Pebble bed reactors' defining characteristics are it's reliance on "pebbles"
of fuel (uranium oxide covered in silicon carbide, pyrolitic carbon, and graphite). Pebbles are added to the top of the reactor
every day, eliminating the shutdown process of a normal nuclear reactor refueling (around 40 days). Helium gas, being inert
and stable, then carried the heat from the reaction and spins the turbine and generates electricity. The downward-moving pebbles would also allow more complete fission since uniform irradiation improves
efficiency. The need for complex piping is removed, thus it is a
small reactor that can be built cheaply and operated safely. The reactor cools by natural circulation.
Pebbles are fireproof and cannot be used to weapons production. It is supposed to survive temperatures of 1650 ° Celsius, hotter than the worst foreseeable
accident.
The helium gas does not dissolve
contaminants or absorb neutrons as water does, so the core has less in the way of radioactive fluids. Spent fuel is
also easy to transport and store.
The reactor is essentially meltdown-proof in
that if the fuel becomes too hot (may raise the temperature of the reactor
to 1600 oC), it absorbs the neutrons, stopping the chain reaction and shutting down the plant.
Active development is ongoing in South Africa as the PBMR design, and in China
whose HTR-10 is the only prototype currently operating (two more are planned). The technology was first developed in Germany, but political and economic decisions were made to
abandon the technology. In various forms, it is currently under development by MIT, the South African company PBMR, General
Atomics (U.S.), the Dutch company Romawa B.V., Adams Atomic Engines, Idaho National Laboratory, and the Chinese company Huaneng. In June 2004, it was announced
that a new PBMR would be built at Koeberg, South
Africa by Eskom, the government-owned electrical utility. There is opposition to the PBMR
from groups such as Koeberg Alert and Earthlife Africa, which has sued Eskom to stop development of the project.
