The fire, which is said to have followed some kind of explosion, started shortly after midnight in a transformer outside reactor number 3 at the Ringhals power plant, on the west coast south of Gothenburg.
A plant spokesman says it never threatened the reactor itself, and there was no risk of a radioactive leak. No one was injured, but the reactor will remain closed for several days.
In July two reactors at the Forsmark nuclear power station were shut down after two backup generators failed during a power failure. They restarted last month after safety improvements were installed.