Only #Uranium^235# can be used to produce energy, but #Uranium^238# is the most abundant variant (99%) of Uranium. Hence uranite (uranium bearing mineral) needs to go through a centrifugal filteration process (Uranium Centrifuge Process) to isolate the #Uranium^235#. #Uranium^238# has a small probability for spontaneous fission or even induced fission with fast neutrons.
There is a third isotope of Uranium called #Uranium^233# which can be produced from natural thorium can also be used by nuclear reactors to generate energy.
Source: https://en.wikipedia.org/wiki/Uranium