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The graphite cores of two ageing nuclear reactors at Hunterston in North Ayrshire have begun to crumble as cracks spread, prompting safety inspectors to impose tough new conditions threatening future operations.

Technical reports released by the UK government’s Office for Nuclear Regulation (ONR) reveal that at least 58 fragments and pieces of debris have broken off the graphite bricks that make up the reactor cores.

According to ONR there is “significant uncertainty” about the risks of debris blocking channels for cooling the reactor and causing fuel cladding to melt. This could cause an accident and a leak of radioactivity.

ONR warns that it will require “more robust arguments” before it agrees to allow the two reactors to restart in 2020. It also highlights concerns about the risk of “fuel snagging” from “multiply cracked bricks” and says that previous predictions have underestimated cracking.

Campaigners fearing a radioactive disaster that could contaminate Scotland’s central belt describe the revelations as “alarming” and “very worrying”. They are demanding that both reactors at Hunterston be decommissioned.

But Hunterston’s operator, the French power company EDF Energy, insists that graphite debris does not “pose a risk to nuclear safety”. ONR’s additional requirements are about “theoretical risks which are extremely unlikely to develop”, it says.

On 20 August 2019 ONR gave permission for reactor four at Hunterston B nuclear power station to restart, but only until December. The reactor had an estimated 209 cracks in its graphite core, and had been shut down since 2 October 2018.

The older reactor three at Hunterston has an estimated 377 core cracks and has been shut down since 9 March 2018. ONR is assessing the safety case determining whether it can be restarted.

ONR has accepted EDF’s argument that it’s safe to relax the limit for the permitted number of cracks. The “operational allowance” for cracks per reactor has been doubled from 350 to 700.

After permitting reactor four to restart for four months ONR posted five detailed technical reports online. One assessing “structural integrity” discloses for the first time that some of the graphite bricks in reactors three and four have begun to disintegrate.

Inspections of reactor three found that “crack opening of fuel bricks was causing other fuel bricks to crack”, the ONR report said. Inspections showed bricks with one or two “full-height” cracks and additional “partial-height” cracks.

ONR warned that there was “the potential for bricks to crack into three or more vertical parts” and become “multiply-cracked bricks” which could cause the uranium fuel to become snagged. The extent of the cracking exceeded EDF’s predictions, and meant it had to revise its safety case.

“Brick cracking was also found in some instances to be generating graphite debris, i.e. small pieces of graphite separating from the brick,” said ONR. EDF inspections of a quarter of reactor three and 10 per cent of reactor four had observed “seven pieces of debris and 51 fragments”.

In one channel of reactor three a triangular fragment 111 millimetres high and 34 millimetres wide “was observed to be missing”, ONR said. “Several pieces of debris were also observed at the bottom of the channel and it was assumed that these were a result of the triangular fragment breaking up.”

EDF Energy did not demonstrate that all barriers to a radiological release were preserved. Office for Nuclear Regulation

ONR concluded that the risk of the debris causing a “significant safety blockage” if reactor four was run for another four months was low. But it added: “Because there is significant uncertainty in determining the true likelihood, a precautionary approach should be taken.”

If reactor three was restarted or reactor four allowed to operate beyond December “it must be expected that debris will become more frequent”, ONR said. ONR inspectors have told EDF it must “provide more robust arguments for mitigating the risks posed by graphite debris, and for the determination of graphite debris production and its migration.”

The ageing of the graphite cores “will provide challenges to making robust safety cases”, the ONR report concluded. EDF will have to address the risks of proliferating cracks and increasing debris before it will be allowed to run the reactors in 2020.

ONR also accused EDF of getting its predictions wrong, and suggested it was safer to rely on the evidence of what had happened in reactor three. The company’s “past predictions of core degradation have been shown in the absence of leading data to be not wholly accurate when data became available,” ONR said.

In a separate report ONR also assessed the faults that could be caused by cracking and debris. These included restricting the movement of uranium fuel and control rods “potentially leading to greater core distortion”.

Another potential problem is disruption of the gas cooling needed to prevent the fuel from overheating. Debris could cause a “partial blockage” leading to “impaired fuel cooling”, ONR said.

According to ONR, EDF “did not demonstrate that all barriers to a radiological release were preserved”. But the company did provide enough evidence that the risk had been “reduced so far as is reasonably practicable”.

ONR concluded, though, that there was “significant uncertainty associated with the point at which fuel clad melt would occur following a partial blockage”. In future EDF would have to establish when melting would occur if it wanted to operate the reactor, ONR said.

These latest revelations show how irresponsible it was to bring reactor four back online this year. Ross Greer, Green MSP

The 50-strong group of Nuclear Free Local Authorities (NFLA) is urging ONR to close down both reactors at Hunterston. “These latest alarming revelations about the graphite reactor cores at Hunterston B starting to crumble and potential issues with the fuel make us even more convinced that reactor three should not be allowed to resume operation,” said NFLA Scotland convener and Glasgow SNP councillor, Feargal Dalton.

“We will be pressing the Office for Nuclear Regulation very hard to examine very carefully any justification which EDF Energy puts forward to reopen reactor four after its initial four month trial, and to be open and transparent about what they find. The precautionary principle would suggest that this reactor too should stay closed.”

NFLA radiation consultant, Dr Ian Fairlie, described ONR’s latest reports as “very worrying”. By considering the melting of fuel cladding the regulatory agency was “getting into even more dangerous matters than before”.

He added: “These reports and their harder language make one wonder why ONR granted a four month extension to reactor four in August. Taken together the new revelations strengthen the calls by local residents to close both reactors at Hunterston B.”

Scottish Green MSP, Ross Greer, argued that Hunterston wasn’t necessary for meeting Scotland’s electricity needs. “These latest revelations show how irresponsible it was to bring reactor four back online this year, given that lack of necessity,” he said.

“Reactor four needs taken back offline immediately, reactor three should never be turned back on and 2019 should be the last year of generation at the plant. EDF’s attempts to drag the plant well beyond its intended lifespan need to be stopped, now.”

The two Hunterston B reactors started generating electricity in 1976 and were originally due to close in 2006, but EDF wants to keep them both going until at least 2023. The company has currently scheduled reactor three to restart on 15 January 2020, though this date has been repeatedly postponed.

According to Friends of the Earth Scotland, Hunterston’s reactors were falling apart. “ONR admits that the resulting debris could cause a major failure, including melting of fuel rods,” said the environmental group’s director, Dr Richard Dixon.

“This problem can only get worse. The uncertainties highlighted by ONR mean that EDF will have a very hard time getting an extension to run reactor four beyond the end of this year and reactor three may already have produced its last few watts of electricity.”

Dixon highlighted the “impressive growth” in electricity from renewable sources such as wind farms. “Running either reactor at Hunterston is effectively only making electricity to export to England or Northern Ireland and to prop up EDF’s falling profits,” he stated.

“Neither of these can be worth the risk of a nuclear accident. Both reactors at Hunterston are years past their design lifetime and should never run again.”

ONR stressed that it had only allowed reactor four to operate for four months. “The longer term condition of the reactor remains uncertain and the licensee will need to justify safe operation beyond this period,” a spokesperson told The Ferret.

“We received the safety case to justify a further operation for reactor three on 17 June 2019. This safety case is being fully assessed by a team of inspectors and permission will only be granted for the reactor to return to service if we are satisfied that it is safe to do so.”

EDF Energy insisted it would be safe to keep the reactors running. “It is important to be clear that graphite debris does not pose a risk to nuclear safety at Hunterston B,” said a company spokesperson.

“The additional evidence asked for by ONR relates to theoretical risks which are extremely unlikely to develop. It is EDF Energy’s job to prove to ONR that we understand how debris may affect the reactors in the future.

“We are confident that we can make a safety case which proves that we can maintain safe operation in all circumstances to end of life.”

This story was altered at EDF’s request at 17.35 on 17 October 2019 to remove a statement that the company had confirmed that there was debris in the reactors. Photo credit: Jonathonchampton | English Wikipedia | CC BY-SA 3.0 | Wikimedia Commons.