23.1.4Quantum and nuclear physics (A Level)
Nuclear Fusion and Fission
5 min read 9 parts Lesson 17 of 23 1 quiz
Lesson summary
23.1.4 Nuclear Fusion and Fission — AS PHYSICS 9702 notes: Describe nuclear fission and fusion, the conditions for each, chain reactions and why both release energy.Fission = the splitting of a large nucleus into smaller nuclei, with the release of energy.Fuel — High-mass nuclei …
- Induced fission
- Energy per fission
- Binding energy
- Controlled
- Control rods
- Proton-proton step
Section 1 of 9Nuclear fission
Fission
Fission the splitting of a large nucleus into smaller nuclei, with the release of energy.
Fuel
- High-mass nuclei such as uranium undergo fission.
Induced fission
- A neutron strikes a nucleus such as uranium-235, which becomes unstable and splits into two or more daughter nuclei.
- Examplen + 235U → 141Ba + 92Kr + 3n + energy.
Energy per fission
- About 200 MeV, mostly as kinetic energy of the fragments, which heat the surrounding material.
Binding energy
- The fragments are higher on the curve than uranium, so they are more tightly bound and total mass decreases.
Neutrons
- Extra neutrons are ejected by the nucleus, not by the fission products.
Section 2 of 9Chain reactions
Chain reaction
- Each fission releases two or three neutrons that can induce further fissions.
- It continues until all the material has fissioned or is halted by a moderator.
Growth
- If more than one neutron per fission causes another fission, the number of fissions grows exponentially.
Controlled
- In a reactor exactly one neutron per fission causes another.
Uncontrolled
- In a weapon the neutron number multiplies without limit and releases energy in a tiny fraction of a second.
Section 3 of 9Reactor components
Reactor components
Fuel rods
- They contain enriched uranium-235.
Moderator
- Water, graphite or heavy water slows neutrons by collisions, because slow neutrons are much more likely to be absorbed by uranium-235.
Control rods
- Boron or cadmium absorbs neutrons, and lowering the rods reduces the reaction rate.
Coolant
- It carries heat to a heat exchanger that makes steam for turbines.
Section 4 of 9Nuclear fusion
1
Fusion
Fusion two nuclei combining to form a single nucleus, releasing energy.
2
Fuel
- Low-mass nuclei such as hydrogen and helium undergo fusion.
3
Proton-proton step
- Two protons fuse to give deuterium, a positron and an electron neutrino, since one proton changes to a neutron by beta-plus decay.
- 1H + 1H → 2H + e+ + ν.
4
Stars
- Four hydrogen nuclei fuse to form a helium nucleus with release of energy, which fuels the star.
5
Deuterium-tritium reaction
- 2H + 3H → 4He + n + 17.6 MeV.
- It is the reaction that research on Earth aims to achieve.
6
Energy per kilogram
- Fusion releases about three to four times as much energy per unit mass as fission.
7
Binding energy
- The lightest nuclei are on the steep rising part of the curve, so the products are bound more tightly per nucleon.
Section 5 of 9Conditions for fusion
High kinetic energy
- Nuclei must get close enough to fuse, within about 10−15 m.
Electrostatic repulsion
- The positively charged protons repel one another, which takes great energy to overcome.
Strong nuclear force
- It acts only over very short distances, so nuclei must be very close for it to take effect.
Temperature
- About 107 K in the Sun's core and over 108 K for reactors on Earth.
- Matter is then a plasma of free nuclei and electrons.
Density
- High pressure or density makes collisions frequent enough.
Confinement
- Gravity confines the Sun's core, and magnetic fields in tokamaks confine plasma on Earth.
Section 6 of 9Comparing fission and fusion
| Feature | Fission | Fusion |
|---|---|---|
| Reaction | Heavy nucleus splits | Light nuclei join |
| Fuel | Uranium-235, plutonium-239 | Hydrogen isotopes |
| Waste | Radioactive fragments | Little long-lived waste |
| Conditions | Slow neutrons, critical mass | Extreme temperature and confinement |
| Energy per kg | Large | Larger still |
Common principle
- The products have greater binding energy per nucleon, so mass is lost and energy is released according to ΔE = Δmc2.
Safety
- Fission fragments are neutron-rich and radioactive for a long time,
- while fusion has no runaway chain reaction because the plasma cools if confinement fails.
Section 7 of 9More from the specification
More from the specification
Nuclear Mass Defect (Δm)
- The mass defect of a nucleus is defined as the difference between the total mass of the separate constituent nucleons (mtotal) and the bound mass of the nucleus, calculated as Δm = Z*mp + (A - Z)*mn - mnucleus.
Mass-Energy Equivalence
- Einstein's mass-energy equivalence equation ΔE = Δm*c² means that mass defect converts directly into nuclear binding energy,
- which is the work required to completely separate nucleons against strong nuclear forces.
Binding Energy per Nucleon Curve
- Calculating binding energy per nucleon (Eb / A) against nucleon number reveals a steep positive gradient for light elements, peaking near iron-56, followed by a gradual negative slope for heavier nuclides.
Energetics of Fusion and Fission
- Both fusion of light nuclei and fission of heavy nuclei release energy because daughter nuclei have a greater binding energy per nucleon, hence greater mass defect and greater stability.
Nuclear Mass Defect (Δm)
- The mass defect of a nucleus is defined as the difference between the total mass of the separate constituent nucleons (mtotal) and the bound mass of the nucleus, calculated as Δm = Z*mp + (A - Z)*mn - mnucleus.
Mass-Energy Equivalence
- Einstein's mass-energy equivalence equation ΔE = Δm*c² means that mass defect converts directly into nuclear binding energy,
- which is the work required to completely separate nucleons against strong nuclear forces.
Binding Energy per Nucleon Curve
- Calculating binding energy per nucleon (Eb / A) against nucleon number reveals a steep positive gradient for light elements, peaking near iron-56, followed by a gradual negative slope for heavier nuclides.
Energetics of Fusion and Fission
- Both fusion of light nuclei and fission of heavy nuclei release energy because daughter nuclei have a greater binding energy per nucleon, hence greater mass defect and greater stability.
Gradient
- Slope of a graph line.
Mass-energy equivalence
- The principle that mass and energy are interchangeable, E = mc2.
Electrostatic force
- Force between charged objects, attractive or repulsive
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Nuclear fission | The splitting of a large nucleus into smaller nuclei with the release of energy. |
| Nuclear fusion | The joining of two light nuclei to form a heavier nucleus with the release of energy. |
| Chain reaction | A self-sustaining series of fissions in which neutrons from one fission cause further fissions. |
| Moderator | A material that slows neutrons so they are more likely to cause fission. |
| Control rod | A rod of neutron-absorbing material used to control the rate of a chain reaction. |
| Plasma | A hot gas of free nuclei and electrons in which fusion can occur. |
Section 9 of 9Exam technique
1
Saying nucleons are destroyed in fission and fusion — they are rearranged into more tightly bound nuclei and the lost mass becomes energy.
2
Saying control rods slow neutrons — the moderator slows neutrons and control rods absorb them.
3
Claiming fusion needs high temperature to overcome gravity — it is the electrostatic repulsion between nuclei that must be overcome.
4
Saying fusion gives more energy per reaction than fission — fission gives about 200 MeV against 17.6 MeV, but fusion gives more per kilogram of fuel.
5
Forgetting that neutrons are released in fission — they make the chain reaction possible.