Something the fungus COULD do (in a hypothetical world) is concentrate radioisotopes along with some moderator to accelerate the fission process and harvest more energy.

Would probably require a lot more time than it would have, however, considering the relatively low amounts of radioisotopes in todays world (due to the halflife of most of them, and the age of our planet).

Several billion years ago it could have been a thing though!

But if it concentrates isotopes to accellerate fission, wouldn't that cause the material to heat up and, ultimately, kill the fungus? Depends on rate of concentration of course, if it just grabs the odd airborne isotope (if that's a thing) then maybe.

Maybe in principle, but neutron radiation from fallout/etc is relatively minimal and you really just have to wait out the decay of those isotopes.

The good news is radiation detectors are insanely sensitive so you can map where the hotspots are and mitigate much of the risk using exclusion zones and / or various cleanup techniques to collect the radioactive material so it can be safety stored.