EDF Energy has again extended an outage at its Hunterston B8 nuclear reactor in Scotland for a further two weeks while the Office for Nuclear Regulation (ONR) assesses whether it is safe for it to restart.

The Hunterston B plant is more than 40 years old. Its two reactors have both been offline since last year after cracks were found in the graphite core during routine inspections. Hunterston B8, which has been offline since last October, had been expected to return to on 30 April. That outage has now been extended to 14 May.

The safety case for ONR must demonstrate that the reactor would be able to operate and shut down safely during normal operation and an unlikely earthquake scenario. Under energy market rules the company must indicate when the reactor is likely to return to service. However, EDF Energy said in a letter to stakeholders: "It is not possible for us to confirm with certainty how long this process will take; our top priority is that it’s completed with appropriate rigour."