An armed British ship believed to be carrying enough plutonium to make about 40 atomic bombs has left a port in eastern Japan to transport the shipment to the U.S. for storage.

Kyodo News agency says the British-flagged nuclear fuel transport ship Pacific Egret left the port in Tokai village, northeast of Tokyo, Tuesday, one day after arriving with another armed ship that had waited off-shore. Tokai is home to Japan Atomic Energy Agency, a research complex where the plutonium had been used for research.

The ship, operated by Pacific Nuclear Transport Ltd., was to take the 331 kilograms (730 pounds) of plutonium to a U.S. government facility in South Carolina under Japan's 2014 pledge.