Pebble-Bed HTGR

From its first conception during 1940s, High Temperature Gas-Cooled Reactors were originally pursued in Germany, the UK, and the USA. Germany focused on pebble-beds with AVR and THTR while the UK and USA focused on prismatic cores with the Peach Bottom Unit 1 and the Ft. Vrain Reactor. After a few demonstrations and in the headwinds from Three-Mile Island and Chernobyl, German and American gas-cooled reactor efforts stalled due to lack of customers and government support. Efforts were made to jump start programs in other countries including South Africa, Japan, and China. 

In Pebble Beds like China's HTR-PM or the very similar X-Energy Xe-100 design, the fuel and graphite are packaged as balls (called pebbles) that are poured into the reactor from the top and emptied at the bottom in a continuous fashion like a gumball machine.

Pebble-beds can also be configured with molten salt coolant as in the Fluoride Salt Reactor (FSR).

For more about the pebble-bed reactors, consult this blog post.