Sen. Mark O. Hatfield (R-Ore.) said yesterday that senior Reagan administration officials have told him that the government has decided to permanently close down the N reactor at the Hanford plant near Richland, Wash., one of two federal facilities used to manufacture nuclear materials for the U.S. atomic arsenal.

Hatfield said that despite ongoing repairs at the reactor, the administration will be unable to certify that the facility is safe, as is required by legislation signed into law last month that prevents the reactor from being restarted until it meets safety requirements. The quarter-century-old reactor has been shut down for the past year.

"The administration simply cannot afford to restart the N reactor," said Hatfield in a statement in which he declined to identify the federal officials who told him the reactor will be mothballed. Hatfield said the decision stems from cost-cutting requirements, classified data that the government does not need the Hanford facility to meet its need for plutonium for nuclear weapons and "the clear message I have received that the Department of Energy is not going to be able to certify that the plant meets the safety requirements . . . ."

Officials at the Office of Management and Budget declined to comment on the report yesterday.

Hanford's N reactor, one of four U.S. reactors used to produce weapons-grade nuclear materials -- the other three are on a federal reservation in South Carolina -- has come under intense scrutiny since the April 1986 nuclear disaster at the Soviet power plant at Chernobyl. Like the Chernobyl reactor, the Hanford facility has a graphite core and weaker containment systems than commercial reactors.

In a report last October, the National Research Council reported that the U.S. weapons production reactors "all display symptoms of acute aging that could affect safety. . . ." The report found that Hanford's 1,700 ton graphite core is expanding, putting stress on cooling tubes that are already brittle and vulnerable to cracking.

Washington state lawmakers could not independently confirm the announcement by Hatfield, the senior Republican on the Senate Appropriations Committee.

But a spokesman for Sen. Brock Adams (D-Wash.) said "the days of the N reactor have been numbered for quite a while."

Barbara Smith, Adams' spokeswoman, criticized the Reagan administration for "not living up to its commitment to the people of Washington state" by helping to diversify the economy surrounding the reservation. About 6,000 jobs would be affected if the N reactor is shut down.