Answer
$N\approx 8\times10^{18}\frac{fission}{s}$.
Work Step by Step
A 240 MW reactor produces $240\times10^6$ joules per second.
Each fission reaction releases $200\;MeV=3.2\times10^{-11}J$ of energy.
Calculate the number of fissions per second.
$$N=\frac{240\times10^6\;J/s}{3.2\times10^{-11}J/fission}=7.5\times10^{18}\frac{fission}{s}$$
$$N\approx 8\times10^{18}\frac{fission}{s}$$
