The Energy Department has under serious study a plan to bury much of the nation's radioactive waste in salt mines near Carlsbad, N.M., and in deep pits outside of Hanford, Wash.

The plan has the approval of Energy Secretary James R. Schlesinger but will undergo further study before it is submitted to the White House for consideration by President Carter. The congressional delegations of both states have been consulted about the plans and are understood to approve them in principle as long as they pose no serious environmental dangers.

Under study is a plan to move the military wastes kept at the plutonium production plant at Savannah River, S.C., to Carlsbad in the next eight years. Once moved to Carlsbad, the wastes would be stored deep below ground in salt mines located about 10 miles outside the city limits.

Under the plan, the military wastes now stored in shallow-ground tanks at Hanford would be buried in pits as deep as 3,000 feet at the Hanford site, which covers 400 square miles in southeast Washington and where most of the plutonium for the nation's atomic weapons arsenal has been produced.

The significance of the plan, if it survives scientific scrutiny and wins White House approval, is that it would be the first solid step by the nation toward a policy on nuclear waste disposal. There has been talk of an atomic garbage policy for the last 20 years but never any action to implement one.

About 70 million gallons of radioactive waste have accumulated at Savannah River and Hanford in the last 25 years, most of it at Handford. An estimated seven million gallons of waste is added to the nation's inventory every year, almost all of it the leftover of the plutonium produced for nuclear weapons at Savannah River and Hanford.

The estimated cost of moving the wastes from Savannah River and burying them in Carlsbad is at least $1 billion, with about $400 million going to construction of the burial site. The Carlsbad site has already been given the codename WIPP, for Waste Isolation Pilot Plant.

The cost of burying the wastes at Hanford would run at least $4 billion, mostly because there is so much more waste at Hanford to bury. The plan under study by the Energy Department talks of burying the Hanford wastes by the turn of the century, not before.

A big part of the cost of both burial plans would be for factories at Savannah River and Hanford to convert the military wastes so far accumulated into 10-foot-long glass "logs" to make burial safer and easier. Most of the 70 million gallons of waste accumulated to date is still in liquid form in steel and concrete tanks, though a program has been under way for several years to evaporate the water in the wastes and convert what is left behind to "salt cake" or grain-like solids.

The plan under study by the Energy Department covers only military wastes built up in the last 25 years of the atomic weapons program. Waste from the civilian nuclear power program has only begun to accumulate and almost all of it is spent fuel kept in "swimming pool" storage sites at the power plants where the fuel was burned.

"We think the entire waste inventory will eventually be moved to deep disposal," one Energy Department source said. "Certainly, most scientists recognize that deep burial is the best way to get rid of nuclear wastes. That way, you don't have to watch it and worry about it all the time."

The radioactive waste from atomic weapons production had been kept in single-walled steel tanks until 1968, when some of the tanks at the Hanford site began to leak. Since then, a program has been undler way to solidify the liquid wastes and store them in double-walled steel tanks to prevent leakage.