AskTheAI: Science and Tech Questions Answered by AI: How Does a Nuclear Chain Reaction Work in an Atomic Bomb?

From Isotope to Inferno: The Science Behind a Chain Reaction

At the heart of every atomic bomb is one of the most terrifyingly elegant physical processes ever harnessed by humans: the nuclear chain reaction. It is not simply an explosion—it’s the rapid unleashing of the energy stored inside the nucleus of atoms. With the right materials and design, this chain reaction becomes unstoppable, releasing more energy in milliseconds than conventional explosives can produce in minutes.

Let’s break down exactly how a nuclear chain reaction works—and how it leads to the devastating force of an atomic bomb.

What Is Nuclear Fission?

A nuclear chain reaction starts with nuclear fission, a process in which the nucleus of a heavy atom—usually uranium-235 or plutonium-239—is split into two smaller nuclei. When this happens, a massive amount of energy is released, along with additional free neutrons.

This splitting occurs when a neutron strikes the nucleus of a fissile atom, destabilizing it. The nucleus elongates, wobbles, and then tears apart, releasing:

Two or more smaller atomic nuclei (called fission fragments)
Two or three high-energy neutrons
A burst of energy—mostly in the form of kinetic energy, heat, and gamma radiation

Each of those emitted neutrons can go on to hit another fissile nucleus, causing it to split as well. This is where the “chain” in chain reaction begins.

The Ingredients: Fissile Materials

Atomic bombs require fissile material—nuclear fuel that is capable of sustaining a fast chain reaction. The two most commonly used are:

Uranium-235 (U-235): A naturally occurring isotope, though only about 0.7% of natural uranium is U-235. The rest is mostly non-fissile U-238. To be useful in a bomb, uranium must be enriched to over 90% U-235.
Plutonium-239 (Pu-239): This is a man-made isotope bred from uranium-238 in a nuclear reactor. It has a higher probability of fission from fast neutrons, making it suitable for weapons.

Each kilogram of U-235 or Pu-239 contains the energy equivalent of approximately 20,000 tons of TNT—if completely fissioned. In reality, only a fraction of the material fissions in a bomb, but even that fraction unleashes incredible power.

The Critical Mass Concept

For a nuclear chain reaction to sustain itself and grow rapidly, the amount of fissile material must reach a certain threshold known as the critical mass. Below this mass, too many neutrons escape the material without causing fission. At or above critical mass, each fission event produces at least one more, and the reaction becomes self-sustaining.

Factors affecting critical mass include:

Type of fissile material (Pu-239 needs less than U-235)
Density of the material (more compression lowers required mass)
Shape (a sphere retains neutrons better than irregular shapes)
Presence of a neutron reflector—a material (like beryllium or tungsten carbide) surrounding the core that bounces escaping neutrons back in

The goal of bomb design is to maintain the material in a sub-critical state until detonation, and then suddenly push it into a supercritical state faster than the chain reaction can start prematurely.

The Two Main Bomb Designs

There are two primary ways to achieve this sudden supercritical assembly in a bomb: the gun-type design and the implosion design.

1. Gun-Type Design

This design was used in the "Little Boy" bomb dropped on Hiroshima. It works by firing one sub-critical piece of uranium-235 into another using a conventional explosive, forming a supercritical mass.

Steps:

Two sub-critical masses of U-235 are kept separate within a long barrel.
An explosive charge fires one piece down the barrel into the other.
Upon joining, the mass becomes supercritical.
A neutron initiator releases neutrons to start the chain reaction at the moment of full assembly.

This method is simple but only works with uranium. Plutonium is unsuitable due to its higher spontaneous neutron emission, which could cause a “fizzle” (premature detonation).

2. Implosion-Type Design

This design was used in the "Fat Man" bomb dropped on Nagasaki and is used in modern weapons. It surrounds a plutonium core with a spherical shell of conventional explosives arranged in carefully shaped lenses.

Steps:

The core is a sub-critical sphere of Pu-239.
When detonated, the explosive lenses create an inward pressure wave that symmetrically compresses the core.
The density increases dramatically, reducing critical mass and making the core supercritical.
A neutron initiator (like a polonium-beryllium trigger) introduces neutrons at peak compression.

Implosion is far more technically complex but more efficient and compact.

Unleashing the Chain Reaction

Once the supercritical mass is formed and neutrons are injected, the chain reaction begins. In an atomic bomb, this reaction unfolds with mind-bending speed:

Each fission releases 2–3 neutrons in about a trillionth of a second.
Each of those neutrons causes more fission reactions—so in microseconds, the number of reactions grows exponentially.
Within less than a millionth of a second, the mass has undergone enough fission to release millions of joules of energy.

This explosive energy is released as intense heat, a pressure wave, and lethal radiation. The rapid heating vaporizes the bomb casing and surrounding air, forming the iconic fireball and shockwave. Temperatures can exceed several million degrees Celsius—hotter than the center of the Sun.

Why It Doesn't Explode Too Soon

The biggest engineering challenge in building an atomic bomb is not starting the chain reaction too early. A premature chain reaction results in a fizzle—low yield and failure. Designers use:

Sub-critical masses separated until detonation
Precise neutron initiators timed at the moment of maximum compression
Explosive lens symmetry to ensure even implosion

This fine balance is what separates a successful weapon from a dud.

Is There a Limit to the Explosion?

Yes. As the chain reaction proceeds, the explosion itself starts tearing apart the fissile material. In milliseconds, the core blows apart and becomes sub-critical again. Only about 1–2% of the material actually fissions before the reaction is halted by the explosion it generates.

That’s why efficiency is a key concern in weapon design—modern bombs aim to maximize the fission before the bomb disassembles itself.

The Bigger Picture: Fusion Bombs

The bombs discussed above are pure fission devices. But thermonuclear (hydrogen) bombs use these fission bombs as triggers to ignite nuclear fusion. Fusion bombs are vastly more powerful and will be covered in another post.

Conclusion

The nuclear chain reaction in an atomic bomb is a terrifying application of basic physics—turning tiny particles into planet-shaking explosions. It’s not magic. It’s the culmination of carefully orchestrated nuclear physics, precise timing, and devastating intent.

At its core is a simple idea: one atom splits, releases energy and more neutrons, and those neutrons split more atoms. Repeat that process in the blink of an eye—and you’ve recreated the fire of stars here on Earth, with consequences that changed history forever.