The Reagan administration, which already is financing the biggest U.S. nuclear weapons building program in 20 years, is seeking additional funds to expand nuclear production facilities that could turn out even larger numbers in the 1990s, according to Department of Energy (DOE) budget documents.

DOE wants to modernize and enlarge facilities that produce tritium, uranium and plutonium, the key nuclear materials for missile warheads, bombs and artillery shells. It also is seeking funds to build sophisticated new laboratories to explore how nuclear explosions could be used for directed-energy weapons as part of President Reagan's Strategic Defense Initiative, the so-called "Star Wars" research program, according to fiscal 1987 budget documents presented to Congress.

At the same time, DOE and the Pentagon's Defense Nuclear Agency are seeking funds for laboratory experiments and underground tests exploring new types of nuclear warheads and missiles for the next decade that would use special explosive effects to destroy hardened Soviet missile silos or black out Moscow's electronic equipment.

These new requests are being made while DOE already has under way a $1.3 billion multiyear program, begun in 1982, to refurbish aging electric generators and other equipment at the seven nuclear weapons production plants, three weapons laboratories and the Nevada Test Site that make up the U.S. nuclear weapons-making complex.

Some of these funds will modernize facilities that date from the Manhattan Project, which built the first atomic bombs in World War II.

Last year, DOE received congressional approval to start a second multiyear program, costing $1.9 billion, to upgrade the research, testing and development facilties of the complex.

DOE is now producing new warheads for the MX intercontinental ballistic missile; Pershing II missile; eight-inch artillery shells; air-, ground- and sea-launched cruise missiles; two versions of a new tactical bomb, and a new strategic bomb.

Under development and approved for future production are a 155 mm artillery shell, the warhead for the Trident II submarine-launched ballistic missile, additional tactical bombs and a new antisubmarine warhead for use on both a missile and a depth bomb.

Government laboratories also are researching a warhead for the Standard Missile SM2 ground-to-air, antiair missile; a warhead for the Midgetman mobile intercontinental missile, and a bomber-carried short-range, air-to-ground missile.

The Reagan administration budget request for nuclear weapons in fiscal 1987 is $8.2 billion, compared with $3.7 billion in the Carter administration's final budget in 1981.

Much of the current weapons production was planned by the Carter administration, while many of the modernization plans have evolved under Reagan.

A measure of the current level of production is illustrated by a March 12, 1983, memorandum to Defense Secretary Caspar W. Weinberger from his top nuclear weapons adviser, Richard L. Wagner Jr., according to informed sources.

The memo reports that unless the weapons complex "is modernized and expanded," it could not handle production of more than 2,000 weapons-per-year, according to sources who have seen the memo. "Projected new builds over the 1980s are at or above that level," sources said the memo states.

All of DOE's major construction and rehabilitation programs are scheduled to be completed by the early 1990s, by which time most current warhead production programs should be finished.

Thus the renovation will allow construction of larger numbers of weapons at the same time Reagan administration officials hope a future administration will decide to proceed with Star Wars, which Reagan said was aimed at making nuclear weapons obsolete.

One example of DOE's expansion relates to tritium, a radioactive gas which decomposes and must be regularly replaced in deployed weapons. Tritium helps trigger a nuclear explosion and also boosts its power. DOE wants funds to start production later this year on a $120 million, "state-of-the-art" tritium loading facility at the department's Savannah River Plant near Aiken, S.C.

The present tritium facility, which has been modernized several times since it was built in 1950, "is not capable of supporting another expansion," DOE told Congress. Such growth is required "to meet scheduled DOE requirements in FY 1990 and beyond," the department said.

DOE is seeking new funds to prepare for construction next year of a $70 million strategic defense facility at the Sandia National Laboratories in Albuquerque, N.M. It would support X-ray laser and particle beam experiments and preparation for underground tests of such directed-energy devices.

Another $40 million simulation technology laboratory is under way at Sandia to project radiation that is now handled in underground nuclear tests.