Theme E — Nuclear and Quantum Physics

E.4 Fission — IB Physics HL Revision Notes & Topical Questions

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E.4 Fission — IB PHYSICS HL notes: Describe spontaneous and induced fission, the energy released, chain reactions, the parts of a nuclear reactor and the management of radioactive waste.Definition — the splitting of a large unstable nucleus into two smaller nucleiFuels — isotopes…

Energy transfer
Spontaneous fission
Uranium-235
Balancing equations
Mass defect
Calculation

Part 1 of 8Nuclear fission

Definition

Definition the splitting of a large unstable nucleus into two smaller nuclei

Fuels

Fuels isotopes of uranium and plutonium are used in nuclear power stations

Products

Products two daughter nuclei, two or three neutrons and gamma rays

Energy transfer

Energy transfer nuclear potential energy becomes the kinetic energy of the fission fragments, which is carried away as heat

Part 2 of 8Spontaneous and induced fission

Spontaneous fission

Spontaneous fission rare, occurring with no additional energy put into the nucleus

Induced fission

Induced fission the unstable nucleus first absorbs a slow-moving thermal neutron

Uranium-235

Uranium-235 its half-life is very long, so it has low activity and releases energy only when neutrons are added
- (about 700 million years)

Mechanism

U-235 absorbs a neutron to form U-236, which is very unstable and splits almost immediately

Balancing equations

Balancing equations mass number and proton number must balance, so missing particles are found from the differences
- (for example, three neutrons)

Part 3 of 8Energy released in fission

Cause

Cause the daughter nuclei have a higher binding energy per nucleon than the parent

Mass defect

Mass defect the difference between parent and daughter masses releases energy, E = Δmc²

Value

Value about 200 MeV per fission of U-235

Calculation

Calculation energy released = total binding energy of the products − binding energy of the parent, with binding energy = binding energy per nucleon × mass number

Energy density

Energy density nuclear fuel has the highest energy density of any currently available fuel

Number of nuclei

N = × N_A
- (mass/molar mass)

Specific energy

Specific energy the energy per kg equals the number of atoms per kg times the energy per fission

Part 4 of 8Chain reactions

Thermal neutrons

Thermal neutrons slow, low-energy neutrons induce fission, because fast neutrons rebound from the nucleus

Neutron production

Neutron production each fission releases two or three fast neutrons

Chain reaction

Chain reaction each released neutron can cause another fission, which releases more neutrons

Controlled reaction

Controlled reaction exactly one neutron per fission goes on to cause another

Critical mass

Critical mass the minimum mass of fuel that maintains a steady chain reaction

Subcritical

Subcritical less than the critical mass, so the reaction dies out

Supercritical

Supercritical more than the critical mass, giving a runaway reaction and possibly an explosion

Part 5 of 8Parts of a nuclear reactor

Parts of a nuclear reactor

Aim

Aim to collect the heat from nuclear reactions while controlling the number and the energy of free neutrons

Control rods

Control rods absorb neutrons and are made of materials like boron or cadmium
Lowering them reduces the rate of fission and raising them increases it

Emergency shutdown

Emergency shutdown lowering the rods fully stops the reaction

Moderator

Moderator slows fast neutrons to thermal speeds by collisions, and is a poor absorber of neutrons such as water or graphite

Coolant

Coolant pumped in cold to remove the heat from the fission reactions

Heat exchanger

Heat exchanger transfers the heat from the coolant to water that becomes steam to drive turbines and generators

Shielding

Shielding a steel and concrete wall nearly 2 m thick absorbs radiation and neutrons

Part 6 of 8Radioactive waste

Low-level

Low-level lightly contaminated clothing, gloves and tools, radioactive for a few years
Encased in concrete and stored a few metres underground

Intermediate-level

Intermediate-level waste between daily items and fuel rods, often from decommissioning
Encased in cement in steel drums and stored underground

High-level

High-level unusable fission products and spent fuel rods
Radioactive for thousands of years and extremely hot

Plutonium-239

Plutonium-239 formed from U-238 and has a half-life of 24 000 years

Treatment of high-level waste

Treatment of high-level waste cooling ponds, harvesting of uranium and plutonium for reuse, vitrification in molten glass, encasing in steel, lead or concrete and storage deep underground

Long half-life

Long half-life it means a lower activity per atom but longer storage

Part 7 of 8Nuclear power in society

Advantages

Advantages no greenhouse gases in operation, high energy density, large fuel reserves and good reliability and safety

Disadvantages

Disadvantages hazardous long-lived waste, risk of catastrophic accidents like Chernobyl, misuse of material and the dangers of mining uranium

Environment

Environment long-lived isotopes must not enter water and food, and burial sites must be geologically stable and secure

Worker safety

Worker safety fuel rods are handled remotely and thick shielding limits exposure

Part 8 of 8More from the specification

Efficiency
- Useful output divided by total input.

Key terms

Term	Meaning
Nuclear fission	The splitting of a large unstable nucleus into two smaller nuclei.
Induced fission	Fission caused when a nucleus absorbs a neutron.
Chain reaction	A reaction in which neutrons from one fission cause further fissions.
Critical mass	The minimum mass of fuel needed to sustain a chain reaction.
Moderator	A material that slows neutrons to thermal speeds.
Control rod	A neutron-absorbing rod used to regulate the fission rate.
Heat exchanger	A device that transfers heat from the reactor coolant to the water that makes steam.

Avoid these mistakes

Saying fast neutrons cause fission in U-235 — thermal neutrons are needed.

Thinking a moderator absorbs neutrons — it slows them.

Confusing the roles of control rods and moderator — rods absorb neutrons and the moderator slows them.

Thinking waste is harmless after one half-life — half the activity remains.

Saying energy is released because the mass of the nucleus increases — the products have less mass.

Last updated: 14 October 2026