The explosion Tuesday that injured one worker and closed the Millstone nuclear plant in Waterford, Conn., was similar to explosions that have damaged or closed atomic plants on 23 previous occasions, according to the Nuclear Regulatory Commission.

Of the 24, Tuesday's explosion was only the second that injured anybody inside a nuclear plant, the NRC said yesterday. The NRC said the injury was probably caused because leaking hydrogen that caused one explosion triggered a second that blew a door off a factory exit on the ground floor.

"The injured worker was near the exit when the door was blown free," said Victor Stello, director of the NRC's division of operating reactors. "Nobody is certain whether he was hit by the door or knocked to the ground when the door was blown out."

The worker, Robert Griswold, 30, of New London, was released yesterday from a hospital where he was kept overnight for the treatment of cuts and observation because of a concussion he suffered in the blast.

He was also treated in special baths to decontaminate him from radiation exposure he suffered in the blast. He was contaminated with radioactive xenon and krypton gas, which were released when the hydrogen effluent from the plant's smokestack exploded inside one of the stack's main ducts.

Stello describe the first and smaller of the two explosions as "unavoidable." He said the second explosion that blew out the plant door was "probably avoidable," though he admitted the cause was unknown.

Hydrogen, which is highly explosive, is exhausted through the smokes-tacks of nuclear power plants when water in the plant's coolant is broken down into hydrogrn and oxygen after it passes through the nuclear core.

Usually, the hydrogen mixes in with oxygen, air and dilute radioactive gases like krypton and blows right out the stack. This time, the hydrogen was ignited inside the stack by built-up static electricity or by a lightning discharge. In five previous explosions, the hydrogen in the exhaust gases was ignited by lightning.

The krypton and xenon that contaminated Griswold posed no health hazard to the worker, the NRC said. They are the least objectionable of the radioactive discharges from a nuclear power plant and are so minute they could not be measured byGeiger counters at the Millstone factory gate.