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Two ships arrived in Japan Monday to transport a shipment of plutonium — enough to make dozens of atomic bombs — to South Carolina for storage under a bilateral agreement.

The British ships arrived at the coastal village of Tokai, northeast of Tokyo, home to the country's main nuclear research facility, the Japan Atomic and Energy Agency, according to the Kyodo News agency and citizens' groups. It will take several hours to load the plutonium-filled casks onto the ships, both fitted with naval guns and other protection.

The Pacific Egret and Pacific Heron, both operated by Pacific Nuclear Transport Ltd., will take the 730 pounds of plutonium to the Savannah River Site, a U.S. government facility, under a pledge made by Japan in 2014.

The plutonium, mostly from Britain, and some from the U.S. and France, had been used for research purposes.

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The Pacific Egret is one of two ships that will transport a cache of plutonium to the U.S. for storage. Iori Sagisawa / AP

The Pacific Egret docked first and appeared to be loading the plutonium, with the second ship standing by off-shore, according to media reports and Japanese and international anti-nuclear groups. Japanese officials refused to confirm details, citing security reasons.

Japan's stockpile and its fuel-reprocessing ambitions to use plutonium as fuel for power generation have been a source of international security concerns.

Japan has accumulated a massive stockpile of plutonium — 11 metric tons in Japan and another 36 tons that have been reprocessed in Britain and France and are waiting to be returned to Japan — enough to make nearly 6,000 atomic bombs.

The latest shipment comes just ahead of a nuclear security summit in Washington later this month, and is seen as a step to showcase both countries' nuclear nonproliferation efforts.

Washington has increasingly voiced concerns about the nuclear spent-fuel-reprocessing plans by Japan and China to produce plutonium for energy generation, a technology South Korea also wants to acquire, saying they pose security and proliferation risks.

The U.S. environmental group Savannah River Site Watch said it recognized the need to secure plutonium, but asked why plutonium of foreign origins had to be brought onto American soil for storage.

In a statement Monday, group director Tom Clemens also urged Washington to "reassess its position at the summit and push hard for Japan to cease reprocessing and plutonium stockpiling due to the proliferation threat those programs pose."

Japan began building a major reprocessing plant with French state-owned company Areva in the early 1990s. The trouble project has been delayed ever since and in November its opening was postponed until 2018 to allow for more safety upgrades and inspections.