COPENHAGEN, Sept 23: A broad power blackout struck southern Sweden and eastern Denmark on Tuesday, leaving up to an estimated five million people without electricity and crippling industry, airports, trains and bridges.

Power went out in the early afternoon including at two Swedish nuclear power plants. The national grids of the two countries began restoring electricity within a few hours, but after five hours two-third of affected Danes were still without electricity.

The blackout, highly unusual in Scandinavia, follows a huge outage that left 50 million North Americans without power for up to two days and a shutdown which paralysed London for several hours, both last month.

Swedish grid Svenska Kraftnat said the exact cause of the failure was unknown, but the company was studying three events, including a storm that hit a power line, that put unusually heavy pressure on the system in a very short time.

The other events that occurred “within a minute or seconds” of each other were a shutdown at the Oskarshamn nuclear power station of a 1,135-megawatt unit and failure of control equipment on one part of Sweden’s grid, an official said.

“We cannot say the exact sequence or interconnection between these events,” Svenska Kraftnat technical director Sture Larsson told Reuters.

The power outage may have hit four to five million consumers, including one to two million in Sweden and between two and three million in Denmark, officials estimated.

Efforts to restore power in both countries were hampered by problems at power stations which had to be taken off stream and where production could not be restored immediately.

“All the power plants are not yet running,” said spokeswoman Karin Tronbeck at Danish power supplier Elkraft.

Operators of the Swedish power stations said it would take 10-12 hours to restore production, though customers were being supplied from other sources and by 1600 GMT power was restored to most customers in southern Sweden, a Sydkraft official said.

Power was earlier cut in southern Sweden and throughout the Zealand island, where the Danish capital Copenhagen is located.

Swedish nuclear safety officials said the outages at the Oskarshamn and Ringhals nuclear power stations, where capacity of about 3,000 megawatts was shut, posed no safety threat.

“The security systems there worked just as they should,” Anders Jorle, chief spokesman at the Swedish Nuclear Power Inspectorate, told Reuters. He said the nuclear units shut down as a safety measure if there were big imbalances in the electricity network and pressure in the network fell suddenly.

The outages at Ringhals and Oskarshamn came while Sweden’s Barseback nuclear station is also out of production for yearly maintenance expected to last into October. Nuclear power accounts for about half of Swedish electricity output.

In late afternoon, Copenhagen airport began checking passengers in for flights after earlier closing to all landings and takeoffs and redirecting incoming flights, the trains started to move, and the Oeresund Bridge linking Denmark to Sweden re-opened.

The economic impact of the outage was unclear.

Several big industrial plants in southern Sweden were forced to halt production due to the power outage. Sodra Cell Morrum, a pulp producer, told Reuters it would lose millions of crowns due to the stoppage.

Sweden’s TT news agency said big companies such as packaging firm Tetra Pak and vehicles maker Scania also stopped production, and telecoms operator TeliaSonera used reserve power to keep its network running.—Reuters