A 1 megaton nuclear warhead detonated over a major city would create a fireball roughly 2 kilometers wide, flatten reinforced concrete buildings within a 6-kilometer radius, inflict third-degree burns on exposed skin up to 13 kilometers away, and spread radioactive fallout across hundreds of square kilometers depending on wind conditions. Here is what happens, step by step, in the seconds and hours after detonation.

The First Microsecond: The Fireball

When a nuclear weapon detonates, it releases its energy in a burst of X-rays that heat the surrounding air to tens of millions of degrees. This creates a fireball — a sphere of superheated plasma that expands outward at nearly the speed of light for the first fraction of a second, then slows as it encounters resistance from the atmosphere.

For a 1 megaton airburst (detonated at optimal altitude, roughly 2,000 meters above ground), the fireball reaches a maximum diameter of about 2.1 kilometers. Everything inside that sphere is vaporized. Not burned, not crushed — vaporized. Steel, concrete, soil, and anything else within the fireball ceases to exist as a solid. It becomes plasma and rises in the developing mushroom cloud.

The fireball is brighter than the sun. Anyone looking directly at it from within 80 kilometers would suffer temporary or permanent flash blindness, even in daylight. At night, the flash would be visible from several hundred kilometers away.

The Blast Wave: 1 to 30 Seconds

The expanding fireball compresses the air around it, creating a shockwave that travels outward at supersonic speed. In the first few seconds, the blast wave moves faster than the speed of sound. It slows as it expands, but it remains destructive over a vast area.

The physics of blast damage are measured in overpressure — the pressure above normal atmospheric levels. Here's what different overpressure levels do at various distances from a 1 megaton airburst:

The blast wave also creates a massive wind. At 5 psi overpressure, the accompanying wind reaches about 260 km/h — stronger than any hurricane ever recorded. This wind is what turns ordinary objects into projectiles and is responsible for much of the destruction beyond the immediate blast zone.

Thermal Radiation: The Silent Killer

About 35% of a nuclear weapon's energy is released as thermal radiation — an intense pulse of heat and light that travels outward at the speed of light. Unlike the blast wave, there's no warning. If you can see the fireball, the thermal radiation has already hit you.

For a 1 megaton weapon, the thermal effects are staggering:

The thermal pulse from a 1 megaton weapon lasts about 10 seconds. That's important because it means you have roughly 10 seconds of continuous, intense heat exposure. Any shadow-casting object — a wall, a car, even a thick tree — provides significant protection. This is why "duck and cover" advice, while often mocked, has a genuine basis in physics. If you're 8 kilometers from a detonation, being behind a solid wall versus standing in the open is the difference between severe burns and relative safety from thermal effects.

Fallout: Hours to Weeks

If the weapon detonates at ground level rather than at altitude, it scoops up millions of tons of soil and debris, irradiates it, and lifts it into the mushroom cloud. This material drifts downwind and settles as radioactive fallout over the following hours and days.

The fallout zone from a 1 megaton ground burst can extend hundreds of kilometers downwind, depending on wind speed and direction. The most dangerous fallout arrives within the first 24 hours. Radiation intensity follows the 7-10 rule: for every sevenfold increase in time after detonation, the radiation level drops by a factor of 10. So if the radiation level one hour after detonation is 1,000 roentgens per hour, it will be roughly 100 R/hr after 7 hours and 10 R/hr after 49 hours.

This is why civil defense planning emphasizes sheltering in place for at least 24-48 hours after a nuclear detonation. The most lethal fallout decays relatively quickly. A basement or interior room with thick walls provides meaningful protection — not perfect, but enough to reduce exposure from lethal to survivable levels.

See the Blast Radius for Your City

Drop a nuclear weapon on any location and see the fireball, blast wave, and fallout zones mapped in real time.

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Why Simulations Matter

Tools like nuclear bomb simulators exist because these numbers are hard to grasp in the abstract. Knowing that 5 psi overpressure extends to 8 kilometers doesn't mean much until you see that radius overlaid on your own city. Suddenly those 8 kilometers include your office, your kid's school, three hospitals, and the entire downtown core.

Alex Wellerstein's NUKEMAP was one of the first tools to make this viscerally clear, and the concept has since been adapted into various interactive formats. The point isn't to frighten people. It's to replace vague anxiety with specific knowledge. When you understand the actual physics — that thermal radiation travels line-of-sight, that fallout is worst downwind, that sheltering dramatically improves survival odds — the situation goes from "hopeless" to "dangerous but navigable."

The same principle applies to other disaster simulations. Understanding how earthquakes propagate, how tornadoes form, or how pandemics spread turns abstract threats into comprehensible systems. And comprehensible systems are ones you can prepare for.