A brushed aluminum plaque, which a decade ago seemed to mark the dawn of a golden era for the American nuclear industry, hangs almost out of sight today on a wall of the Tarapur atomic power station.

"This cooperative project between India and the United States marks the introduction of nuclear power into India and, as the first nuclear power plant to be built in any developing nation, it demonstrates the practicality of the developing nations sharing in this benefit of nuclear energy."

Signed by Glenn T. Seaborg, then chairman of the U.S. Atomic Energy Commission, the plaque stands as mute testimony to a one-time American policy of encouraging the sale of atomic power stations to developing countries.

The U.S. nuclear industry, moreover, dominated by General Electric -- which built the Tarapur plant -- and Westinghouse, was poised to dominate the hundred-billion-dollar export market envisaged in the decades ahead.

In 1971, according to the Atomic Industrial Forum, Westinghouse and GE sold eight reactors outside the United States. Their European competitors sold only one.

But in the mid-1970s, the Ford and Carter administrations began taking another look at U.S. nuclear export policies and started rewriting the rules in an effort to prevent the spread of atomic weapons.

The shifting American nuclear policy had one immediate result. Almost over night, it practically eliminated Westinghouse and GE from the world market.

"They killed us," one Westinghouse official remarked bitterly. That does not appear to be much of an exaggeration.

U.S. manufacturers in 1976 sold one nuclear reactor overseas, compared with nine for Europeans. In 1977, the Europeans won four new contracts -- the Americans none.

Brazil, which bought its first small atomic power station from Westinghouse, decided to buy its next eight large plants from West Germany's Kraftwerk Union. Iran, which had been negotiating with both Westinghouse and GE, finally bought two reactors from Germany and two from France's Framatome.

The sales were particularly bitter medicine for Westinghouse. Both Kraftwerk Union and Framatome began building enriched uranium reactors as Westinghouse licensees.While Framatome still pays royalties, Kraftwerk Union -- which has altered its reactor design -- pays none.

While the U.S. government's unwillingness to let sensitive technology be transferred to Brazil led to Brasilia's decision to buy power plants from Germany, a more fundamental problem for the American nuclear industry is the uncertainty the Carter administration has created over fuel for power stations.

When a manufacturer sells an atomic power plant to a foreign purchaser the contract generally includes supply of the enriched uranium needed to keep it running for at least the first 10 years -- and in some cases for the life of the reactor.

With enactment of the Nuclear Nonproliferation Act this past spring, however, the Carter administration is threatening to break contractual agreements to supply fuel if countries like India and South Africa do not agree to meet new guidelines.

Irrespective of the merit of the new guidelines, the doubts the Carter administration has created by indicating that contracts may be broken every time a new president takes office has had a devastating effect on the sales efforts of the U.S. nuclear industry.

"The Carter administration seems to be saying that a contract signed with the United States is valid until the U.S. decides to change it," a high-ranking Indian official said.

A top South African Foreign Ministry official, even more bitter over the prospect that the United States will not honor a 1972 contract to provide fuel for two atomic power stations under construction at Koeburg, declared: "You cannot rely on the United States in the long term for such a vital ingredient to your economy."

The surprising discovery one makes in touring the developing countries, however, is how many key people -- despite the difficulties in doing business with the U.S. government -- favor buying future atomic power stations from the American nuclear industry.

Whether the country is Brazil, or Iran, or South Korea, many of the top nuclear scientists went to universities in the United States. Many of the engineers who are returning home as project managers worked for U.S. companies like Bechtel and favor American methods of construction.

"The situation is still wide open here for U.S. companies," a top Brazilian official said. Echoed a key member of the Korean nuclear program: "American companies could still have virtually the whole nuclear industry worldwide."

But even Korea, which gave the U.S. industry a small boost by buying two more nuclear power plants this year from Westinghouse, is talking seriously about shifting the bulk of its future purchases to European suppliers if the Carter administration continues to pursue a negative policy on such advanced technologies as breeder reactors and reprocessing.

"We have a commitment with the U.S. government that we will follow the principles that they set forth," said Bong Suh Lee, assistant minister for planning in the Ministry of Energy and Resources. "But if other countries seem more liberal, we may have to consider again whether it is wise to follow the U.S. policy."