The Reagan administration has authorized the export of a computerized control system for a "sensitive" facility crucial to Argentina's aim of nuclear independence, it has been learned.

The move, approved by Energy Secretary James B. Edwards last year after an interdepartmental review, is another in a series of reversals of Carter administration policies on nuclear exports, and has seriously undercut a longtime U.S. effort to pressure Argentina to put all of its nuclear program under international safeguards.

The U.S. government refuses to make public applications by companies for export licenses under section 57b of the Atomic Energy Act on the grounds that this information might damage the applicant's competitive position.

The system that was approved for export by the Foxboro Co. of Foxboro, Mass., will become the "brains" of a large heavy-water plant. A heavy-water plant--like uranium enrichment and plutonium reprocessing facilities--is classified as a "sensitive" technology because it is the critical element in one route to production of materials that could be used in nuclear bombs.

Argentina's acquisition of such a plant, which is under construction at Arroyitos and expected to come on line in 1984, would appear to take on even more significance in light of recent remarks by Adm. Carlos Castro Madero, chairman of Argentina's Atomic Energy Commission, since the war with Britain over the Falkland Islands.

"Until today, the safeguard agreements signed by Argentina have not permitted it to undertake nuclear energy development for military purposes," Castro Madero said in a radio interview three weeks ago. "Argentina will comply with all its obligations, but from now on reserves the right for itself to undertake the development of euphemistically so-called non-proscribed military uses."

The 1978 Nuclear Nonproliferation Act bans U.S. cooperation with any country that has not placed all its nuclear activities under International Atomic Energy Agency safeguards. But the Reagan administration was able to authorize the export to Argentina because the control system was ostensibly sold to a Swiss firm that has the overall contract for the plant.

The Foxboro Co., in seeking approval of the sale, made no effort to deceive the administration. Its application specified that it proposed to sell "a process control system to Sulzer Brothers in Switzerland for ultimate end-use in an Argentinean heavy-water production facility."

The Carter administration had made a major effort to pressure Switzerland into refusing to sell Argentina the heavy-water plant until the Argentines agreed to accept safeguards on all their nuclear facilities. The Swiss refused to make such sweeping demands, though their agreement calls for safeguards on the $300 million plant. Some of Argentina's nuclear plants are under international safeguards, but some key facilities are not open to international inspection.

The Reagan administration has repeatedly emphasized that it felt the Carter administration policies were too rigid, and has contended that the United States can only exercise influence on other countries' nuclear programs by reestablishing America's position as a leading nuclear exporter.

The export decision thus would seem to suggest a willingness by the Reagan administration to exploit loopholes in the Nuclear Nonproliferation Act. The Energy Department later turned down a request by Masoneilan International, a subsidiary of McGraw Edison, to sell valves for a heavy-water plant directly to Argentina.

Senior officials of both Masoneilan and Foxboro agreed the valves are a far less critical part than the process control system that would run them.

Rep. Jonathan B. Bingham (D-N.Y.), who has introduced legislation that would amend the Nuclear Nonproliferation Act to eliminate loopholes that permit exports like these to countries not eligible for direct sales, held hearings two weeks ago at which administration officials testified they would soon announce a more restrictive nuclear export policy.

"If these changes are not carried out in such a way as to keep nuclear technology out of the hands of countries like Argentina that are bent on making nuclear weapons," Bingham said last week, "then the only recourse is for Congress to pass legislation forcing the administration to act responsibly."

Argentina has steadfastly refused to sign the 1968 Nuclear Nonproliferation Treaty, which Castro Madero has denounced as inherently discriminatory, and Argentina also has refused to ratify the Treaty of Tlateloco, which would ban nuclear weapons from Latin America.

The Congressional Research Service, in a mid-May assessment of Argentina's atomic program, concluded that Argentina was "getting closer to an ability to make nuclear weapons" and "might be able to test a nuclear explosive by the mid-1980s." The heavy-water plant that Sulzer Brothers is building for Argentina will be the only significant facility of its kind in the developing world, and will give Argentina an independent source of the liquid needed to operate its three natural uranium power reactors.

Since these power reactors are fueled with natural uranium--which is plentiful in Argentina--and do not require the enriched uranium used by nuclear power plants in the United States, completion of the heavy-water production plant will significantly reduce Argentina's dependence on foreign suppliers. It recently has been purchasing its heavy water from the Soviet Union.

Heavy-water reactors like Argentina's also produce plutonium that is better suited for use in nuclear weapons than the plutonium found in the burned-up fuel of American commercial power reactors. Argentina currently has a pilot facility capable of reprocessing small amounts of plutonium, and is constructing a larger reprocessing plant scheduled to go into operation in 1983.

Gerald H. Gleason, vice president of Foxboro, said the process control equipment that his firm is providing for the Argentine heavy-water facility "has not yet left our Dutch factory." He described the equipment as a digital system that controls the temperature, pressure, flow and density of the fluids moving through a heavy-water plant.

"While our equipment will be used in a nuclear facility, it is not nuclear in nature," Gleason said. "It could be controlling an oil refinery, a chemical plant, or in this case some part of a nuclear process. Our direct competitor for this order was Siemens in West Germany."

B.J. Rayburn, president of Masoneilan International of Norwood, Mass., whose contract to provide $1 million worth of valves to Argentina for a heavy-water plant was disapproved by the Reagan administration early this year, said he was surprised when he learned the Foxboro Co.'s order had been approved.

"That's the system that tells our valve what to do. It is the brain of the system," Rayburn said. ". . . . If I had known they had approved that, I think I would have screamed a little louder."