MIDDLETOWN, Pa. - Amid renewed criticism from Gov. 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 that the ventilation system had 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 DeSantis described as "invisible dust," into the containment building.

A top U.S. Nuclear Regulatory Commission inspector said yesterday that although the agency continued to investigate the incident, the company's hypothesis likely was 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. "There's no reason to believe they are wrong. This seems to be the most probable reason."

The name of 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 many years afterward, there were no applications to build new plants in the United States.

Last weekend's incident at TMI's sister Unit 1 comes amid signs of resurgence in the nuclear-power industry. In the last 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. One such proposal is for a site called Bell Bend, in Luzerne County.

Some companies have also taken steps to refurbish older reactors with new equipment and extend their life. 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 employees were exposed to no 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 NRC reported "minor contamination of about 20 workers" in the Saturday incident. The worker exposed to the most radiation received the equivalent of about two dental X-rays, the NRC's 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.

Numerous other reactor owners have replaced their generators without incident - which in Lyman's view raised a question about why Exelon was unable to follow their example.

"You can say that accidents do happen," said Lyman, a senior scientist with the group in Cambridge, Mass. "But contaminating a number of workers like this is a big blunder. . . . No level of contamination should be regarded as acceptable."

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

In September, Three Mile Island's enormous new generators were ferried through southern Chester County on their way to the plant, traveling at the deliberate speed of 3 m.p.h. Crews moved phone and electric cables aside, and spectators gawked at the procession.

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, Rendell, in a letter to John W. Rowe, chairman and chief executive 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 did not contact state officials directly until more than an hour after that.

The governor said he did not consider such a delay in reporting of "an incident of this magnitude" to be "timely or appropriate."

"In fact," he wrote yesterday, the company's news release about the incident didn't reach the state Emergency Management Agency "until after the local TV station aired your interview."

Exelon's DeSantis said last night of 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 ensure that we have open lines of communication."

Eric Epstein, chairman of a Harrisburg antinuclear group, TMI-Alert Inc., said, "Hopefully, TMI will not send workers into harm's way before identifying, isolating, and defeating the root cause of the contamination."

Epstein also chided Exelon for not alerting state and local emergency officials sooner, saying, "The Pony Express delivers messages faster."

Contact staff writer Mario F. Cattabiani at 717-787-5990 or mcattabiani@phillynews.com.