The Geiger counter sat on the table today, going tick, pause, tick-tick, tick-tick, pause, tick - registering nothing but sunbeams.

When it was registering the level of radiation emitted during the Three-Mile Island nuclear power plant accident in March, there were a couple more ticks per minute. But when a reporter put his luminous wristwatch on it today, the device sounded off like a machine gun.

It was show-and-tell about radiation, nuclear power and Three Mile Island at the hheaquarters of the Babcock & Wilcox Co., which built the Pennsylvania plant. Officials maintained once again that the accident was largely the fault of the operators.

"We don't believe we have blame in the Three Mile Island accident," said company vice president John H. MacMillian. "We believe that inappropriate operator action was the significant factor."

About 45 reporters and photographers were flown to this southwestern Virginia town, mostly from New York and Washington, to hear B&W's version of Three Mile Island. "It's the first time we've spoken out in public except for formal testimony" before Congress or the Nuclear Regulatory Commission, MacMillan said before the meeting.

A glowing reactor diagram pulsed in the background and a plastic model of a reactor emptied and refilled with water several times to help the company make its points. "If proper operators actions had been taken, there would have been no core uncovering and no reactor damage," MacMillan said. "Of the six significant factors, five involved . . . inappropriate operator actions."

The sixth factor, the premature shutdown of critical reactor cooling pumps, was correct in light of operator training, even though it made the situation worse, MacMillan said. The only B&W equipment failure was a pressure relief valve that stuck open, and operators should have realized it was open and closed it earlier than they did, he said.

A gleaming stainless steel piece of modern art, curved and domed and knobbed, about 14 by 8 by 24 inches, stood beside the podium. It was a pressure relief valve.

The other four significant factors at Three Mile Island, MacMillan added were the closure before the accident began, "for reasons yet to be understood," of two emergency water line valves; an operator's fixation on one gauge that misled him into thinking everything was all right; the failure to shut off automatic pumps that took radioactive water to a neghboring building and let it escape into the atmosphere, and an operator's decision to shut off a high-pressure pump just when it was needed most.

The Nuclear Regulatory Commission has also listed those six factors as the most crucial events.

"Yes, there was an accident and it was unfortunate," said L. M. Favret, vice president for B&W's power generation unit. "However, there was no meltdown - no 'China syndrome' - " and emissions from the plant were "extremely low."

Radiation from the accident may add one cancer death and one genetic defect to the 325,000 fatal cancers and 60,000 such defects that will occur anyway, said cancer and health physics supervisor John W. Cure. Smoking two packs of cigarettes a day shorten one's life by 10 years, living in a city costs five years and Three Mile Island may cost 10 seconds, he said.

All of these figures, taken from Environmental Protection Agency and National Academy of Sciences studies, have been challenged by nuclear industry critics.

The Geiger counter shifted to a disco beat when it got close to a chunk of Virginia granite. Brick houses, Cure said, may cause four cancers a year. But the radium dial of his 1960-vintage wristwatch, and that of at least one reporter, pushed the counter off the scale. Modern watches no longer use radium.

"Any questions?" Cure asked. "Yeah," said a reporter. "Why are you wearing that wristwatch?"

It was just for demonstration purposes. B&W remains a firm defender of nuclear power and its safety in comparison to other energy sources. Although the industry will be operating "at less than half capacity for the foreseeable future" because of lagging orders, the company has no intention of leaving the field, said B&W President George G. Zipf.

"The company has been in the [power equipment] business for 112 years, and we expect to be here a lot longer," he said.