Neutronium seems to have this very strange radiation blocking pattern. I don't think this is as intended. It says that it is 100% blocking but in fact it is letting 100% of the radiation through. Even worse, is that it also infects the neighboring tiles. So building some lead metal tiles connected to the neutronium will cause the lead to let all the radiation through.
Radiation seems to take on this striped pattern. In the first screenshot I highlight a black line, indicating 0 radiation. In the second I highlight the tile just above indicating 5k radiation. The radiation source in this case is the reactor that is just below. So if the first tile blocked everything, the second should not show radiation. The radiation is clearly visible also in the pincha pepper farm above the neutronium.
Make a research reactor close to some neutronium, and observe the way it blocks. I've included my save file for those who want to have a look. Go to "Topiarazon" to the bottom left and find the research reactor.