After starting their first atomic fission reaction, officials at the long-delayed Seabrook power plant said they know how Thomas Edison must have felt when he lit the first electric lightbulb in 1879.

"We certainly haven't reinvented the lightbulb," Seabrook's top executive, Edward Brown, said Tuesday. "But we believe there is great significance in our success."Have we changed history? Some people are saying so," Brown said. "Seabrook Station has faced one of the longest, most convoluted licensing processes that any commercial nuclear power plant has ever encountered in this world."

The plant symbolizes the troubled state of the nation's nuclear power industry. Originally planned for commercial operation 10 years ago at a cost of $973 million, the plant suffered repeated regulatory delays and cost overruns that drove its pricetag up to $6 billion.

The plant also became a focus of anti-nuclear activism. Protests since construction began in 1976 have resulted in more than 3,200 arrests, including 734 arrests in two days of civil disobedience earlier this month.

Despite its woes, the seaside plant was completed in July 1986 and last month won its low-power testing license, which allows it to conduct tests at up to 5 percent of full power.

Three faulty safety valves set back the start of fission by about a week, but at 5:23 p.m. Tuesday, Seabrook operators started an atomic chain reaction in the plant's 100 tons of uranium fuel.

At a news conference a half hour later, Brown said Edison's associates cheered when he turned up the voltage and the lightbulb grew brighter.

In Seabrook's control room, there was no apparent sign that nuclear operations had begun, but about 30 plant workers broke into applause and shook hands when told the reactor had split its first atoms.

"We have crossed the threshold," said Joseph Grillo, the operations manager. "We are in the homestretch toward full-power operation, and everyone here is excited about it."

Operators plan to conduct tests for about seven days at power levels ranging from a fraction of 1 percent to 3 percent of capacity.

Still unresolved is whether Seabrook will ever operate commercially. It must win federal approval for evacuation plans for crowded beaches and communities up to 10 miles away - including six communities in Massachusetts, which refuses to cooperate.

Seabrook has developed its own evacuation plans for Massachusetts, and Nuclear RegulatoryCommission licensing board hearings on them are expected to finish later this month. Seabrook officials hope to win a commercial license this fall.