President Ferdinand Marcos' wife said yesterday that the Soviet Union has offered to replace a planned U.S.-built nuclear plant whose design and financing arrangements are being questioned by the Americans.

Imelda Marcos made the disclosure as a group of U.S. scientists raised the latest criticism of the power project, whose main developer is Westinghouse International.

The Union of Concerned Scientists said the design for the 620-megawatt, $1.1 billion plant 45 miles northwest of Manila is unreliable and probably unsafe. Foundation work has begun on the project, scheduled for completing in the early 1980s.

U.S. news reports recently implied irregularities in the financial arrangements for the plant, saying Marcos' golfing partner, Herminio T. Disini, made millions of dollars on the deal as the Westinghouse representative and as owner of several companies with subcontracts. Marcos subsequently ordered a probe of Disini's involvement in the deal.

Westinghouse and Disini, who is also the husband of a cousin of Mrs. Marcos, denied any wrongdoing.

The project was also attacked in a Washington hearing conducted last week by Rep. Clarence D. Long (D-Md.), chairman of the House Appropriations subcommittee on foreign operations. Long criticized State Department and Export-Import bank officials for promoting and guaranteeing a $644 million loan for the power plant without investigating its safety and ownership.

Mrs. Marcos said that Soviet Ambassador Valerian E. Michaelov offered to replace the Westinghouse plant last month. She said Michaelov offered "a complete plant, even a reactor, to replace the Westinghouse one. He even offered to help us develop our uranium resources."

She said her husband has not decided whether to accept the Soviet offer.