Answer
$3.1\times 10^{10}~atoms/s$
Work Step by Step
We can find the energy from the fission of one atom:
$E = (200\times 10^6~eV)(1.6\times 10^{-19}~J/eV)$
$E = 3.2\times 10^{-11}~J$
We can find the required rate to generate 1.0 W:
$\frac{1.0~W}{3.2\times 10^{-11}~J} = 3.1\times 10^{10}~atoms/s$
