EDF Energy shut two reactors at Heysham 1 in June due to a malfunction at one of its eight boiler units. The reactor remains out of operation in order to determine the nature of the defect.

"Although routine inspections of other boiler spines have not previously indicated any similar defects, EDF Energy has taken the conservative decision to shut down Heysham 1 Reactor 2 and Hartlepool Reactors 1 and 2 that are of similar design over the next few days to carry out further inspections in order to satisfy itself and the regulator that the reactors can be safely returned to service," the company said in the statement, published on its website.

“Until the results of the further inspections are known it is not possible to advise exact return-to-service dates for these four reactors; however, an initial estimate is that these investigations will take around eight weeks," the company said, adding that Heysham 1 Reactor 1 "is likely to continue to operate on reduced load when it returns to service until a suitable repair strategy can be implemented."

In 2013, EDF signed an agreement with UK authorities to build the Hinkley Point nuclear power plant together with France’s Areva and the Chinese CGNPC and CNNC companies.