Do the digestive processes of the fungi somehow alter the atomic composition so the fungi not become radioactive?
...
Nope. The melanin in the fungi absorbs ionizing radiation and converts it into energy like a solar cell converts light into energy.
All we have to do is line our cars with mushrooms and run on nukes
here is the word they call it Transduction ;
Melanin, Radiation, and Energy Transduction in Fungi.
https://www.ncbi.nlm.nih.gov/pubmed/28256187
pretty amazing stuff
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2677413/
Thank you.