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 NuclearFuel 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.
控制棒会吸收路子减慢链式反应，降低核反应的能量释放，进而关掉反应堆。
The control rods soak up the neutrons that mediate the chain reaction which produces most of a reactor's energy, shutting it down.
乏燃料由完全耗尽铀-235的旧燃料棒组成，核反应堆的链式反应就是由铀-235激发的。
It is made up of old fuel rods that have become sufficiently depleted of uranium-235, the isotope that drives a reactor’s main chain reaction, to be no longer useful.
乏燃料由完全耗尽铀-235的旧燃料棒组成，核反应堆的链式反应就是由铀-235激发的。
It is made up of old fuel rods that have become sufficiently depleted of uranium-235, the isotope that drives a reactor’s main chain reaction, to be no longer useful.
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