Pebble bed reactor
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. The basic design of pebble bed reactors features 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 235U) surrounded by a coated ceramic layer of silicon carbide for structural integrity and fission product containment. 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, which does not react chemically with the fuel elements.
This type of reactor is claimed to be passively safe; that is, it 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 survive in accident scenarios, which may raise the temperature of the reactor to 1,600 °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 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.
The concept was first suggested by Farrington Daniels in the 1940s, but commercial development did not take place until the 1960s in the German AVR reactor by Rudolf Schulten. but this system was plagued with problems and political and economic decisions were made to abandon the technology. The AVR design was licensed to South Africa as the PBMR and China as the HTR-10, the latter currently the only such design operational. In various forms, other designs are under development by MIT, University of California at Berkeley, General Atomics (U.S.), the Dutch company Romawa B.V., Adams Atomic Engines, and Idaho National Laboratory.
Pebble bed design 
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.
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. Uranium-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 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. The machinery can be repaired or the fuel can be removed. These safety features were tested (and filmed) with the German AVR reactor. All the control rods were removed, and the coolant flow was halted. Afterward, the fuel balls were sampled and examined for damage and there was none.
PBRs are intentionally operated above the 250 °C annealing temperature of graphite, so that Wigner energy is not accumulated. This solves a problem discovered in an infamous accident, the Windscale fire. One of the reactors at the Windscale site in England (not a PBR) caught fire because of the release of energy stored as crystalline dislocations (Wigner energy) in the graphite. The dislocations are caused by neutron passage through the graphite. At Windscale, a program of regular annealing was put in place to release accumulated Wigner energy, but since the effect was not anticipated during the construction of the reactor, and since the reactor was cooled by ordinary air in an open cycle, the process could not be reliably controlled, and led to a fire. The 2nd generation of UK gas-cooled reactors, the AGRs, also operate above the annealing temperature of graphite. The continuous refueling means that there is no excess reactivity in the core. Continuous refueling also permits continuous inspection of the fuel elements. The design and reliability of the pebbles is crucial to the reactor's simplicity and safety, because they contain the nuclear fuel. The pebbles are the size of tennis balls. Each has a mass of 210 g, 9 g of which is uranium. It takes 380,000 to fuel a reactor of 120 MWe. The pebbles are mostly high density graphite which keeps its structural stability at the maximum equilibrium temperature of the reactor. The graphite is the moderator for the reactor, and are strong containment vessels. In fact, most waste disposal plans for pebble-bed reactors plan to store the waste within the spent pebbles..
Berkeley professor Richard A. Muller has called pebble bed reactors "in every way... safer than the present nuclear reactors, and arguably safer than the global-warming danger posed by fossil fuels".
Most pebble-bed reactors contain many reinforcing levels of containment to prevent contact between the radioactive materials and the biosphere.
- Most reactor systems are enclosed in a containment building designed to resist aircraft crashes and earthquakes.
- The reactor itself is usually in a two-meter-thick-walled room with doors that can be closed, and cooling plenums that can be filled from any water source.
- The reactor vessel is usually sealed.
- Each pebble, within the vessel, is a 60 millimetres (2.4 in) hollow sphere of pyrolytic graphite.
- A wrapping of fireproof silicon carbide
- Low density porous pyrolytic carbon, high density nonporous pyrolytic carbon
- The fission fuel is in the form of metal oxides or carbides
Pyrolytic graphite is the main structural material in these pebbles. It sublimates at 4000 °C, more than twice the design temperature of most reactors. It slows neutrons very effectively, is strong, inexpensive, and has a long history of use in reactors. Its strength and hardness come from anisotropic crystals of carbon. Pyrolytic graphite is also used, unreinforced, to construct missile reentry nose-cones and large solid rocket nozzles. It is nothing like the powdered mixture of flakes and waxes in pencil leads or lubricants.
Pyrolytic carbon can burn in air when the reaction is catalyzed by a hydroxyl radical (e.g., from water). Infamous examples include the accidents at Windscale and Chernobyl—both graphite-moderated reactors. Some engineers insist that pyrolytic carbon cannot burn in air, and cite engineering studies of high-density pyrolytic carbon in which water is excluded from the test. However, all pebble-bed reactors are cooled by inert gases to prevent fire. All pebble designs also have at least one layer of silicon carbide that serves as a fire break as well as a seal.
The fissionables are also stable oxides or carbides of uranium, plutonium or thorium which have higher melting points than the metals. The oxides cannot burn in oxygen, but have some potential to react via diffusion with graphite at sufficiently high temperatures; the carbides might burn in oxygen but cannot react with graphite. The fission materials are about the size of a sand grain, so they are too heavy to be dispersed in the smoke of a fire.
The layer of porous pyrolytic graphite right next to the fissionable ceramic absorbs the radioactive gases (mostly xenon) emitted when the heavy elements split. Most reaction products remain metals, and reoxidize. A secondary benefit is that the gaseous fission products remain in the reactor to contribute their energy. The low density layer of graphite is surrounded by a higher-density nonporous layer of pyrolytic graphite. This is another mechanical containment. The outer layer of each seed is surrounded by silicon carbide. The silicon carbide is nonporous, mechanically strong, very hard, and also cannot burn. However, at temperatures greater than 1300 °C it starts to break down in air, as experiments indicate. A drawback of SiC is its poor retention capability for certain metallic fission products, e.g. Ag, Cs and Ru, at high operation temperatures. Thus, He-temperatures of at maximum 750 °C are recommended for current fuel, which however excludes applications as hydrogen generation by water splitting.
Pebble bed reactors do not have a pressure retaining containment (cost reasons). US-NRC has announced that the presence of a full containment as in all other types of reactors would facilitate PBR licensing. Many authorities consider that pebbled radioactive waste is stable enough that it can be safely disposed of in geological storage thus used fuel pebbles could just be transported to disposal.
Production of fuel 
Most authorities agree (2002) that German fuel-pebbles release about three orders of magnitude (1000 times) less radioactive gas than the U.S. equivalents. All kernels are precipitated from a sol-gel, then washed, dried and calcined. U.S. kernels use uranium carbide, while German (AVR) kernels use uranium dioxide.
The precipitation of the pyrolytic graphite is by a mixture of argon, propylene and acetylene in a fluidized-bed coater at about 1275 °C. The fluidized bed moves gas up through the bed of particles, "floating" them against gravity. The high-density pyrolytic carbon uses less propylene than the porous gas-absorbing carbon. German particles are produced in a continuous process, from ultra-pure ingredients at higher temperatures and concentrations. U.S. coatings are produced in a batch process. Although the German carbon coatings are more porous, they are also more isotropic (same properties in all directions), and resist cracking better than the denser U.S. coatings.
The silicon carbide coating is precipitated from a mixture of hydrogen and methyltrichlorosilane. Again, the German process is continuous, while the U.S. process is batch-oriented. The more porous German pyrolytic carbon actually causes stronger bonding with the silicon carbide coat. The faster German coating process causes smaller, equiaxial grains in the silicon carbide. Therefore, it may be both less porous and less brittle. Some experimental fuels plan to replace the silicon carbide with zirconium carbide to run at higher temperatures with melting point SiC 2300° and ZrC 3552° but pyrophoric, means no burning protection.
Criticisms of the reactor design 
The most common criticism of pebble bed reactors is that encasing the fuel in combustible graphite poses a hazard. When the graphite burns, fuel material could potentially be carried away in smoke from the fire. Since burning graphite requires oxygen, the fuel kernels are coated with a layer of silicon carbide, and the reaction vessel is purged of oxygen. While silicon carbide is strong in abrasion and compression applications, it does not have the same strength against expansion and shear forces. Some fission products such as xenon-133 have a limited absorbance in carbon, and some fuel kernels could accumulate enough gas to rupture the silicon carbide layer. Even a cracked pebble will not burn without oxygen, but the fuel pebble may not be rotated out and inspected for months, leaving a window of vulnerability.
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, the current emphasis on reactor safety means that any new design will likely have a strong reinforced concrete containment structure. Also, any explosion would most likely be caused by an external factor, as the design does not suffer from the steam explosion-vulnerability of some 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. Current US legislation requires all waste to be safely contained, therefore 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, typically in a deep geological repository, reprocessed, transmuted in a different type of reactor, or disposed of by some other alternative 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. Proponents[who?] point out that this is a plus, as it is difficult to re-use pebble bed reactor waste for nuclear weapons.
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 (see THTR-300 section). This accident released radiation into the surrounding area, and probably was one reason for the shutdown of the research program by the West German government.
In 2008, a report about safety aspects of the AVR reactor in Germany and some general features of pebble bed reactors have drawn attention. The claims are under contention. Main points of discussion are
- No possibility to place standard measurement equipment in the pebble bed core, i.e. pebble bed = black box
- Contamination of the cooling circuit with metallic fission products (Sr-90, Cs-137) due to the insufficient retention capabilities of fuel pebbles for metallic fission products. Even modern fuel elements do not sufficiently retain strontium and cesium.
- improper temperatures in the core (more than 200 °C above calculated values)
- necessity of a pressure retaining containment
- unresolved problems with dust formation by pebble friction (dust acts as a mobile fission product carrier)
Rainer Moormann, author of the report, requests for safety reasons a limitation of average hot Helium temperatures to 800 °C minus the uncertainty of the core temperatures (which is at present at about 200 °C).
The pebble bed reactor has an advantage over traditional reactors in that the gases do not dissolve contaminants or absorb neutrons as water does, so the core has less in the way of radioactive fluids. However, the pebbles generate graphite particulates that can blow through the coolant loop and will absorb fission products if fission products escape the TRISO particles.
There is significantly less experience with production scale Pebble Bed Reactors than Light Water Reactors. As such, claims made by both proponents and detractors are more theory-based than based on practical experience.
The first suggestion for this type of reactor came in 1947 from Prof. Dr. Farrington Daniels at Oak Ridge, who also created the name "pebble bed reactor". The concept of a very simple, very safe reactor, with a commoditized nuclear fuel was developed by Professor Dr. Rudolf Schulten in the 1950s. The crucial breakthrough was the idea of combining fuel, structure, containment, and neutron moderator in a small, strong sphere. The concept was enabled by the realization that engineered forms of silicon carbide and pyrolytic carbon were quite strong, even at temperatures as high as 2000 °C (3600 °F). The natural geometry of close-packed spheres then provides the ducting (the spaces between the spheres) and spacing for the reactor core. To make the safety simple, the core has a low power density, about 1/30 the power density of a light water reactor.
A 15 MWe demonstration reactor, Arbeitsgemeinschaft Versuchsreaktor (AVR translates to experimental reactor consortium), was built at the Jülich Research Centre in Jülich, West Germany. The goal was to gain operational experience with a high-temperature gas-cooled reactor. The unit's first criticality was on August 26, 1966. The facility ran successfully for 21 years, and was decommissioned on December 1, 1988, in the wake of the Chernobyl disaster and operational problems. During removal of the fuel elements it became obvious that the neutron reflector under the pebble bed core had cracked during operation. Some hundred fuel elements remained stuck in the crack. During this examination it became also obvious that the AVR is the most heavily beta-contaminated (strontium-90) nuclear installation worldwide and that this contamination is present in the worst form, as dust. In 1978 the AVR suffered from a water/steam ingress accident of 30 metric tons, which led to contamination of soil and groundwater by strontium-90 and by tritium. The leak in the steam generator, leading to this accident, was probably caused by too high core temperatures (see criticism section). A re-examination of this accident was announced by the local government in July, 2010.
The AVR was originally designed to breed uranium-233 from thorium-232. Thorium-232 is about 400 times as abundant in the Earth's crust as uranium-235, and an effective thorium breeder reactor is therefore considered valuable technology. However, the fuel design of the AVR contained the fuel so well that the transmuted fuels were uneconomic to extract—it was cheaper to simply use natural uranium isotopes.
The AVR used helium coolant. Helium has a low neutron cross-section. Since few neutrons are absorbed, the coolant remains less radioactive. In fact, it is practical to route the primary coolant directly to power generation turbines. Even though the power generation used primary coolant, it is reported that the AVR exposed its personnel to less than 1/5 as much radiation as a typical light water reactor.
The localized fuel temperature instabilities mentioned above in the criticism section resulted in a heavy contamination of the whole vessel by Cs-137 and Sr-90. Some contamination was also found in soil/groundwater under the reactor, as the German government confirmed in January, 2010. Thus the reactor vessel was filled with light concrete in order to fix the radioactive dust and in 2012 the reactor vessel of 2100 metric tons will be airlifted to an intermediate storage. There exists currently no dismantling method for the AVR vessel, but it is planned to develop some procedure during the next 60 years and to start with vessel dismantling at the end of the century. In the meantime, after transport of the AVR vessel into the intermediate storage, the reactor buildings will be dismantled and soil and groundwater will be decontaminated. AVR dismantling costs will exceed its construction costs by far. In August 2010 the German government published a new cost estimate for AVR dismantling, however without consideration of the vessel dismantling: An amount of 600 million € ( $750 million) is now expected (200 million € more than in an estimate of 2006), which corresponds to 0.4 € ($0.55) per kWh of electricity generated by the AVR. Consideration of the unresolved problem of vessel dismantling is supposed to increase the total dismantling costs to more than 1 bn €. Construction costs of AVR were 115 million Deutschmark (1966), corresponding to a 2010 value of 180 million €. A separate containment was erected for dismantling purposes, as seen in the AVR-picture.
Thorium High Temperature Reactor 
Following the experience with AVR, a full scale power station (the Thorium High Temperature Reactor or THTR-300 rated at 300 MW) was constructed, dedicated to using thorium as the fuel. THTR-300 suffered a number of technical difficulties, and owing to these and political events in Germany, was closed after only four years of operation. One cause for the closing was an accident on 4 May 1986 with a limited release of the radioactive inventory into the environment. Although the radiological impact of this accident remained small, it is of major relevance for PBR history. The release of radioactive dust was caused by a human error during a blockage of pebbles in a pipe. Trying to restart the pebbles' movement by increasing gas flow led to stirring up of dust, always present in PBRs, which was then released, radioactive and unfiltered, into the environment due to an erroneously open valve.
In spite of the limited amount of radioactivity released (0.1 GBq 60Co, 137Cs, 233Pa), the THTR management tried to hide the accident, possibly because this accident pointed to some specific problems with pebble bed reactors, mostly pebble flow and radioactive dust. The management might have thought that the emission would not be detectable due to the Chernobyl fallout happening at the same time. They continued to blame the Chernobyl fallout for all of the contamination found in the surroundings, until the presence of Pa-233 in the vicinity was detected. 233Pa is not formed in uranium reactors, such as Chernobyl, but only in thorium reactors (and also by natural spontaneous fissions with thorium nearby). Thus, step by step, the THTR management report lost all credibility. The radioactivity in the vicinity of the THTR-300 was finally found to result 25% from Chernobyl and 75% from THTR-300. The handling of this minor accident severely damaged the credibility of the German pebble bed community, and pebble bed reactors lost a lot of support in Germany.
The reactor also suffered from the unplanned high destruction rate of pebbles during normal operation, and the resulting higher contamination of the containment structure, and problems with compact pebble allocations, which caused deformations in the control rods and of the side reflector arrangement. Ammonia, which was added to helium as lubricant for core rods moving in the pebble bed, was found to cause unacceptable corrosion on metallic components. Pebble debris and graphite dust blocked some of the coolant channels in the bottom reflector, as was discovered during fuel removal some years after final shut-down. A failure of insulation required frequent reactor shut-downs for inspection, because the insulation could not be repaired. Further metallic components in the hot gas duct failed in September 1988, probably due to thermal fatigue induced by unexpected hot gas currents. This failure led to a long term shut-down for inspections. In August, 1989 the THTR company almost went bankrupt, but was financially rescued by the government. Because of the unexpected high costs of THTR operation, and this accident, there was no longer any interest in THTR reactors. The government decided to terminate the THTR operation at the end of September, 1989. From 1985 to 1989 the THTR-300 registered 16,410 operation hours and generated 2,891,000 MWh of electrical power. This corresponds to 14 months of full power operation.
Current designs 
China has licensed the German technology and is actively developing a pebble bed reactor for power generation. The 10 megawatt prototype is called the HTR-10. It is a conventional helium-cooled, helium-turbine design. The program is at Tsinghua University in Beijing. The first 250-MW plant is scheduled to begin construction in 2009 and commissioning in 2013. There are firm plans for thirty such plants by 2020 (6 gigawatts). By 2050, China plans to deploy as much as 300 gigawatts of reactors of which PBMRs will be a major component. If PBMRs are successful, there may be a substantial number of reactors deployed. This may be the largest planned nuclear power deployment in history.
Tsinghua's program for Nuclear and New Energy technology also plans in 2006 to begin developing a system to use the high temperature gas of a pebble bed reactor to crack steam to produce hydrogen. The hydrogen could serve as fuel for hydrogen vehicles, reducing China's dependence on imported oil. Hydrogen can also be stored, and distribution by pipelines may be more efficient than conventional power lines. See hydrogen economy.
South Africa 
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. In February 2010 the South African government stopped funding of the PBMR because of a lack of customers and investors. PBMR Ltd started retrenchment procedures and stated the company intends to reduce staff by 75%.
On the September 17, 2010 the South African Minister of Public Enterprises announced the closure of the PBMR. The PMBR testing facility will likely be decommissioned and placed in a "care and maintenance mode" to protect the IP and the assets.
Mobile power systems 
Pebble-bed reactors can theoretically power vehicles. There is no need for a heavy pressure vessel. The pebble bed heats gas that could directly drive a lightweight gas turbine.
Romawa B.V., Netherlands, promotes a design called Nereus. This is a 24 MWth reactor designed to fit in a container, and provide either a ship's power plant, isolated utilities, backup or peaking power. Romawa has neither produced nor is licensed to produce a nuclear reactor at this time.
It is basically a replacement for large diesel generators and gas turbines, but without fuel transportation expenses or air pollution. Because it requires external air, Romawa's design limits itself only to environments in which diesel engines can already be used.
Romawa's reactor heats helium, which in turn heats air that drives a conventional gas turbine that are well-developed for the aircraft and stationary power industries. The Romawa design reduces the size and expense of heat exchangers by operating at very high temperatures, and should therefore be small, inexpensive and efficient. The design exhausts the air from the turbine, avoiding the large, inefficient, expensive low-temperature heat exchanger that would otherwise be necessary to cool the turbine's exhaust.
The air passing through the turbine never passes through the reactor, and is therefore never exposed to neutron flux, and therefore particles and gasses cannot become radioactive. The turbine is likewise not part of the primary loop, and uses air as its working fluid. The technology is therefore very standard. Most moving parts do not touch the primary loop, and therefore service should be relatively easy and safe. Romawa proposes two types of throttling. For vehicular power, they advocate a valve between the turbine and reactor while for efficient utility-style throttling, they advocate a system that reduces the pressure of helium in the coolant loop that connects the reactor to the turbine.
Romawa proposes a refueling and maintenance plan, based on "pool service." Users of large gas turbines customarily pool their repair resources to minimize expensive equipment, spares and training. By shipping entire reactors, Romawa plans to eliminate on-site service, and provide all service in one or a few centralized, specialized workshops.
Adams Atomic Engines 
AAE went out of business in December 2010. Their basic design was self-contained so it could be adapted to extreme environments such as space, polar and underwater environments. Their design was for a nitrogen coolant passing directly though a conventional low-pressure gas turbine. US 5309492, [|Adams, Rodney M.], "Control for a closed cycle gas turbine system", published 1994-05-03, issued 1993.Expired on May 3, 2006 due to failure to pay maintenance fees (http://www.uspto.gov/web/offices/com/sol/og/2006/week26/patexpi.htm). and due to the rapid ability of the turbine to change speeds, it can be used in applications where instead of the turbine's output being converted to electricity, the turbine itself could directly drive a mechanical device, for instance, a propeller aboard a ship.
Like all high temperature designs, the AAE engine would have been inherently safe, as the engine naturally shuts down due to Doppler broadening, stopping heat generation if the fuel in the engine gets too hot in the event of a loss of coolant or a loss of coolant flow.
Other issues 
Both Romawa and AAE plan to use neutron reflectors (graphite) and radiation shields (heavy metals) that are bins of balls. This means that the shielding need not have complex ducting to cool it.
See also 
- Next Generation Nuclear Plant
- Very high temperature reactor
- Generation IV reactor
- Nuclear fuel
- Nuclear safety
- Rainer Moormann
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- http://www.world-nuclear-news.org/NN-PBMR_postponed-1109092.html World Nuclear News 11 September 2009
- Pebble Bed Modular Reactor Company is Contemplating Restructuring Measures
- IAEA HTGR Knowledge Base
- AVR, experimental high-temperature reactor : 21 years of successful operation for a future energy technology ISBN 3-18-401015-5
- High Temperature Reactor 2006 Conference, Sandton, South Africa
- MIT page on Modular Pebble Bed Reactor
- Research on innovative reactors in Jülich
- Differences in American and German TRISO-coated fuels
- NPR's Living on Earth (February 24, 2004) Living on Earth: Pebble Bed Technology -- Nuclear promise or peril?
- "Nuclear China" - Australian science documentary about China's pebble-bed reactor, Feb 2007
- Let a Thousand Reactors Bloom - article by Spencer Reiss in Wired magazine about China's pebble bed technology
- "'Pebble-bed' cracker to begin construction", China Daily, Feb 2006
- Nuclear Now! - article by Peter Schwartz (futurist) and Spencer Reiss in Wired magazine about "How clean, green atomic energy can stop global warming"
- "Nuclear activists radiate with anger", 2002
- Idaho National Laboratory - United States
- Modular Pebble Bed Reactor Project, University Research Consortium Annual Report 2000
- A Preliminary Study of the Effect of Shifts in Packing Fraction on k-effective in Pebble-Bed Reactors 2001
- Modular Pebble-Bed Reactor Project: Laboratory-Directed Research and Development Program FY 2002 Annual Report
- Matrix Formulation of Pebble Circulation in the PEBBED Code 2002
- Conceptual Design of a Very High Temperature Pebble-Bed Reactor 2003
- NGNP Point Design - Results of the Initial Neutronics and Thermal-Hydraulic Assessments During FY-03, Rev. 1
- New Generation Nuclear Plant (NGNP) Project, Preliminary Point Design 2003
- The Next Generation Nuclear Plant - Insights Gained from the INEEL Point Design Studies 2004
- Computation of Dancoff Factors for Fuel Elements Incorporating Randomly Packed TRISO Particles 2005
- South Africa
- Coalition Against Nuclear Energy South Africa
- PBMR (Pty.) Ltd.
- Pebble Bed Modular Reactor - PBMR - Home
- Atomic Energy in South Africa
- Earthlife Africa: Nuclear Energy Costs the Earth campaign
- Science in South Africa, June 2003, "South Africa's nuclear programme"
- EIA, December 2003, Nuclear Power in South Africa
- Christian Science Monitor, 23 September 2003, "South Africa Looks to Next-Generation Nuclear Power: But last week, opponents filed papers against a new pebble-bed reactor near Cape Town"
- Steve Thomas (2005), "The Economic Impact of the Proposed Demonstration Plant for the Pebble Bed Modular Reactor Design", PSIRU, University of Greenwich, UK
- NPR (April 17, 2006) NPR: South Africa Invests in Nuclear Power