Here's a hypothetical planet with complex, sapient life and a very weak magnetic field. Could they have Uranium-based blood, much as our blood is based on iron and crab's is based on copper? Deinococcus radiodurans can resist radiation extremely well. Could this hypothetical life do the same? Or would their blood accidentally cause nuclear fission?
I would say it’s very unlikely. There is a difference between tolerating radiation and incorporating unstable isotopes in your organism. In metazoa, the metal ion serves the purpose of binding to another molecule (a ligand, oxygen) which needs to be moved around within the organism. There is no reason why a rare element with an unstable nucleus would be useful for performing that task. A more abundant, easier to manage element would be selected for.
SciFi speculation: I could envision a cellular mechanism able to harvest energy from the nuclear decay of unstable nuclides. On a planet where the material is much more abundant than it was here on earth during the hadean. This is purely speculative.