Fission of one atom of ${}_{92}{}^{235}\mathrm{U}$ releases 200 MeV of energy. What mass of ${}_{92}{}^{235}\mathrm{U}$ (in kg) would be required in a reactor to supply one million kW power per year?

Energy released in one year $=\left({10}^6\times {10}^3\right)\left(3.154\times {10}^7\right)$ Joule

Number of ${}_{92}{}^{235}\mathrm{U}$ nuclei fissioned

$=\frac{{10}^9\left(3.154\times {10}^7\right)}{200\left(1.6\times {10}^{-13}\right)}=9.856\times {10}^{26}$

Mass of ${}_{92}{}^{235}\mathrm{U}$ required in the reactor in one year 

$=\frac{\left(9.856\times {10}^{26}\right)235}{6.023\times {10}^{23}}=384.5\mathrm{kg}$