Nuclear Fission and Fusion

Nuclear fission and fusion release large amounts of energy by rearranging nucleons toward more tightly bound configurations, splitting heavy nuclei or merging light ones.

Definition

Nuclear fission is the division of a heavy nucleus into lighter fragments, usually accompanied by the emission of neutrons and energy, while nuclear fusion is the combination of light nuclei into a heavier nucleus with the release of energy; both convert nuclear binding-energy differences into usable energy.

Scope

This topic covers the splitting of heavy nuclei such as uranium and plutonium into lighter fragments with the release of neutrons and energy, and the fusion of light nuclei such as hydrogen isotopes into heavier nuclei. It treats the liquid-drop model of fission, chain reactions and criticality, the Coulomb barrier that fusion must overcome, and the conditions for controlled and explosive release of nuclear energy.

Core questions

• How does a heavy nucleus split, and how is the released energy and number of neutrons determined?
• What conditions are required to sustain a controlled chain reaction?
• How can light nuclei overcome their mutual electrostatic repulsion to fuse?
• Why does fusion power stars while remaining difficult to achieve on Earth?

Key concepts

• Fission fragments and neutron emission
• Chain reaction and criticality
• Fission barrier
• Coulomb barrier in fusion
• Proton-proton chain and CNO cycle
• Energy release and binding-energy curve

Key theories

Liquid-drop theory of fission

Bohr and Wheeler modeled fission as the deformation and splitting of a charged liquid drop, explaining the competition between surface tension and Coulomb repulsion that sets the fission barrier.

Stellar fusion cycles

Bethe identified the proton-proton chain and the carbon-nitrogen-oxygen cycle as the fusion reactions powering stars, converting hydrogen into helium with the release of energy.

Clinical relevance

Fission powers nuclear reactors and weapons and produces medical and industrial isotopes, while fusion drives the Sun and stars and is pursued as a potential large-scale clean energy source in magnetic and inertial confinement experiments.

History

Nuclear fission was discovered chemically by Hahn and Strassmann in 1938 and interpreted by Meitner and Frisch in 1939, with Bohr and Wheeler providing the theoretical mechanism the same year, leading rapidly to reactors and weapons. In parallel, Bethe explained in 1939 that fusion powers the stars, and the pursuit of controlled terrestrial fusion has continued ever since as a major scientific and engineering challenge.

Key figures

• Lise Meitner
• Otto Frisch
• Niels Bohr
• Hans Bethe

Seminal works

• meitner1939
• bohrwheeler1939
• bethe1939

Frequently asked questions

What is a chain reaction?

In fission, each splitting nucleus releases neutrons that can induce further fissions. If on average at least one released neutron triggers another fission, the reaction sustains itself as a chain reaction, the basis of reactors and weapons.

Why is fusion harder to achieve than fission?

Fusion requires bringing positively charged nuclei close enough to fuse, which demands very high temperatures and pressures to overcome their electrostatic repulsion. Fission, by contrast, can be initiated by a slow neutron with no such barrier.