Shaul Horev, the head of Israel's Nuclear Energy Commission, revealed the return of the material at the International Atomic Energy Agency ministerial conference on nuclear safety in Vienna Monday, Haaretz reported.
Horev did not specify the exact amount returned but some estimates suggest Israel has sent back hundreds of pounds of 93 percent enriched uranium used to power the Sorek reactor, the newspaper said.
The return took place under a special U.S. government program to prevent nuclear waste -- which can be recycled and used to manufacture nuclear weapons -- from falling into the hands of terrorist organizations, Haaretz reported.
The Sorek research reactor is a five-megawatt facility donated to Israel by the United States under former president Dwight Eisenhower's "Atoms for Peace" program, along with nuclear fuel to power it.
The United States stopped supplying enriched uranium for the Sorek reactor as early as 1977 following a law passed by Congress and because Israel was not a signatory to the Nuclear Non-Proliferation Treaty, Haaretz said.