President Carter yesterday removed the chairman of the Nuclear Regulatory Commission and promised further agency reorganization in pursuit of nuclear safety.

But he reiterated his administration's commitment to atomic power, saying, "We cannot shut the door on nuclear energy."

Responding for the first time to the report of the presidential commission that investigated the March accident at Three Mile Island, Carter also urged the NRC to wait no more than six months before ending its self-imposed pause in licensing of new nuclear power plants.

The president thus outlined a position consistent with his earlier pro-nuclear views and clearly different from those of both of his Democratic challengers for the presidency, who have called for a strong licensing moratorium.

As Carter spoke, NRC staff members outlined an "action plan" to implement changes made necessary by Three Mile Island. Sources said these changes could cost as much as $25 million at each existing plant, or about $2 billion nationwide.

In replacing Joseph M. Hendrie as NRC chairman with Commissioner John F. Ahearne, Carter said he was acting in the spirit of the recommendation from his advisory commission that the NRC be reorganized under a single administrator. Administration officials said it had become clear that such a change would have no support in Congress, which would have had to implement it.

Instead, the president chose to strengthen the powers of the chairman as chief executive officer to take action in case of nuclear emergencies. A further reorganization plan to clarify the chairman's powers in personnel and other decisions will be sent to Congress early next session, Carter said.

A five-member advisory committee of experts also will be named to report to the president and Congress on the progress of change within the industry and at the NRC.

Ahearne's appointment is temporary until June, when Commissioner Richard T. Kennedy's term expires and a vacancy will allow the president to appoint an outsider as chairman. Other officials confirmed that Kennedy's name would not be resubmitted.

Hendrie said he will remain on the commission, that Carter had not asked him to resign and that he had never considered resigning.

"The call from the [president's advisory] commission for new and outside leadership was very strong and clear," Hendrie said. "I think he acted for that reason and not because he was looking for someone to tar and feather. Ahearne is going to be a first-class chairman. I very much applaud his selection."

Ahearne returned the compliment. "Joe is a gentleman in all senses and we will have no problem working together," he said.

As interim chairman, Ahearne said, his first priority will be keeping the agency moving. "It will be a very difficult period. . . . We will all have to work extremely hard to carry out the actions we've committed ourselves to and to live up to the expectations a large number of people have of us," he said.

Carter accepted most other recommendations of the advisory commission, which was headed by Dartmouth College President John G. Kemeny and made its report Oct. 30.

Kemeny said he was delighted with Carter's action, which he called "more comprehensively supportive than anything I had dared hope for." He added that, although Carter's details were slightly different, "they are completely responsive to the goal we were trying to achieve."

Carter called on the nuclear industry, which supplies 13 percent of American electricity, to beef up its training program for reactor operators and to modernize, standardize and simplify reactor control rooms as much as possible. "I challenge our utility companies to bend every effort to improve the safety of nuclear power," he said.

As Carter was speaking, NRC staff members were briefing the NRC's Advisory Committee on Reactor Safeguards (ACRS) on the agency's "action plan," its list of changes utilities will be required to make as a result of what was learned from the Three Mile Island accident last March.

Sources at the agency said the list of proposed changes, indexed into 200 categories, could cost each power plant as much as $25 million to implement. The sources also said gearing up to oversee the changes could cost the NRC $50 million or more. The overall implementation could take two to four years.

The action plan, expected to be explained further to the NRC next week, does not include recommendations either from the NRC's special task force on the accident, headed by Washington attorney Mitchell Rogovin, or from a Senate investigation now in progress. Those results are expected in January, but are not expected to differ radically from previous recommendations, such as those of the Kemeny commission.

Still, the list is expected to be reduced simply to be financially realistic. "It just couldn't be implemented as it is," one ACRS member said.

While the most pressing changes are being put into effect, the NRC has imposed a freeze on the issuing of new licenses for construction or new plant operation. Carter endorsed the pause, but added, "I urge the NRC to complete its work as quickly as possible, and in any event no later than six months from today."

He also took pains to note that the NRC has authority to proceed with licensing on a case-by-case basis. The Edison Electric Institute, while expressing general satisfaction with Carter's backing for nuclear power, pointed out that the nine plants ready to be licensed now or within two years could produce electricity equal to 100 million barrels of oil a year.

Carter also raised that theme. "Every domestic energy source, including nuclear power, is critical if we are to free our country from its overdependence on unstable sources of high-priced foreign oil," he said. "We do not have the luxury of abandoning nuclear power or imposing a lengthy moratorium on its future use."

However, Carter added, he has always seen nuclear power as "an energy source of last resort." By that he meant that reaching the goals of conservation, coal use, solar power and synthetic fuel development would allow use of nuclear power to be minimized, Carter said. His aides said later that meant that the 95 power plants now under construction will be built.

Among the dozens of memos the president's staff collected from everyone concerned with nuclear power in the weeks before yesterday's announcement, there were several warning that dumping Hendrie as chairman would make him a scapegoat for the sins of the commission, the NRC staff and the industry in general.

But Hendrie seemed almost cheerful. "The president did what he had to do," he said."Commissioners are less restrained than the Chairman, and this may give me a chance to speak my mind from time to time. It's going to be fun." CAPTION: Picture, JOSEPH M. HENDRIE . . . looking forward to speaking out