To put things into perspective, there was about 6.4 kg of Plutonium-239 on Fat Man. Assuming all of it reacted to produce it’s ≈20-21 kt TNT equivalent blast, that is about 0.00000000000545 joules per atom. To boil a pot of water (1kg) from STP, you would need to split 77quadrillion atoms to produce enough energy. (More when accounting for losses)
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