Investigations have been launched into four safety incidents at the Hunterston B nuclear power station in North Ayrshire, according to the UK government’s nuclear watchdog.
The plant’s operator, EDF Energy, is examining three incidents that took place between April and June. Cooling for a reactor was lost, a shutdown pump failed and smoke came from a control room panel.
The company has also been investigating a fourth incident from earlier in the year when a power failure prevented cooling gas from being circulated around a reactor.
The two reactors at Hunterston B have been closed down since last year because of proliferating cracks in their ageing graphite cores. EDF has repeatedly postponed their planned restart dates as the safety implications of the cracking are assessed.
Campaigners said that losing reactor coolant could be “extremely dangerous” and urged that the reactors remain closed. But EDF described the incidents as “very minor” and insisted the reactors could be safely restarted.
The four incidents have been disclosed in the latest quarterly safety reports on Hunterston B from the UK government’s Office for Nuclear Regulation (ONR). The dates on which the incidents occurred are not given.
The report covering April to June 2019 described a problem during operations to fill cooling systems for reactor three with carbon dioxide. “A gas circulator low level alarm was received on the lubrication oil tank which caused the gas circulators to trip,” said the report.
“This caused a loss of forced cooling to the reactor. The remaining available gas circulators were started up and the tripped gas circulators were returned to service following top up of the oil tank level.”
The report added: “Reactor three has been shut down for a considerable time and the brief loss of gas circulation did not challenge reactor three cooling requirements.”
A second incident in the same quarter saw a nitrogen pump fail “due to a valve configuration issue”. This led to one of three pumps in a system used to shut down reactors in an emergency to become unavailable.
“The reactors are shutdown with all control rods inserted, thus the significance of the event was minor as the two other nitrogen pump trains were available and therefore operating rule requirements continued to be met,” the ONR report said.
In a third incident between April and June smoke was seen coming from an electrical panel in a control room. “The event occurred due to an electrical coil that had burnt out within a control panel,” said ONR.
“The panel was isolated and there was no further risk of smoke or fire. A site fire muster was carried out and the fire service attended. There were no nuclear safety consequences from this event.”
The electrical supply to the gas circulators was lost resulting in loss of forced circulation to the reactor for short period. Report, Office for Nuclear Regulation
A fourth incident involving a power failure at reactor three was outlined in the ONR report covering January to March. “The electrical supply to the gas circulators was lost resulting in loss of forced circulation to the reactor for short period whilst supplies were re-established,” it said.
“Investigations identified that a switch semaphore in the control room had indicated that an interconnector was closed when in fact the interconnector was open resulting in an interruption to electrical supplies when the supplies were switched.”
There had been a failure by EDF “to adequately risk assess the operation,” the ONR report added. “There were no radiological consequences from this event as reactor three has been shut down for a year and the cooling requirements were much reduced.”
Reactor three at Hunterston – the subject of a series of revelations by The Ferret since 2017 – is the oldest of its kind in the UK. It was was shut down on 9 March 2018 and EDF currently plans to restart it on 1 October 2019.
According to ONR, there are an estimated 377 cracks in the graphite blocks that make up the radioactive core of reactor three. That is based on EDF inspections of 28 per cent of the core, and breaches the reactor’s operational safety limit of 350 cracks.
Hunterston reactor four has an estimated 209 cracks, based on an examination of 11 per cent of the core. It was shut down on 2 October 2018.
EDF announced on 2 August that the planned restart date for reactor four had again been delayed from 5 to 30 August. The Ferret understands this is because ONR wants to find out more about the risks of cracks opening wider than 1.2 centimetres.
The two reactors started generating electricity in 1976 and can provide up to 20 per cent of Scotland’s electricity. They were originally due to close in 2006, but EDF has said that it wants to keep them both going until at least 2023.
Pete Roche, editor of No2NuclearPower, was worried there were “too many incidents” at Hunterston. “Where they involve a potential loss of cooling they could be extremely serious,” he told The Ferret.
“It looks as though we have escaped a very dangerous situation only because reactor three was shutdown. Best leave it that way.”
Anti-nuclear campaigners from Cumbria handed in a letter to EDF at Hunterston on 3 August calling for reactors three and four to stay shut. They fear that the plant risks an accident similar to that at Chernobyl in Ukraine in 1986.
Friends of the Earth Scotland urged governments to abandon nuclear power in favour of renewable energy. “These most recent incidents raise serious questions about the safety of Hunterston B and whether the reactors should ever be re-opened,” said the environmental group’s head of campaigns, Mary Church.
For too long now these ageing and now cracking nuclear reactors have had their lifetimes repeatedly extended. Ross Greer, Green MSP
According to the Scottish Greens, the community around Hunterston had felt for years that safety concerns had been ignored. “These latest alarming revelations will compound those concerns,” said West of Scotland Green MSP, Ross Greer.
“For too long now these ageing and now cracking nuclear reactors have had their lifetimes repeatedly extended, rather than effort being put into the jobs transition the community so clearly needs for its post-nuclear future.”
EDF Energy stressed that it had an open reporting culture. “We brought these incidents to the ONR’s attention as soon as they happened as part of the usual processes,” said a company spokesperson.
“Each of the issues were very minor and did not pose a risk to workers, the public, or the plant itself. We have strength in depth on all our safety systems, and our back-systems remained available and maintained the safety of the plant at all times.”
The spokesperson added: “We investigate incidents thoroughly so we can learn from them and improve our stations’ operations in the future.”
We will only allow the re-start of either reactor at Hunterston B if we are satisfied that it is safe to do so. Spokesperson, Office for Nuclear Regulation
The Office for Nuclear Regulation pointed out that the four recently reported incidents were “not connected” to the graphite core cracking. “Nuclear plants are designed and operated to deal with unexpected events that occur and there are multiple safety barriers to stop those events becoming more serious,” said an ONR spokesperson.
“Due to this defence in depth, the type of incidents in question were relatively minor and there was no risk of harm to people or the environment. However we are not complacent and we consider every event and will take appropriate regulatory action where necessary to ensure the safety of workers and the public.”
ONR is continuing to examine the safety cases made by EDF for restarting reactors three and four. “We will not rush in reaching our regulatory judgement and our specialist inspectors continue to examine the detailed evidence presented,” the spokesperson added.
“We should also re-emphasise that we will only allow the re-start of either reactor at Hunterston B if we are satisfied that it is safe to do so.”
ONR is not expected to make a decision about restarting reactor four for several weeks. A decision on reactor three is thought to be months away.
Photo thanks to David Autumns. This story was updated at 11.55 on Monday 5 August with a new header image, and to record the delivery of a letter by campaigners on 3 August.