If one wants to contain all the radioactive material in the bulb, then cool hydrogen shouldnt even get into the bulb, but if not then the superhot core could easily touch the walls and melt them. The temperatures stated are about 25k K for core and I think fused quarz melts at about 1-2K K.
Nothing is hard, just a little bit of a superconductor and a duct tape, and you get a reactor.
But its surely fascinating to think about what can be done if it would be possible to build a rocket like that.
