The Pebble Bed Reactor is an advanced nuclear reactor design. This technology, under development by MIT and the South African power utility Eskom, claims a dramatically higher level of safety and efficiency. Instead of water, it uses helium as the coolant, at very high temperature, to drive a turbine directly. This eliminates the complex steam management system from the design, and increases the transfer efficiency (ratio of electrical output to thermal output) to about 50%.
Instead of shutting down for weeks to replace fuel rods, pebbles are placed in a bin from which spent pellets are removed from the bottom and new ones added to the top (actually, each pellet goes through the cycle several times).
The pebbles are the size of tennis balls, and it takes 150,000 of them in the inner core and 380,000 in the outer core (annulus) to fuel a reactor of 120 MW. Each pebble contains carbon and uranium, and is surrounded by hard silicon carbide. Even if the helium coolant were to leak away, it would take weeks before meltdown would even be a possibility.
A 15MW demonstration reactor was built in Germany in the 1960s. It ran successfully for 21 years, until the project was axed in the wake of the Chernobyl disaster.
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A prototype pebble bed reactor is proposed for Koeberg, South Africa.
