High temperature gas-cooled reactors (HTGR) come in basically two flavors: prismatic cores and pebble-bed cores. Both use graphite moderator and TRISO fuel particles to make the core which allows the reactor to reach high temperatures and handle accidents with relative ease. Both use helium to cool it, which allows thermal applications up to 950°C and power conversion efficiencies of 40%-55%, compared to just 300°C and 30% in water-cooled reactors. They differ in the geometry and form factor of the moderator and fuel which leads to significant differences in size, operations, and technology roadmaps.
In prismatic cores like the Ultra Safe MMR or Japan's HTTR, graphite is formed into chair sized hexagonal blocks with holes for cylindrical fuel pellets and separate holes for coolant channels. The core is maximally packed and nothing moves. Refueling the reactor involves swapping out the graphite blocks.