There also need to be enough fissionable atoms about for the neutrons to bumpinto—in other words, a critical mass.
同时还需要足够多的、可裂变的原子核用于中子进行撞击，换句话说，需要一个临界质量。
In a hole in the center of this system is placed a subcritical sphere of fissionable material.
在这个系统的中心圆洞中放置一枚次临界的球形可裂变物质。
The reaction is slowed by a combination of depletion of the fissionable material and expansion of the core.
由于裂变物质的耗尽以及由于核心的膨胀，才使反应减慢下来。
The system has enhanced capabilities to recognize the elemental content of a container which can be used to detect concealed explosive and fissionable materials.
该系统具有识别集装箱的基本内容的增强能力，可用于检测隐藏的爆炸物质和裂变物质。
The system has enhanced capabilities to recognize the elemental content of a container which can be used to detect concealed explosive and fissionable materials.
该系统具有识别集装箱的基本内容的增强能力，可用于检测隐藏的爆炸物质和裂变物质。
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