The authorities have launched an investigation into last week's power blackout at the Forsmark nuclear power station on the east coast.

The blackout led to the immediate shutdown of a reactor at Forsmark, amid allegations that a potentially catastrophic reactor meltdown had been avoided by sheer luck.

A second Forsmark reactor was stopped at the weekend.

Then on Wednesday, the Oskarshamn nuclear power station on the southeastern coast shut down two reactors. It said their safety could not be guaranteed until the Forsmark incident had been cleared up.

A fifth reactor, at Ringhals power station in southern Sweden, was closed down for routine maintenance on Thursday.

That left the country with just five of its 10 reactors functioning.

Nuclear power accounts for nearly half of Sweden's electricity production and the shutdowns sparked a record rise in Swedish energy prices. The price per kilowatt hour reached a high of 0.543 Swedish kronor (0.06 euros, 0.08 dollars) on the Nordic spot market.

The Swedish Nuclear Power Inspectorate (SKI) said it was not planning any more closures.

But it did say it was scrutinising reports from the power stations about what might have gone wrong at Forsmark and whether it was safe to bring the reactors back on stream.

"Several of our groups have been asked to analyse the incoming security reports to make sure that we agree with their conclusions," SKI spokesman Anders Bredfell told the TT news agency.

A decision on restarting the reactors would only be taken once the Forsmark incident had been properly investigated, he said.

Sweden's energy market appeared to be coping with the reduced output, helped by warm summer weather. Experts said the energy shortfall would have had serious consequences had it occured in winter, when Sweden's energy needs rise sharply.

Sweden has shut two of its 12 nuclear reactors since 1999 as part of a plan to phase out nuclear power over the next 30 or so years, or when the reactors' lifespan expires.

The Forsmark plant has three reactors, as does Oskarshamn, while Ringhals has four.

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