A type of nuclear reactor that is cooled by helium and is championed as being inherently fail safe. The fuel assembly consists of about 400,000 tennis ball-sized carbon shells filled with small grains of ceramic coated uranium fuel. If the reactor suddenly loses all of its coolant, the carbon shells will absorb enough neutrons to keep the temperature of the reactor core below 3600 degrees, the melting point of the fuel, thus preventing a meltdown. Spent fuel spheres are sucked out of the bottom of the reactor core using pumps and replaced by new ones from above.