How is uranium contained in the nuclear power plant?

In most reactor designs all the uranium is contained within the core, which is heavily shielded.

There are a few designs that have been experimented with, like the homogenous slurry reactor where the uranium and coolant circulate as a slurry through the primary coolant loop. But all the moderator is inside the core, so the reaction stops when the slurry leaves the core. None of these designs ever left experimental stage.