But in court documents discussed today at the hearing and reported last week by the Cincinnati papers, Government officials acknowledged for the first time that ''the Government knew full well that the normal operation of the Fernald plant would result in emissions of uranium and other substances'' into water supplies and into the atmosphere. A Health Hazard

The uranium was released in the form of particles. The court documents on Fernald, dated Sept. 30, when they were filed in Ohio by the department in response to a suit filed by residents, did not say whether there had been injuries or how many workers or local residents might have been exposed to potentially dangerous levels of radiation. But nuclear experts from outside the Federal Government testifying today said the discharges represented a health hazard.

The danger, say health specialists, is that the particles emit low levels of radiation. Although they are essentially harmless outside the body, they can be deadly if inhaled. If the particles become lodged in the lungs, the radiation can penetrate surrounding cells, altering biochemical processes and possibly leading to malignancies. If the particles end up in drinking water, as they have in Ohio, the particles can be lodged in other body tissues, posing the same danger.

In written testimony for the hearing, Dr. Richard Shank, director of Ohio's Environmental Protection Agency estimated that about 12.7 million pounds of uranium waste had been disposed of in pits since the plant opened in 1951. He said an additional 167,000 pounds had been discharged into the Great Miami River and 298,000 pounds had been discharged into the air.

The acknowledgment by the Energy Department comes after more than two weeks of revelations about unsafe conditions, accidents, radioactive contamination and deficient management at two weapon plants in South Carolina and Colorado that are now shut down.