The power obtained in a reactor using ${\mathrm{U}}^{235}$ disintegration is 1000 kW. The mass decay of ${\mathrm{U}}^{235}$ per hour is

$\mathrm{E}={\mathrm{mc}}^2$

$\mathrm{m}=\frac{\mathrm{E}}{{\mathrm{c}}^2}$

So, mass decay per second

$\frac{\mathrm{dm}}{\mathrm{dt}}=\frac{1}{{\mathrm{c}}^2}\frac{\mathrm{dE}}{\mathrm{dt}}=\frac{1}{{\mathrm{c}}^2}\text{ (Power in watt) }=\frac{1}{{\left(3\times {10}^8\right)}^2}\times 1000\times {10}^3$

and mass decay per hour $=\frac{dm}{dt}\times 60\times 60$

$\begin{array}{l}=\frac{1}{{\left(3\times {10}^8\right)}^2}\times {10}^6\times 3600=4\times {10}^{-8}\mathrm{kg}\\ =40\text{ microgram }\end{array}$