The pebble bed reactor ( PBR ) is a graphite-moderated, gas-cooled, nuclear reactor. It is a type of Very high temperature reactor (VHTR) , one of the six classes of nuclear reactors in the Generation IV initiative. Like other VHTR designs, the PBR uses TRISO fuel particles, which allows for high outlet temperatures and passive safety.
The base of the PBR's unique design is the spherical fuel elements called "pebbles". These tennis ball-sized pebbles are made of pyrolytic graphite (which acts as the moderator), and they contain thousands of micro fuel particles called TRISO particles. These TRISO fuel particles consist of a fissile material (such as U 235 ) surrounded by a coated ceramic layer of SiC for structural integrity and fission product containment. In the PBR, 360,000 pebbles are amassed to create a reactor core, and are cooled by an inert or semi-inert gas such as helium, nitrogen or carbon dioxide.
This type of reactor is also unique because its passive safety removes the need for redundant, active safety systems. Because the reactor is designed to handle high temperatures, it can cool by natural circulation and still remain intact in accident scenarios, which may raise the temperature of the reactor to 1600°C. 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 advantage that the gases do not dissolve contaminants or absorb neutrons as water does, so the core has less in the way of radioactive fluids. A number of prototypes have been built. Active development is ongoing in South Africa as the PBMR design, and in China whose HTR-10 is the only prototype currently operating.
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, the latter of which has sued Eskom to stop development of the project. In September 2009 the demonstration power plant was postponed indefinitely.
One proposed design of nuclear thermal rocket uses pebble-like fuel containers in a fluidized bed to achieve extremely high temperatures.
Pebble bed design
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 bin or can. 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 that it operates 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. The reactor design is such that it is power-limited or inherently self controlling due to Doppler broadening.
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.
Safety features
When the nuclear fuel increases in temperature, the rapid motion of the atoms in the fuel causes an effect known as Doppler broadening. The fuel then sees a wider range of relative neutron speeds. U 238 , which forms the bulk of the uranium in the reactor, is much more likely to absorb fast or epithermal neutrons at higher temperatures. This reduces the number of neutrons available to cause fission, and reduces the power of the reactor. Doppler broadening therefore creates a negative feedback because as fuel temperature increases, reactor power decreases. All reactors have reactivity feedback mechanisms, but the pebble bed reactor is designed so that this effect is very strong and does not depend on any kind of machinery or moving parts. Because of this, its passive cooling, and because the pebble bed reactor is designed for higher temperatures, the pebble bed reactor can passively reduce to a safe power level in an accident scenario. This is the main passive safety feature of the pebble bed reactor, and it makes the pebble bed design (as well as other very high temperature reactors) unique from conventional light water reactors which require active safety controls.
The reactor is cooled by an inert, fireproof gas, so it cannot have a steam explosion as a light-water reactor can. The coolant has no phase transitions—it starts as a gas and remains a gas. 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.
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
Snoozer Round Pet Beds
Snoozer Bolstered Pet Bed - Extra Large
Pet Beds At Pet Bed Warehouse - Pet Beds - All Beds - Round Fleece ...
Pet Bed Warehouse carries many varieties of pet beds including dog beds and cat beds. Feel free to browse the web site to find specialized pet beds for things such as extra large ...
Outdoor Chair Cushions - Round Ottomans, Designer Pet Bed, Beach ...
Buy outdoor chair cushions, seat cushions, large round ottomans, luxury designer pet beds, patio lounge chairs, beach chairs, outdoor furnitures etc available at Kushease.com. Buy ...
Pet Beds At Pet Bed Warehouse - Pet Beds - All Beds - Round Gusseted ...
Pet Bed Warehouse carries many varieties of pet beds including dog beds and cat beds. Feel free to browse the web site to find specialized pet beds for things such as extra large ...
Snoozer Waterproof Round Pet Bed - Dog Beds & Home Living
Your outdoor dog will just love this waterproof bed! Ideal for giving your pet a dry place to relax after a dip in the pool! The banding on this bed allows for stability and the ...
Overstuffed Round Pet Beds - Frontgate
Frontgate - Find luxury home decor, indoor furnishings, kitchen and bath essentials, and electronics at Frontgate - we outfit America's finest homes with high-end furnishings
Kiko Small Round Pet Bed | Dog Bed | Portable Pet Beds | Designer Dog ...
Shop affordable kiko small round pet bed, dog bed and other designer dog beds in Florida USA. For more information call our experts today before the stock exhaust
SuperSoft Round Pet Beds
Quality cat beds and cushions for your pet at discount prices from PetMarket.com. foam, cedar, plush, crate mats, snoozers, heated, etc
Snoozer Lamb Round Pet Beds
Quality pet beds and cushions for your dog or cat at discount prices from PetMarket.com. foam, cedar, plush, crate mats, snoozers, heated, etc
Luxury Round Pet Beds - Frontgate
Frontgate - Find luxury home decor, indoor furnishings, kitchen and bath essentials, and electronics at Frontgate - we outfit America's finest homes with high-end furnishings