A Department of Energy advisory panel has recommended construction of a $4 billion nuclear reactor at the Savannah River Plant in South Carolina to produce the tritium needed to increase the nation's stockpile of hydrogen warheads, it was learned yesterday.

The proposed government plant -- a larger, higher-powered version of existing reactors at Savannah River that produce all the plutonium and tritium used in America's nuclear weapons -- would be the first new reactor built for the U.S. military program in more than two decades.

The advisory panel, which submitted its classified report Monday to Energy Secretary Donald P. Hodel, said that after evaluating the adequacy of future supplies of tritium and plutonium, it agreed with earlier studies that concluded a new production reactor is needed "to assure an adequate supply of strategic nuclear materials in the 1990s and beyond."

The increased need for tritium, a radioactive gas used in thermonuclear weapons, stems in part from U.S. plans to build thousands of new warheads for the MX, Trident and cruise missiles. It also is needed for the neutron warheads that will replace older tactical atomic warheads. In addition, because the isotope decays, the tritium in existing warheads periodically must be replenished.

Hodel, whose department is in charge of all of the facilities that produce materials for nuclear weapons, is expected to act on the panel's recommendation by March 1, an Energy Department spokesman said yesterday.

If Hodel approves the recommendation, funds for initial planning would be added to the Energy Department's fiscal 1984 budget request, a source said. If funds are approved, construction of the reactor would begin in 1985.

In recommending construction of the ZEPHWR (zero electric power heavy-water reactor) at the Savannah River Plant, the special advisory panel headed by former atomic energy commissioner T. Keith Glennan passed over all six reactor types and three alternative sites that had been proposed by the Energy Department.

These included a proposal that the Energy Department take over a partially completed commercial atomic power plant in Washington, where the utility spent $804 million before terminating construction, and convert this reactor to the production of plutonium and tritium.

The panel said it decided this proposal involving the Washington Public Power Supply System (WPPSS) reactor was the least desirable because of "the potential for adverse public perception of converting a plant designed for commercial production of electricity to production of weapons material."

The advisory group also cited concern "over use of commercial technology to produce weapons material" in strongly advising against construction of a pressurized light-water reactor that would be virtually identical to many of the atomic power plants operating in this country and overseas.

The panel also rejected the option of constructing the new tritium production reactor at the Nevada nuclear weapons test site, noting that the underground tests conducted there raised "safety questions" involving earthquakes.

Glennan said in a telephone interview yesterday the panel felt constructing a new reactor similar to those that have been operating at the Savannah River Plant since the 1950s was the best way to meet the objective of producing needed materials on a timely basis.

"Essentially, you have a reactor that has been proven over many, many years, and the Savannah River Plant is operated by an enormously reliable contractor, Du Pont," Glennan said. "Those factors played a major part in our decision."

Of the five production reactors built by Du Pont at the Savannah River Plant in the 1950s, three produce plutonium and tritium for the weapons program, and a fourth -- which was shut down in 1968 -- is being refurbished and is scheduled to be put back into operation next September. The fifth was shut down in 1964, and there are no plans to reactivate it.

The panel said if the administration decided it was essential that the new production reactor generate as a byproduct electricity that could be sold to a local utility, it still would recommend that such a plant be built at the Savannah River Plant site and that it be similar to the reactors already operating there.

The group said if the administration felt it was "essential" that the nation's entire production of tritium for hydrogen warheads not be concentrated at one location, it would recommend that a replacement for the aging N-reactor be built at Hanford, Wash.

The Hanford Reservation was the site of all the plutonium production for America's atomic weapons program during the 1940s. At one point, nine production reactors were operating there. But all have been shut down for over a decade except for the N-reactor, which in recent years has been largely used to produce fuel-grade plutonium for the Energy Department's breeder research program.