During a single fission event of uranium-235 in a nuclear reactor the total mass lost is 0.23 u - AQA - A-Level Physics - Question 31 - 2018 - Paper 2

Since the reactor is 25% efficient, the useful energy output per fission event is:

$E_{useful} = 0.25 imes E = 0.25 imes 3.43 imes 10^{-11} J = 8.575 imes 10^{-12} J$.

Power is defined as energy per unit time. Therefore, the total energy required per second to generate 900 MW is:

$P = 900 ext{ MW} = 900 imes 10^6 ext{ W} = 900 imes 10^6 ext{ J/s}$.

To find the number of fission events per second, we can divide the total power by the useful energy per fission event:

$N = \frac{P}{E_{useful}} = \frac{900 imes 10^6 ext{ J/s}}{8.575 imes 10^{-12} J} \approx 1.05 \times 10^{20}$ events per second.

Based on our calculations, the answer approximates to $1.1 \times 10^{20}$ events per second, which corresponds to option C.