Uranium is obtain in nature in three isotops U234 = 0.0058%; U235= 0.715%;   U238 =99.28% 
U235 is us as fule, it is enrichd by gaseous diffusion uranium enriched process. 
use of U235 bcoz it will usually capture the neutron and split extremely quickly and requied low energy to absorbtion, posibility of chain reaction in u-235 is more compare to other. 
 the energy can be calculate by 
so for m= 1mg ⇒ 1×10^-6 kg (m is measure in kg)
          c= speed of light ⇒ 3×10^8 m/s
E= [1×10^-6] * [3×10^8]^2 
  = 9×10^10 joules