Prime Minister John Vorster of South Africa reassured President Carter in a personal letter earlier this month that South Africa did not intend to develop any nuclear explosive device. White House press secretary Jody Powell said yesterday.

Powell made public a portion of Vorster's Oct. 13 letter to Carter after questions were raised by a statement Vorster made Sunday during a television interview on "Issues and Answers" (ABC, WJLA).

Vorster was asked about a promise he was said to have made to the President that South Africa did not intend to develop any nuclear explosive devices, including weapons. Without specifically mentioning explosive devices, he replied:

"I am not aware of any promise that I gave to President Carter. I repeated a statement which I have made very otten, that, as far as South Africa is concerned, we are only interested in peaceful development of nuclear facilities."

According to Powell in the personal letter to the President almost two weeks ago. Vorster had told Carter:

"You will recall that pursuant to representations made by the United States government, we formally advised it in August that South Africa did not have, nor did it intend to develop, a nuclear explosive device for any purpose, peaceful or otherwise, that the so-called Kalahart [desert] facility was not a testing ground for nuclear explosives; and that there would not be any nuclear explosive testing of any kind in South Africa."

Powell said the original message from the South African government, referred to in Vorster's letter was the basis for a statement the President made at a news conference Aug. 23. At that news conference. Carter announced that South African officials had told him: "They do not have and do not intend to develop nuclear explosive devices for any purposes, either peaceful or as a weapon . . ."

The Vorster interview taped in Pretoria Oct. 17, appeared to contradict the President's news conference statement, prompting the White House to release a portion of the Oct. 13 letter.

Powell said that despite Vorster's television comment "we are certainly not questioning the assurances given us by the South Afrcian government" in August.

The United States, the Soviet Union and other nuclear nations were alarmed in early August by reports that the white minority government of South Africa planned to test a nuclear device. They worked in concert to prevent it, and Carter clearly took great satisfaction when he announced on Aug. 23 the assurances he had received from South Africa.

Much of the President's day yesterday revolved around the questions of energy and oil. In the morning, he hosted a breakfast for congressional leaders to discuss his energy legislation, and later met with Foreign Minister Prince Saud of Saudi Arabia, telling Saud he did not think world oil prices should be raised in the coming year.

According to a White House statement, the two men also discussed the Middel East negotiations and the President assured Saud that the United States will continue to help Saudi Arabia "meet [its] leguimate delense needs."

In other developments yesterday. Carter signed legislation increasing in the Medicare and Medicaid programs.

The President also sent to Congress his nomination of Samuel D. Zagoria to be a member of the Federal Election Commission. Zagoria, now director of the Labor-Management Relations Service of the U.S. Conference of Maryors, is a former reporter and editor for The Washington Post and a former member of the National Labor Relations Board.