Free E.4 Fission questions for IB Physics (SL & HL), part of E Nuclear & quantum physics. Each question has the answer and the marking points an examiner would look for.
Generate a full E.4 Fission worksheet →All E Nuclear & quantum physics questionsEach fission of a uranium-235 nucleus releases 200 MeV. Molar mass of U-235 = 235 g mol−1.
What energy is released by the complete fission of 1 kg of uranium-235?
Answer: C
What are the functions of the moderator and the control rods in a thermal fission reactor?
| moderator | control rods | |
|---|---|---|
| A | slows neutrons down | absorb neutrons |
| B | slows neutrons down | transfer thermal energy |
| C | absorbs neutrons | slow neutrons down |
| D | absorbs gamma rays | absorb neutrons |
Answer: A
A nuclear power station has an electrical output of 1500 MW and an overall efficiency of 35%. Each fission of uranium-235 releases 200 MeV.
Calculate the number of fissions per second.
Determine the mass of uranium-235 used in one year.
Outline the role of the moderator and the control rods.
Explain why the fission products are usually β− emitters.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | 1.3 × 1020 s−1 thermal power = 4.3 × 109 W ✓ n = 1.3 × 1020 s−1 ✓ | 2 |
| (b) | 1600 kg nuclei per year = n × 3.16 × 107 ✓ mass = 1600 kg ✓ | 2 |
| (c) | slow neutrons; absorb neutrons moderator slows neutrons so they are more likely to cause fission ✓ control rods absorb neutrons to control the rate of reaction ✓ | 2 |
| (d) | neutron-rich fission products have too many neutrons for their proton number ✓ β− decay converts a neutron into a proton, moving towards stability ✓ | 2 |
In a nuclear reactor, uranium-235 absorbs a neutron: 23592U + 10n → 14456Ba + 8936Kr + x 10n. Each fission releases about 200 MeV.
Determine x.
Explain, with reference to binding energy per nucleon, why energy is released.
The reactor produces 1200 MW of electrical power with an efficiency of 32%. Calculate the number of fissions per second.
Calculate the mass of uranium-235 used in one year.
Outline the roles of the moderator and the control rods.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | 3 x = 3 (nucleon numbers balance) ✓ | 1 |
| (b) | products more tightly bound the products have a greater binding energy per nucleon ✓ so the total binding energy increases and the mass decreases ✓ | 2 |
| (c) | 1.2 × 1020 s−1 thermal power = 3750 MW ✓ 200 MeV = 3.20 × 10−11 J ✓ 1.2 × 1020 s−1 ✓ | 3 |
| (d) | 1443 kg moles per year = fissions ÷ NA ✓ 1443 kg ✓ | 2 |
| (e) | slow neutrons; absorb neutrons moderator slows neutrons so they are more likely to cause fission ✓ control rods absorb neutrons to control the reaction rate ✓ | 2 |
A U-235 nucleus absorbs a neutron and splits into Ba-141, Kr-92 and three neutrons, releasing 173 MeV.
Explain why energy is released, with reference to binding energy per nucleon.
Calculate the mass defect in kg.
Explain the role of the moderator in a nuclear reactor.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | products more tightly bound products have higher binding energy per nucleon ✓ total binding energy increases, mass decreases ✓ | 2 |
| (b) | 3.1 × 10−28 kg 173 MeV = 2.77 × 10−11 J ✓ Δm = E/c2 = 3.1 × 10−28 kg ✓ | 2 |
| (c) | slows neutrons by collisions neutrons lose kinetic energy in collisions with moderator nuclei ✓ slow (thermal) neutrons are more likely to cause fission ✓ | 2 |
This question is about the operation of a nuclear reactor.
Explain the role of the control rods.
State what is meant by critical mass.
Explain why spent fuel rods must be stored for a long time.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | absorb neutrons they absorb neutrons ✓ so that on average one neutron per fission causes a further fission / the rate is controlled ✓ | 2 |
| (b) | minimum mass for sustained chain reaction the minimum mass of fuel for a self-sustaining chain reaction ✓ | 1 |
| (c) | long half-life products fission products are highly radioactive ✓ some have very long half-lives ✓ | 2 |
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Meet Riyaz →Original IB-style practice questions written for mentorriyaz.in. These are not IB past paper questions, and this site is not affiliated with or endorsed by the International Baccalaureate Organization. Updated 2026-11-04. Paper generator · mentorriyaz.in