Nuclear chain reaction

A nuclear chain reaction occurs when a single nuclear event triggers one or more subsequent nuclear events, creating a self-sustaining sequence of reactions. In fission systems, this sequence depends on the release of neutrons from splitting nuclei and the absorption of those neutrons by other fissile nuclei, enabling the reaction to propagate. Chain reactions form the basis of both nuclear power generation and nuclear weapons, though the two applications differ dramatically in design, fuel characteristics, and rate of reaction.

Basic Mechanism

Nuclear fission of heavy nuclei—most notably uranium-235 (²³⁵U) and plutonium-239 (²³⁹Pu)—produces:

• two or more fission fragments,
• a large amount of binding energy, and
• 2–3 free neutrons on average.

If at least one of these neutrons induces another fission event, the chain reaction becomes self-sustaining. The dynamics depend on neutron interactions:

• Absorbed neutrons may trigger further fission.
• Non-fissile isotopes, such as uranium-238 (²³⁸U), may absorb neutrons without fissioning.
• Escaping neutrons reduce the number available to propagate the chain.

A controlled chain reaction is essential for power production; an uncontrolled, rapidly accelerating chain reaction underlies the operation of nuclear weapons.

Historical Development

Early Concepts

The idea of chain reactions emerged first in chemistry. In 1913, Max Bodenstein proposed the concept of chemical chain reactions, which helped explain explosive increases in reaction rates. This conceptual framework later informed nuclear theory.

Szilárd’s Insight

On 12 September 1933, Leó Szilárd conceived the idea of a nuclear chain reaction after learning about experiments in which accelerated protons split lithium nuclei. Coupled with the recent discovery of the neutron by James Chadwick in 1932, Szilárd deduced that if a nuclear reaction emitted neutrons capable of initiating further reactions, a self-propagating process could be possible. He filed a patent in 1934 describing a simple reactor concept, though the mechanism of nuclear fission had not yet been discovered.

Discovery of Fission

In 1938, Otto Hahn and Fritz Strassmann experimentally observed nuclear fission of uranium. Soon after, Lise Meitner and Otto Robert Frisch provided the theoretical explanation and coined the term “fission.” In early 1939, further work by Hahn and Strassmann predicted additional neutron emission during fission, a prerequisite for chain reactions.
Shortly thereafter, Frédéric Joliot-Curie, Hans von Halban, and Lew Kowarski demonstrated neutron multiplication in uranium, experimentally confirming the feasibility of a chain reaction. Patents filed in May 1939 described both power production and explosive applications.

The First Artificial Chain Reaction

The global implications prompted Szilárd and Einstein to warn U.S. President Franklin D. Roosevelt in the Einstein–Szilárd letter. Research accelerated as part of the Manhattan Project.
On 2 December 1942, Enrico Fermi, Szilárd, and colleagues achieved the first controlled, self-sustaining nuclear chain reaction—Chicago Pile-1—beneath Stagg Field at the University of Chicago. This milestone marked the start of practical nuclear reactor technology.

Natural Chain Reactions

In 1956, Paul Kuroda theorised that natural reactors could have existed in Earth’s early history due to higher proportions of ²³⁵U. This was confirmed in 1972 with the discovery of ancient natural fission reactors at Oklo in Gabon, where geological conditions allowed sustained reactions two billion years ago.

Process of a Fission Chain Reaction

A chain reaction requires:

• Fissile isotopes such as ²³⁵U, ²³⁹Pu, or ²³³U;
• Free neutrons to initiate fission; and
• The correct moderation, geometry, and material composition to maintain neutron availability.

Each fission event releases neutrons whose number and energy depend on nuclear physics of the system. Some neutrons are absorbed by structural materials or by isotopes that capture without fissioning. Others escape the system entirely. The chain continues only if, on average, each fission event produces at least one additional fission, a condition known as criticality.

Controlled vs. Uncontrolled Reactions

• Nuclear reactors maintain a steady (critical) or adjustable (sub- or supercritical) rate of fission through neutron moderators, control rods, coolant, and engineered system design.
• Nuclear weapons exploit a prompt, supercritical configuration to achieve an exponentially growing reaction that leads to massive, uncontrolled energy release.

The contrast lies in precision: reactors deliberately slow and regulate neutrons, while weapons concentrate fissile material into supercritical arrangements that cannot be controlled after initiation.

Fuel Types

Uranium-235 (²³⁵U)

²³⁵U is the primary fissile isotope in natural uranium, comprising about 0.7% of its total mass. For reactor use, it is typically converted into uranium dioxide (UO₂) and formed into ceramic fuel pellets. Because natural concentrations are low, uranium is often enriched to increase the proportion of ²³⁵U for efficient reactor performance.

Plutonium-239 (²³⁹Pu)

²³⁹Pu is produced inside reactors when ²³⁸U absorbs neutrons and undergoes two beta decays. It is fissile with slow neutrons and widely used both in reactors and certain weapon designs. Natural quantities are negligible; modern plutonium is synthetic.

Uranium-233 (²³³U)

²³³U can be bred from thorium-232. Though not commercially implemented as of 2021, this thorium fuel cycle remains an area of research interest.

Enrichment

Enrichment is the process of increasing the proportion of fissile isotopes—typically ²³⁵U—for use in power reactors or weapons. Low-enriched uranium fuels reactors, while highly enriched uranium is used in nuclear weapons. The enrichment stage is crucial for determining how the chain reaction behaves under specific conditions.