1·To make fuel for electricity-generating reactors, uranium is enriched to less than 5 percent of the isotope U-235.
为将燃料做成电能反应堆，铀被浓缩至U-235同位素含量5%以下。
2·The prime example is the element uranium (U), which occurs naturally as two isotopes-the lighter, fissionable U-235 and the heavier, non-fissionable U-238.
铀（U）元素就是一个很好的例子。 天然铀有两种同位素； 较轻的负235可以裂变，较重的铀238则不会裂变。
3·Only three fissile materials are known at present, there being U-233, U-235, and plutonium.
目前已知有三种裂变材料，它们是铀233，铀235，以及钚。
4·The officials did not know the exact size of the Chinese ships but described them as being smaller than the 235-foot long U. S. vessel.
官员没有点明中国船只的规模，但描述该船比235英尺的美国船只要小。