铀或钚的同位素被称为裂变材料，即我们可以利用自由中子去轰击它们来产生裂变反应。
The uranium or plutonium isotopes are referred to as "fissile," which means we can use them to induce fission by bombarding them with free neutrons.
这个过程能压缩钚，并与此同时钚也会受到由钋组成的触发器散发出的中子的轰击，这样就引发了第二个阶段。
That compresses the plutonium while it is simultaneously bombarded with neutrons from a trigger made of polonium, and thus begins the second stage.
铀或钚的同位素被称为裂变材料，即我们可以利用自由中子去轰击它们来产生裂变反应。
"The uranium or plutonium isotopes are referred to as" fissile, "which means we can use them to induce fission by bombarding them with free neutrons."
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