The 331 kg (730 lb) of plutonium, enough to make about 50 nuclear weapons, was taken from a nuclear research centre in the port town of Tokai Mura, and left on a British ship, the Pacific Egret, for transport to the U.S. Department of Energy’s Savannah River Site in South Carolina, Greenpeace said.
The website www.vesselfinder.com said the ship is a nuclear fuel carrier.
The Japan Atomic Energy Agency (JAEA), which is the government agency responsible for the nuclear material, has not commented on the Greenpeace statement. An agency spokesman cited security reasons.
The U.S. embassy in Tokyo also declined to comment.
Shipments of plutonium are highly sensitive because it can be used in nuclear weapons or to make a so-called dirty bomb. In Japan, public sensitivity is also high because it is the only country that has been attacked with nuclear bombs.
The shipment is a tiny portion of the nearly 50 tonnes of plutonium Japan holds. Most of it comes from reprocessing spent nuclear fuel burned in its reactors at power stations.
All but two of Japan’s reactors have been shut since a meltdown at a nuclear plant in Fukushima, north of Tokyo, after a powerful earthquake and tsunami in 2011.
The plutonium being shipped was supplied by the United States, Britain and France for the JAEA’s Fast Critical Assembly project in Tokai Mura, according to the International Panel on Fissile Materials.
The agreement to transfer the material to the United States was reached in March 2014, the panel said on its website.