The short-circuit that nearly caused a meltdown

from 12 August 2006

(by Diet Simon)

Work on high-tension power lines outside a Swedish power station caused a short-circuit that cut its electricity supply and nearly caused a nuclear meltdown. The plant’s emergency batteries and diesel generators failed to take over.

That’s how Lars-Olov Höglund, the former head of the construction division of the reactor, described the near-disaster on 25 July at the Forsmark I nuclear power plant about 150 km north of Stockholm to a German newspaper. He told the Hamburger Abendblatt:

"They tried desperately to start up one of the diesel generators from the control room to keep the pumps for the cooling water for the reactor going. But there was no power to give the electric impulse. On top of that several measuring instruments had stopped working.”

According to Höglund the tension in the control room worsened. Steam had escaped in the turbine building, which the fire alarm read as fire, responding by automatically switching on the sprinkler system and sirens.

"The automatic cameras monitoring the plant didn’t work – there was no power – and neither did the public address system which is supposed to warn people in the station in an emergency,” says Höglund.

In their desperation the control room crew had phoned for help from the neighbouring nuclear power station, Forsmark II, which was off the grid for maintenance.

“The colleagues ran over,” says Höglund. One of the engineers called in to help had found a way to get start-up power to two diesel generators.

"From then on things worked. But they simply tried things out. It was not the outcome of safety analysis and training. The breakdown was not foreseeable nor was it foreseeable whether the counter-measures would work,” criticises Höglund. Afterwards the control room crew had been “so finished” that they stopped work before the end of their shift and needed psychological counselling, says the engineer.

Högland told the Hamburg paper that in the time between the loss of power in the plant and the start-up of the emergency diesel generator the control crew were in panic.

“It was the worst situation since Chernobyl and Harrisburg. We were horrifyingly close to a really dangerous situation.” A meltdown had only just been avoided. The way Höglund tells it, there was 23 minutes of high drama in that control room.

What makes the breakdown so serious for him is the phenomenon Högland calls "common cause failure" which figures largely in the safety discussion about nuclear power stations. “It means simultaneous malfunctions happening in systems which support and replace each other. It must not be allowed to happen that one cause puts backup systems out of action.”

In this case it means the short-circuit should not have been able to prevent four generators starting up and four battery systems switching coming on. At least two should have worked.

The former employee of Vattenfall, the state-owned power company that owns the power station, says he had access to the official papers about the breakdown. “The more I read about it, the more seriously I regard it.”

Höglund explains that Swedish nukes have two emergency systems to make up for loss of power. There are four battery systems independent of each other, each of which can deliver half the electricity the plant needs. And there are four diesel generators that also deliver half. Theoretically two of the eight units suffice to keep the entire plant supplied.

"This self-supply is a construction condition for nuclear power stations. Usually the systems start up automatically. They didn’t in this case. None of the systems came on. That is very unusual and must not happen,” says Höglund. It was an unknown failure and so was not practised. "The seven people in the control room just didn’t know how to respond to it and panicked."

"Nobody plans for such failures,” says Höglund. "To keep the likelihood of something like that happening as low as possible, four diesels from different makers should be installed. But that’s not done to keep costs under control and train staff on one system."

This is risky, says Höglund, because the power supply is the second most important component in a nuclear power station. "The most important is the fast shut-down and the management of the control rods. If they can’t be lowered back into the reactor, a meltdown is inevitable. There are also backup systems for that, which did function in Forsmark.

“But immediately after that ranks the power supply. Without electricity I’ve got no way to pump cooling water because the pumps are electrically powered. If the cooling water turns to steam, you get over-heating and uncontrolled reaction by the core. That’s why power must not be lost.”

Höglund argues that the work on the high-tension grid near the plant should never have been done while the nuke was in operation because it entails the risk of a breakdown.

“If in so important a matter as power supply coincidence determines at the end of the day whether there’s going to be a catastrophe or not, the entire reactor safety is called into question. Then I have to ask myself how many unknown mistakes are still lurking and how reliable all the safety calculations are.”

"Vattenfall say quite rightly that everything ended well. But that was pure luck. It can’t be on that the safety of atomic power is a matter of luck.”

Vattenfall contradict Höglund’s assertion that a meltdown was close. “That was definitely not the case,” the corporation’s spokesman, Martin May, told the Abendblatt.

He says both the Swedish supervisory authority SKI and Stockholm Technical University had investigated the incident. They had found that only two of the four diesels had started up, but that had been enough for the emergency cooling of the reactor.

The other two failing generators were to have powered the monitor screens in the control room. They could only be switched on by hand after 20 minutes.

According to the spokesman, the SKI had found no danger to the environment.

Bearbeitet am: 12.08.2006/ad


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