Vice President Mondale, not President Carter, will address the U.N. special session on disarmament, it was announced yesterday.

The White House said Mondale's participation "underscores the importance the United States attaches" to the five-week session. Nevertheless, the failure of Carter to deliver the U.S. speech next week as had been expected must be considered "pretty significant," as one arms control expert put it.

U.N. Secretary General Kurt Waldheim has described the session as the "largest, most representative gathering ever convended" to discuss arms control.

Heads of state from France, Britain, India, West Germany and more than a dozen Third World countries are expected to participate.

White House press secretary Jody Powell said yesterday Carter was too busy to appear at the United Nations. He added that Carter would make a major address before the North Atlantic Treaty Organization summit in Washington May 29, and also is scheduled to deliver the commencement address at the U.S. Naval Academy at Annapolis on June 7.

Opinions flowed freely among Washington officials yesterday as to why the President decided to skip the U.N. event.

"It reflects a political assessment that he is too close to arms control," was the way one administration official, critical of Carter's decision, put it.

A slightly different view came out of the State Department. An official there said neither Carter nor Secretary of State Cyrus R. Vance wanted to make the speech because they were focusing on strategic arms limitation negotiations with Soviet Foreign Minister Andrei Gromyko the following week.

"The speech had to be one of generalities," this official said, "so they turned to Mondale, the most ranking official with the least substantial public position on disarmament."

The Mondale speech is expected to consist primarily of a report on the various arms control negotiations presently under way.

One administration effort at a new initiative, suspending production by all countries of special nuclear materials used to make weapons, has been tied up in interagency discussions, according to Arms Control and Disarmament Agency aides. It was opposed by the Departments of Energy and Defense, which are asking Congress for funds to increase plutonium production next year because of projected new weapons production.