Fission and Fusion
5 marks · NormalBoth fission and fusion reactions release energy. The conditions under which these reactions happen are very different.
State one condition required for fission to occur, and one required for fusion.
Explain why a chain reaction happens more readily with a large mass of uranium-235 than with a small mass.
Suggest one reason why nuclear fusion reactors might be safer than fission reactors.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) —
(b) —
(c) —
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States that fission needs absorption of a slow (thermal) neutron OR critical mass; and fusion requires very high temperature/pressure.
B2 (a) · 1 mark · Both conditions need to be different and correct.
M1 (b) · 1 mark · Explains that more neutrons are likely to hit another nucleus in a large mass, sustaining chain reaction.
A1 (b) · 1 mark · Mentions with small mass, neutrons escape and chain reaction stops.
B1 (c) · 1 mark · Gives one valid reason (no/less radioactive waste, not self-sustaining, can’t run out of control, etc).
Worked solution
a) Fission requires absorption of a slow (thermal) neutron by a fissile nucleus (such as uranium-235), or a critical mass of fuel. Fusion requires extremely high temperatures and/or pressures to occur.
b) When a large mass of uranium-235 is present, the neutrons produced in fission are more likely to strike other uranium-235 nuclei and continue the chain reaction. In a small mass, too many neutrons escape without causing further fissions and the chain reaction stops.
c) Fusion reactors produce little or no radioactive waste, and fusion cannot result in a runaway chain reaction as in fission, making them inherently safer.