Fission and Fusion
5 marks · HardScientists want to develop fusion reactors as a future energy source, while fission reactors are already in use. The diagram represents a simplified nuclear reactor core.
Describe the function of: (i) the moderator (ii) the control rods in a nuclear fission reactor.
Explain why a continuous chain reaction occurs in a fission reactor but not in a fusion reactor, even though both reactions can produce neutrons.
Fusion occurs in stars. State two conditions required for fusion to occur and explain why these are necessary.
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(a) The moderator slows down neutrons to increase probability of fission; control rods absorb neutrons to control or stop the chain reaction. · must show Moderator slows neutrons, Control rods absorb neutrons
(b) A fission reactor can sustain a chain reaction because released neutrons cause further fission; in fusion, the conditions and products do not sustain a chain reaction in the same way. · must show Released neutrons sustain chain, Fusion does not produce chain-fission in same way
(c) Very high temperature and pressure are required for fusion. These are needed to overcome the electrostatic repulsion between positively charged nuclei. · must show High temperature, High pressure, Overcome repulsion
Common mistakes: Confusing roles of moderator/control rods; Thinking chain reactions always occur in fusion
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Marking points
B1 (a) · 1 mark · Correct function of moderator: slows neutrons so they are more likely to be absorbed by U-235.
B2 (a) · 1 mark · Correct function of control rods: absorb neutrons and control chain reaction.
B1 (b) · 1 mark · Explains that in fission chain reactions, neutrons are produced that can be absorbed by other nuclei to sustain the reaction.
B2 (b) · 1 mark · Explains that in fusion, reactions do not lead to further reactions in the same way.
B1 (c) · 1 mark · States two required conditions (high temperature, high pressure) and why needed (to overcome repulsion).
Worked solution
a) (i) The moderator slows down fast neutrons produced in fission, increasing the chance that they will be absorbed by another U-235 nucleus, allowing the chain reaction to continue. (ii) The control rods absorb some of the released neutrons, reducing the number available to cause further fission; by raising or lowering them, operators control or stop the chain reaction.
b) In a fission reactor, each fission event produces neutrons, which can go on to split more nuclei, sustaining the chain reaction. In a fusion reactor, though neutrons may be produced, the process does not lead to repeated splitting or further fusion in the same way, so a self-sustaining chain reaction does not occur.
c) Fusion requires very high temperatures (to give nuclei sufficient kinetic energy) and very high pressures (to increase the chance of collision). Both are needed to overcome the electrostatic repulsion between the positively charged nuclei so that they can come close enough for the strong nuclear force to make them fuse.