The concern is more spreading Earth life. NASA's Planetary Protection team (which is a delightful job title) is largely concerned with sterilizing stuff we send out so any discovery of microbes on Mars doesn't turn out to be hitchhikers.

Even a fully fleged nuclear reactor isn't gonna do much damage to Europa and potential life. Swimming in a nuclear reactor's fuel pool is quite safe; water's some of the best shielding we have. https://what-if.xkcd.com/29/