SEABROOK, N.H. (AP) _ The Seabrook nuclear power plant began its first atomic chain reaction Tuesday, a pivotal event for a $6 billion project vexed by years of delays, cost overruns and protests that made it a focus of the nation's anti-nuclear movement.

Seabrook operators began a sustained nuclear fission reaction in the plant's 100 tons of uranium fuel at 5:23 p.m., plant operations manager Joseph Grillo said.

''We have crossed the threshold,'' Grillo told control room workers and reporters over a closed-circuit television hookup. About 30 Seabrook workers in control room applauded.

''This event today has brought lumps to our throats and I'm very, very proud of it,'' Edward Brown, president of New Hampshire Yankee, said at a news conference. New Hampshire Yankee operates the plant for the 12 utility-owners.

''Seabrook station has faced one of the longest, most convoluted licensing processes that any nuclear power plant has ever encountered in this world,'' he said. ''We've reached the last milestone before full-power operation.''

Completed in July 1986, Seabrook received its federal low-power license on May 26, a day after a federal appeals court declined to block the tests.

Three faulty safety valves delayed initial atomic operations by about a week. Final preparations for the first fission reaction resumed Monday.

Then plant operators withdrew control rods from the reactor's fuel core and slowly began diluting the boron concentration in the reactor coolant water. Boron absorbs neutrons, preventing them from striking and splitting uranium atoms to release more neutrons, triggering a chain reaction.

Seabrook originally was planned for commercial operation in 1979. A twin reactor was abandoned, 25 percent complete. Both reactors initially were to have cost less than $1 billion.

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.

Low-power tests must be completed successfully before a nuclear power plant may operate commercially. But before Seabrook may win its full-power license, it also must gain final approval for evacuation plans for crowded nearby beaches and communities up to 10 miles away.

Massachusetts has six communities within 10 miles of Seabrook but refuses to cooperate, saying evacuation of the area is not feasible. Federal hearings are underway on evacuation plans Seabrook has developed for Massachusetts.

Federal regulators say approved evacuation plans are not needed for low- power operation because the tests pose minuscule safety risks.

Seabrook's owners also may have to prove they are financially qualified to run the plant commercially. Lead owner Public Service Co. of New Hampshire is under federal bankruptcy protection, and some of the other 11 New England utilities with Seabrook shares also face financial problems.

But the Nuclear Regulatory Commission ruled in December that those troubles pose no safety problem for low-power tests as long as the owners posted a bond to pay to decommission the reactor after the tests should it never operate commercially. Owners posted the bond last month.

Critics had argued that Seabrook should not be allowed to start the tests and become contaminated by radioactive materials before commercial operation is assured and dumps are available for the plant's nuclear waste.

Plant officials maintained that contamination would be minimal outside the reactor vessel. However, with the start of a fission reaction, spent uranium fuel would be classified as high-level radioactive waste.

Political and technical problems have delayed construction of a national high-level nuclear dump, while New Hampshire has been shut out of the nation's three low-level dumps because it has not met federal deadlines for becoming responsible for its own radioactive waste.