Had the title read "Safe Nukes" and you did not put any thought or prior knowledge at all into the title, then sure, oxymoron. Fine.
The title, however, reads "Safer nukes" the fact that it says safer automatically shows that it is being compared to something. When you use a word such as that, and you are comparing the safeness of something, even if it is deadly, it isn't an oxymoron. One is safer than the other. It doesn't contradict itself.