MIDDLETOWN, Pa. -- Amid renewed criticism from Gov. Ed Rendell for a five-hour delay in telling emergency officials of a weekend radiation incident, Exelon Corp. said its Three Mile Island nuclear plant was "back to normal" yesterday.

Ventilation fans probably caused the release of a small amount of radiation inside one building on Saturday afternoon, Exelon officials said.

They said the ventilation system has since been modified, and that the 150 workers stationed in the building had all returned to work.

"Things are back to normal," site vice president Bill Noll said yesterday. No contamination was found outside the containment building, and the event never posed a threat to the public, company and federal officials reiterated yesterday.

Three Mile Island's Unit 1 was shut down last month for refueling and to replace two massive steam generators. Workers were cutting cooling-system pipes when an unexpected change in air pressure stirred up radiation particles, said Ralph DeSantis, an Exelon spokesman.

Ventilation fans then blew the particles, which Mr. DeSantis described as "invisible dust," into the containment building.

A top U.S. Nuclear Regulatory Commission inspector said yesterday that, although the agency continues to investigate the incident, the company's hypothesis likely is right.

"That's the theory they are working under right now," said John White, the NRC's supervisor for radiation, protection and inspection activities in the region. "This seems to be the most probable reason."

The name Three Mile Island is forever linked to the partial meltdown that occurred in 1979, at the plant's Unit 2, which remains shut down. For decades afterward, there were no applications to build new plants in the U.S.

Yet last weekend's contamination incident at TMI's sister Unit 1 comes amid signs of resurgence in the nuclear power industry. In the past few years, the federal government has received applications for 26 new reactors at 17 sites around the country. That's because electricity demand continues to rise, and advocates say nuclear power -- which does not emit greenhouse gases -- must be part of the solution to climate change.

Just last month, the NRC granted Exelon's request for a 20-year renewal of its license to operate the Three Mile Island site. Two days before Saturday's incident, Exelon announced it was beginning a two-year, $2.2 million project to replace all 96 emergency sirens within the 10-mile radius of the site, on the Susquehanna River south of Harrisburg.

Saturday's incident came to light when monitors detected airborne contamination, and about 150 workers stationed in the building were sent home. Tests over the weekend confirmed that no employee was exposed to more radiation than they might get in a typical day's work in the building, the company said.

Exelon began calling those employees back to work on Sunday and as of yesterday morning, the company said, all had returned. The worker exposed to the most radiation received the equivalent of about two dental X-rays, Mr. White said.

Replacing steam generators "is one of the most radiologically delicate operations," said Edwin Lyman, a nuclear expert at the nonprofit Union of Concerned Scientists, a frequent critic of the industry.

He said it was too soon to say whether the plant owner was at fault.

Generally, such generators have been replaced at nuclear plants because they are made from an alloy that corroded sooner than expected.

Exelon and NRC officials said Saturday's incident didn't come close to tripping the federal requirement that the company notify state emergency management officials within 15 minutes.

Nonetheless, Mr. Rendell, in a letter to John W. Rowe, chairman and chief executive officer of Chicago-based Exelon, called the delay "totally unacceptable."

State emergency officials first heard of the incident secondhand -- from Dauphin County officials, at 9:30 p.m. Saturday, more than five hours after it occurred. And the company didn't contact state officials directly until more than an hour after that.

Mr. DeSantis said last night of Mr. Rendell's letter, "We appreciate the governor's concern on this issue and we are committed to continuing to work with federal, state and local officials to insure that we have open lines of communication."

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