The Nuclear Regulatory Commission yesterday fined the owners of the Salem, N.J., nuclear power plant a record $850,000 for violations that resulted in an accident in February described as the most serious incident "with implications for reactor safety since Three Mile Island."

The fine, which exceeds by $250,000 the highest penalty levied by the commission, was imposed for "violations of NRC requirements which resulted in the failure of the Salem 1 plant to automatically shut down on Feb. 22 and 25."

Public Service Electric and Gas Co., owner of the two-unit plant on the Delaware River, 20 miles southeast of Wilmington, said it would have "no comment on the proposed fine until the company has had a chance to review the terms of the proposal."

The incidents at Salem mark the only time a commercial atomic power reactor in the United States has failed to shut down on automatic command. An NRC investigation subsequently determined that the electrical breakers which should have shut down the reactor had been maintained improperly.

In both instances, operators in the plant's control room manually shut down the reactor within 25 seconds.

NRC officials said at hearings on the incidents that if an atomic plant was running at full power, failure of the automatic system could lead to a meltdown unless an operator recognized what was happening and shut the reactor down manually in less than 90 seconds.

One of the reasons such a large fine was imposed was because the Salem operators "failed to recognize that the electrical breakers had failed" to shut the reactor down automatically during the first incident, the NRC said.

The operators on that occasion believed the reactor had shut down automatically despite signals to the contrary, even though they turned the manual shutdown switch as a precaution, according to testimony.

Beyond this, the NRC said it viewed the fact that the reactor was put back into service the day after the first incident "with both electrical breakers inoperable" as a Level 1 violation, the most serious category.

The company, which was given NRC approval to restart the plant a week ago after taking steps to improve maintenance and operation procedures, said yesterday it had been delayed in putting the reactor back into operation because oysters were growing in the cooling water system.

"We expect to be back in operation next week," said company spokesman Brian Smith.

NRC spokesman Karl Abraham said that at this time of year, small oysters, clams and mussels tend to be sucked into the plant's cooling water intake from the Delaware River.

"Since the reactor has been shut down, you don't have a rapid water flow through the pipes so they are able to find places to breed and as they grow, they reduce the amount of water flowing through the pipes," Abraham said.

"So now they are trying to decide what combination of measures to take to clean them out," he said.

The Salem plant, which has been bedeviled the past year by a variety of security problems and sabotage attempts, also was victim Wednesday of a bomb threat.

The threatening call, according to NRC sources, was made on an inside-plant line to the security office shortly after noon. Salem officials conducted a search and at 1 p.m. declared an "unusual event," but did not find any explosive device and concluded that the call was a hoax, NRC sources said.

The largest previous fine levied by the NRC was a $600,000 penalty imposed in February on Carolina Power and Light Co. for failing to test various safety systems at its Brunswick atomic power station at Southport, N.C.