A chain reaction requires several kilograms, densely packed, if I'm not mistaken (https://en.wikipedia.org/wiki/Critical_mass#Bare_sphere https://en.wikipedia.o

A chain reaction requires several kilograms, densely packed, if I'm not mistaken (https://en.wikipedia.org/wiki/Critical_mass#Bare_sphere https://en.wikipedia.org/wiki/Critical_mass#Bare_sphere). So that's already a tall order for a fungus. But the radio-active material stays in place. These fungi absorb the radiation.