I’m trying to understand why there is more energy output (3-4 times) with nuclear fusion when fission of U-235 releases 200 MeV and D-T fusion only below 20 MeV (14.9 / 17.6).
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So, you’re right that the energy *per event* is greater in nuclear fission. That much is clear.
But what we’re really more interested in is the energy [*per unit mass*](http://www4.hcmut.edu.vn/~huynhqlinh/olympicvl/tailieu/physlink_askexpert/ae534.cfm.htm) — in this case, per nucleon (proton or neutron). Pound for pound, nuclear fusion releases about 6.2 MeV per reacting nucleon, while fission only releases 0.7 MeV. Your mass goes a lot farther with fusion, and the raw materials are relatively easier to come by.
Well the energy output you see that has fusion releasing more than fission refers to energy output per gram of fuel.
Uranium atoms are far far heavier than hydrogen and helium.
In bombs fusion gives you more power because you can’t have too much fission fuel in the same spot or you get runaway chain reactions, molten fuel, and lots of radiation. Limits the ability to squish it into one spot for long enough to maintain the reaction. Fusion doesn’t have that self starting problem.
In power plants the output per gram doesn’t matter as much really, as much as the availability of fuel.
But power wise fusion plants output more power because in the workable designs we have it’s go big or go home to get net positive power. It just takes a ton of energy to heat and compress it to even start the reaction. Fission plants are basically just carefully controlled and shielded hot chunks of metal, and you can make them pretty small. The ones that go on subs are maybe like 30ft tall by 10ft diameter cylinders on the small end?
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