Fission and Fusion
5 marks · NormalThe diagrams below show two types of nuclear reaction used to generate energy on Earth and in stars.
Describe how energy is released in a nuclear fission reactor. Include the roles of the moderator and control rods in your answer.
Explain why nuclear fusion reactions occur readily in stars but are harder to achieve on Earth.
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(a) — · must show Mass is converted into energy in fission; most energy as kinetic; moderator slows neutrons; control rods absorb neutrons and control rate
(b) — · must show High temperature and pressure needed to overcome repulsion; stars have these conditions; Earth reactors lack them easily
Common mistakes: Stating only that fission splits atoms without mentioning mass/energy loss; Not mentioning requirement for high temperature/pressure in fusion
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Marking points
M1 (a) · 1 mark · Describes that energy is released as mass is converted to energy in fission.
B1 (a) · 1 mark · Mentions role of moderator in slowing/neutrons.
B2 (a) · 1 mark · Mentions control rods absorb neutrons and control chain reaction.
M2 (b) · 1 mark · States high temperature and pressure are needed for fusion.
A1 (b) · 1 mark · Explains why these conditions exist in stars but not on Earth.
Worked solution
a) In a nuclear fission reactor, energy is released because the mass of the products is less than the mass of the original nucleus. This lost mass is converted to energy, mainly as the kinetic energy of the fission fragments. The moderator slows down fast neutrons, making them more likely to cause further fission. Control rods absorb neutrons to regulate the rate of the chain reaction and prevent it from running out of control.
b) Nuclear fusion reactions require extremely high temperatures and pressures to overcome the electrostatic repulsion between the positively charged nuclei. These conditions exist in the core of stars, allowing fusion to occur naturally there. On Earth, it is difficult to achieve and maintain these conditions, which is why fusion reactors are very challenging to build.