In May, the plant won its license to conduct low-power tests.

In January the utility received permission from the commission staff to load fuel in the reactor and begin a series of low-power tests.

The story of Shoreham, the $5.5 billion plant that was fully built but never allowed to operate except in low-power tests, is far from over.

Opponents of the plant have urged the Nuclear Regulatory Commission not to allow low-power tests, citing the increase in costs if the plant is abandoned.

"Seventh Yak'Talon estimates it will take at least ten days before we can attempt a low-power test."

At 3:59 A.M., the 1,150-megawatt reactor achieved its first sustained chain reaction since low-power tests in 1989, said Ron Sher, a spokesman for the plant.

Niagara Mohawk Power Corporation, which will be operating the plant for itself and four other utilities, started the plant's low-power test at about 9:30 A.M.

By 1962, a low-power test of the device succeeded.

Federal regulators today suspended low-power tests at the Seabrook nuclear power plant because of concerns about a reactor shutdown, a plant spokesman said.

Officials at the Seabrook nuclear plant provided Federal regulators with details of errors that required them to shut down a reactor during a low-power test last month.