Moving around radioactive material doesn't affect its activity, unless you're specifically talking about collecting it into a near-critical mass or something like that. Presumably that's what GP was thinking about wrt neutron reflectors. And I'm pretty sure that only works even in principle if the isotope in question can be stimulated into activity by absorbing neutrons (or other radiation I suppose), which is not the case for all of them. Bio-accumulating a critical mass of radioactive material ion by ion... well, it sure is sci-fi.