A problem with the cooling system prompted the PPL Corp.'s Susquehanna nuclear power plant to declare an "unusual event" and reduced the power output at one of the two units yesterday, but the problem was repaired quickly, the utility said.

An electrical failure occurred in a panel that powers the cooling system and a system that removes gases from the turbine's main condenser, without which the unit cannot operate at full power, officials said.

As a result, power at the Luzerne County plant's Unit 2 was reduced by about 20 percent, officials said. The utility declared an "unusual event," the lowest of the four emergency classifications for nuclear power plants, at 1:25 p.m. and declared an end to the event at 3:52 p.m.

Plant operators had begun to return the Unit 2 reactor to full power, PPL officials said last night.