The Reagan administration is considering a proposal to let Japan and West Germany finance and participate in a full-scale test of a nuclear fuel reprocessing plant in South Carolina capable of producing weapons-usable plutonium, the Energy Department confirmed yesterday.

The proposed "cold test" of the Barnwell reprocessing plant--which would begin in late September and run for about 10 days--would not involve any fission products, but would demonstrate how safeguards at the plant would function if it were turning out plutonium that could be used in the manufacture of nuclear bombs.

The plant's owner, Allied General Services Inc.--a consortium made up of Allied Chemical Corp., Gulf Oil Corp., and Royal Dutch Shell--also has been soliciting British and French involvement in a test of its idle $300 million facility.

Only Japan and West Germany expressed sufficient interest in putting up money for the $1.5 million test, however. They are included in Allied General's application to DOE for permission to let foreign scientists and engineers observe a demonstration involving a "sensitive nuclear technology" that the United States has previously not shared with other countries.

The reprocessing plant, which needs an estimated $400 million in related facilities to go into full operation, has been in limbo since 1977 when President Carter, concerned about the risk of nuclear proliferation, ordered an "indefinite deferral" of commercial reprocessing in this country.

The Reagan administration has been seeking to revive the plant, however, and DOE recently prepared for the White House a recommendation that the goverment underwrite the plant by agreeing to purchase its output of plutonium and by promising to buy out investors if U.S. policy changes again in the future.

The proposal, however, has run into stiff opposition in Congress, and a Congressional Research Service study sent to the House Energy and Commerce Committee yesterday sharply criticized the recommendation for not fully addressing the implications of "domestic and world use of plutonium as a nuclear fuel" in terms of "the possible future spread of nuclear weapons."

The information contained in the proposal "is inadequate for a top-level decision on a national policy of this import," the CRS study said. The study also notes the administration plan envisions foreign investment in Barnwell--most likely by West Germany--and criticizes the fact that the DOE report "is silent on the access of foreign investors to the Barnwell reprocessing technology."

Rep. Richard L. Ottinger (D-N.Y.), chairman of a House energy subcommittee and sponsor of a bill that would toughen the 1978 Nuclear Non-Proliferation Act, raised the question of protecting sensitive nuclear technology yesterday and called for an investigation of whether foreign participation in tests at Barnwell would "circumvent the Atomic Energy Act.

"These tests may well transfer sensitive reprocessing and safeguards information not now available to Japan and West Germany," Ottinger said. "This is yet another example of the Reagan administration's willingness to jeopardize our security by allowing the spread of dangerous nuclear technology for the benefit of a few special interests."

Ottinger said he was sending a letter to Energy Secretary James B. Edwards inquiring about the Barnwell proposal, which congressional sources said has been circulating in classified form for several days. A DOE spokesman confirmed yesterday that such a request was under consideration but said no recommendation had yet gone to Edwards, who under the Atomic Energy Act must grant specific approval.

Dr. James Buckham, president of Allied General, said yesterday the consortium that owns Barnwell expected government approval of foreign participation in the safeguards test "shortly."

Buckham said, however, it now appeared that more likely only the Japanese would put up the financing and participate in the demonstration. He said that to proceed with the test in September, Allied would need a commitment from the Japanese by the end of the month.

"The reason the Japanese are interested is they are initiating the design of a large reprocessing plant like Barnwell, and if they are going to have a safeguards system like the one we use, it would be much better to fit it in at the beginning of the design rather than much later," Buckham said.

Buckham said permitting the Japanese to observe the safeguards system at Barnwell would not enable them "to acquire any technology they don't already have in reprocessing" as a result of operating a small reprocessing plant at Tokaimura. A number of U.S. nonproliferation experts, however, disagree and suggest that a Japanese scientific and engineering team could probably learn everything there was to know about Barnwell through watching it operate for 10 days.