What is fuel pebble?

The basic design of pebble-bed reactors features spherical fuel elements called pebbles. These tennis ball-sized pebbles (approx. 6.7 cm or 2.6 in in diameter) are made of pyrolytic graphite (which acts as the moderator), and they contain thousands of micro-fuel particles called TRISO particles.

How does a pebble bed reactor work?

The fresh pebbles are loaded in the reactor like a gumball machine and helium is pumped down through the pebble bed to extract the heat into a steam generator that produces electricity.

What happens if a pebble bed reactor fails?

A pebble-bed reactor thus can have all of its supporting machinery fail, and the reactor will not crack, melt, explode or spew hazardous wastes. It simply goes up to a designed “idle” temperature, and stays there. In that state, the reactor vessel radiates heat, but the vessel and fuel spheres remain intact and undamaged.

What kind of coolant is used in a pebble reactor?

In the PBR, thousands of pebbles are amassed to create a reactor core, and are cooled by a gas, such as helium, nitrogen or carbon dioxide, that does not react chemically with the fuel elements. Other coolants such as FLiBe (a molten salt) have also been suggested for implementation with pebble fuelled reactors.

How does the moderator work in a pebble reactor?

Similarly, the moderator is solid carbon; it does not act as a coolant, move, or have phase transitions (i.e., between liquid and gas) as the light water in conventional reactors does. Convection of the gas driven by the heat of the pebbles ensures that the pebbles are passively cooled .

Why are pebble beds used in nuclear power plants?

Because of its design, its high temperatures allow higher thermal efficiencies than possible in traditional nuclear power plants (up to 50%) and has the additional feature that the gases do not dissolve contaminants or absorb neutrons as water does, so the core has less in the way of radioactive fluids .