President Reagan released a report Thursday calling for increased spending of $81 billion to modernize the nuclear weapons complex over the next 20 years and the closing of four major facilities.

Reagan said the 31-page report "will enable the administration and the Congress to adopt a long-term approach to modernization rather than a piecemeal response to the problems within the nuclear weapons complex."It recommended stopping nuclear fuels processing at four sites where problems have been highly publicized: the Rocky Flats Plant near Denver, the Hanford Site near Richland, Wash., the Feed Materials Production Center at Fernald, Ohio, and the Mound Plant in Ohio.

"Waste management and environmental restoration at these sites will continue until completion," the report said.

It was not clear whether research activity would continue at Hanford, which produced the material for early atomic weapons during the 1940s.

The report also said that the only three U.S. reactors capable of producing tritium, a critical element in modern thermonuclear weapons, could not be restarted until late 1989. The three reactors, all at the Savannah River Plant, in Aiken, S.C., have been shut down since last April undergoing changes to make them safer.

"In spite of these improvements and increasingly extensive maintenance, the reactors may not be able to achieve acceptable production efficiencies," said an unclassified version of the report, released at the White House.

To meet tritium needs, the report repeated previous administration recommendations for the construction of a $3.2 billion heavy-water reactor at Savannah River and a $3.6 billion new design of a gas-cooled reactor at the Energy Department facility at Idaho Falls, Idaho.

The budget for fiscal 1990 includes money for the design of those new reactors.

The report also addresses the problem of cleaning up nuclear contamination around the 15 largest facilities in 12 states, as well as modernizing the plants and closing down four of the most troubled operations.