National security adviser Samuel R. "Sandy" Berger made an unusual two-hour, closed-door appearance before members of the Senate Select Committee on Intelligence yesterday to answer questions about his handling of allegations of Chinese espionage at the nation's nuclear weapons laboratory.

"He told his story with no visible impairment to his position," said one source familiar with the meeting. Presidential advisers such as Berger normally do not testify before committees.

Committee Chairman Richard C. Shelby (R-Ala.) had no comment on the meeting, a spokesman said. Shelby was among several members of Congress who had said that Berger should resign because of his failure to warn President Clinton in April 1996, after receiving his first Energy Department intelligence briefing on the alleged espionage.

At one point in the session, Senate Majority Leader Trent Lott (R-Miss.) appealed for a bipartisan effort to bolster security and operation of the nuclear laboratories "for the nation's good," one source said. Republican senators and Energy Secretary Bill Richardson are at odds over how to reorganize the nuclear weapons research and production complex. The senators support a plan, also backed by a presidential panel headed by former senator Warren B. Rudman (R-N.H.), to make the complex a semi-autonomous agency inside Energy. Richardson wants to create a new undersecretary to run the complex directly but not make it "a new fiefdom."

Berger told the senators he stands by Richardson's efforts to reach a compromise.

Meanwhile, administration and congressional sources said a new FBI investigation was underway into possible economic espionage by a contractor at one of the Energy Department's civilian laboratories. Although the sources would not identify the lab, the report illustrates the need for a department-wide counterintelligence effort rather than one primarily within the nuclear weapons complex, according to Energy officials.

The case, initially uncovered by an Energy Department internal audit, stemmed from questionable "irregular" expenditures by a contractor that led to suspicions that proprietary government information was being used for the contractor's private business, according to sources.

"On a scale of one to 10, this is a one," an Energy official said yesterday. But, he added, "it shows that this is an area that needs further exploration."

Rep. F. James Sensenbrenner Jr. (R-Wis.), chairman of the House Science Committee, which has authority over Energy's non-weapons laboratories, has asked for a briefing on the matter. He has been exploring whether all the department's research facilities should be contained within the semi-autonomous entity being proposed by Senate Republicans for the weapons complex only.