单词uranium-239
the commonest isotope of uranium; it is not fissionable but when irradiated with neutrons it produces fissionable plutonium 239.
铀最常见的同位素，不能裂变，但与中子一起被照射后会产生可裂变的钚。
An atomic bomb uses conventional explosives to slam together two lumps of fissionable material, usually uranium-235 or plutonium-239.
原子弹利用常规爆炸让铀235和钚239这两块分裂性原料相互撞击。
Nuclear fission can only occur in some types of heavy and unstable nuclei such as in the uranium -235 and plutonium -239.
核裂变只能在某种类型的重核及不稳定核素内发生，如铀-235及钚-239。