The US Navy and the Japanese government have confirmed that water containing a small amount of radiation leaked from a US nuclear-powered submarine that stopped by Japan earlier this year.
Japan's Foreign Ministry, citing information provided by the US government, said the leak was too small to have any impact on the environment, and was found during an inspection of the nuclear-powered Houston in Hawaii.
The incident comes at a time when the United States has been trying to allay fears over the planned stationing of
nuclear-powered aircraft carrier George Washington in Japan, the only country to have suffered nuclear attacks.
A civic group opposed to the deployment of the George Washington, planned for December, said the leak raises doubts about the safety of the US Navy's nuclear-powered vessels.
"The claim...that the US Navy's nuclear-powered vessels are safe and are not susceptible to radiation leaks has completely broken down," Masahiko Goto, a lawyer representing the group said in a statement.
He called on the US Navy to disclose the causes of the radiation leak and urged the Japanese government to seek complete disclosure.
Lieutenant Commander David Benham, spokesman for Commander Submarine Forces, Pacific in Pearl Harbor said the radiation leak from the Houston came to light after an incident in July, when the submarine was in dry dock.
About a gallon of water that contained a small amount of radioactivity spilled out from a pipe onto a crew member of the Houston, Benham said. But no contamination was found on the sailor, he said.
It was unclear when the radiation leak had started, and whether there had been any leak while the Houston was in Sasebo in southern Japan from late March to early April, he said.
"But seeing as we couldn't rule it out, we notified the government of Japan," Benham said.
"We're talking about an extremely minute amount of radiation...it's about half as much radiation as you would find in a 50 pound bag of fertiliser," he said.
Twenty-four hour radiation monitoring conducted by the Japanese government during the Houston's stay in March and April had revealed no irregularities, the Japanese Foreign Ministry said.