The US has admitted a nuclear-powered submarine has leaked small amounts of radiation at three Japanese ports, as well as Guam and Pearl Harbor. The leak on the USS Houston was discovered last month after two years. One of the Japanese ports was just thirty miles southwest of Tokyo. The Pentagon says the leak was too small to cause any harm. The disclosure comes amidst controversy over US plans to deploy another nuclear-powered warship in Japan, the USS George Washington. And it comes just as Japan marked the sixty-third anniversary of the US atomic bombing of Hiroshima. Japan is the only country to have ever suffered a nuclear attack.