The problem at the Philippsburg plant near the city of Karlsruhe was rated at the second-lowest level, 1 for an "anomaly," on the International Nuclear Event Scale (INES).

The state environmental affairs ministry said falling pressure had been noticed Thursday night in the outer vessel of reactor 1 at the site while the reactor was being powered up and the cause was being sought.

There was no evidence of any release of radioactivity into the vicinity.

The vessel, which encloses all the main parts of the reactor, is supposed to maintain 20 millibars more pressure than the air outside, but had been losing 1 millibar per hour, presumably because of a leak.