---
title: "Nuclear fission, fusion and fusion requirements"
subject: "Energy Resources"
chapter: 7
key: "semester_6/energy_resources:nuclear-fission-fusion-requirements"
source: https://www.bctengineer.com/q/semester_6/energy_resources/nuclear-fission-fusion-requirements
site: BCT Engineering
---

# Nuclear fission, fusion and fusion requirements

Energy Resources · Chapter 7: Nuclear energy

- Asked in: year not given, 8 marks
- Video: [Fusion Power Explained – Future or Failure](https://www.youtube.com/watch?v=mZsaaturR6E) — Kurzgesagt – In a Nutshell
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/energy_resources/nuclear-fission-fusion-requirements
- Whole chapter: https://www.bctengineer.com/md/semester_6/energy_resources/chapter-7

## Question

Differentiate between nuclear fission and nuclear fusion. State the requirements for a controlled fusion reaction, including the Lawson criterion, and explain the two approaches to confinement.

## Short answer

Fission splits heavy nuclei (U-235) releasing ~200 MeV; fusion combines light nuclei (D-T) releasing 17.6 MeV, as in sun. Fusion requires temperature ~10^8 K, density n, confinement time τ, and nτ ≥ 10^20 s/m³ (Lawson criterion). Confinement: magnetic (tokamak) and inertial (laser compression of pellets).

## Answer

**Nuclear fission** is the splitting of a heavy nucleus (U-235, Pu-239) after absorbing a neutron into two medium nuclei, with 2–3 neutrons and about 200 MeV of energy per fission. **Nuclear fusion** is the joining of two light nuclei (isotopes of hydrogen) into a heavier nucleus with release of energy, as in the sun.

| Point | Fission | Fusion |
|---|---|---|
| Process | Heavy nucleus splits | Light nuclei combine |
| Fuel | U-235, Pu-239 | Deuterium, tritium (lithium) |
| Energy per reaction | about 200 MeV | 17.6 MeV (D-T) |
| Energy per kg of fuel | about 8×10¹³ J | about 3×10¹⁴ J (D-T) |
| Condition | Neutron, ordinary temperature | 10⁸ K plasma |
| Control | Chain reaction, controlled by rods | Stops easily if conditions fail |
| Waste | Long-lived radioactive waste | Little long-lived waste; neutron activation |
| Status | Commercial | Experimental (tokamak, ITER) |

Main reaction: $^2_1D + {}^3_1T \rightarrow {}^4_2He + {}^1_0n + 17.6\ \text{MeV}$

#### Requirements for fusion
Nuclei are positively charged and repel each other (Coulomb barrier). To fuse they must have very high kinetic energy, which gives these requirements:
1. **High temperature:** about 100–150 million K for D-T (10–15 keV) so the fuel is a fully ionised plasma.
2. **Sufficient density** $n$ of the plasma, so that collisions are frequent.
3. **Confinement time** $\tau$: the plasma must be held away from walls long enough.
4. **Lawson criterion:** to produce more energy than is spent in heating, the product $n\tau \ge 10^{20}\ \text{s/m}^3$ for D-T at about 10 keV (triple product $nT\tau \approx 3\times10^{21}\ \text{keV s/m}^3$).
5. Fuel supply of deuterium (from seawater) and tritium (bred from lithium in the blanket), and purity of plasma.

#### Confinement approaches
- **Magnetic confinement:** the plasma is held in a magnetic field in a doughnut-shaped chamber (tokamak, stellarator). Low density, seconds-long confinement.
- **Inertial confinement:** a small fuel pellet is compressed and heated by lasers or ion beams to extremely high density for a very short time (about 10⁻¹¹ s), so the fusion burns before the pellet blows apart.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which correctly differentiates fission and fusion?
   A. Fission joins light nuclei; fusion splits heavy nuclei
   B. Fission splits a heavy nucleus after neutron absorption; fusion joins light nuclei
   C. Both need 10^8 K
   D. Fusion is the commercial process today
   Answer: B. Fission splits U-235 or Pu-239; fusion combines isotopes of hydrogen. (Key point: Fission vs fusion)

2. Why must a fusion plasma be at 100-150 million K?
   A. To overcome Coulomb repulsion between positively charged nuclei
   B. To melt the container
   C. To cool the neutrons
   D. To make tritium solid
   Answer: A. Nuclei repel each other, so they need very high kinetic energy to fuse. (Key point: Why high temperature)

3. What does the Lawson criterion state for D-T fusion?
   A. Temperature must be below 1 keV
   B. n x tau must be at least about 10^20 s/m3 to get more energy out than spent heating
   C. Confinement time must be zero
   D. Density must be as low as possible
   Answer: B. The product of density and confinement time must exceed about 10^20 s/m3 at about 10 keV. (Key point: Lawson criterion)

4. How do magnetic and inertial confinement differ?
   A. Magnetic uses a field in a tokamak at low density for seconds; inertial compresses a pellet with lasers for about 10^-11 s
   B. Magnetic uses lasers; inertial uses magnets
   C. Both hold plasma for hours at high density
   D. Inertial uses a doughnut chamber
   Answer: A. Magnetic confinement is long and low-density; inertial is very short and extremely dense. (Key point: Confinement approaches)

## Related in Energy Resources

- [Energy from D-T fusion and U-235 fission](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Afusion-fission-energy-mass-defect) — Energy Resources, chapter 7

## Related in other subjects

- [Notes and summary: nuclear fission](https://www.bctengineer.com/question.md?key=semester_5%2Fcommunication_english%3Anotes-summary-nuclear-fission-chain-reaction) — Communication English, chapter 1
- [Two basic approaches of structural analysis](https://www.bctengineer.com/question.md?key=semester_4%2Ftheory_of_structures_i%3Atwo-basic-approaches) — Theory of Structures I, chapter 1, asked 3×
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- [Two reaction model of salient pole machine](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Atwo-reaction-model-salient-pole) — Electrical Machines, chapter 6