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Pebbles |
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The most common criticism of
pebble bed reactors is that encasing the fuel in potentially flammable
graphite poses a hazard in and of itself. Were the graphite to burn,
fuel material could potentially be carried away in smoke from the
fire. To prevent this, the reaction vessel is purged of oxygen,
usually being replaced with helium. Oxygen entering the vessel would
cause the graphite in the fuel pebbles to burn since the reactor
operating temperature is around 1500 degrees Celsius.
Some designs for pebble bed reactors lack a containment building,
potentially making such reactors more vulnerable to outside attack and
allowing radioactive material to spread in the case of an explosion.
However, an explosion would most likely be caused by an external
factor, as the design does not suffer from the steam-explosion
vulnerability of water-cooled reactors.
Since the fuel is contained in graphite pebbles, the volume of
radioactive waste is much greater, but contains about the same
radioactivity when measured in becquerels per kilowatt-hour. The waste
tends to be less hazardous and simpler to handle. As current
legislation requires all waste to be safely contained, pebble bed
reactors would increase existing storage problems. Defects in the
production of pebbles may also cause problems. The radioactive waste
must either be safely stored for many human generations, reprocessed,
transmuted in a different type of reactor, or disposed of by a method
yet to be devised. The graphite pebbles are more difficult to
reprocess due to their construction, which is not true of the fuel
from other types of reactors.
The strength and hardness of silicon carbide are known from use in
abrasion and compression applications. However, it does not have the
same strength against expansion and shear forces. Since some fission
products such as xenon-133 have a limited absorbance in carbon, some
of the fuel pebbles could accumulate enough gas to rupture the silicon
carbide layer of the fuel pellet.
Critics also often point out an accident in Germany in 1986, which
involved a jammed pebble damaged by the reactor operators when they
were attempting to dislodge it from a feeder tube. This accident
released radiation into the surrounding area, and led to a shutdown of
the research program by the West German government. |
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