Not if you use the best fat-nucleus element available - the nuclear fuel itself. Design a ship whose uranium or plutonium can double as shielding. There will need to be some additional shielding against radiation from the fuel itself, but that's minimal.
If you brought together a layer of fissionable uranium heavy enough to stop the radiation of some presumably localized reaction, the shield would itself be well over criticality and so couldn't even be built. Plutonium? Same thing, except it's also a devil of a material to deal with structurally (goes through something like five phases in the solid state depending on temperature).