The Inferior Department will not try to stop construction of a nuclear power plant next to the Indiana Dunes National Lakeshore, Secretary Cecil D. Andrus said yesterday.

The nuclear project had been opposed by former Secretary Thomas S. Kleppe, who said it would pollute the air and water of the lakeshore, one of the few urban parks in the nation. However, Kleppe did the suit to stop its construction.

Andrus told a group of reporters yesterday, "We would have been better off if the plant had not been licensed. But the company has spent $80 million. Until there is new evidence, we do not feel we can ask for a rehearing" before the Nuclear Regulatory Commission.

The $705 million plant is supported by its sponsor, the Northern Indiana Public Service Co., which says it is needed to provide electricitiy. It is opposed by environmentalists, by the city of Gary, Ind., and by the state of Illinois. They contend it would damage the park and endanger visitors.

The utility has just completed escavation for the plant on a site next to Lake Michigan, between the Bethlehem steel complex and the park.

The park, an 8,500-acre expanse of beaches, sand dunes and marshes, is in the middle of an industrial area. It is heavily used by residents of Gary, five miles away, and Chicago, 30 miles away.

In answer to a question yesterday, Anrus said. "Frankly, I have been a proponent of nuclear power. The very first reactor was built in Idaho (where Andrus was governor). I've been around it most of my life, and I don't have a fear of it."

Andrus has been under considerable pressure from Rep. Sidney R. Yates (D-Ind.), chairman of Interior's appropriations subcommittee, to go to court to stop the plant's construction.

Yates said yesterday Andrus

Yates said yesterday Andrus' decision was "unfortunate" and that the plant "interferes with the public's right ot have the lakeshore for its enjoyment and use.

Opponents of the project led by a Chicago group called Businessmen for the Public Interest, had pinned their hopes on the Interior Department after a federal court refused to revoke the plant's license last year.

They maintained that the U.S. Circuit Court of Appeals in Chicago virtually invited the Interior Department to oppose the project by pointing out that it could not deny the license when the department was not involved in the suit.

Kleppe's response was a sharply worded letter to Marcus A. Rowden, chairman of the Nuclear Regulatory Commission. The site of plant is "most inappropriate . . . (and) does not make sense ecolgically or aesthetically," Kleppe wrote, pointing out that Interior had opposed it since 1972.

The public would be restricted from using the park within a mile and a half of the reactor, Kleppe said, because of the possibility of a "catastrophic accident and discharge of radioactive wastes."

The 450-foot-high cooling towers would create an "unacceptable . . . visual impact and aesthetic intrusion as well as acid air pollution from the combination of vapor with the plume of a nearby coal-fired plant, Kleppe said.

The company is already engaged in a dispute with Interior becaus ponds to store ash next to its coal plant seep into the park. Andrus said the utility has 60 days to clean up the ponds.

A utility spokesman said the park will be protected from any adverse effects by a 300- to 500-foot "greenbelt buffer zone" around the nuclear plant.

Andrus said, "In the future, where we have the opportunity, we will be more aggressive than my predecessor" in protecting public parks from adjacent nuclear plants.