I dunno about nuclear fusion, but nuclear fission smells like dust and metal shavings. At least that's what it smelled like inside the reactor compartment when we went in to clean it.

The sad part is, we really have gone in to clean reactor compartments. We even wrote our names in the lead shielding.

Hydrogen (whatever isotope) and helium don't have a smell, so in one respect, the answer would be `nothing'. On the other hand, if you're in an oxygen-containing atmosphere near nuclear fusion, there's probably some ozone floating about, so the answer may be `laser printers'. I think the best answer, though, is. . .

ARGH! MY NOSE IS BURNING DUE TO THE LARGE AMOUNT OF ELECTROMAGNETIC RADIATION BEING RELEASED IN ITS VICINITY!

In the radioactive parts of the US DOE lab that I used to work at, it smells a lot like a machine that cuts plastic. Sort of that smell in a machine shop when someone is cutting plexiglass. Mix in laser printer, or maybe one of those really old copiers also, but a lot stronger. Thanks to neil for helping me place that part of the smell. There is also something else, something bad, but I can't relate it to anything else in the world. It is a bit like spent model rocket engines, but different.