Re: computational fluid dynamics (CFD)
Err, when DARPA write "incompressible CFD" that means the easy stuff - with fluids like water whose compressibility can be ignored.
The fluid in a nuclear fusion reactor is very, very, compressible which makes the fluid dynamics much harder. Not only that, it is a plasma, so the particles are all charged, and if you have moving charged particles you need to consider magnetism. There are people who try and model magneto-hydro-dynamics, but it's ... not easy.
Of course, that doesn't nullify your conclusion "quantum computers won't solve nuclear fusion" - if they can't do the easy stuff, what hope have they got with the really messy stuff?