Four years after it was completed and 14 years after construction began, the Seabrook nuclear plant supplied power to New England customers for the first time.

The plant was operating at 15 percent capacity and producing enough electricity to supply 65,000 homes when it was hooked into the New England Power Pool's grid Tuesday night, officials said.The milestone occurred during tests to ready the $6.5 billion plant for full commercial operation by mid-summer. At full power, Seabrook will generate 1,150 megawatts, enough for 1 million homes.

The plant was to be disconnected from the power grid Wednesday for equipment tests. Operators will repeatedly connect and disconnect the generator during the next several weeks of testing, an official of Public Service Co. of New Hampshire said.

Seabrook has become a symbol of the nuclear industry's troubles and a target of anti-nuclear activists. Construction began in 1976 and took 10 years. Even then, the plant was unable to get an operating license until this past March.

Because of delays, costs soared from less than $1 billion and forced PSNH, the plant's lead owner, into bankruptcy court in 1988.

Critics say Seabrook is unsafe and evacuation plans inadequate.

The plant produced electricity during tests in 1985, but that was non- nuclear power generated by using the heater and pumps.