original_query: What is the basic design of pebble-bed reactors?
query: What is the basic design of pebble-bed reactors?
answer: The basic design features spherical fuel elements called pebbles, about the size of a tennis ball, made of pyrolytic graphite containing thousands of micro-fuel particles called TRISO particles.  These TRISO particles contain fissile material (like Uranium-235) surrounded by a ceramic layer. Thousands of pebbles form the reactor core, cooled by a gas like helium.