Pleading guilty yesterday to environmental crimes at a plutonium plant in Rocky Flats, Colo., Rockwell International Corp. waived its contractual right to recover $18.5 million in fines from the Energy Department which owns the plant, U.S. Attorney Michael J. Norton said.

Details of the plea agreement were revealed in a federal court in Denver and, if accepted by Chief U.S. District Judge Sherman G. Finesilver, will mark the second-largest criminal environmental penalty ever paid.

In the past, contractors at nuclear weapons factories have passed through to the Energy Department the relatively small fines and penalties assessed against them for violations of environmental or safety regulations.

When news of the impending plea agreement emerged Wednesday, several members of Congress and environmental groups expressed concern that Rockwell might invoke clauses in its contracts allowing such pass-throughs as a normal cost of producing weapons for the Energy Department.

But Norton, chief federal prosecutor in Colorado, said Rockwell's agreement to plead guilty to five felony and five misdemeanor violations of federal environmental laws "prohibits Rockwell from seeking payment from DoE under the indemnification provisions of its various contracts . . . as it would otherwise be entitled to do."

Rockwell was the principal contractor at Rocky Flats, the nation's only facility for making plutonium triggers used to set off nuclear warheads, when FBI agents raided the plant in June 1989 to investigate reports of environmental violations. Federal agents analyzed more than 3.5 million pages of documents and interviewed more than 800 people, Norton said. A special grand jury heard testimony from 110 witnesses.

The inquiry found mostly technical violations of federal laws governing disposal of hazardous waste and sewage treatment. It produced "no evidence" of any threat to residents near the plant or of environmental damage not already well-known, Norton said.

Norton said the 10 offenses to which Rockwell pleaded guilty were the most that could be proved, and the proposed fines for them would be the maximum permitted under the law.