Nuclear fusion

This lesson covers: 

  1. The mechanism behind nuclear fusion, which involves the merging of atomic nuclei.
  2. The significant energy input required for initiating fusion reactions.
  3. The immense energy output resulting from fusion processes.
  4. The comparison of energy efficiency between fusion and fission, highlighting fusion's superior energy yield per unit mass.
  5. The technical and scientific hurdles in achieving practical fusion energy generation.

Understanding nuclear fusion

Nuclear fusion is the process where lightweight atomic nuclei merge to form a heavier nucleus, simultaneously releasing a large amount of energy.

A common example of this process is the fusion of hydrogen nuclei into helium:


11​H + 11​H →24​He + energy


Key points:

  • Fusion combines lighter nuclei to create heavier ones.
  • High temperatures and pressures are essential to overcome the electrostatic forces repelling the nuclei.
  • Fusion reactions predominantly occur in the plasma state, a high-energy phase of matter present within stars.

Energy released during fusion

The energy liberation in fusion is substantial due to the fact that the resulting nucleus:

  • Exhibits a stronger nuclear binding energy per nucleon compared to the reactants.
  • Has a slightly lower total mass than the sum of the original reacting nuclei.

This mass difference is converted into energy, as described by Einstein's equation (E=mc2).

Worked example: - Calculating the energy released in a nuclear fusion reaction

Determine the energy released when two deuterium nuclei (12​H) fuse to form a helium-4 nucleus (24​He). The mass of a deuterium nucleus is 2.014102 u, while the helium-4 nucleus mass is 4.002602 u.


Step 1: Calculate the mass defect

Δ m = mass of reactants - mass of product

Δ m = 2(2.014102 u) - 4.002602 u = 0.025602 u


Step 2: Convert u to kg

1 u = 1.661 x 10-27 kg

0.025602 u = 4.25 x 10-29 kg


Step 3: Calculate the energy released

E = Δm c2

E=4.25×10−29×(3.00×108)2=3.83×10−12 J

Developing fusion power

Advantages of fusion power include:

  • No long-lived radioactive waste produced.
  • Abundance of fuel sources, such as hydrogen.


Achieving a self-sustaining fusion reaction that generates a net energy output is currently beyond our reach due to:

  • The need for extremely high temperatures, exceeding 100 million °C, to achieve hydrogen isotope fusion.
  • The complexity in designing and maintaining confinement systems capable of containing the resultant plasma state.