The Carter administration plans to ask Congress for $500 million over the next ive years to modernize the nation's nuclear weapons building complex, according to informed sources.

The first $100 million for the program is expected to be included in the fiscal 1980 Department of Energy budget, which is now being completed at the Office of Management and Budget, the sources said.

The new money comes in the face of budget cuts for many existing programs.

The government-owned nuclear weapons complex consists of laboratories, test facilities and manufacturing and assembly plants spread from California to Florida.

Some of the key facilities date back to the start of the first atomic bomb program during World War II. Others began operation in the 1950s when the nuclear weapons program went into high gear.

A 1977 DOE study found that 14.4 percent of the nuclear warhead production equipment was "in 'poor' physical condition and/or 'inadequate' technology status at the end of 1976."

A report that same year by a House armed Services subcommittee said that "putting off the modernization and upgrading of the national security nuclear weapons facilities . . . could be a form of unilateral disarmament."

The decision to finance an upgrading of the complex comes at a time when the United States is about to undertake its most ambitious nuclear weapons building program in 20 years. Over the next five years, the government plans to add to the stockpile a new submarine-launched Trident ICBM, a new land-based Minuteman III warhead and a new air-launched cruise missile.

In addition, new strategic and tactical nuclear bombs are being developed along with new nuclear artillery and battlefield missiles.

A major focus in the new upgrading program will be the plant at Oak Ridge, Tenn., where the DOE study found serious deterioration in the utilities and equipment. Some 43 percent of the utilities at the plant were labeled "poor and/or inadequate" in the study.

Oak Ridge reprocesses and fabricates uranium parts of nuclear weapons

Another facility that will receive extra attention will be the Pantex plant at Amarillo, Tex., where all U.S. nuclear warheads are assembled.

Pantex already has received extra DOE funds to make its World War II structures safer. An explosion in March 1977 at a high-explosive research facility at Pantex killed three persons.

At congressional hearings last year, Herman Roser, manager of the national DOE nuclear weapons complex, said he believed the Pantex facility should be replaced "with a new and modern plant" but that it would take eight to 10 years to do that without interrupting production.

Meanwhile, he said, upgrading of the plant is now needed because "we have come to the conclusion that [it] doesn't meet the latest safety or safeguard criteria."

The Savannah River plant, where the Department of Energy manufactures plutonium and tritium used in hydrogen weapons, was termed a situation of "galloping obsolescence" in the House Armed Services study.

Sources involved in the DOE nuclear weapons program said recently that despite the current lid on new budget programs, no difficulty had been encountered in getting OMB support for the program of upgrading the production facilities.

OMB officials contacted yesterday refused to comment on the program.