Calculate the amount of \({ }^{235} \mathrm{U}\) remaining in number of atoms/cc after an irradiation period of 1 year for a \(3 \%\) enriched \(\mathrm{UO}_{2}\) fuel in a thermal reactor with a flux of \(1 \times 10^{13} \mathrm{n} / \mathrm{cm}^{2} / \mathrm{s}\). Also estimate the fuel burnup in atom percent (assume the absorption cross section for \({ }^{235} \mathrm{U}\) to be 680 barns).