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。
2
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。
3
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.