There are different types of radiation and body's response to them are different. Elevated levels of simple gamma rays (and maybe also protons) are reasonably tolerated - the body has mechanisms to recover from that.
However, a steady dose of relativistic heavy nuclei is entirely different matter. Gun wound vs bulldozed over. Cells have mechanisms for repairing local damage, even in the cell nucleus. However, a fact heavy nucleus wrecks so much that the repair mechanisms have no chance - a cell that got hit simply dies (or, at least, is never going to be able to divide again). The damage is cumulative, does not get repaired.
In game parlance: Gammas reduces your HP (that can come back over time, if the hits do not occur too fast). Fast heavy nucleus reduces your max UP. You are doomed, given enough time.
However, a steady dose of relativistic heavy nuclei is entirely different matter. Gun wound vs bulldozed over. Cells have mechanisms for repairing local damage, even in the cell nucleus. However, a fact heavy nucleus wrecks so much that the repair mechanisms have no chance - a cell that got hit simply dies (or, at least, is never going to be able to divide again). The damage is cumulative, does not get repaired.
In game parlance: Gammas reduces your HP (that can come back over time, if the hits do not occur too fast). Fast heavy nucleus reduces your max UP. You are doomed, given enough time.