation’, but said it will begin decommissioning the facility by January 2022 at the latest.
EDF Energy said in a statement it is planning for a further, final, run of six months for each of the two reactors. This will be subject to regulatory approval.