The company said it discovered that "possibly a few" of the 80,000 uranium fuel rods inside the reactor were "defective."

Vattenfall has blamed the plant manager for failing to install discharge detectors on a transformer as promised to German authorities.

The incident last weekend is the second major mishap after a 2007 electrical transformer fire at the Kruemmel power station near Hamburg.

Although the latest malfunction did not involve the reactor itself, it has suddenly turned nuclear safety into an election issue in Germany and thrown an uncomfortable spotlight onto Vattenfall, which operates two of Germany's 17 nuclear plants.

The conservative state premier of Schleswig-Holstein, Peter Harry Carstensen, has threatened Vattenfall with the permanent closure of Kruemmel, which is in his state, if the utility does not get its act together. Carstensen and his party, Chancellor Angela Merkel's Christian Democrats, generally support the use of nuclear power.

German Foreign Minister Frank-Walter Steinmeier, the ranking Social Democrat who is campaigning to unseat Merkel in the September elections, has called for Kruemmel to be shut down for good.

Kruemmel was in operation for only two weeks after two years of repairs when Saturday's shutdown occurred and this means the plant will be off the power grid for many more months while two transformers are replaced.

Vattenfall now faces intense scrutiny

Tuoma Hatakka, the chief executive of Vattenfall Europe, has insisted that "Kruemmel is safe" and gave assurances at a press conference in Berlin that "all technical and organizational processes will be re-examined."

Vattenfall claims that there was "no danger to the public at any point," but Hatakka admitted the mishap was a "setback", and had "once again reduced public trust."

He conceded that nuclear regulators had not been promptly informed after Kruemmel's shutdown last weekend knocked out more than 1,500 traffic lights in Hamburg and cut off electricity to some shopping malls.

This week, Vattenfall's image took another beating after Swedish media reported that Sweden's nuclear safety authority was considering stricter supervision of the company's operations in its home country.

The possible action follows the accumulation of 60 safety incidents, two of them described as very serious, at its Ringhals power plant near Gothenburg in southern Sweden.

gb/dpa/AFP/ARD

Editor: Chuck Penfold