Nuclear Fuel Cycle

A critical number you must memorise for calculations:

$\boxed{1 \text{ watt (thermal)} = 3.1 \times 10^{10} \text{ fissions per second}}$

This comes from the fact that each fission releases approximately 200 MeV of energy, and 1 W = 1 J/s = $6.242 \times 10^{12}$ MeV/s, so:

$\frac{6.242 \times 10^{12}}{200} \approx 3.1 \times 10^{10} \text{ fissions/s/W}$

Note: Some sources use $3.3 \times 10^{10}$ fissions/W/s. The difference comes from how much of the 200 MeV is assumed to be deposited locally as recoverable heat. Use whichever value is given in the question. If no value is stated, use $3.1 \times 10^{10}$.