Carolyn Kaster, Associated Press Cooling towers for the Three Mile Island nuclear power plant are reflected in the Susquehanna River last night. Click photo for larger image.

HARRISBURG -- The reactor at Three Mile Island, site of the nation's worst nuclear accident, shut down yesterday after a faulty instrument reading triggered the automatic safety precaution, company and federal officials said.

The plant operator, AmerGen Energy Co. LLC, issued a statement saying there was no release of radiation as a result of the shutdown.

The reading came from a condenser on the non-nuclear side of the operation, and the problem posed no public threat, said Neil Sheehan, a spokesman for the federal Nuclear Regulatory Commission.

"Now the company will need to take a careful and deliberate look at what went wrong," Mr. Sheehan said. "It appears this was an uncomplicated, smooth shutdown."

An AmerGen spokesman, Ralph DeSantis, said it was the first shutdown of Unit 1 since 1997. The unit, which shut off at 1:35 p.m., remained down yesterday evening.

Mr. DeSantis said other power plants in the regional electricity grid would ensure that customers experienced no power outage. Workers were examining what went wrong and it was unclear when the reactor would resume operation, he said.

Sheehan said the reactor's condenser signaled a low vacuum condition, which triggered the unit's shut-off. Adequate vacuum conditions increase the efficiency of the process of cooling steam back into water.

Unit 1 opened in 1974.

TMI, located in Middletown, about 10 miles southeast of Harrisburg, was the site of the nation's worst nuclear accident when a partial meltdown occurred in the Unit 2 reactor in March 1979.