Castle Bravo on Tokyo
15 Mt thermonuclear weapon · Japan · Population 13,960,000 · Density 6,300/km²
About this scenario
This page calculates what would happen if the Castle Bravo (USA, 1954) detonated over Tokyo (Japan). Most powerful US nuclear test.
The capital of Japan and the world's largest metropolitan area; Greater Tokyo holds over 37 million people. With an urban-core density of about 6,300 people per km², even a relatively small detonation over the city center would affect a large population.
The Castle Bravo delivers 15 Mt of explosive yield — 1,000× more powerful than the Hiroshima bomb. The tables below show calculated effect radii for an air burst (optimized for blast spread) and a surface burst (which produces massive fallout).
Air-burst effects (Castle Bravo over Tokyo)
| Effect zone | Radius | Est. affected |
|---|---|---|
| Fireball (vaporization, 100% fatal) | 6.79 km | ~912,220 |
| Severe blast (20 PSI, ~98% fatal) | 11.51 km | ~1,658,358 |
| Moderate blast (5 PSI, ~50% fatal) | 24.60 km | ~4,677,492 |
| Light blast (1 PSI, glass injuries) | 69.98 km | ~4,247,462 |
| 3rd-degree thermal burns | 34.54 km | — |
| 2nd-degree thermal burns | 61.86 km | — |
Estimated total fatalities: ~7,248,070 · Estimated total affected (inside 1 PSI light-blast radius): ~96,927,251.
Surface-burst effects (with fallout)
A surface burst trades blast spread for radioactive fallout — much smaller blast radii but a large lethal fallout plume drifting downwind. This is what would happen if the Castle Bravo struck ground level rather than detonating optimally above Tokyo.
| Effect zone | Radius (surface burst) |
|---|---|
| Fireball | 5.43 km |
| Severe blast (20 PSI) | 6.33 km |
| Moderate blast (5 PSI) | 13.53 km |
| Light blast (1 PSI) | 38.49 km |
| 3rd-degree thermal burns | 20.72 km |
| Lethal fallout zone | ~561.8 km |
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See the actual blast zones overlaid on a map of Tokyo with population-density-based casualty estimates updated in real time as you move the detonation point.
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FAQ
What would happen if the Castle Bravo detonated over Tokyo?
In an air burst over Tokyo, the Castle Bravo (15 Mt) would produce a fireball about 6.79 km in radius. The 5 PSI moderate-blast zone — where most residential buildings collapse — would extend to 24.60 km. Light blast damage and shattered windows would reach 69.98 km. Given Tokyo's urban density (~6,300/km²), this scenario yields an estimated 7,248,070 immediate fatalities and about 4,247,462 additional injured.
How many people would die in Tokyo from a Castle Bravo strike?
An air burst of the Castle Bravo over Tokyo could cause an estimated 7,248,070 immediate fatalities and 4,247,462 additional injuries. The fireball alone (radius 6.79 km) would kill approximately 912,220 people; the severe-blast zone (20 PSI, radius 11.51 km) would add 1,658,358; the moderate-blast zone (5 PSI, radius 24.60 km) would add 4,677,492 more. Real numbers depend heavily on time of day, sheltering, weather, and altitude of detonation.
What is the blast radius of the Castle Bravo on Tokyo?
For an air burst over Tokyo: fireball 6.79 km, severe blast (20 PSI) 11.51 km, moderate blast (5 PSI) 24.60 km, light blast (1 PSI) 69.98 km. Thermal radiation causes 3rd-degree burns out to 34.54 km. A surface burst would shrink the blast radii by roughly 40 percent but generate massive radioactive fallout extending ~562 km from ground zero.
Is the Castle Bravo bigger than the bomb that hit Hiroshima?
The Hiroshima bomb (Little Boy) had a yield of about 15 kilotons. The Castle Bravo at 15 Mt is 1,000× more powerful than the Hiroshima bomb.