Technical problems have compelled the temporary shut down of four reactors in the UK, leaving a third of the country's nuclear capacity offline.

EDF Energy has announced that four reactors at its Heysham 1 and Hortlepool nuclear plants in the north of England will be temporarily shut down after inspectors discovered a problem with a boiler unit.

A fault in the boiler pump of one of the reactors was uncovered by chance during routine inspections earlier this year, compelling EDF Energy to suspend the operation of its Heysham 1 plant in Lancashire in January.

EDF Energy has since decided to take the advanced precaution of shutting down other reactors fitted with similar boiler units and deploying over 300 staff to conduct safety tests for all of them.

“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 safety returned to service,” said EDF Energy in a statement.

The French-owned energy giant has spent the past several months conducting staff training programs for the specific purpose of undertaking the current round of inspections.

The reactors are expected to remain shut for roughly two months, and will be reopened once EDF has convinced the Office of for Nuclear Regulation that it has “an acceptable safety case.”

According to the ONR, EDF had uncovered “unexpected cracking” in one of the boiler spines, which is a forged metal tube bearing the weight of the entire boiler. The spine is a critical piece of equipment, and its failure could cause water to enter the reactor vessel.

In addition to the safety inspections, ONR further expects EDF to fit equipment on the boilers which will permit monitoring of the condition of their spines.

The suspensions of the two plants for inspection purposes will put a significant dent in British power generation, as their combined capacity of more than 2.3 gigawatts is the equivalent of around four per cent of the UK’s peak winter demand.

In tandem with the suspension of two other EDF plants in Scotland and Kent, the shut down of Heysham 1 and Hartlepool will leave a third of Britain’s nuclear capacity temporarily offline.