HARRISBURG (AP) _ Technicians at the Three Mile Island nuclear plant planned to shut down the Unit 1 reactor Friday night to repair leaks in the cooling system, a spokeswoman said.

Lisa Robinson, a spokeswoman for GPU Nuclear Corp. which operates the plant, said it would be out of service for about 30 days.

The shutdown would be the second ordered since a five-month maintenance program ended in March.

The plant's Unit 2 was the site of the nation's worst accident commercial nuclear power plant, in March 1979. That unit remains closed.

During the scheduled repairs on Unit 1, workers will replace two systems used to cool reactor water before processing. Both of the so-called letdown coolers were replaced during the recent maintenance program, but have since developed leaks, Ms. Robinson said.

The leaks were contained and did not present an immediate safety or operation risk, but the company decided to replace them to stave off any potential problems, she said.

Engineers are uncertain why the units malfunctioned so soon after installation, Ms. Robinson said.

Robert Pollard, an engineer with the Washington D.C.-based Union of Concerned Scientists, said the letdown system could be isolated if necessary and that leaks in the system were not a major safety concern.

Unit 1 was shut down May 1 so mineral deposits in the plant's steam generators could be redistributed. The shutdown allowed the plant, which had been limited to about 85 percent power, to reach 100 percent capacity.