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 efficiency.
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 pebble goes through the cycle several times.
The pebbles are the size of billiard balls and are coated with a layer of
graphite.
Within this graphite layer are approximately 15,000 coated particles. Each particle
contains several layers of coatings and the 0.5mm diameter uranium dioxide fuel
kernel. The inner zone of reactor contains approximately 1,85,000 graphite
spheres and the outer zone contains approximately 3,70,000 fuel elements. Even if
the Helium coolant were to leak away, it would take weeks before melt down,
would be even a possibility.