Nuclear Processes - A Level Standard Questions
Q2.
(a)
(i) Explain what is meant by the term binding energy for a nucleus.
Binding energy is the work done on nucleons to separate nucleons completely
[or the energy released by nucleons when nucleus is formed from separated nucleons] 
(ii) Sketch a graph of the average binding energy per nucleon against nucleon number A, giving approximate values of the scale on each axis.
curve: correct shape, maximum at A between 40 – 60
sharp rise from <50 (binding energy) max
gradual fall to > 60% (binding energy)max
scales: binding energy per nucleon to 8 – 10 MeV
A to > 220 max 5
(b) Use your graph to explain why energy is released when a neutron collides with a U nucleus causing fission.
Uranium is low on the right hand side of the curve. It splits into two fragments resulting in fission fragments with higher
binding energy per nucleon (higher on the curve) (causing energy release)
(2)
(c) Neutrons are released when nuclear fission occurs in U. Some of these neutrons induce further fission, others are absorbed without further fission and others escape from the surface of the material. The average number of neutrons released per fission is 2.5, of which at least one must produce further fission if a chain reaction is to be sustained.
Explain how a chain reaction can occur only if the piece of uranium has a certain minimum mass (the critical mass).
Critical mass is the smallest mass needed to maintain a chain reaction. The number of neutrons escaping is proportional to surface area
As mass increases a smaller fraction escapes because the surface/volume ratio decreases hence the fraction producing fission increases as mass increases max 3
(3)
(Total 10 marks)
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