What initial mass of 235U is required to operate a 650-MW reactor for 1 year? Assume 40% efficiency.

1 answer

The amount of heat that must be generated by the fission reaction is 650*10^6/0.4 Watts. Multiply that by the number of seconds in a year to get the number of Joules of heat required.

Then divide that by the amount of heat released in a typical U235 fission.
This website may help:
http://hyperphysics.phy-astr.gsu.edu/HBASE/nucene/u235cs.html#c1

That will tell you how many U235 atoms must fission. Convert that number to mass, using the mass of a U-235 atom, which is 235/Nav grams. Nav is Avogadro's number