literally more non-dangerous

literally "safer"

Safer has a connotation of safety whereas ungefährlicher has a connotation of danger.

If something that would have killed you before, is less likely to do so now you would call it ungefährlicher or less dangerous but you wouldn't call it safer unless you're intending to be funny.

Native German here: ungefährlich means "not dangerous" also "safe" or "harmless".

so "=== gefahrlos" ?

Not really a good explanation. Ungefärlich is more like "harmless" in English. Yes, it contains the word "harm", but there's a modifier that inverts its meaning.

it's ambiguous in German as well, but both: a fusion reactor has very little radioactive waste (only the components that get hit by neutrons from the fusion reaction are activated, and hard-to-activate materials are being used there), and it's also less radioactive than the waste produced by a fission reactor.