Worked out Example
Q.
A typical fission reaction of uranium –235 is:
    23592 U + 10 n   →   13957 La + 9542 Mo + 210 n + 7 0–1 e
Calculate the total energy released by the fission of 0.5 kg of uranium –235, neglecting the masses of beta–particles (electrons). Given: Mass of uranium –235 atom = 235.124 u, Mass of neutron = 1.009 u,Mass of lanthanum –139 atom = 138.955 u, Mass of molybdenum –95 atom = 94.946 u
Sol.
We will first calculate the total mass of reactants as well as products, separately.
Total mass of reactants = Mass of U + Mass of 1 neutron
= 235.124 + 1.009 = 236.133 u.....(1)
Total mass of products = Mass of La + Mass of Mo + Mass of 2 neutrons
= 138.955 + 94.946 + 2 × 1.009
= 138.955 + 94.946 + 2.018 = 235.919 u....(2)
Loss of mass during fission = Mass of reactants – Mass of products
= 0.214 u . . . . . . . (3)
This mass of 0.214 atomic mass units is now to be converted into kilograms.
We know that: 1 u = 1.66 × 10–27 kg
So, 0.214 u = 1.66 × 10–27 × 0.214 kg
= 0.355 × 10–27 kg
Thus, the mass lost (m) from 1 nucleus (or 1 atom) of uranium –235 is 0.355 × 10–27 kg.
We will now calculate the energy released by the loss of this much mass.
Now, Energy, E = m × c2  =  [0.355 × 10–27] × [(3 × 108)2] joules  = 3.195 × 10–11 joules
This much energy is released by the fission of one uranium –235 nucleus.
We will now calculate the energy released by the fission of 1 mole of uranium –235 nuclei or 6.023 × 1023 uranium –235 nuclei.
Energy released by the fission of 1 U – 235 nucleus = 3.195 × 10–11 J
So, Energy released by the fission of 6.023 × 1023 U –235 nuclei  =  3.195 × 10–11 × 6.023 × 1023 J = 19.24 × 1012 J.
This much energy is released by the fission of 1 mole of uranium –235 which is equal to gram atomic mass of uranium –235 or 235.124 grams of uranium –235.
But we have to calculate the energy released by the fission of 0.5 kilogram (or 500 grams) of uranium.
Now, Energy released from 235.124 g of uranium  =  19.24 × 1012 J.
So, Energy released from 500 g of uranium =  =  4.09 × 1013 J.
Thus, the energy released by the fission of 0.5 kilogram of uranium –235 is 4.09 × 1013 joules.
We will now calculate the electrical power generated by the fission of 0.5 kilogram of uranium –235 per day.
We know that:

Power = =  =  (∵ 106 W = 1 MW) = 473 MW