Question

why is uranium-235 the only isotope of uranium that can be used as fuel for a nuclear weapon?

Answer 1

Uranium-235 is a fissile isotope, meaning that it can split into lower molecules when a lower energy neutron is fired at it.

While, other isotopes like uranium-238 are fissionable isotopes meaning that, they will split into smaller molecules but the energy of the fired neutron must be much higher in order for fission to take place.

Uranium-235 is very well suited for producing nuclear power because, it decays naturally by a process known as Alpha radiation. Another reason is that it can undergo induced fission. When a free neutron collides with the U-235 molecule the nucleus will capture the neutron and split extremely quickly. The splitting of single U-235 atom can release roughly 200 MeV.