Castle Bravo on London
15 Mt thermonuclear weapon · United Kingdom · Population 9,002,488 · Density 5,500/km²
About this scenario
This page calculates what would happen if the Castle Bravo (USA, 1954) detonated over London (United Kingdom). Most powerful US nuclear test.
The capital of the United Kingdom and most populous city in Western Europe, on the River Thames. With an urban-core density of about 5,500 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 London)
| Effect zone | Radius | Est. affected |
|---|---|---|
| Fireball (vaporization, 100% fatal) | 6.79 km | ~796,382 |
| Severe blast (20 PSI, ~98% fatal) | 11.51 km | ~1,447,773 |
| Moderate blast (5 PSI, ~50% fatal) | 24.60 km | ~4,083,525 |
| Light blast (1 PSI, glass injuries) | 69.98 km | ~3,708,101 |
| 3rd-degree thermal burns | 34.54 km | — |
| 2nd-degree thermal burns | 61.86 km | — |
Estimated total fatalities: ~6,327,680 · Estimated total affected (inside 1 PSI light-blast radius): ~84,619,028.
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 London.
| 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 London 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 London?
In an air burst over London, 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 London's urban density (~5,500/km²), this scenario yields an estimated 6,327,680 immediate fatalities and about 3,708,101 additional injured.
How many people would die in London from a Castle Bravo strike?
An air burst of the Castle Bravo over London could cause an estimated 6,327,680 immediate fatalities and 3,708,101 additional injuries. The fireball alone (radius 6.79 km) would kill approximately 796,382 people; the severe-blast zone (20 PSI, radius 11.51 km) would add 1,447,773; the moderate-blast zone (5 PSI, radius 24.60 km) would add 4,083,525 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 London?
For an air burst over London: 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.