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Exclusive: Dungeness nuclear power station quietly taken offline for five months over fears of Fukushima-style flood disaster

An internal report by operator EDF found that the shingle bank sea defences at the site in Kent were 'not as robust as previously thought'

The energy giant EDF has been accused of playing down the threat of flooding at Dungeness after it emerged that one of the nuclear power plant’s reactors was quietly shut down for five months last year after experts identified risk of a Fukushima-style disaster.

EDF closed the reactor on the Kent coast on 22 May to allow work on a new flood protection wall, after alerting the Office of Nuclear Regulation that without urgent work the site was at risk of being inundated by sea water.

The reactor – which should provide power for about 750,000 homes – did not reopen again until 15 October.

The closure of the 550-megawatt reactor – one of two at Dungeness – followed an internal EDF report which found that the shingle bank sea defences were “not as robust as previously thought”, raising fears that they could be overwhelmed in extreme weather, according to the ClickGreen website, which first reported the closure.

EDF marked the closure of the reactor with a short statement to local media saying: “Unit 22 at Dungeness station was taken offline on 20 May for maintenance work which includes completing improvements to flood defences for extreme events.” Five months later, the company said: “Unit 22 at Dungeness B power station resynchronised to the Grid at 0.522am on Tuesday 15 October.”

There was no clear explanation of the remarkable length of the outage, which was not widely reported.

Stephen Thomas, professor of energy policy at the University of Greenwich, said: “EDF should have made more of an announcement. If a plant closes for five months it is not just fiddling about, it is something serious and EDF can’t pretend it’s not.

“I think there is a bad attitude in this country that we must not frighten the horses. But playing it down is the wrong way – we need to be told the truth,” Professor Thomas added. He calculates that the five-month closure could have cost EDF around £100m in lost electricity revenue, while the group would have saved very little in the way of expenses, still having to pay wages and maintain the reactor.

Friends of the Earth climate campaigner Guy Shrubsole added: “It’s astounding that the shutdown of EDF’s reactor wasn’t better publicised and calls into question the transparency of the nuclear industry.”

The EDF review that led to the flooding work was carried out with the Environment Agency and meteorological experts based on new modelling in the wake of the Fukushima disaster, which involved assessments of extreme still-water levels, wave heights and historical tsunamis.

EDF notified the Office for Nuclear Regulation (ONR) in December 2012 that it “no longer had confidence” in its primary sea defence. It committed to boosting flood protection by “taking due cognisance of the need for a margin against more severe events”, ONR documents show.

This involved upgrading the defence from one that could protect against a one in 1,000-year weather event to one that occurs every 10,000 years.

According to a briefing to local community representatives by the station director, EDF originally boosted the shingle flood defences early last year. However, a further review of the site in May concluded that “the flood protection work already completed needed to be extended… to further enhance the plant resilience to this extreme hazard”, the station director said.

The defence consists of “a permanent flood protection wall around the site” and is expected to be finished by the end of this month.

Doug Parr, policy director at Greenpeace UK, said: “Whilst EDF are happy to publicise interviews with community radio stations on their website, it does not feel the need to tell the wider community about serious safety worries over flooding.”

Dungeness is one of eight operational nuclear power plants in the UK and the Government has plans to build 12 new reactors as it looks to switch to lower-carbon energy. But none of the proposed projects has been finalised.

The meltdown at the Fukushima plant in Japan in March 2011 occurred after a tsunami overwhelmed a 10m-high seawall, flooding the rooms which housed the emergency generators needed to ensure the cooling systems protected the reactors from meltdown.

An EDF spokesman said the decision to shut down the reactor was taken “to improve the flooding defence: not because the prior work was not sufficient or ‘bodged’”.

The spokesman said the closure was part of its “response to events in Japan which caused serious damage to Fukushima Daiichi [while] an extensive programme of analysis, modelling and physical testing has been carried out to review and update the assessments of potential flooding around our sites”.

He added: “We are continuously updating and improving the plant to ensure it is operating safely.”

Martin Pearson, station manager at Dungeness B, said: “The recent adverse weather has had no impact on existing infrastructure and the power station has operated normally in recent high tides and stormy weather.” A spokesperson for the Department for Energy and Climate Change declined to comment.