TOKYO - Japan has concluded that the tremors detected in North Korea were a nuclear explosion, marking the sixth atomic test by Pyongyang since 2006. 'After examining the data we concluded that it was a nuclear tests,' Foreign Minister Taro Kono said at a briefing broadcast by public broadcaster NHK following a meeting of Japan's National Security Council. Japan's Ministry of Defense said it had dispatched at least three military jets from bases in Japan to test for radiation.