(a) Name two isotopes of uranium which are fissionable.

(b) Which of the isotope mentioned in part (a) above is easily fissionable? Give reason.

(c) State whether the neutron needed for fission reaction of the isotope mentioned in part (b) above, is slow or fast?

(a) The ore of uranium found in nature contains two isotopes (${}_{92}^{238}\text{U}${92}^{238}\text{U} and ${92}^{235}\text{U}$). Both are fissionable.

(b) Although both the isotopes are fissionable, but experimentally it is found that the isotope ${}_{92}^{235}\text{U}${92}^{235}\text{U} is more fissionable than the isotope ${92}^{238}\text{U}$.
The reason is that the fission of ${}_{92}^{238}\text{U}${92}^{238}\text{U} nucleus is possible only by fast neutron, while the fission of ${92}^{235}\text{U}$ nucleus can occur even by the slow neutrons.

(c) Either of slow or fast neutron can be used for the fission reaction of $_{92}^{235}\text{U}$.