By Nina Chestney

LONDON (Reuters) – A reactor at EDF Energy’s Hunterston B nuclear power plant in Scotland will remain offline for additional safety checks after cracks were found in its core, Britain’s Office for Nuclear Regulation (ONR) said.

Ageing reactors generate just over 20 percent of Britain’s power but almost half of this capacity, including Hunterston, is due to go offline by 2025, prompting the government to plan new plants.

ONR was informed in March about keyway root cracks found during planned inspections of graphite bricks in the core of Reactor 3 at Hunterston.

Graphite bricks ensure reactors can be cooled and thousands of them are used in reactor cores.

“Inspections confirmed the expected presence of new keyway root cracks in the reactor core and also identified these happening at a slightly higher rate than modeled,” EDF Energy said in a statement.

The reactor has been offline since March and was due to come back online this month, but EDF Energy has extended the outage until later this year.

“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 modeling,” it said.

Hunterston B in North Ayrshire, Scotland, has been generating electricity since 1976. Last year, it produced enough electricity for 1.8 million homes.

CRACKS

In 2015, EDF Energy said routine inspections had revealed cracks in part of the graphite core at a Hunterston B nuclear reactor. It said three of 6,000 bricks had cracked, something that had been expected to begin happening at that point in the power station’s life.

Two of EDF Energy’s nuclear power plants in Britain – Heysham 1 and Hartlepool – were offline for months in 2014 for inspections after a crack was found on a boiler spine at Heysham 1.

In Belgium, the regulator ordered production to be stopped at two nuclear reactors in 2012 after finding indications of tiny cracks in core tanks.

The cracks turned out to be particles of hydrogen that were trapped inside the tanks when they were made by a Dutch company in the early 1980s.

EDF said it expects Hunterston B’s Reactor 3 to return to service “before the end of 2018”. EDF Energy’s outage website shows an expected return date of Oct. 4.

Its Reactor 3 and Reactor 4 are both Advanced Gas-cooled Reactors. The outage will reduce its 2018 output by 3 terawatt hours, the company said.

EDF Energy said the operation of its other UK reactors was not affected.

(Reporting by Nina Chestney; Editing by Jason Neely and Mark Potter)