Theme E — Nuclear and Quantum Physics
E.4.2 Energy Released in Fission Reactions — IB Physics HL Revision Notes & Topical Questions
5 min read 8 parts Lesson 33 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.4.2 Energy Released in Fission Reactions — IB PHYSICS SL notes: Calculate the energy released in fission from mass differences or binding energies and state where it goes.The daughter nuclei have a higher binding energy per nucleon than the parent nucleus.The mass defect betwee…
- The mass defect between parent and daughters appears as energy
- For uranium-235 the products are about 0.19 u lighter
- Chemical reactions release only a few eV per molecule
- With atomic masses the electron numbers must balance
- Free neutrons are excluded
- Rough estimate
Part 1 of 6Why energy is released
Why energy is released
The daughter nuclei have a higher binding energy per nucleon than the parent nucleus.
The mass defect between parent and daughters appears as energy
- Q = (Σmreactants − Σmproducts)c2.
Nuclear fission fuel has the highest energy density of any fuel available now.
- Uranium releases several million times more energy per kilogram than coal.
Part 2 of 6Energy from mass difference
1Q (MeV)
Δm (u) × 931.5 — with masses in unified atomic mass units.
2For uranium-235 the products are about 0.19 u lighter
- For uranium-235 the products are about 0.19 u lighter giving about 175 to 200 MeV per fission.
3Chemical reactions release only a few eV per molecule
- Chemical reactions release only a few eV per molecule so fission releases about a million times more per atom.
4With atomic masses the electron numbers must balance
- With atomic masses the electron numbers must balance so they cancel.
Part 3 of 6Energy from binding energies
Energy from binding energies
Read the binding energy per nucleon for the parent and each fragment from the curve.
Multiply each by its nucleon number to get total binding energy.
Q = Σ binding energy of products − Σ binding energy of reactants.
Energy from binding energies
Free neutrons are excluded
- Free neutrons are excluded they have zero binding energy.
Rough estimate
- Rough estimate uranium at 7.6 MeV per nucleon and fragments at about 8.5 MeV per nucleon give about 0.9 × 235, roughly 210 MeV.
Check
- Check an answer should lie in the range 170–210 MeV (about 3 × 10-11 J).
Part 4 of 6Where the energy goes
| Form | Energy per fission (MeV) |
|---|---|
| Kinetic energy of fission fragments | about 168 |
| Kinetic energy of fission neutrons | about 5 |
| Prompt gamma rays | about 7 |
| Beta particles and gamma rays from fragment decay | about 13 |
| Antineutrinos (escape, not recoverable) | about 10 |
1The fragments are heavy and charged
- The fragments are heavy and charged they travel a tiny distance, colliding with atoms and heating the fuel.
2This thermal energy is what a power station uses
- This thermal energy is what a power station uses antineutrinos carry their energy away.
Part 5 of 6Energy density and power
Energy density and power
Energy density
Energy density energy released per unit volume of fuel; specific energy = energy per kilogram.
- Number of nuclei = (mass / molar mass) × NA.
1 kg of uranium-235 holds about 2.6 × 1024 nuclei
- 1 kg of uranium-235 holds about 2.6 × 1024 nuclei complete fission releases about 8 × 1013 J.
- Petrol releases only about 4.6 × 107 J kg-1.
Power
Power fissions per second × energy per fission.
- A 3.0 GW thermal reactor needs about 9 × 1019 fissions each second.
Output is controlled by adjusting the number of fissions per second
- Output is controlled by adjusting the number of fissions per second by controlling the neutron population.
Real fuel is only 3–5% uranium-235 and not all is fissioned
- Real fuel is only 3–5% uranium-235 and not all is fissioned so the energy extracted per kilogram of fuel is far lower than the ideal figure.
Part 6 of 6More from the specification
Uranium fuel half-life in years
- Uranium fuel half-life in years fissile uranium-235 isotopes decay very slowly over hundreds of millions of years before neutron-induced fission.
High-level waste
- Highly radioactive, heat-generating waste such as spent fuel.
Unstable nucleus
- A nucleus with an imbalance of protons and neutrons that may decay.
Radioactive
- Describes a nucleus that decays spontaneously and randomly.
Spontaneous
- A process which cannot be influenced by environmental factors such as temperature or pressure.
Efficiency
- Useful output divided by total input.
Shielding
- Material that absorbs radiation.
Moderator
- Material that slows neutrons.
Induced fission
- Fission caused by absorption of a neutron.
Radioactive waste
- Unwanted material that emits ionising radiation.
Chain reaction
- Neutrons from one fission trigger further fissions.
Key terms
| Term | Meaning |
|---|---|
| Q value | Energy released in a nuclear reaction. |
| Mass defect | Decrease in rest mass in a reaction. |
| Fragment kinetic energy | Largest share of fission energy, converted to heat. |
| Antineutrino | Carries away about 10 MeV per fission; not recoverable. |
| Energy density | Energy released per unit volume of fuel. |
| Specific energy | Energy released per kilogram of fuel. |
Avoid these mistakes
1
Including free neutrons in a binding-energy sum — they have zero binding energy.
2
Forgetting to convert u to MeV by multiplying by 931.5.
3
Counting antineutrino energy as usable heat — it escapes.
4
Using masses in grams in E = mc2 — use kilograms.
5
Assuming all fuel mass is uranium-235 — only 3–5% is.
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