Repair workers at the nuclear-powered Limerick Generating Station, about 20 miles northwest of Philadelphia, kept a reactor shut down Saturday to replace a main steam valve, a plant spokeswoman said.

Dana Melia said a slight temperature rise indicated a possible malfunction in the valve Friday, and it can be replaced only when the Unit 2 reactor is off line. She said a valve repair usually requires a shutdown of a couple of days.

The Limerick plant shut down its Unit 1 reactor July 18 for an electrical problem. That repair was labeled an "unusual event," the lowest of four emergency classifications. Unit 1 began operating at full power again last week.

- Mark Fazlollah