Originally posted here by foxyloxley if 1Kg pluotonium = 20 Ktons, it would mean they need 500 Kg of plutonium for the 10 Megaton device................. not realistic. The Soviet 'City Killer' was 10 Megaton+, but more bang, requires LOTS more maintenance. Also Soviet technology required a bigger bang as they couldn't guarantee accuracy within 1 mile........ US tech can now drop within 10 METERS, hence US require 'only' kiloton warheads.....

tech note: for max efficiency ???????? air-burst at 5000 meters will give a 'better' kill zone than a ground burst, which is why the tech race was won and lost on delivery systems.
A ground burst will generate a lot more fall out ( irradiated particles of ground zero) but will make the initial blast zone smaller. 100's of meters instead of 1000's of meters.
Check this out Tsar Bomba
A dirty device derived from the Bassoon was weaponized to create the highest yield weapon the U.S. ever fielded, the 25 megaton Mk-41. The Tsar Bomba design was for a fission-fusion-fission bomb with a staggering yield of at least 100 megatons!