E.4 Fission
Practice exam-style IB Physics questions for Fission, aligned with the syllabus and grouped by topic.
Question 1
A heavy nucleus undergoes spontaneous fission. What is meant by spontaneous fission?
A.
A beta particle causes the nucleus to become unstable.
B.
The nucleus splits without being struck by another particle.
C.
The nucleus splits after absorbing a slow neutron.
D.
Two light nuclei combine to form a heavier nucleus.
Question 2
In a steady chain reaction in a nuclear reactor, the average number of neutrons from each fission that cause further fission is
A.
greater than one.
B.
zero.
C.
less than one.
D.
one.
Question 3
What is the principal role of the moderator in a thermal nuclear reactor?
A.
To increase the number of protons in the fissile nuclei
B.
To absorb gamma photons before they leave the core
C.
To slow down neutrons by collisions without absorbing many of them
D.
To convert the kinetic energy of fission fragments into electrical energy
Question 4
Control rods are lowered further into a reactor core. The immediate effect is that
A.
fewer gamma photons are emitted by each fission event.
B.
more neutrons are absorbed and the fission rate decreases.
C.
more neutrons are moderated and the fission rate increases.
D.
the fuel becomes enriched with uranium-235.
Question 5
A possible fission reaction is
[
^{235}{92} ext{U}+^{1}{0} ext{n}\rightarrow ^{140}{54} ext{Xe}+^{94}{38} ext{Sr}+2^{1}_{0} ext{n}+\Delta E.
]
State the quantity conserved that allows the proton numbers to be checked.
[1]
State the quantity conserved that allows the nucleon numbers to be checked.
[1]
Question 6
In the fission of uranium-235, most of the energy released immediately appears as
A.
kinetic energy of the fission fragments.
B.
rest energy of antineutrinos only.
C.
gravitational potential energy of the products.
D.
chemical energy in the moderator.
Question 7
A fission reaction has a mass decrease of 0.20 u. The energy released is closest to
A.
186 MeV
B.
47 MeV
C.
93 MeV
D.
466 MeV
Question 8
A fission product is neutron-rich compared with stable nuclei of similar mass. The decay most likely to move it toward stability is
A.
alpha decay.
B.
beta-minus decay.
C.
spontaneous fusion.
D.
gamma absorption.
Question 9
A neutron-induced fission reaction is
A.
4
B.
2
C.
3
D.
1
Question 10
Atomic masses may be used instead of nuclear masses in many fission energy calculations because
A.
electrons have no mass in nuclear reactions.
B.
the electron masses cancel when total proton number is conserved.
C.
fission changes only electron energy levels.
D.
neutrons and electrons have equal masses.
Question 11
A fission reaction has a mass decrease of (0.215, ext{u}).
Calculate the energy released in MeV. Use (1, ext{u}=931.5, ext{MeV},c^{-2}).
[1]
State one form in which this energy is released immediately.
[1]
Question 12
Fast neutrons are produced in the fission of uranium-235. In many reactors these neutrons pass through a moderator.
State what is meant by a thermal neutron.
[1]
Explain why moderation helps sustain a chain reaction in uranium-235 fuel.
[1]
Question 13
Control rods are moved in a reactor core.
State the material property needed for control rods.
[1]
Outline the effect of inserting the control rods further into the core.
[1]
Question 14
Fission fragments repel each other strongly just after a heavy nucleus splits.
State the force responsible for this repulsion.
[1]
Explain how this repulsion leads to heating of the reactor core.
[1]
Question 15
The graph shows binding energy per nucleon against nucleon number for a range of stable nuclei.
State the approximate region of nucleon number where binding energy per nucleon is greatest.
[1]
Describe how the binding energy per nucleon changes when a very heavy nucleus fissions into two medium-mass nuclei.
[1]
Explain why this change leads to energy release.
[1]
Question 16
The graph shows the activity of two radioactive fission products after they are removed from a reactor.
Identify which isotope has the shorter half-life.
[1]
Describe how the activity of each isotope changes with time.
[1]
Explain why recently removed spent fuel must be cooled even after fission has stopped.
[1]
Question 17
What is the purpose of a heat exchanger in a pressurized-water nuclear power station?
A.
To increase the binding energy per nucleon of the fuel
B.
To enrich uranium by separating uranium-235 from uranium-238
C.
To transfer energy between coolant circuits while keeping the fluids separate
D.
To absorb excess neutrons when the reactor is shut down
Question 18
The binding energy per nucleon of the fission products is greater than that of the original heavy nucleus. The best interpretation is that the products have
A.
the same total rest mass as the reactants, so only momentum changes.
B.
smaller total rest mass than the reactants, so energy is released.
C.
lower binding energy, so the strong force has no role.
D.
greater total rest mass than the reactants, so energy is absorbed.
Question 19
A reactor has electrical output (P_e), efficiency (\eta), and energy released per fission (\Delta E). The fission rate (R) is
A.
\(R=\dfrac{\eta\Delta E}{P_e}\)
B.
\(R=\dfrac{P_e}{\eta\Delta E}\)
C.
\(R=\dfrac{\Delta E}{\eta P_e}\)
D.
\(R=\eta P_e\Delta E\)
Question 20
Spent fuel continues to produce heat after control rods have stopped the chain reaction because
A.
control rods emit neutrons when fully inserted.
B.
the moderator stores elastic collision energy for many years.
C.
uranium-238 fissions only after shutdown.
D.
radioactive fission products continue to decay.
Question 21
Hydrogen nuclei are effective at slowing neutrons in a moderator mainly because
A.
they convert neutrons into gamma photons.
B.
they absorb all fast neutrons before fission occurs.
C.
they have a charge opposite to that of the neutron.
D.
their mass is similar to the neutron mass.
Question 22
A nuclide in high-level waste has a long half-life and emits penetrating gamma radiation. The main implication for waste management is that it requires
A.
release into groundwater after one half-life.
B.
isolation and shielding over long timescales.
C.
short-term storage only, because its activity is initially zero.
D.
conversion into chemical waste by heating.
Question 23
The reason a neutron can induce fission in uranium-235 more readily than a proton of the same kinetic energy is that the neutron
A.
has no momentum before entering the nucleus.
B.
has a much greater positive charge than a proton.
C.
is converted into two protons when absorbed.
D.
is not repelled by the positive charge of the nucleus.
Question 24
The diagram shows the main energy-transfer circuits in a nuclear power station.
Name the component labelled X.
[1]
Explain why the primary and secondary water circuits are kept separate.
[1]
State the role of the turbine-generator system.
[1]
Question 25
Spent nuclear fuel is first stored under water in cooling ponds.
State what is meant by decay heat.
[1]
Explain two reasons why water is suitable for initial spent-fuel storage.
[1]
State why remote handling is required.
[1]
Question 26
Nuclear fission can contribute to low-carbon electricity generation, but it also creates waste.
State one reason fission power can reduce carbon dioxide emissions during operation.
[1]
Outline two difficulties associated with long-term storage of high-level nuclear waste.
[1]
State what is meant by decommissioning a nuclear power plant.
[1]
Question 27
The atomic masses in a fission reaction are:
initial atoms and neutron: (236.0526, ext{u})
final atoms and neutrons: (235.8380, ext{u})
Calculate the mass decrease in u.
[1]
Calculate the energy released in MeV.
[1]
State why atomic masses may be used in this calculation.
[1]
Question 28
A heavy nucleus can be stable only because of nuclear binding.
State why gravitational attraction cannot hold a nucleus together.
[1]
Explain why fission becomes energetically possible for very heavy nuclei.
[1]
Question 29
Many fission products undergo beta-minus decay followed by gamma emission.
State the particle, in addition to an electron, emitted in beta-minus decay.
[1]
Explain why the beta-particle energies form a continuous range.
[1]
State why nuclear gamma photons usually have much greater energy than photons from atomic transitions.
[1]
Question 30
Distinguish between reprocessing and long-term geological disposal of spent nuclear fuel.
State the purpose of reprocessing.
[1]
State the purpose of geological disposal.
[1]
State one concern associated with reprocessing.
[1]
Question 31
The graph shows the number of fissions per second in a small reactor during three different control-rod positions.
Identify the interval during which the chain reaction is approximately steady.
[1]
State what is meant by a steady chain reaction in terms of neutrons.
[1]
Explain the change in fission rate when the control rods are inserted further.
[1]
Suggest why control rods must be able to move rapidly during an emergency.
[1]
Question 32
The table compares three possible moderator materials for a thermal reactor.
| Material | Mean energy loss / % | Relative absorption | Thermal/chemical suitability |
|---|---|---|---|
| Light water (H2O) | 50 | 1.0 | Liquid; good heat transfer |
| Heavy water (D2O) | 36 | 0.002 | Liquid; good heat transfer |
| Graphite (C) | 16 | 0.02 | Solid; stable at high T |
Identify the material that would slow neutrons most effectively.
[1]
Identify the material least likely to remove neutrons by absorption.
[1]
Explain why the best moderator is not determined only by how much energy a neutron loses per collision.
[1]
Suggest one other practical property required of a moderator in a reactor core.
[1]
Question 33
The chart shows the relative abundances of fission fragments produced by neutron-induced fission of uranium-235.
State whether fission most often produces two fragments of equal mass.
[1]
Identify the approximate pattern in fragment masses.
[1]
Explain why many fission fragments are radioactive.
[1]
State one implication of this distribution for waste management.
[1]
Question 34
A nuclear power station has electrical output (1.1, ext{GW}). The overall efficiency is 0.34. Each fission releases (3.2 imes10^{-11}, ext{J}).
Calculate the thermal power produced by fission.
[1]
Calculate the number of fissions per second.
[1]
State one reason the efficiency is less than 1.
[1]
Question 35
The average binding energy per nucleon of uranium-235 is (7.59, ext{MeV}). It fissions into products with total nucleon number 236 after absorbing a neutron. The average binding energy per nucleon of the final products is (8.44, ext{MeV}).
Estimate the increase in total binding energy.
[1]
State what this increase represents physically.
[1]
State one reason this estimate may not equal the useful electrical energy obtained per fission.
[1]
Question 36
Natural uranium contains a small fraction of uranium-235 and mostly uranium-238.
State what is meant by enriched uranium.
[1]
Explain why enrichment can help sustain a chain reaction in a thermal reactor.
[1]
State why uranium-238 may reduce the probability of a sustained thermal chain reaction.
[1]
Question 37
A proposed moderator material slows neutrons effectively but also has a significant probability of absorbing slow neutrons.
Explain why slowing neutrons is useful in a uranium-235 reactor.
[1]
Suggest why excessive neutron absorption by the moderator is undesirable.
[1]
Question 38
The table gives atomic masses for one possible fission reaction of uranium-235.
[
^{235} ext{U}+n\rightarrow ^{141} ext{Ba}+^{92} ext{Kr}+3n+\Delta E
]
| Species | Mass / u |
|---|---|
| ²³⁵U | 235.0439 |
| ¹⁴¹Ba | 140.9144 |
| ⁹²Kr | 91.9262 |
| neutron | 1.0087 |
Use the table to determine the mass decrease.
[1]
Calculate the energy released in MeV.
[1]
Explain why the result is an energy release rather than an energy input.
[1]
Question 39
The graph shows the electrical output of a nuclear power station over one day. The station efficiency is given on the graph. Each fission releases (3.2 imes10^{-11}, ext{J}).
Determine the approximate average electrical power from the graph.
[1]
Calculate the corresponding average fission rate.
[1]
Suggest why the fission rate is adjusted when electricity demand changes.
[1]
State one limitation of assuming a constant efficiency over the day.
[1]
Question 40
The table compares two proposed strategies for managing spent nuclear fuel: reprocessing followed by vitrified waste storage, and direct geological disposal.
| Strategy | HLW volume / m³ t⁻¹ fuel | Relative cost | Pu proliferation concern | Chemical processing hazard | Isolation time / yr |
|---|---|---|---|---|---|
| Reprocess + vitrify | 0.3 | High | High: Pu separated | High | ≥10000 |
| Direct geological disposal | 2.5 | Medium | Lower: Pu in spent fuel | Low | ≥100000 |
Identify which strategy gives the smaller final volume of high-level waste according to the table.
[1]
Identify which strategy has the greater proliferation concern according to the table.
[1]
Explain one scientific reason why both strategies require long-term isolation.
[1]
Evaluate, using the table, whether the lower waste volume alone is sufficient to choose one strategy.
[1]
Question 41
The graph shows the decay heat power from spent fuel as a fraction of its operating reactor power after shutdown.
Describe the trend in decay heat power after shutdown.
[1]
State why the decay heat is not zero immediately after control rods are inserted.
[1]
Suggest two design features needed for safe spent-fuel management based on the graph.
[1]
Question 42
A thermal nuclear reactor uses uranium fuel, a moderator, control rods, coolant, a heat exchanger and shielding.
Outline the role of the moderator and the control rods.
[1]
Explain how energy from fission is transferred to electrical energy while reducing the spread of radioactive material.
[1]
Question 43
Nuclear fission is sometimes proposed as part of a strategy to reduce climate change.
Outline two advantages of nuclear fission for electricity generation.
[1]
Discuss scientific and technological issues that must be considered when deciding whether to use fission power.
[1]
Question 44
A uranium-235 nucleus absorbs a neutron and fissions into two medium-mass nuclei and several neutrons.
State two conservation laws used to balance the nuclear equation.
[1]
Explain why energy is released in this fission reaction and identify the main immediate forms of the released energy.
[1]
Question 45
The graph shows the probability of uranium-235 fission and uranium-238 neutron absorption as a function of neutron kinetic energy.
Identify the neutron-energy region in which uranium-235 fission is most probable.
[1]
Describe how the probability of uranium-238 absorption changes in the same region.
[1]
Explain why a moderator and enriched fuel are both useful in a thermal reactor.
[1]
Question 46
A country is comparing two options for high-level nuclear waste: extended surface storage in engineered containers and deep geological disposal.
Outline why high-level nuclear waste remains hazardous after removal from a reactor.
[1]
Evaluate the suitability of deep geological disposal compared with extended surface storage.
[1]
Question 47
A nuclear power station produces electrical power using uranium-235 fission. Each fission releases about (200, ext{MeV}).
Show that the energy released per fission is about (3.2 imes10^{-11}, ext{J}).
[1]
Explain how binding energy, reactor efficiency and neutron control determine the rate of electrical energy production.
[1]
Question 48
Radioactive products from fission include short-lived and long-lived isotopes.
Outline how activity changes with time for a radioactive isotope.
[1]
Compare and contrast the hazards and management requirements of short-lived and long-lived fission products.
[1]
Question 49
The energy released in fission can be described using binding energy, mass defect and conservation laws.
Define mass defect and binding energy.
[1]
Discuss how conservation laws and changes in binding energy account for neutron-induced fission of uranium-235, including the subsequent radiation from fission products.
[1]
Question 50
A reactor designer must choose a moderator and control system for a uranium-235 thermal reactor. One candidate moderator slows neutrons very effectively but absorbs more neutrons than graphite. Another slows neutrons less per collision but absorbs very few neutrons.
Explain why slow neutrons are important in a uranium-235 thermal reactor.
[1]
Evaluate the factors that should determine the choice of moderator and control-rod system.
[1]
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