Two key congressmen are attempting to block foreign participation in a test of a plutonium reprocessing plant in South Carolina pending a study of "implications of this decision on our efforts to halt the further spread of nuclear weapons," it was learned yesterday.

Sen. Gary Hart (D-Colo.) and Rep. Richard L. Ottinger (D-N.Y.) said in a letter to Energy Secretary James B. Edwards that a proposal to permit Japan and West Germany to finance and participate in a "cold test" of the Barnwell reprocessing plant clearly involves "the export of sensitive nuclear technology."

The plant's owner, Allied-General Nuclear Services--a consortium of Allied Chemical Corp., Gulf Oil Corp. and Royal Dutch Shell--has asked Edwards to approve a demonstration for Japanese and German scientists of how the safeguards system at the $300 million facility would work if the plant were turning out plutonium that could be used in manufacture of nuclear weapons.

The Barnwell plant, designed to reprocess spent fuel from civilian atomic power plants and separate the reusable plutonium, has been in limbo since President Carter--concerned about the risk of nuclear proliferation--ordered an "indefinite deferral" of commercial reprocessing in 1977. The Reagan administration has been seeking to revive the plant.

The owners of the plant have been eager to defuse the nonproliferation argument by demonstrating the facility's sophisticated safeguards system, designed to make it easier for international inspectors to detect diversion of plutonium for possible clandestine use in manufacturing nuclear weapons.

But Hart and Ottinger, who have sponsored legislation that would tighten the 1978 Nuclear Nonproliferation Act, expressed concern that by demonstrating Barnwell's system for keeping track of plutonium, sensitive technology might be transferred "from the United States to nuclear scientists and engineers from foreign countries."

The test was originally planned for late September. But since none of the foreign parties has agreed to put up the $1.5 million needed to conduct the 10-day demonstration, the test probably could not be held before early next year.