Pebble Bed Reactor Schematic
A pebble bed power plant combines a gas-cooled core and a novel packaging of the fuel that dramatically reduces complexity while improving safety.
The uranium, thorium or plutonium nuclear fuels are in the form of a ceramic (usually oxides or carbides) contained within spherical pebbles a little smaller than the size of
a tennis ball and made of pyrolytic graphite, which acts as the primary neutron moderator. The pebble design is relatively simple, with each sphere consisting of the nuclear
fuel, fission product barrier, and moderator (which in a traditional water reactor would all be different parts). Simply piling enough pebbles together in a critical geometry
will allow for criticality.
The pebbles are held in a vessel, and an inert gas (such as helium, nitrogen or carbon dioxide) circulates through the spaces between the fuel pebbles to carry heat away from the
reactor. If helium is used, because it is lighter than air, air can displace the helium if the reactor wall is breached. Pebble bed reactors need fire-prevention features to keep
the graphite of the pebbles from burning in the presence of air although the flammability of the pebbles is disputed. Ideally, the heated gas is run directly through a turbine.
However, if the gas from the primary coolant can be made radioactive by the neutrons in the reactor, or a fuel defect could still contaminate the power production equipment, it
may be brought instead to a heat exchanger where it heats another gas or produces steam. The exhaust of the turbine is quite warm and may be used to warm buildings or chemical
plants, or even run another heat engine.
Much of the cost of a conventional, water-cooled nuclear power plant is due to cooling system complexity. These are part of the safety of the overall design, and thus require
extensive safety systems and redundant backups. A water-cooled reactor is generally dwarfed by the cooling systems attached to it. Additional issues are that the core irradiates
the water with neutrons causing the water and impurities dissolved in it to become radioactive and that the high pressure piping in the primary side becomes embrittled and requires
continual inspection and eventual replacement.
In contrast, a pebble bed reactor is gas-cooled, sometimes at low pressures. The spaces between the pebbles form the "piping" in the core. Since there is no piping in the core
and the coolant contains no hydrogen, embrittlement is not a failure concern. The preferred gas, helium, does not easily absorb neutrons or impurities. Therefore, compared to
water, it is both more efficient and less likely to become radioactive.
A large advantage of the pebble bed reactor over a conventional light-water reactor is in operating at higher temperatures. The reactor can directly heat fluids for low pressure
gas turbines. The high temperatures allow a turbine to extract more mechanical energy from the same amount of thermal energy; therefore, the power system uses less fuel per kilowatt-hour.
A significant technical advantage is that some designs are throttled by temperature, not by control rods. The reactor can be simpler because it does not need to operate well at the varying
neutron profiles caused by partially withdrawn control rods. For maintenance, many designs include control rods, called "absorbers" that are inserted through tubes in a neutron reflector
around the reactor core. A reactor can change power quickly just by changing the coolant flow rate and can also change power more efficiently (say, for utility power) by changing the
coolant density or heat capacity.
Pebble bed reactors are also capable of using fuel pebbles made from different fuels in the same basic design of reactor (though perhaps not at the same time). Proponents claim
that some kinds of pebble-bed reactors should be able to use thorium, plutonium and natural unenriched uranium, as well as the customary enriched uranium. There is a project in
progress to develop pebbles and reactors that use MOX fuel, that mixes uranium with plutonium from either reprocessed fuel rods or decommissioned nuclear weapons.
In most stationary pebble-bed reactor designs, fuel replacement is continuous. Instead of shutting down for weeks to replace fuel rods, pebbles are placed in a bin-shaped reactor.
A pebble is recycled from the bottom to the top about ten times over a few years, and tested each time it is removed. When it is expended, it is removed to the nuclear waste area,
and a new pebble inserted. The core generates less power as its temperature rises, and therefore cannot have a criticality excursion when the machinery fails, it is power-limited or
inherently self controlling due to Doppler broadening. At such low power densities, the reactor can be designed to lose more heat through its walls than it would generate. In order
to generate much power it has to be cooled, and then the energy is extracted from the coolant.