Theoretically yes, as long as the isotopes are themselves fissile and susceptible to chain reactions. E.g. U-235 is (obviously, since it's fission reactor fuel) but, say, Iodine-131 undergoes beta decay. That electron can't get into another I-131 atom and cause another decay there like neutrons do in U-325. So piling up I-131 won't get it going faster.

In principle if fungi could somehow concentrate enough fissionable material (say uranium), you could get something like the Oklo reactor going, but it would have to be a truly gigantic, probably unphysical amount of fungi to have access to that much environmental uranium in the first place and it would then have to be concentrated very strongly to get any measurable effect. You won't see anything at all if you just move a few atoms a few mm, so it would need to have very long range hyphae. You also need it to be basically one huge organism in order to collect the uranium to one place - billions of small fungi just doing a few square inches each won't work. It's unlikely the fungus could survive to become so huge on only the promise of fractionally higher future radiation, so it would need to eat something else too.

And then it would decay into daughter isotopes that don't further benefit from the concentration so it might not help a lot anyway if you're looking for cleanup. Plus you've covered your cleanup site in, presumably, millions of tonnes of fungus which might or might not be an improvement.

> Might that be plausible?

Not really. You're talking about a fungus creating essentially a nuclear reactor inside of its cells, and creating it out of fuel that's not good enough to make a nuclear reactor in the first place (it at one time was, but now it's a mess of decay products and nonsense).

Reactors also take a certain amount of mass. You can't just squish two tiny microgram particles together and hope to get anything going.

A chain reaction requires several kilograms, densely packed, if I'm not mistaken (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.