A nuclear detonation kills people through four distinct physical mechanisms: the fireball, the blast wave, thermal radiation, and radioactive fallout. They have different ranges, different timings, and different survival probabilities. Understanding the four mechanisms separately is the foundation of how the simulator computes effects — and of how civil-defense planning has historically been organized.
The fireball
The fireball is the sphere of plasma created in the first microseconds of detonation. Inside the fireball, temperatures exceed 10 million degrees Celsius — hotter than the surface of the Sun. Everything within the fireball radius is vaporized: humans, buildings, soil. There are no survivors.
Fireball radius scales with yield to the 0.4 power. For a 15 kt Hiroshima-class weapon, the fireball is about 0.18 km in radius. For a 1 Mt strategic warhead, it is about 0.74 km. For Tsar Bomba (50 Mt), it was nearly 4 km wide — fireball-large enough to be visible from 1,000 km away.
The blast wave (overpressure)
A nanosecond after detonation, the rapidly expanding fireball drives a shock wave outward through the surrounding air at supersonic speeds. The shock wave is felt as overpressure — an instantaneous spike in air pressure above atmospheric, measured in pounds per square inch (PSI). Overpressure damages structures and kills people through wind effects, building collapse, and direct lung injury.
Damage is conventionally categorized by overpressure level. The 20 PSI zone destroys reinforced concrete buildings; the 5 PSI zone collapses most residential structures; the 1 PSI zone shatters windows and causes light injuries from flying glass. A 100 kt weapon produces a 5 PSI radius of about 4.8 km in an air burst — enough to flatten a city center.
The blast wave arrives a few seconds after detonation at typical urban ranges — roughly 5 seconds for the 5 PSI shock at 5 km. People who hear or feel the blast have already been exposed to thermal radiation, which travels at the speed of light and arrives instantly.
Thermal radiation
About 35% of a nuclear weapon's energy is released as a brief, intense flash of light and infrared radiation lasting a few seconds. At close range this thermal pulse causes fatal burns through clothing; at longer range it causes 3rd-degree burns to exposed skin; at much longer range it can ignite paper, fabric, and wood, starting fires that may merge into a city-wide firestorm.
Thermal radiation propagates faster than blast — it scales with yield to the 0.41 power instead of 0.33. For a 1 Mt warhead, the 3rd-degree burn radius is about 13 km; the 2nd-degree burn radius is about 17 km. People looking at the flash within these distances can suffer flash blindness or permanent retinal damage.
The Hiroshima firestorm killed tens of thousands of people who survived the initial blast — burned alive in collapsed wooden buildings as fires spread. Modern construction (steel and concrete) reduces firestorm risk, but does not eliminate it.
Fallout
Surface bursts vaporize soil along with everything else in the fireball. The vaporized soil is irradiated by the bomb's neutron flux, becoming radioactive. As the fireball cools and rises, this radioactive debris condenses into particles and is lofted to the upper atmosphere, where it drifts downwind and falls back to earth as fallout — anywhere from minutes to weeks after the detonation.
Fallout intensity depends strongly on whether the weapon is detonated at the surface or in the air. Air bursts produce essentially no local fallout because the fireball never touches the ground. Surface bursts produce massive fallout plumes — for a 1 Mt surface burst, the lethal fallout zone can extend 200-400 km downwind, depending on wind speed.
Fallout dose rates decay rapidly: at one hour after detonation, dose rates may be 1,000 times higher than at one day, and 10,000 times higher than at one week. People who can shelter for the first 48 hours after fallout arrives can avoid most of the lethal dose. The "rule of seven" — a 7-fold increase in time means a 10-fold decrease in dose rate — captures this decay roughly.
Timing and overlap
The four mechanisms arrive in a predictable sequence. At time zero: the prompt nuclear radiation pulse (gamma rays and neutrons) and the thermal flash arrive together at the speed of light. Within a few seconds: the fireball reaches its maximum extent. Within 5-30 seconds: the blast wave arrives at urban ranges. Within minutes to hours: fallout begins to deposit downwind of surface bursts.
Survival depends on which mechanisms reach you. Within the fireball: nothing survives. Inside the 20 PSI radius: 98% of unsheltered people die from blast. Inside the 5 PSI radius: 50% die. Inside the 3rd-degree burn radius (often outside the 5 PSI radius): 95% of unsheltered people die from burns.
Beyond all of these, survival comes down to fallout. Air bursts mean negligible fallout: the people who survive blast and thermal effects largely survive long-term. Surface bursts mean potentially lethal fallout out to hundreds of kilometers — and surviving the immediate detonation is no guarantee of surviving the radioactive plume that arrives hours later.
Why the simulator visualizes them as concentric circles
The familiar "rings" view of a nuclear blast — fireball, severe blast, moderate blast, light blast, thermal — is an idealization but a useful one. It captures the dominant kill mechanism at each range. Inside the fireball, vaporization. Just outside, severe blast. A bit further, thermal burns dominate. Further still, moderate blast. Out at the edge, glass injuries.
In practice the boundaries blur — a building might collapse from blast and then burn from thermal ignition, or a person might survive blast but die later from acute radiation. But the concentric-ring simplification is good enough to give a clear picture of total casualties, which is what the simulator computes. See the methodology page for the underlying formulas.