Four of Sweden's 10 nuclear reactors have been shut down, following an electricity failure.
Forsmark supplies one-sixth of Sweden's electricity
The Swedish Nuclear Power Inspectorate, SKI, has been holding an emergency meeting, following the incidents at the Oskarshamn and Forsmark plants.
The firm operating Oskarshamn, in southern Sweden, stopped two of its three reactors late on Wednesday, citing safety concerns.
Last week, two reactors were also shut down at Forsmark, north of Stockholm.
The two reactors in Oskarshamn - about 250km (150 miles) south of Stockholm - were shut down after the operator said their safety could not be guaranteed.
Last week's shutdown in Forsmark - some 75km (46 miles) north of Stockholm - came after a short-circuit in a unit supplying power to the reactors.
SKI spokesman Anders Bredfell told the BBC News website that the Forsmark incident ranked as a number two on the 0-7 scale used by the International Atomic Energy Agency (IAEA) to classify nuclear incidents.
"We can't say how long the reactors will remain shut down," he said.
He stressed there was "no danger of a meltdown" at Forsmark. He said two of the four backup generators had failed to start there, but two were sufficient to run the plant's cooling system. In addition, the plant has gas turbines that can be used to supply power in an emergency, he said.
On Friday SKI asked all of Sweden's other nuclear plants to prove that the same failure could not happen to them, Mr Bredfell explained.
"The Oskarshamn reactors were shut down because they couldn't prove that the same thing couldn't happen there," he said.
The IAEA was automatically informed about the Forsmark incident, and Sweden's response showed that the procedures for such incidents were working, he added.
The environmental group Greenpeace in Sweden has urged the government to stop all nuclear reactors in the country.
Sweden plans to phase out all of its reactors in the next few years.