OxfordAQA International A Level Physics
OxfordAQA International A Level Physics 3.12 Nuclear Energy Exam Questions
Free OxfordAQA International A Level Physics revision for 3.12 Nuclear Energy: what you need to know, common mistakes and exam-style questions with answers. Covers nuclear radius, closest approach, mass defect, binding energy per nucleon, induced fission, thermal reactors, reactor safety and nuclear fusion.
Practise 3.12 Nuclear Energy with past papers
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About OxfordAQA International A Level Physics 3.12 Nuclear Energy
Nuclear energy is part of Unit 4 (PH04), Energy and energy resources, in the OxfordAQA International A2 year. It builds on the AS work on atomic structure and radioactivity. You estimate nuclear radius, show that nuclear density is constant, use E = mc^2 to calculate mass defect and binding energy, and interpret the binding energy per nucleon curve to explain why fission and fusion release energy. You then study the thermal fission reactor reactor safety and waste, and the conditions needed for fusion. Examiners often ask for binding energy calculations in MeV, explanations of reactor components, and evaluation of the risks and benefits of nuclear power.
What you need to know
- 3.12.1 Nuclear radius Closest approach of alpha particles gives an upper estimate of nuclear radius. R = R0 A^(1/3), where R0 is about 1.2 fm (check the value you are given). Because volume is proportional to A, nuclear density is the same for all nuclei.
- 3.12.2 Mass and energy E = mc^2. Mass defect is the difference between the mass of the separated nucleons and the mass of the nucleus. Binding energy is the energy needed to separate a nucleus into its nucleons. 1 u is equivalent to 931.5 MeV.
- Binding energy per nucleon The curve peaks near iron-56. Fusion of light nuclei and fission of heavy nuclei both move towards the peak, increasing binding energy per nucleon and releasing energy.
- 3.12.3 Induced fission A thermal neutron is absorbed by uranium-235, which splits into two daughter nuclei and two or three fast neutrons. A chain reaction needs at least critical mass. The moderator slows neutrons by elastic collisions, control rods absorb neutrons, and the coolant removes heat.
- 3.12.4 Safety Reactors use thick concrete shielding, remote handling of fuel and emergency shut-down by inserting control rods fully. High-level waste is highly radioactive and hot, so it is stored under water then vitrified and buried; evaluate risks against benefits.
- 3.12.5 Fusion Light nuclei such as hydrogen isotopes must overcome electrostatic repulsion, so very high temperatures are needed to give them enough kinetic energy. In the Sun, hydrogen is converted to helium.
Common mistakes in 3.12 Nuclear Energy
- Saying binding energy is energy stored in the nucleus that is released when it forms bonds. It is the energy needed to separate the nucleus into free nucleons.
- Saying control rods slow neutrons down. The moderator slows neutrons; control rods absorb them.
- Using total binding energy rather than binding energy per nucleon to compare stability.
- Forgetting to subtract the mass of the nucleus from the total mass of protons and neutrons, or using atomic masses inconsistently with electron masses.
- Saying fusion needs high temperature to break nuclei apart. The high temperature gives nuclei enough kinetic energy to overcome electrostatic repulsion and get close enough to fuse.
Try three 3.12 Nuclear Energy exam-style questions
1. The mass defect of a helium-4 nucleus is 0.0304 u. Calculate its binding energy per nucleon in MeV. (2 marks)
Show the mark scheme answer
Binding energy = 0.0304 x 931.5 = 28.3 MeV; per nucleon = 28.3 / 4 = 7.08 MeV.
2. Using R = R0 A^(1/3) with R0 = 1.2 fm, calculate the radius of an aluminium-27 nucleus. (1 mark)
Show the mark scheme answer
R = 1.2 x 10^-15 x 27^(1/3) = 1.2 x 10^-15 x 3 = 3.6 x 10^-15 m.
3. Explain the role of the moderator in a thermal nuclear reactor and why a suitable moderator has light nuclei. (3 marks)
Show the mark scheme answer
Fission produces fast neutrons but U-235 is more likely to absorb slow (thermal) neutrons; the moderator slows the neutrons through repeated elastic collisions; light nuclei have a mass close to the neutron so more kinetic energy is transferred per collision.
3.12 Nuclear Energy FAQs
What does OxfordAQA International A Level Physics 3.12 Nuclear Energy cover?
It covers nuclear radius and density, mass defect and binding energy, the binding energy per nucleon curve, induced fission and thermal reactors, reactor safety and waste, and nuclear fusion.
Which unit is 3.12 Nuclear Energy examined in?
It is examined in Unit 4 (PH04), Energy and energy resources.
How do I get binding energy calculations right?
Find the mass defect in u, multiply by 931.5 to get MeV, then divide by the number of nucleons if you need binding energy per nucleon. If working in kg, use E = mc^2 and convert to MeV by dividing by 1.60 x 10^-13.
