The United States intends to increase its production of weapons-grade plutonium and tritium at Department of Energy-owned facilities more than 70 percent by 1985 to meet the demands of the nation's expanding nuclear weapons program, according to government sources.

This higher level of production at government-owned facilities is expected to continue to satisfy weapons needs into the 1990s, sources said, thereby making it unnecessary for Washington to turn to spent fuel from domestic nuclear power reactors as a source of weapons-grade plutonium.

The heightened production of weapons-grade plutonium, however, will create a shortage of fuel-grade plutonium. The administration is looking to Great Britain for a temporary solution to that problem.

Although an expansion of plutonium production for weapons has been in the works since the closing days of the Carter administration, the scope of the expansion has been "substantially increased" under the long-term guidance provided by the Reagan administration, sources said.

The presidential stockpile memorandum, the annual planning document that lists the numbers of weapons to be built and retired over the next eight years, is in the final stages of preparation for President Reagan, sources said last week.

Last year's stockpile memorandum, which was signed last October by then-President Carter, forecast that the Department of Energy, which supervises U.S. nuclear weapons building, could not meet its "nuclear materials requirements . . . beyond 1985" without "new production initiatives," according to material presented to Congress earlier this year.

In December, 1980, the Carter administration began the program that the Reagan people have developed. When completed, it is expected to cost close to $1 billion and include:

* Renovating and increasing output from the government's three operating production reactors at Savannah River, S.C.

* Restarting a fourth Savannah River reactor that has been down since 1968.

* Changing the output of a fifth government reactor at Richland, Wash., from fuel-grade to weapons-grade plutonium.

* Reopening a radiochemical reprocessing plant at Richland to change that reactor's plutonium output into metallic form that can be used in weapons. The fuel-grade plutonium that has been produced and stored at Richland for the past eight years will be processed and mixed with "pure" plutonium so it, too, can be used to make weapons.

Negotiations are under way with the British government for the purchase of some of their fuel-grade plutonium to make up for the government-owned, fuel-grade plutonium that is being switched to the weapons program.

The prime need for such fuel-grade plutonium now is for the Clinch River fast-breeder reactor, which President Reagan last week declared would be completed and put into operation.

The actual production figures for plutonium and tritium are highly classified. A recent congressional hearing, however, gave a hint of what is involved.

The conversion of the reactor at Richland to making weapons-grade plutonium would provide "an additional 750 kilograms per year," according to a DOE official. A nuclear scientist suggested that about four kilograms of plutonium can make an atomic bomb.

Present plans, however, call for production of thousands of new nuclear bombs over the next eight to 10 years.

The nuclear weapons assembly plant at Amarillo, Tex., is now producing new warheads for the Minuteman III missile, warheads for the submarine-launched Trident I missile, a new family of B61 tactical and strategic bombs and a warhead for the air-launched cruise missile.

Assembly of new eight-inch neutron artillery shells and Lance warhead missiles is expect to begin shortly, and production has been approved for a ground-launched cruise missile and the Pershing II missile, both of which are to be deployed in Western Europe. In addition, a special strategic bomb designed originally for delivery from the B1 has also gone into the final phase of development.

Reagan administration initiatives include revival of a new shell for the 155-millimeter gun and the submarine-launched cruise missile, both of which had been held up by the Carter administration.

In addition, nuclear materials will be needed in coming years for the MX intercontinental ballistic missile and the planned Trident II submarine-launched missile.

Because it takes five to 10 years to complete production of the major weapons programs, the needs for nuclear materials must be planned long in advance.

Weapons-grade plutonium as used in this country is primarily plutonium 239, a radioactive metal used in making both atomic and hydrogen bombs. It has impurities, one of which is another plutonium element, plutonium 240. The plutonium 240 is not wanted in weapons because it gives off more radioactive emissions, enough so that some of the weapons could become dangerous to the people required to fire them.

Therefore, the standard used is that weapons-grade plutonium must consist of no more than 6 percent plutonium 240.

Fuel-grade plutonium, on the other hand, has 12 percent plutonium 240. Fuel-grade plutonium is used in the rods that make nuclear power reactors work.

Tritium is a radioactive form of hydrogen, used in most hydrogen bombs. It is a gas and inserted in weapons in a capsule, which has to be replaced every five to eight years.