Simple question - how is the energy release of a nuclear weapon detonation defined? We often hear numbers like 50 kilotons or 5 megatons and so on. Now, when a nuclear weapon explodes (lets say it's a simple fission bomb), we have U-235 or some other fissile material fissioning in a chain reaction an being converted into fission products, which are themselves more often than not radioactive. These decay and so on, right through the decay chains until eventually all nuclei present in the initial explosion reach a stable isobar, yes? OK then, so when we talk about a 5 megaton weapon, does that mean that the total energy release from the whole process, from the first instant of fission right down through the thousands of years until the last unstable nuclei decays into a stable state, is equal to 5 megatons of TNT exploding? Or does the 5 megatons come in as some sort of measure of the 'initial blast strength' - i.e. 5 million tons of TNT and the equivalent size nuclear weapon, sitting next to each other, would yield the same size explosion/impact/damage etc, with the energy release from the later radioactive decay of nuclear fission products ignored? It would seem to me that there would be a significant difference in initial blast size described by the label '5 megaton', depending on the definition used... Any knowledgeable people out there feel like chiming in?