Castle Bravo was a ground burst and produced a crater about 2Km across. The debris from the crater ended up as fallout.
Tsar Bomba was detonated at an altitude of 4Km, the shock wave prevented the fireball from reaching the ground and producing a crater, so it produced less fallout.
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Tsar Bomba was detonated at an altitude of 4Km, the shock wave prevented the fireball from reaching the ground and producing a crater, so it produced less fallout.