Sub Critical

In light water reactors the new fuel has about 4 to 5 percent U-235, which is the fissionable part, the rest being U-238. In some countries mixed oxide fuel is used (MOX) which contains some Plutonium as well as U-235, but the fissionable content is much the same. Heavy water or graphite reactors can use natural uranium, which contains 0.7 percent U-235.

Yes, this isotope is very fissionable and is used in nuclear reactors and nuclear weapons.

The fissionable isotope is required for the nuclear reactor operation. The fissionable isotope when fissions it give energy due to the mass difference according to Einstein formula E = mc2

Fissionable substances.