A federal grand jury in Harrisburg is investigating allegations that employes of the Three Mile Island nuclear plant deliberately falsified the results of required tests for months before the accident that crippled the Pennsylvania reactor in March, 1979.

Sources familiar with the case said federal prosecutors presenting evidence to the grand jury recommended seeking a perjury indictment against one TMI control room operator in August because of conflicts in his testimony on the test results. But D. Lowell Jensen, head of the Justice Department's Criminal Division, declined to approve it at the time because he felt there were legal weaknesses in the case.

The name of the operator facing possible prosecution could not be learned, but sources said the investigation is continuing. It is possible that he will be allowed to return to the grand jury to clear up conflicts between his testimony there and what he told Nuclear Regulatory Commission investigators earlier.

Carlon O'Malley, the U.S. attorney in Harrisburg, declined to comment on the case. When asked about Jensen's action, he said, "I'm not aware of anything that could be considered sinister."

If the allegations are proved, they could have an adverse impact on General Public Utilities Corp., the $4 billion corporation that owns TMI and wants NRC permission to restart a companion reactor.

The grand jury investigation has been going on for more than a year with little public notice. It stems from allegations made by Harold W. Hartman Jr., a former TMI control room operator, to the NRC and a New York television station.

Hartman told NRC investigators in May, 1979, that safety valves at the plant were leaking for at least three months before the accident, according to a summary of a staff interview. He said the test results "had to be fudged every time we got, just about every time we got it, we had to do something to make it right."

When he told his supervisors about the problem he was told to "get a good leak rate." Otherwise, the reactor might have to be shut down.

A GPU spokesman said yesterday that the company would have no comment on Hartman's allegations because of the grand jury proceeding. He said GPU considered the investigation "non-company" because it was looking only at Hartman's charges about fellow operators.

The NRC staff said in a report that its preliminary investigation into Hartman's charges "identified a number of apparent problems related to procedure adherence." But,pending completion of the grand jury inquiry,NRC said that so far "there appears to be no direct connection" to the TMI accident.

Some congressional TMI investigators said they feel there is a connection, because the allegedly false test reports were describing part of the same system that failed when TMI was crippled.

Victor Stello Jr., director of the NRC's office of inspection and enforcement, said he forwarded information about Hartman's allegations to the Justice Department in early 1980 because of the possibility that plant records had been falsified.