Assuming the energy released per fission on  $U^{235}$ is 200 MeV, the energy released in the fission of 1kg of  $U^{235}$ is

$E=200MeV=200\times {10}^6\times 1.6\times {10}^{-19}=3.2\times {10}^{-11}$

 Number of atom in 1kg uranium
 $n=\frac{mNA}{m}=\frac{1000\times 6.023\times {10}^{23}}{235}$
$=2.56\times {10}^{24}$
Total energy released
$$\begin{gathered} nE=2.56\times {10}^{24}\times 3.2\times {10}^{-11} \\ =8.19\times {10}^{13}J \end{gathered}$$