Technicians at the Ringhals plant, near Gothenburg, say it was simply the large difference in temperatures between the hot water in the pipes of a cooling system and the cold air outside.
They hope to be able to isolate the problem and get the reactor connected to the national grid again on Tuesday.
The fault follows a series of problems at the Forsmark nuclear plant, north of Stockholm, over the past few months, which have led to the site’s Chief Executive resigning and a prosecutor investigating the handling of an emergency shutdown last summer.
Sweden’s ten nuclear reactors provide around half of the country’s electricity supplies. Two others have already been closed as part of a plan to gradually phase out the use of nuclear power, following a referendum on the issue in 1980.