These particular fungi don't actually 'feed' on the radioactive material - it's more like photosynthesis, so they don't absorb any of the radioactive isotopes. That makes them more likely to become a non-toxic food source, but prevents them from actively breaking down radioactive waste.

However, there's another research group looking at using fungi to clean up depleted uranium - converting the uranium into uranyl phosphates, which locks it in the soil and prevents plants and microbes from taking it up.

Maybe the old saying saying should be ammended to "give me a lever long enough, a fulcrum strong enough and an army of genetically modified fungi, and I can move the world..."