10¹⁶ fissions per second are taking place in a nuclear reactor having efficiency 25%. The energy released per fission is 200 MeV. The power output of the nuclear reactor is 

$\mathrm{P}=\frac{\frac{25}{100}\times \mathrm{nE}}{\mathrm{t}}$
Energy released in one sec
$={10}^{16}5200\mathrm{MeV}={10}^{16}\times 200\times {10}^6\times 1.6\times {10}^{-19}\mathrm{J}$
Given that efficiency of the reactor is 25%. 
$\therefore$Power output of the nuclear reactor
$={10}^{16}\times 200\times {10}^6\times 1.6\times {10}^{-19}\times \frac{25}{100}=80 \mathrm{kW}$