Sweden's reactor 3 in Oskarshamn on Wednesday was forced to close down again, the Swedish Daily newspaper Svenska Dagbladet reported.
"The third reactor in Oskarshamn's nuclear power plant stopped again at lunch time on Wednesday," the report said.
The reactor is the one which has most problems in recent years. It stopped before Christmas and started again after the festival.
The problem this time was a reduced pressure in the reactor's inside stop part. It is not clear how serious the problem is, but according to information officer Per Sangrud, it can resume operation again on Saturday.
"The investigation into the problem is underway," he said.
According to the UN nuclear body IAEA, the Swedish nuclear power plant in 2009 stopped the most times in the world.