An exceptional five-year, $7.5 million research project on the Soviet Union, conducted by academics with federal financing, was disclosed publicly yesterday, two years after being launched with great confidentiality to preserve its objectivity.

The study taps what many specialists regard as one of the deepest wells of information on the Soviet Union available in the West, the more than 250,000 Soviet citizens who have emigrated from the Soviet Union since 1970.

About 130,000 of them reached the United States. They are the prime source of data being systematically examined for the first time to seek answers to these questions:

* How does the Soviet system really work?

* How do Soviet citizens work the system?

* Where is the Soviet system going?

Project spokesmen said approximately 85 percent of these Soviet emigres were statistically classed as Jewish, although many were only nominally Jewish or non-Jews in mixed marriages or children of such marriages; 10 percent are Armenian, and the remainder are of other ethnic origin.

From this pool of emigres, a scientifically selected sampling of 2,800 has been selected for interviews, to minimize bias caused by above-average education and income, skilled employment and ethnic background. Officials said unusual precautions have been taken to limit knowledge of the interviewees' identity to non-governmental scholars.

In the early 1950s, Harvard University sponsored a successful project that drew on Soviet Armenian emigres. Broader opportunities were created by the surge of Soviet emigres that began in the late 1960s, peaked in 1979 and was severely curtailed after the U.S.-Soviet clash over the Soviet invasion of Afghanistan in December of that year.

A U.S. attempt in 1972 to tap the emigre flow into Israel, State Department officials recalled yesterday, foundered on Israeli concern that it might dry up the refugee flow.

That prompted the present study, first championed by Carter administration Soviet specialist Marshall Shulman, and official Washington's prime proponent of federally supported research on the Soviet Union, Andrew W. Marshall of the Pentagon.

Paul K. Cook, special assistant for Soviet and East-West affairs in the State Department's Bureau of Intelligence and Research, said that, while U.S. scholars have drawn on the Soviet emigre community for specialized research projects, the project now under way is "to my knowledge the largest single social science project the U.S. government has ever financed."

He said that $3.7 million of the estimated $7.5 million has been allocated and that nearly two-thirds of the amount is from the Defense Department, less than one-third from the CIA's "overt side" and "5 to 10 percent from State," mostly in services.

The project is officially funded by the National Council for Soviet and East European Research, an independent nonprofit corporation created in 1979, and final responsibility for it rests with academics, not government officials.

Research is directed by a nine-member group of scholars, headed by Prof. James R. Millar, professor of economics at the University of Illinois. The project employs 92 bilingual interviewers, mainly U.S. graduate and post-graduate students in Soviet affairs. First published results are expected in about a year.