The reactor is at the other end of the rocket far away from the payload in the high radiation environment of space. At worse, a small block of material should be enough of a shield. The radiation will dissipate at square the distance from the reactor. Then it had to penetrate the liquid hydrogen in the tank. As for the tank, a shiny thin Mylar sheet would be enough to reflect the rays of the sun. The Vacuum of space makes for a perfect insulator.