Gas-cooled high-temperature reactor with a core of spherical fuel and moderator (graphite) elements. The decommissioned nuclear power plants AVR in Jülich and THTR-300 in Uentrop had a pebble bed reactor. The THTR-300 contained about 600,000 spherical fuel and moderator elements. The fuel elements consist of a kernel of U-235 and thorium surrounded by a graphite matrix of a diameter of 6cm.

Schematic representation of the spherical fuel element of a pebble bed reactor


Nuclear power plant with pebble bed reactor