The Reagan administration, in a reversal of U.S. policy, has approved the sale of 143 tons of "heavy water" to Argentina for its nuclear reactors even though that country has refused to open all its atomic facilities to international inspection, administration officials said yesterday.

Approval of the sale was announced in a two-paragraph item in the Federal Register Aug. 2. There was no effort to obtain the concurrence of the Nuclear Regulatory Commission, as has generally been done in the past.

Some NRC officials said that they would have raised objections to the sale if asked.

Administration officials, however, said that NRC approval is not required by law.

The heavy water, a liquid used in controlling nuclear chain reactions, is owned by West Germany. But U.S. approval of the $100 million sale is required because it was produced in the United States.

The Carter administration resisted nuclear sales to Argentina. It pressured West Germany and Canada not to sell Argentina a heavy water nuclear plant unless that country agreed to place all its facilities under international safeguards.

Argentina, which has not signed the nuclear Non-Proliferation Treaty or ratified the Treaty of Tlatelolco, which declared Latin America a nuclear-free zone, is building an unsafeguarded reprocessing plant that could provide plutonium for atomic weapons. That plant does not use heavy water, however.

Because the sale of the heavy water is not a "direct export," an NRC license does not have to be obtained for the sale, as it would if the heavy water were being purchased directly from the United States.

"But clearly, since this is U.S.-origin heavy water, the sale should meet the same criteria that would apply to a direct export," a senior NRC staff member said yesterday.

"We had indicated enough concern over this that we expected to have an opportunity to review it and express our opinions," another NRC official said. "That's how this always has worked. And there are things going on in Argentina we would have taken into consideration."

Deputy Assistant Secretary of State James B. Devine said, however, that the administration decided to approve the sale after Argentina agreed that the heavy water would be used in safeguarded electric power reactors as opposed to other atomic facilities and that "there would be no use of any items exported to Argentina in nuclear explosives."

Devine said that while the Reagan administration thinks nuclear-supplier countries should require that buyers place all their nuclear facilities under international safeguards "as a condition of receiving significant new supplies, I don't think we consider this transaction a significant new supply."

While Switzerland has sold Argentina a heavy water plant that will have a 250-ton-a-year heavy-water capacity when completed in 1985, Argentina has had difficulty in recent years obtaining sufficient supplies. At one point, it bought 11 tons of heavy water from the Soviet Union.

Argentina has one of the most advanced nuclear industries in the Third World and has insisted on its right to stage peaceful nuclear explosions in addition to using nuclear plants for electric power.

U.S.-Argentine relations have been extremely sensitive since the Reagan administration took Britain's side in the war over the Falkland Islands last year.

NRC officials complained that they learned that the administration intended to approve the sale when the Federal Register announced it.

"Then, instead of giving us a chance to express our opinions, they went ahead and approved the re-transfer the very next day," an NRC official said. "I mean we are talking about Argentina. What's the hurry beyond the fact that Congress isn't in session and half the NRC commissioners aren't in town?"

Sen. Gordon J. Humphrey (R-N.H.) also expressed concern at the way the matter was handled, saying that "it had not been publicly announced."

"If our national policy is not to export nuclear materials to any country that has not signed the Non-Proliferation Treaty, we ought not be continually making exceptions as we are with India and now this Argentine thing," Humphrey said.

Devine yesterday said he was "a little bit perplexed" by the NRC complaints over the handling of the Argentine deal. He said that the request for approval of the transfer was first made in 1981 when the heavy water was jointly owned by Germany and Britain.

"We are not obliged to get the the NRC's views on re-transfers, but they knew we were getting the assurances from Argentina," Devine said. "We didn't hear one word from them."

In the interval between the time the re-transfer was proposed and its approval two weeks ago, Britain sold its share of the heavy water to Germany. Thus, Britain is not involved in aiding Argentina's nuclear program.