Both U 235 92 and P 94 u 239 are fissionable, but P 94 u 239 is used more in bombs and reactors. Why?

(i) The concentration of U 235 required for the fission in nuclear reactor is about 60 % The separation of U 235 92 from natural uranium (in which it is only 0 . 7 % ) is a very difficult process and this would lead to depletion of uranium reserves. (ii) P 94 u 239 can be produced from abundant U 238 92 in fast breeder reactors. (iii) The critical size of P 94 u 239 is smaller compared to that of U 235 92 for chain reaction. Hence, its use is economical than of U 235 92 .

Why is it difficult to separate U 235 92 from U 238 92 ?

Isotopes can be separated only by some physical process based on atomic-mass, like diffusion. In natural uranium, the ratio of U 235 and U 238 is 1 : 145 , and the ratio of their atomic masses is very close to 1 . So, the difference in their atomic masses is very close to 1 and correspondingly the difference in the root-mean-square values of velocities of the atoms of these isotopes is very small v 1 r m s v 2 r m s = m 2 m 1 . Hence, to separate them, some gaseous compound of uranium U F 6 is taken, and it has to be diffused thousands of times in ultracentrifuges in series to get the required concentration of U 235