Three environmental groups Friday sued the Energy Department to try to block construction of a $1.2 billion plant in Idaho that would use a new technology to produce weapon-grade plutonium.

The suit, filed in federal district court in Washington, seeks to stop the department from going ahead later this year with initial preparations for construction of the Special Isotope Separation plant on federal land near Idaho Falls.The environmental groups - the Natural Resources Defense Council, the Snake River Alliance and the Palouse-Clearwater Hanford Watch - contend the department failed to fully assess the potential impact of the project on the local environment.

The suit also contends the department did not give enough consideration to alternatives such as building the proposed plant on other federal sites or not building it at all.

"DOE simply has not lived up to its obligation to look before it leaps into a potentially unnecessary and dangerous plant," said Dan W. Reicher, an attorney for the Natural Resources Defense Council.

A department spokesman, Will Callicott, said there would be no official comment on the suit.

The plant, to be built at the Idaho National Engineering Laboratory by 1995, would use laser technologies to make weapon-grade plutonium out of reactor-grade plutonium that is recovered from commercial nuclear power plants, the suit alleges.

The technology is under development at the government's Lawrence Livermore National Laboratory in California.

Some private analysts of the government's nuclear weapons program contend there is no demonstrated need for the Idaho facility because there already is more than enough plutonium to last for the foreseeable future. The size of the nation's plutonium inventories is classified.

"In light of the plutonium surplus, SIS appears to be a $3 billion pork-barrel project," Paul Leventhal, president of the Nuclear Control Institute, said in a letter to congressional leaders last month. He was referring to the estimated $1.2 billion construction cost and $1.9 billion operating cost.

The Energy Department, in a report earlier this year on plans for modernizing its nuclear weapons complex, labeled the Idaho project "time-critical and essential" and ranked it among its top defense program priorities.

Energy Secretary James D. Watkins, who is reassessing the Reagan administration's long-range plans for the weapons program, has told Congress he strongly supports building the Idaho plant.

"The Special Isotope Separation facility in Idaho should be constructed to provide flexibility and technological diversity to a production complex burdened by reactor operating constraints," he told a House panel earlier this month.