The staff of the Nuclear Regulatory Commission yesterday recommended that all operating nuclear power plants constructed by Babcock & Wilcox Co., the firm that built the plants at Three Mile Island, be shut down.

The commission postponed a decision until this afternoon.

The staff recommended closure of four plants: Oconee I, II, and III of the Duke Power Co. in Charlotte, N.C., and the Rancho Seco unit of the Sacramento, Calif., Municipal Utility District. Five other B&W reactors already closed down would remain shut under the recommendation.

Duke Power President Bill Lee told the commission that closure would cost the American public about $100 million per month. Any shutdown, he and the staff agreed, would last at least one to two months.

Staff director Harold Denton gave four reasons for recommending the closures:

B&W plants are particularly sensitive to the kind of accident - loss of feedwater - that caused the March 28 near-disaster at Three Mile Island in Pennsylvania.

Control systems for the feedwater operations are "more complex than in other kinds of reactors" and have been responsible for 10 to 15 percent of feedwater system malfunctions at B&W plants in the past.

The control system may overcompensate for a feedwater accident in a manner still not totally understood.

The system for manually controlling the situation needs upgrading.

"Absent prompt and remedial action by the control system," Denton said, "the steam generator will dry out." That could lead to a disastrous meltdown.

Denton said he had met with representatives from all the utilities with B&W reactors on Tuesday and yesterday morning, but got little from them in the way of proposed remedial actions for these problems. "I frankly had expected a more positive response," he said.

Pressed by the commissioners to set a priority of danger among the plants, Denton refused. "I was not able to distinguish among them," he said.

The NRC staff recommendation came as President Carter swore in his Commission on the Accident at Three Mile Island, telling them, in a morning ceremony at the White House, that their most urgent task is to advise the nation on how such accidents can be prevented in the future.

"When you make your recommendations to me, I intend to carry them out," Carter told the 11 members. "The public is eager to hear from you and, of course, so am I."

They responded by promising publicly to pursue their charter to find out "everything there is to know" about the Three Mile Island accident.

At the NRC meeting, meanwhile-a remarkable, jam-packed affair that lasted well into the evening-the five commission members agonized publicly over the recommendation as utility executives and even some members of the NRC staff attacked the idea of shutdown.

Dennis Rathbun of the NRC's policy evaluation office said that electricity supply problems in the Virginia-Carolinas (VACAR) power grid area "could be rather serious. . . barely okay at best." The Mid-South Utilities area would be more than 2,000 megawatts below its summer demand level, he said.

"This doesn't necessarily mean that the lights go out but that something must be done to buy power from someone else," he said.

Duke Power president Lee said "it is highly probabe that we will go into rotating blackouts" with the advent of warmer weather. He said any shutdown of the Oconee units would cost his customers $1.2 million a day in replacement fuel.

Changes implemented since Three Mile Island provide "several orders of magnitude of additional assurance" against another incident like it, Lee added. He complained of a "communications gap" with the staff of the NRC, saying, "Their concerns relate to a lack of flow of official paperwork."

Commissioner Richard T. Kennedy worried that the staff evaluation had been "very, very quick. Are we moving too fast?" he asked.

NRC Chairman Joseph M. Hendrie observed that some short-term steps yet undiscovered might avoid the need to close. "I'm unhappy that decisions have to be made before these have a chance to be implemented," he said.

Denton agreed that although one or two more days posed little risk, "the legal system forces you to make some sort of decision, black or white." He said he would probably have known already of any substantive change that could be done in less than a month. "We believe that these plants should be shut down now," he said.

Officials of the other utilities joined Lee in outlining steps they had taken and in urging the commission to keep their reactors running. B&W's vice president for nuclear operations, John MacMillan, warned against "hasty, illconceived [design] changes without consideration of their effect on all aspects of plant operation."

The NRC made public a telegram from California Gov. Edmund G. (Jerry) Brown Jr. again asking that the Rancho Seco plant in that state be closed. Ti also was learned yesterday that five lawsuits have been filed in Harrisburg, Pa., by area residents charging damages from the Three Mile Island accident. CAPTION: Picture, President Carter meets with newly sworn-in members of his commission on the Three Mile Island accident. By Frank Johnston-The Washington Post