Nuclear Physics Exam Question Help Needed

Just came across a 1 mark question on Paper 2 June 2018 and I am struggling to find the answer. Could you provide some hints that will help me reach the answer please?

Question :

During a single fission event of uranium-235 in a nuclear reactor the total mass lost is 0.23 u. The reactor is 25% efficient.

How many events per second are required to generate 900 MW of power?

My first thought is mass energy equivalence - using E=mc^2 to work out the energy of 0.23u then multiply that by 0.25 as the reactor is only 25% efficient.

Then divide 900MW by that number and you get the number of ‘events’ per second (I think?)

Hope this helps : ))

Thank You that really helped but can you tell me why did you divided 900MW by that number please?

I think about physics in terms of units

Power is in watts which are J s^-1. Your energy calculated from E=mc^2 x 0.25 is measured in joules (J). Do the division (J s^-1)/J, the Js cancel, and you get s^-1, which is the number of events ‘per second’ ie what the question is asking for.

Basically you have a total energy and the energy of 1 nucleus, so doing total/(1 nucleus) = number of nuclei : ))

oooooooooh makes sense. THANK YOU

Haha np.

Silly question. Is that the right answer lol? Do you have the mark scheme?