(b) Energy released on bombarding $U^{235}$ by neutron=200 MeV
Power output of atomic reactor = 1.6 MW
$\therefore$ Rate of fission= $\frac{1.6\ \times \ 10^{6}}{200\ \times \ 10^{6} \times 1.6\ \times \ 10^{- 19}}$
$\ \ \ = \ 5\ \times \ 10^{16}\ s^{- 1}$