WJEC GCSE Physics
WJEC GCSE Physics 2.9 Nuclear Decay and Nuclear Energy Exam Questions
Free WJEC GCSE Physics revision for 2.9 Nuclear Decay and Nuclear Energy: what you need to know, common mistakes and exam-style questions with answers. Covers nuclear fission, uranium-235, chain reactions, moderators, control rods, fission products, nuclear fusion, balancing nuclear equations and containment problems.
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About WJEC GCSE Physics 2.9 Nuclear Decay and Nuclear Energy
This final topic of Unit 2 explains how energy is released from the nucleus. You need to describe how slow neutrons cause uranium-235 nuclei to undergo fission, releasing energy and more neutrons that can sustain a chain reaction. You must explain the roles of the moderator and control rods in a reactor and why fission products are radioactive with a range of half-lives. The topic covers nuclear fusion of deuterium and tritium, balancing fission and fusion equations using the neutron symbol, and the problems of containing reactors and fusion plasmas. It links to generating electricity in 1.2 and stars in 2.5. Examiners often ask for chain reaction explanations.
What you need to know
- Fission A slow neutron is absorbed by a uranium-235 nucleus, which becomes unstable and splits into two smaller nuclei, releasing energy and two or three neutrons.
- Chain reaction Neutrons released by one fission can cause further fissions. In a reactor this is controlled so, on average, one neutron from each fission causes another.
- Moderator and control rods The moderator (e.g. graphite or water) slows down neutrons so they are more likely to cause fission. Control rods (e.g. boron) absorb neutrons; lowering them reduces the rate of fission.
- Fission products The daughter nuclei are radioactive with a wide range of half-lives, so spent fuel must be stored safely for a long time.
- Fusion Light nuclei such as deuterium and tritium join to form a heavier nucleus (helium), releasing energy and a neutron: 2/1 H + 3/1 H → 4/2 He + 1/0 n. Fusion needs extremely high temperatures and pressures.
- Containment Fission reactors need thick shielding and a strong pressure vessel. Fusion plasma is so hot that it cannot touch any container, so it is held by strong magnetic fields, which is a major technical challenge.
Common mistakes in 2.9 Nuclear Decay and Nuclear Energy
- Saying the moderator absorbs neutrons. The moderator slows neutrons down; control rods absorb them.
- Describing fission as nuclei joining. Fission is splitting a heavy nucleus; fusion is joining light nuclei.
- Forgetting to count the neutrons when balancing fission equations. Multiply the neutron nucleon number by the number of neutrons released.
- Saying fusion power stations are already in use. Fusion is still being developed because of the difficulty of containing the hot plasma.
Try three 2.9 Nuclear Decay and Nuclear Energy exam-style questions
1. Complete the equation: 235/92 U + 1/0 n → 141/56 Ba + ?/36 Kr + 3 1/0 n (2 marks)
Show the mark scheme answer
Nucleon numbers: 236 = 141 + A + 3; A = 92 (krypton-92, 92/36 Kr).
2. Explain how a chain reaction occurs in a nuclear reactor and how it is controlled. (4 marks)
Show the mark scheme answer
Neutron absorbed by uranium-235, which splits releasing energy and 2 or 3 neutrons; these neutrons cause further fissions; control rods absorb some neutrons; so only about one neutron per fission causes another fission.
3. Explain why nuclear fusion is difficult to achieve on Earth. (2 marks)
Show the mark scheme answer
Very high temperatures needed to overcome repulsion between positive nuclei; hot plasma is difficult to contain (needs magnetic fields).
2.9 Nuclear Decay and Nuclear Energy FAQs
What does WJEC GCSE Physics 2.9 Nuclear Decay and Nuclear Energy cover?
It covers nuclear fission of uranium-235, chain reactions, the moderator and control rods, radioactive fission products, nuclear fusion of deuterium and tritium, balancing nuclear equations and containment problems.
Which unit is Nuclear Decay and Nuclear Energy examined in?
It is examined in Unit 2: Forces, Space and Radioactivity, a 1 hour 45 minute written paper worth 45% of the GCSE, sat at foundation or higher tier. The Unit 3 practical assessment draws on the same practical skills.
What should I learn for nuclear energy?
Learn the fission sequence, the roles of the moderator and control rods, a balanced fusion equation and the reasons fusion is difficult. Practise balancing both nucleon and proton numbers.
