DENVER (AP) _ A former engineer at the Rocky Flats nuclear weapons plant, fired for his whistleblower activities, filed a $1 billion lawsuit against Rockwell International, claiming it cheated the federal government.

James S. Stone and his attorney, Hartley Alley, said Tuesday the lawsuit on behalf of Stone and the government was filed July 5, 1989, but was sealed by U.S. District Judge Jim Carrigan for security reasons.

Carrigan lifted the seal Tuesday because of the government's inability to investigate the charges. In his order, the judge said the government had demonstrated no substantial progress.

Alley told reporters that he and Stone will pursue the lawsuit ''for as long as it takes.''

Rockwell, based in El Segundo, Calif., operated Rocky Flats for the Department of Energy from 1975 until January when the contract was taken over by EG&G Inc.

The plant manufactures plutonium triggers for nuclear weapons. The Energy department has suspended plutonium production at the plant in order to clean it up, including removal of plutonium dust from ventilation ductwork.

Kevin Gallagher, a spokesman for C.T. Corp., which represents lawyers for Rockwell, said the company had not been notified about the lawsuit, but that he expected it would be.

Rockwell International could pay $1 billion or more, since treble damages, plus attorney fees, are possible under the Federal False Claims Act, under which citizens can file against firms that defraud or cheat the federal government, Alley said.

The U.S. attorney general's office has not made an application to join in the lawsuit, Alley said.

Stone's lawsuit alleges wrongful conduct consisting of the manufacture of false records or statements, conspiracy to defraud the government, concealing of deficient deliveries of property and inducing the government to pay billions of dollars in violation of the False Claims Act.

The billions of dollars include bonuses Rockwell received from the Energy Department after claiming excellent performances in environmental protection, employee health programs, and so forth, Alley said.

Stone also is claiming damages, including reinstatement of employment, stemming from what he says are vindictive acts by Rockwell in retaliation for his call for an investigation of incidents at the plant north of Denver.

Stone, a professional engineer now living in New Jersey, worked for Rockwell from November 1980 until he was fired March 17, 1988. He was the principal engineer in the Utility Design Department at the plant.

Stone claimed he warned of the buildup of plutonium in the plant's ventilating ducts months before he was fired. EG&G officials confirmed the problem last year.

He decided to sue because he was ''disturbed at what I believe was the unconscionable action by management in place at Rocky Flats.''

''These people need to be held accountable, the funds recovered, and (the money) could be used to clean up the problem,'' he said. ''They have really screwed up Denver, and they need to be held accountable.''

Alley said that after serving papers to Rockwell officials, the next step in the case will be to subpoena evidence the FBI collected from the Department of Energy about Rockwell's operation of the plant.

Rockwell designs and manufactures military weapons and electronics equipment, space systems and rocket engines. It is one of the nation's largest industrial companies, with about $12 billion in sales and about $800 million in profits for 1988, records show.