American industry stands to lose as much as $6 billion in export sales of nuclear reactors over the next two years to foreign firms -- notably French -- whose governments more generously subsidize such exports, according to some U.S. business leaders.

They cite President Reagan's planned reduction of direct lending authority by the Export-Import Bank as the main reason for a disadvantaged competitive situation, affecting nuclear power plants and exports of heavy machinery, aircraft and other capital equipment.

But the money pinch on the Ex-Im Bank would have the most dramatic impact on the ability of American companies to participate in the nuclear power boom getting under way in the Third World and in some small industrial countries, desperate for new sources of energy.

At stake, according to Claude Hobbs, vice president and senior counselor of Westinghouse -- one of four major American producers -- are 14 nuclear power projects. There are two each in Taiwan, Mexico, Egypt, Korea, the People's Republic of China, Spain and Italy, ranging in cost from $500 million to $1.1 billion.

If Reagan's proposed limitations on the Export-Import Bank budget are adopted by Congress -- as now seems inevitable -- "for all practical purposes, we are going to be out of that market for the foreseeable future," Hobbs said. He adds that the Third World is the only market that can keep the nuclear power plant industry alive in this country, because other major industrial producers won't buy equipment they can make themselves.

The overriding fear articulated by U.S. manufacturers is that their know-how and highly specialized work force will dwindle slowly without foreign orders, leaving the the nation vulnerable at some point down the road when fears engendered by the Three Mile Island accident could wane, allowing a full-scale return to the expansion of nuclear-power capability here.

At the moment, the Ex-Im Bank is bumping up the annual ceiling for new loan commitments, scheduled to be set by the new budget at around $5 billion for this year. The Ex-Im Bank has already made preliminary commitments for two and one-half times that amount, and is not thinking of accepting any new business. Industry officials say that this $5 billion ceiling needs to be boosted to around $7.5 billion to meet the competition from abroad for nuclear power plant sales.

But the Reagan administration argues that while it is committed to a strong export policy, including an important role for the Ex-Im Bank, it believes that its basic recovery program will do more to increase U.S. exports than doubling or tripling the Ex-Im budget would do.

For example, in congressional testimony April 28, acting deputy Assistant Treasury Secretary John D. Lange Jr. noted that the bank's outlays amounted to 3 3/4 precent of capital goods exports last year.

Nonetheless, several high administration officials, including Commerce Secretary Malcolm Baldridge, have expressed sympathy privately with the industry position. They are believed to have some support among high White House aides. But while the budget is still moving through the congressional process, one industry man said, "They're not going to compromise -- after all, this is the 'even-handed' part of their budget cut."

Yet, for each 1,000-megawatt plant costing upwards of $1 billion, industry sources say, there's a return of $600 million to the U.S. government directly for "enrichment services" for the nuclear fuel. Thus for the 14 plants up for grabs in the next two years, involving about 7 million megawatts, there would be a $4 billion return directly to the U.S. government, according to the industry.

In addition, production in the United States of the turbines and other equipment for nuclear plants sold abroad is a source of substantial employment and jobs -- much of it in the hard-pressed Northeast, industry sources claim. Beyond the original orders, the private companies that get the business -- Westinghouse, General Electric, Combustion Engineering and Babcock & Wilcox -- also get the income from reloading or replacing part of the original core.

One of the large American producers also claims that the turbine generator business here has been depressed because other makers -- the West Germans and Swiss primarily -- have open access to the U.S. market with only a small duty to pay, but those same countries, and Japan, have in place an absolute ban on the purchase of American turbine generators.

Reagan proposed a budget that would have required a cutback from President Carter's recommendation of $5.5 billion in new direct loan authority money for fiscal 1981 to $4.75 billion, with a further drop to $4.0 billion in 1982. The compromise Senate-House budget appears to have trimmed Reagan's figure back to an even $5 billion, which in effect becomes the ceiling on new loans for this year.

The real crunch is obviously even more severe, according to the industry, because fiscal 1981 is about half over. In addition to direct loans, the Ex-Im bank can make loan guarantees, or provide insurance to help make U.S. products competitive with foreign products, within the scope of an over-all ceiling which is not yet in jeopardy.

In effect, the Ex-Im bank lends money to overseas customers who then purchase American products. The money, supporters of a bigger Ex-Im role point out, never actually leaves the United States, and the foreign borrowers repay the Ex-Im bank with interest.The bank has earned a profit every year since it was begun in 1934.

But the system works for American companies only when some other government doesn't offer a better deal. Although the Carter administration has complained, in past economic summit meetings and at the Paris-based Organization of Economic Cooperation and Development (OECD), that France pursues unfair subsidy practices, no agreement has been reached on uniform practices. New discussions along these lines began last week in Paris, but not much is expected to result from them. "The Reagan people sacrificed their clout," says company lawyer, "when they cut the Ex-Im budget."

Until interest rates began to soar in this country last year, Ex-Im rates apparently were just about competitive with those offered by other countries. But with the prime rate at 19 1/2 percent, American companies despair of meeting the kind of terms offered by foreign governments without more generous subsidies from the Ex-Im bank.

A Westinghouse company memordandum alleges, for example, that in obtaining a recent contract for the sale of two nuclear plants to the Korea Electric Co., a French firm had 85 percent of the price financed by the French government over a 22 year period at a 7.6 percent interest rate, inclusive of fees.

As against that deal, recent Ex-Im bank guidelines would have provided for only 65 percent financing at an 8 3/4 interest rate, plus a 1/2 percent commitment fee on the undisbursed portion, the Westinghouse memorandum says. The additional 20 percent of the loan would have cost at least 13 3/4 percent of finance.

According to the Westinghouse calculation, when the differentials offered the Koreans are applied to a $1 billion project, the interest cost-saving to the customer over a 22-year period amounts to $199,750,000. What burns American businessmen up is the conviction that the French do not have a technological edge, "only a subsidized financing edge." In addition, sources say, France also offered its Korean customers certain fuel technology that the United States would not provide, in view of its effort to prevent the spread of bomb-grade nuclear materials.

Although American sources cite France as their principle competitor in Third World markets, other countries also are active in subsidizing nuclear power exports. It was recently reported in the British press that a British company won a $1.2 billion contract from Hong Kong's China Power and Light Co. with the help of "the biggest export subsidy ever made by the British government." Other producers include West Germany, Canada and, to a certain extent, Sweden.

Separately, the industry complains that another 11 nuclear plants either finished or nearly completed are caught in a licensing logjam at the Nuclear Regulatory Commission, an outgrowth of the workload caused by the Three Mile Island nuclear accident. The industry is demanding a speed-up, citing "unnecessary delays."