The government’s energy policy is under renewed pressure after the prolonged closure of one of Britain’s oldest nuclear reactors because of cracks in its graphite core raised questions over the future of six other plants built in the 1970s and 1980s.

The temporary shutdown of reactor three at Hunterston B in Scotland is also expected to burn an estimated £120m hole in the revenues of its owner, EDF Energy. The firm said this week that it was taking the reactor offline for six months after inspections revealed more cracks than expected.

The graphite core is used to moderate the neutrons in a nuclear reaction, but over time the irradiation degrades the graphite, ultimately leading to cracks. These cause a series of knock-on effects that can impair control of the nuclear reaction.