Half life of the fuel of the fission reactor, $t_{\frac{1}{2}}=5$ years $=5 \times 365 \times 24 \times 60 \times 60 s$

We know that in the fission of $1\; g$ of $_{92}^{235} U$ nucleus, the energy released is equal to $200 MeV$.

$1$ mole, i.e., $235 \;g$ of $_{92}^{235} U$ contains $6.023 \times 10^{23}$ atoms.

$\therefore 1 \;g\;\; _{92}^{235} U \quad \frac{6.023 \times 10^{23}}{235}$ atoms contains

The total energy generated per gram of $_{92}^{235} U$ is calculated as:

$E=\frac{6.023 \times 10^{23}}{235} \times 200 MeV / g$

$=\frac{200 \times 6.023 \times 10^{23} \times 1.6 \times 10^{-19} \times 10^{6}}{235}=8.20 \times 10^{10} J / g$

The reactor operates only $80 \%$ of the time. Hence, the amount of $_{92}^{235} U$ consumed in $5$ years by the $1000 MW$ fission reactor is calculated as

$\frac{5 \times 80 \times 60 \times 60 \times 365 \times 24 \times 1000 \times 10^{6}}{100 \times 8.20 \times 10^{10}}\, g$

$\approx 1538 \,kg$

Initial amount of $_{92}^{235} U=2 \times 1538=3076\, kg$