What Would a Nuclear Bomb Actually Do to Your City?

It is a question most people have thought about at least once, usually late at night, usually after reading the news. What would actually happen if a nuclear weapon detonated over your city? Not in the abstract, historical, black-and-white-footage sense, but in the concrete, where-you-live, your-specific-neighborhood sense. The answer is physics, and the physics is terrifying in its precision.

A modern strategic nuclear warhead, the kind mounted on an intercontinental ballistic missile, typically yields around 800 kilotons of TNT equivalent. For context, the bomb dropped on Hiroshima in August 1945, code-named Little Boy, yielded about 15 kilotons. A modern warhead is roughly 50 times more powerful. But yield alone does not tell you what happens. For that, you need to understand the zones.

The Anatomy of a Detonation

A nuclear detonation unfolds in distinct stages, each with its own radius of destruction. These zones expand outward from the hypocenter like concentric rings, each one defined by a different physical mechanism.

The fireball forms first. For an 800-kiloton airburst, the fireball reaches a diameter of roughly 1.8 kilometers. Within this sphere, temperatures exceed 100 million degrees Celsius, hotter than the core of the sun. Everything inside the fireball does not burn; it vaporizes. Buildings, vehicles, people, the pavement itself, all converted instantly into superheated plasma. There is no shelter, no survival, and nothing recognizable left behind.

The thermal radiation pulse extends much further. At distances up to 10 to 13 kilometers from an 800-kiloton blast, the flash of thermal energy is intense enough to cause third-degree burns on exposed skin. This pulse travels at the speed of light, arriving before any sound, shockwave, or warning. People facing the blast outdoors at these distances would suffer severe burns in under a second. Combustible materials, dry leaves, paper, dark-colored clothing, can ignite spontaneously.

The shockwave follows the thermal pulse. It is a wall of compressed air moving at supersonic speed, gradually decelerating as it expands outward. At 20 psi of overpressure, about 3.4 kilometers from ground zero for an 800-kiloton blast, even reinforced concrete structures are destroyed. At 5 psi, extending to roughly 6 kilometers, most residential and commercial buildings collapse. The shockwave also generates winds exceeding 250 miles per hour near the center, flinging debris that becomes lethal shrapnel.

The Scaling Problem

One of the most counterintuitive aspects of nuclear weapons is how blast radius scales with yield. You might expect that a bomb 50 times more powerful than Hiroshima would have a blast radius 50 times larger. It does not. Blast radius scales with the cube root of yield. This means a weapon 50 times more powerful produces a blast radius only about 3.7 times larger.

This cube-root scaling is why nuclear strategists talk about "overkill." Doubling the destructive area of a single weapon requires increasing its yield eightfold. At a certain point, it becomes more efficient to use multiple smaller warheads than one enormous one. This is part of the logic behind MIRVed missiles, which carry multiple independently targetable reentry vehicles, each with its own warhead, fanning out to strike different targets.

Fallout: The Slow Killer

If a weapon detonates at ground level rather than as an airburst, the fireball scoops up millions of tons of earth, irradiates it, and lofts it into the atmosphere as radioactive fallout. This debris rains downwind over hundreds of kilometers, depositing lethal doses of radiation on areas that might otherwise have escaped the blast entirely.

The intensity of fallout depends on wind patterns, yield, and burst height. A ground burst produces far more fallout than an airburst because the fireball contacts the earth and irradiates surface material. Airbursts, by contrast, produce relatively little local fallout because the radioactive products are carried into the upper atmosphere and dispersed globally over weeks and months.

The most dangerous fallout isotopes have half-lives measured in days to weeks. The general rule of thumb is the "7-10 rule": for every sevenfold increase in time after detonation, radiation intensity decreases by a factor of 10. After 49 hours, radiation is one-hundredth of its initial level. After two weeks, one-thousandth. Sheltering in a basement or interior room during the first 48 hours dramatically improves survival odds.

From Testing to Taboo

Between 1945 and 1996, nations conducted over 2,000 nuclear test detonations worldwide. The United States alone detonated 1,032 devices. The Soviet Union tested 715. These tests ranged from small tactical devices to the Tsar Bomba, the largest nuclear weapon ever detonated, which yielded 50 megatons, over 3,000 times the Hiroshima bomb. Its fireball was 8 kilometers in diameter, and windows were shattered 900 kilometers from the blast site.

The Comprehensive Nuclear-Test-Ban Treaty of 1996 effectively ended atmospheric and underground testing for most nations. But the data gathered during those five decades of testing forms the basis of everything we know about nuclear weapon effects, and it is the data that drives modern simulation tools.

Why We Think About It

There is a reason nuclear detonation simulators consistently rank among the most-used tools on the internet. It is not morbidity for its own sake. It is a form of threat comprehension, a way of making the abstract concrete and the unthinkable thinkable. When you see the blast radius overlaid on your neighborhood, when you can identify which of your familiar landmarks fall inside the fireball versus the moderate damage zone, the weapon stops being an abstraction and becomes something you can reason about.

Understanding the physics of nuclear weapons is not about cultivating fear. It is about replacing fear with knowledge. The fireball has a defined radius. The shockwave attenuates at a predictable rate. Fallout follows wind patterns that meteorologists can model. None of this makes nuclear weapons less horrifying, but it does make them less mysterious, and mysteries are always more frightening than understood dangers.