Nuclear Regulatory Commission member Thomas M. Roberts should resign because of conduct that "falls far short of the ethical behavior we should expect," the chairman of a Senate panel said yesterday.

Roberts, already under fire from several members of Congress over the leak of a commission document to officials of a troubled nuclear plant, "has not been forthcoming" in repeated hearings on the issue, said Sen. John B. Breaux (D-La.).

Breaux, chairman of the Senate Environment and Public Works subcommittee on nuclear regulation, joined the heads of six other House and Senate panels who have demanded that Roberts step down.

Roberts, who also is under investigation by the Justice Department, has steadfastly denied any wrongdoing. In a letter hand-delivered to Breaux yesterday, he again declined to resign and said he was "disappointed" that Breaux would suggest it.

"With all due respect, I am convinced that the facts relating to my conduct do not warrant my resignation," he wrote.

The controversy revolves around the discovery by the NRC's Office of Investigations in 1985 that a confidential 1983 memorandum about problems at the Waterford nuclear plant in Taft, La., had been leaked to the plant's operators. Markings on the document indicated it came from Roberts' office.

When Roberts was questioned about the matter in April by Sen. John Glenn (D-Ohio), chairman of the Governmental Affairs Committee, the commissioner said he thought "someone was trying to set me up."