一些如铀一般的元素，通常在核电厂用作核燃料，都是有放射性的；这些元素由于自身的性质是没有无放射性状态的。
Some elements such as uranium, commonly used as fuel in nuclear power plants, are always radioactive; there is no nonradioactive form of them that naturally occurs in nature.
日本在1999年就曾发生过一次4级核事故（又称“东海村核事故”——译注），当时在日本核燃料转换公司（Japan Nuclear Fuel Conversion）的一个燃料厂内，三名正在为反应堆准备燃料的工人引发了一次链式核反应，使反应堆达到临界状态。
Japan itself had a Level 4 accident in 1999, when three workers preparing fuel for a reactor at a Japan Nuclear Fuel Conversion plant triggered a nuclear chain reaction, called a criticality.
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