What if the fungus accumulated radioactive particles in vesicles? Might they create chained reactions and thus deplete the radioactivity faster than spatially separated particles? Might that be plausible?

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 concentrate enough fissionable material, 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.

And then it would decay into daughter isotopes that don't further benefit from it so it might not help a lot anyway.

> 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.