Nuclear fission

Nuclear fission is a fundamental nuclear reaction in which the nucleus of a heavy atom divides into two or more smaller nuclei, releasing energy, neutrons, and gamma radiation. This process underpins both nuclear power generation and the destructive capability of nuclear weapons. Fission can occur spontaneously in certain heavy isotopes or be induced when a nucleus absorbs an external neutron. Its discovery in 1938–39 transformed modern physics, energy production, and global geopolitics.

Discovery and Early Understanding

The discovery of nuclear fission is attributed to chemists Otto Hahn and Fritz Strassmann, who in December 1938 demonstrated experimentally that bombarding uranium with neutrons resulted in the formation of lighter elements such as barium. The theoretical explanation was provided soon afterwards by Lise Meitner and Otto Robert Frisch, who recognised that the process involved the splitting of the uranium nucleus. Frisch introduced the term “fission” by analogy with cell division in biology. Their interpretation also highlighted the significant energy released due to the rearrangement of nuclear binding energies.
In February 1939, Hahn and Strassmann correctly predicted that fission events produce additional neutrons, opening the possibility of a sustained nuclear chain reaction. This insight made clear that both immense peaceful power applications and highly destructive weaponry were feasible.

Mechanisms and Physical Principles

Nuclear fission results when a heavy nucleus absorbs energy sufficient to overcome the fission barrier, a combination of nuclear surface tension and electrostatic repulsion between protons. For neutron-induced fission, the absorbed neutron briefly converts the nucleus into an excited compound system—for example, the absorption of a neutron by uranium-235 forms uranium-236 in a highly unstable state.
The nucleus then deforms and divides into two daughter nuclei, commonly with a typical mass ratio of around 3:2, releasing:

• Two or three free neutrons
• Gamma-ray photons
• Large amounts of kinetic energy in the fragments

These fragments account for roughly 85% of the total energy released, with neutrons, gamma rays, and beta-decay products contributing the remainder. The energy per fission event is extraordinarily high: the splitting of a single uranium nucleus releases around 180 MeV, and the energy from fissioning one gram of uranium far exceeds the output of large quantities of chemical fuels.
Fission can be binary, producing two major fragments, which is the most common pathway, or ternary, in which a smaller third particle—often a high-energy helium-4 nucleus (“long-range alpha”)—is emitted. Ternary fission occurs only a few times per thousand events but contributes to the build-up of gases such as helium and tritium within reactor fuel.

Types of Fission and Nuclear Fuels

Fission reactions fall broadly into two categories:

• Spontaneous fission, a form of radioactive decay in which some very heavy isotopes divide without external input. This phenomenon was first observed in 1940 by Georgy Flyorov, Konstantin Petrzhak, and Igor Kurchatov.
• Induced fission, triggered by absorbing a neutron.
  • Fissile isotopes (e.g., uranium-235, plutonium-239, uranium-233) readily fission when struck by thermal (slow) neutrons.
  • Fissionable isotopes require fast neutrons to induce fission.

Because induced fission generally releases more neutrons than it consumes, a self-sustaining chain reaction can occur. This reaction can be controlled in a reactor or left uncontrolled, resulting in a nuclear explosion.

Early Experiments and Technological Milestones

The study of fission quickly led to significant scientific, military, and industrial developments:

• Chicago Pile-1, the world’s first artificial nuclear reactor, achieved controlled chain reaction in 1942.
• Experiments such as the demon core explored criticality behaviour, showing the importance of safety in neutron-reflective environments.
• The Trinity test in 1945 demonstrated the first fission-based nuclear weapon.
• The largest pure-fission explosive test ever conducted, Ivy King (1952), showcased the maximum theoretical yield for fission-only devices.

Meanwhile, research reactors and power reactors were developed to harness the heat output from fission for electricity generation. Modern power stations, such as the Cattenom Nuclear Power Plant, demonstrate the high thermal output achievable from controlled fission.

Natural Fission and Its Rarity

Although fission is usually associated with engineered systems, natural reactors can form under exceptional geological conditions. The most famous example is the Oklo natural reactor in Gabon, Africa, where around two billion years ago conditions permitted sustained fission reactions. These rare occurrences demonstrate that fission is a valid natural process, even if it has negligible effects on the evolution of the universe compared with fusion.

Fission Products, Radioactivity, and Waste

Fission produces a wide variety of radioactive daughter nuclei. Their unpredictable distribution distinguishes fission from decay processes with fixed outcomes, such as alpha decay. Fission products are typically much more radioactive than the original heavy nucleus, although only seven long-lived isotopes contribute most of the long-term radiological hazard.
Neutron capture processes that do not lead to fission produce:

• Plutonium isotopes, chiefly from uranium-238
• Minor actinides, including neptunium, americium, and curium

These nuclides possess high radiotoxicity and contribute significantly to the challenge of long-term waste management. Concerns about accumulation of these materials influence policy debates on nuclear power.
The thorium fuel cycle offers potential advantages, as it produces much less plutonium and fewer minor actinides, though its key isotope uranium-233 produces intense gamma emissions through decay chains, presenting engineering challenges.
A closed fuel cycle, achieved through nuclear reprocessing, aims to recycle actinides back into reactors, reducing waste and extending fuel resources.

The Liquid Drop Model and Theoretical Insights

The foundational theory of fission was developed by Niels Bohr and John Wheeler, who modelled the nucleus as a charged liquid drop. Using insights from surface tension, Coulomb forces, and the semi-empirical mass formula, they demonstrated how excitation leads to deformation, elongation, and eventual scission. The model remains a cornerstone of nuclear physics, providing a conceptual framework for understanding fission energetics and fragment distribution.

Chain Reactions and Energy Production

A chain reaction occurs when at least one emitted neutron from each fission event induces another fission. Power reactors maintain the chain reaction at a controlled rate using moderators, control rods, and coolant systems. Weapons, in contrast, rely on rapid supercritical assembly to release energy on microsecond timescales.
The energy released by fission is immense: per unit mass, it far exceeds that of chemical fuels or even hydrogen fuel cells. This makes fission a powerful source of heat for electricity generation but also explains its destructive potential when uncontrolled.

Applications and Implications

Nuclear fission has diverse applications:

• Electricity generation in power reactors
• Naval propulsion in submarines and aircraft carriers
• Isotope production for medicine, industry, and scientific research
• Space applications, including radioisotope power systems
• Weapons, forming the basis of early nuclear bombs and components of thermonuclear devices