Theme E — Nuclear and Quantum Physics
E.4 Fission — IB Physics HL Revision Notes & Topical Questions
5 min read 10 parts Lesson 31 of 45 1 quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
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 = × NA
- (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
1
Saying fast neutrons cause fission in U-235 — thermal neutrons are needed.
2
Thinking a moderator absorbs neutrons — it slows them.
3
Confusing the roles of control rods and moderator — rods absorb neutrons and the moderator slows them.
4
Thinking waste is harmless after one half-life — half the activity remains.
5
Saying energy is released because the mass of the nucleus increases — the products have less mass.
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