(i) Explain what is meant by the term binding energy for a nucleus.
(ii) Sketch on the axes below, a graph of the average binding energy per nucleon against nucleon number A, giving approximate values of the scale on each axis.
(b) Use your graph to explain why energy is released when a neutron collides with a nucleus causing fission.
(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).
(Total 10 marks)