In October 2014 it was reported that cracks had been found in one of the reactors at the plant following routine inspections which began in August 2014.
Two of about 3,000 graphite bricks in the core of reactor four at Hunterston were affected.
The plant's operator, EDF Energy, said the cracking was predicted to occur as the station ages and said that the issue would not affect the safe operation of the reactor.

In October 2016, it was announced that super-articulated control rods would be installed in the reactor because of concerns about the stability of the reactors' graphite cores.
The Office for Nuclear Regulation (ONR) had raised concerns over the number of fractures in keyways that lock together the graphite bricks in the core.
 An unusual event, such as an earthquake, might destabilise the graphite so that ordinary control rods that shut the reactor down could not be inserted.
Super-articulated control rods should be insertable even into a destabilised core.

In early 2018, a higher rate of new keyway root cracks than modelled was observed in Reactor 3 during a scheduled outage, and EDF announced in May 2018 "While Hunterston B Reactor 3 could return to operation from the current outage, it will remain offline while the company works with the regulator to ensure that the longer term safety case reflects the findings of the recent inspections and includes the results obtained from other analysis and modelling."

In December 2018, EDF pushed back their estimated return to service date to March 2019 for Reactor 4 and April 2019 for Reactor 3, to allow for further modelling work and a new seismic analysis.
In March 2019 pictures of the cracking were released with EDF stating that it intended to seek permission from the ONR to restart reactor 3 by raising the operational limit for the number of cracks.
About 370 fractures have been discovered, on average 2mm wide, in about 10% of the graphite bricks in the reactor core.
This was above the operational limit of 350 fractures, and EDF intends to present a new safety case for an operational limit of 700 cracks.

One reactor was restarted on 25 August 2019 then shut down again on 10 December 2019.
In August 2020, EDF received regulatory approval to restart the two reactors, in August and September 2020 respectively, before moving to defuel and decommission the plant starting no later than 7 January 2022.
In October 2014 it was reported that cracks had been found in one of the reactors at the plant following routine inspections which began in August 2014. Two of about 3,000 graphite bricks in the core of reactor four at Hunterston were affected. The plant's operator, EDF Energy, said the cracking was predicted to occur as the station ages and said that the issue would not affect the safe operation of the reactor.
In October 2016, it was announced that super-articulated control rods would be installed in the reactor because of concerns about the stability of the reactors' graphite cores. The Office for Nuclear Regulation (ONR) had raised concerns over the number of fractures in keyways that lock together the graphite bricks in the core.  An unusual event, such as an earthquake, might destabilise the graphite so that ordinary control rods that shut the reactor down could not be inserted. Super-articulated control rods should be insertable even into a destabilised core.
In early 2018, a higher rate of new keyway root cracks than modelled was observed in Reactor 3 during a scheduled outage, and EDF announced in May 2018 "While Hunterston B Reactor 3 could return to operation from the current outage, it will remain offline while the company works with the regulator to ensure that the longer term safety case reflects the findings of the recent inspections and includes the results obtained from other analysis and modelling."
In December 2018, EDF pushed back their estimated return to service date to March 2019 for Reactor 4 and April 2019 for Reactor 3, to allow for further modelling work and a new seismic analysis. In March 2019 pictures of the cracking were released with EDF stating that it intended to seek permission from the ONR to restart reactor 3 by raising the operational limit for the number of cracks. About 370 fractures have been discovered, on average 2mm wide, in about 10% of the graphite bricks in the reactor core. This was above the operational limit of 350 fractures, and EDF intends to present a new safety case for an operational limit of 700 cracks.
One reactor was restarted on 25 August 2019 then shut down again on 10 December 2019. In August 2020, EDF received regulatory approval to restart the two reactors, in August and September 2020 respectively, before moving to defuel and decommission the plant starting no later than 7 January 2022.
Hunterston B nuclear power station