The United States and Japan have completed the removal of all highly enriched uranium and separated plutonium fuels from Japan's Fast Critical Assembly research project that is due to be sent to South Carolina, the countries said on Friday.
Announced alongside the Nuclear Security Summit in Washington, the transfer makes good on a 2014 agreement at a previous non-proliferation summit to move the material from the site in Tokai Mura, Japan, to the United States, the countries said.
In a joint statement, the countries said the removal furthers a mutual goal of reducing the amount of these nuclear materials held worldwide.
"This is the largest single nuclear material removal in the history of this summit process," U.S. Energy Secretary Ernest Moniz told reporters.
"This process will permanently remove any risk of this material falling into the wrong hands," he added.
Moniz and Japan's Deputy Chief Cabinet Secretary Koichi Hagiuda left immediately after delivering statements at the summit and would not take questions.
The shipment has stirred some controversy over its transfer to the United States, specifically to a Department of Energy site in South Carolina where leaders and environmental activists have balked at receiving the weapons-grade plutonium.
Such shipments are highly sensitive because the material can be used in nuclear weapons or to make a so-called dirty bomb.
Last month, South Carolina Governor Nikki Haley asked the U.S. Department of Energy for the shipment to be turned back or sent elsewhere.
A Japanese government spokesman declined to provide details of the shipment's status, citing security concerns.
On Tuesday, Moniz called Haley and agreed to eventually store some of the plutonium now at South Carolina's Savannah River Site at a facility in New Mexico, the Associated Press reported.
U.S. Energy Department representatives did not respond to a request for further details on the move.
South Carolina has sued over the issue, and Haley has pledged to keep the pressure on the federal government.
"We will not back down from our lawsuit until the DOE pays the $1 million a day fine they are required to under federal law," Haley said in a statement provided to Reuters.
In its statement, the United States said it will now "downblend" the materials for use in civilian activities or for final disposition.
The countries also said they were working to move material from another site, the Kyoto University Critical Assembly, to the United States.
"This is one more important step in the ongoing program to convert research reactors to low-enriched uranium," Moniz said of the Kyoto site.