Stung by disclosures of its secret deliberations about problem-plagued atomic power plants, the Nuclear Regulatory Commission changed its policy yesterday to keep more of its discussions out of view of the public and Congress.

On a 3-to-2 vote, the commission changed the definition of "meeting" that it has used for eight years under the federal Sunshine Act.

The new definition would exclude briefings by its staff on technical problems common to several power plants, general discussions and "brainstorming sessions."

Even before the vote, the chairman of a House subcommittee that oversees the NRC's activities scheduled hearings next week on the change, which he said reflects a "bunker mentality."

"I cannot comprehend that it is in anybody's best interest for the NRC to rush to draw down a lead curtain around its deliberations," said Rep. Edward J. Markey (D-Mass.), chairman of the House Energy and Commerce Committee's subcommittee on conservation and power.

The NRC had the authority to meet privately and has held dozens of such meetings annually. But in the past, the agency always had transcripts of those sessions.

Some of those transcripts, released by congressional committees or leaked to reporters, have subjected the NRC to lawsuits in the past two years over its handling of licenses for the Shoreham nuclear plant in New York and the Diablo Canyon plant in California.

Under the new rule, the commission would not have to make transcripts or even keep minutes of closed sessions because they would be excluded from the NRC's definition of a "meeting."