> 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.