Japan's Nuclear and Industrial Safety Agency said Tuesday that an explosion was heard at the No. 2 reactor of Fukushima No.1 nuclear power plant at 6:10 a.m. Tuesday (2110 GMT).
The suppression pool of the reactor might have been damaged at No. 2 reactor, said the safety agency.
日本经济产业省原子能安全和保安院15日说，东京电力公司福岛第一核电站二号机组当天上午发出爆炸音，可能是核反应堆中的控制压力池出现损坏。
A rise in radiation was monitored near the nuclear plant shortly after the blast, said the agency. The radiation fell to 882 micro sievert after briefly rising as high as 965.5 micro sievert, topping the legal limit of 500.
But the agency said that the radiation will not immediately affect people's health at the levels of above figures.
原子能安全和保安院当天召开记者会宣布，福岛第一核电站二号机组的核燃料棒露出水面约2.7米，露出长度差不多是核燃料棒的一半。发出爆炸音后不久，福岛第一核电站四周监测到965.5毫希的辐射，之后下降到882毫希。原子能安全和保安院说，上述辐射的数值不会立即对人体产生影响。