WASHINGTON – A U.S. civic group said Friday two ships have arrived in Kobe to transport the massive plutonium stash Japan agreed in 2014 to return to the United States.

The British-flagged ships will pick up the cargo in the village of Tokai, Ibaraki Prefecture, and take it during a 52-day voyage to the Savannah River Site, a U.S. government nuclear facility in South Carolina, according to Savannah River Site Watch.

The Pacific Heron and the Pacific Egret, anchored in Kobe, will transport 331 kg of the highly toxic material, including weapons-grade versions, from the Fast Critical Assembly run by the Japan Atomic Energy Agency in Tokai.

It will be the largest shipment of plutonium to be transported by sea since 1993, when Japan’s Akatsuki Maru carried 1 ton of the material from France to Japan, said Tom Clements, head of the civic group monitoring nuclear issues.

It is believed that much plutonium is enough to make 50 nuclear bombs.

The U.S. provided the plutonium, including some from Britain and France, to Japan for research during the Cold War.

In March 2014, at the Nuclear Security Summit in The Hague, Prime Minister Shinzo Abe agreed with U.S. President Barack Obama to return the plutonium, and to also send highly enriched uranium kept at the FCA.

Once it arrives in the U.S., the plutonium will be disposed of, according to the bilateral agreement.

Obama, who was awarded the Nobel Peace Prize for his policy of seeking a world free of nuclear weapons, will host the next round of the biennial Nuclear Security Summit on March 31 and April 1 in Washington.