Investigations launched into four safety incidents at the Hunterston B nuclear power station in North Ayrshire, UK, said the country’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 old graphite cores. EDF has repeatedly postponed their planned restart dates as the safety implications of the cracking are assessed.

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Detractors said losing reactor coolant could be “extremely dangerous” and urged 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,” the report said.

“This caused a loss of forced cooling to the reactor,” the report said. “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,” ONR said.

“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.”

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