Pebble bed reactor

The Pebble Bed Reactor is an advanced nuclear reactor design. This technology claims a dramatically higher level of safety and efficiency. Instead of water, it uses pyrolytic graphite as the neutron moderator, and an inert or semi-inert gas such as helium, nitrogen or carbon dioxide as the coolant, at very high temperature, to drive a turbine directly. This eliminates the complex steam management system from the design and increases the transfer efficiency (ratio of electrical output to thermal output) to about 50%.

The technology was first developed in Germany but made obsolete through political and economic decisions. In various forms, it is currently under development by MIT, the South African power utility Eskom, General Atomics (U.S.), the Dutch company Romawa B.V. (http://www.romawa.nl), Adams Atomic Engines, a U.S. Company, and the Chinese company Chinergy, working with Tsinghua University.

Contents

1 Basic design

1.1 Stationary designs and history
1.2 Mobile power systems

2 Safety Features

2.1 Production of Fuel

3 Criticism

4 External links

4.1 Anti-PBMR news and links

Basic design

The reactor provides heat, which is used to turn a generator. However, there are a number of different design choices.

The uranium, thorium or plutonium fuels are in oxides (ceramic form) contained within spherical pebbles made of pyrolitic graphite (see discussion below).

The pebbles are in a bin or can. An inert gas, helium, nitrogen or carbon dioxide, circulates through the spaces between the fuel pebbles to carry heat away from the reactor.

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, it may be instead brought to a heat exchanger, where it heats another gas, or 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.

The primary advantage of pebble bed reactors is that it can be designed to be inherently safe. As the reactor gets hotter, the rate of neutron capture by U-238 increases, reducing the number of neutrons available to cause fission. This places a natural limit on the power produced by the reactor. The reactor vessel is designed so that without mechanical aids it loses more heat than the reactor can generate in this idle state. The design adapts well to safety features (see below). In particular, most of the fuel containment resides in the pebbles, and the pebbles are designed so that a containment failure releases at most a 0.5 mm sphere of radioactive material.

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 systems to get more mechanical energy from the same amount of thermal energy; therefore, 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 with 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 core.

If throttled by temperature, the reactor can change power quickly, just by changing the coolant flow rate (this is patented). A coolant-throttled design can also change power efficiently (say, for utility power) by changing the coolant density or heat capacity.

Another advantage is that fuel pebbles for different fuels might be used 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 the plutonium from surplus or expired nuclear explosives.

Stationary designs and history

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 concept was invented by Professor Dr. Rudolf Schulten in the 1950s. The basic concept was to make a very simple, very safe reactor, with a commoditized nuclear fuel. The crucial breakthrough was the idea of combining fuel, structure, containment, and 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. 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.

The core generates less power as its temperature rises, and therefore cannot have a criticality excursion when the machinery fails. 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 power extracted from the coolant.

A 15 megawatt (electric) demonstration reactor, Arbeitsgemeinschaft Versuchsreaktor (AVR - roughly translated to working group test reactor), was built at the nuclear research center Kernforschungszentrum in Jülich, West Germany. The goal was to gain operational experience with a high-temperature gas-cooled reactor. The unit obtained criticality 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.

The AVR was originally designed to breed uranium 233 from thorium 232. Thorium is about three times as abundant in the Earth's crust as uranium, and an effective thorium breeder 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.

China has licensed the technology of the AVR, and is actively developing a pebble-bed modular 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 200 megawatt production plant is planned for 2007. 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. 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 such a reactor to crack steam to produce hydrogen. The hydrogen could serve as fuel for vehicles, reducing China's dependance on imported oil. Hydrogen can also be stored, unlike electricity, and distribution by pipelines may be more efficient than conventional power lines in some circumstances.

Eskom in South Africa may be the current technology leader. It is developing a modular pebble-bed reactor. On June 25, 2003, the South African Republic's Department of Environmental Affairs and Tourism approved ESKOM's prototype 110MW pebble-bed modular reactor for Koeberg, South Africa. Eskom also has approval for a pebble-bed fuel production plant in Pelindaba. The uranium is to be imported from Russia through the S. African port of Durban.

The modular design allows a small reactor to be mass-produced, reducing the life-cycle costs of safety-certification and design qualification. Sites that require larger generation capacity can simply install more reactors. The system to cool the turbine's exhaust must be adapted to the site. The module is 165 MWe. The reactor could be a significant export item for South Africa.

Eskom's primary coolant is helium. The helium directly turns low-pressure turbomachinery, without intervening losses from heat-exchangers. Helium is well-favored because it is chemically inert, and neutrons do not transmute it to a radioactive element. This means that the turbomachinery does not become radioactive, even though it operates on primary coolant. One disadvantage is that the turbine must be somewhat larger, and therefore more expensive.

The turbine's compressors are decoupled from the turbine, which permits the turbine's pressurization to be decoupled from the generator speed. Utility generators must be synchronized to the power grid. The prototype test of the closed-cycle helium system including compressors, turbine and recuperator has been developed in the engineering lab at Potchefstroom University.

Helium is lighter than air, so 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. Luckily, these are not difficult.

Eskom's reactor's design can be throttled in real time to meet peak electric power loads just like conventional reactors, where power follows steam demand in seconds. The modular design also supports the speculation that it will be useful in building peak load plants. South Africa lacks fossil fuels for the gas turbines that normally power peak loads, but it exports uranium and thorium.

Eskom has also said that the reactor was designed to desalinate seawater, to help with South Africa's continuing lack of fresh water.

An inherently safe modular reactor that can provide peaking-power and fresh water would be a genuinely useful addition to the market, and a valuable export item. If the trial is successful, Eskom says it will build up to ten local PBMR plants on South Africa's seacoast. Eskom also wants to export up to 20 PBMR plants per year. The estimated export revenue is 8 billion rand (roughly US$ 1.2 billion)/yr, and could employ about 57,000 people. The program's total cost is about US$ 1 billion, and the developers estimate that about 30 plants will need to be produced to break even.

The environmental group Earthlife Africa (website) (http://www.earthlife.org.za/) filed a court challenge (http://www.elaw.org/resources/text.asp?ID=1867) to the EIA approval of the Koeberg PBMR in September 2003. The Cape Town city government and other civic and environmental groups also say they oppose the plant. In July 2003, following the approval of the environmental impact assessment, there were public demonstrations against the project in both Johannesburg and Cape Town. Earthlife Africa also opposes (http://wildnetafrica.co.za/wildlifenews/2001/07/1552.html) the Pelindaba fuel plant.

Mobile power systems

Pebble-bed reactors can plausibly power vehicles. There is no need for a heavy pressure vessel. The pebble bed produces gas hot enough that it could directly drive a lightweight gas turbine.

Romawa B.V., the Netherlands, promotes a design called "Nereus". This is an 8 MW (very small) reactor designed to fit in a container, and provide either a ship's power plant, isolated utilities, backup or peaking power. The reactor heats helium, which in turn heats air that drives a conventional gas turbine. Romawa has a business agreement with Adams Atomic Engines (http://www.atomicengines.com/) in the U.S.

The Romawa design cleverly reduces the size and expense of heat exchangers. The main heat exchangers, the reactor and air-heater, operate at very high temperatures, and should therefore be small, inexpensive and efficient. The large, inefficient, expensive low-temperature steam condenser (the largest part of a light water reactor—the big cooling tower) is avoided by exhausting the air from the turbine. The design is as safe as a light water reactor, because only the helium passes through the reactor. The air passing through the turbine never passes through the reactor, and is therefore never exposed to neutron flux.

Because it requires external air, Romawa's design limits itself only to environments in which diesel engines are already possible.

Romawa proposes two types of throttling. For vehicle power, they advocate a reliable, quick-acting, inexpensive valve between the turbine and reactor. For efficient utility-style throttling, they advocate a system that reduces the density of helium in the coolant loop that connects the reactor to the turbine.

Romawa also proposes a clever 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, very capable workshops.

Adams Atomic Engines (AAE) of the U.S. also has a proposed design. AAE's engine is completely self-contained, and therefore adapts to dusty, space, polar and underwater environments. The primary coolant loop uses nitrogen, and passes it directly though a conventional low-pressure gas turbine. Nitrogen and air are almost identical, so a turbine designed for air should work well almost without changes. Though AAE's design might require a larger secondary condenser, this might not be a practical problem with a sea-water-cooled condenser, or a small stationary installation that can afford a small cooling tower.

AAE holds the patent on direct throttling of a turbine heated by a pebble-bed reactor.

Both Romawa and AAE plan to use neutron reflectors (graphite) and shields (heavy metals) that are bins of balls. This means that the shielding need not have complex ducting to cool it.

One proposed design of nuclear thermal rocket uses pebble-like fuel containers in a fluidized-bed to achieve extremely high temperatures.

Safety Features

When a pebble-bed reactor gets hotter, the more rapid motion of the atoms in the fuel increases the probability of neutron capture by U-238 atoms through an effect known as Doppler broadening. This reduces the number of neutrons available to cause U-235 fission, reducing the power output by the reactor. This natural negative feedback places an inherent upper limit on the temperature of the fuel without any operator intervention.

The reactor is cooled by an inert, fireproof gas, so it cannot have a steam explosion as a light-water reactor can.

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 issues are not just theory. This exact test was performed (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. There was none.

PBRs are intentionally operated above the annealing temperature of graphite, so that Wigner energy is not accumulated. This solves a problem discovered in a famous 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, the process could not be reliably controlled and led to a fire.

The continuous refueling means that there is no excess reactivity in the core. The system also permits continuous inspection of the fuel elements.

Most pebble-bed reactors contain seven levels of containment. First, most reactor systems are enclosed in a containment building designed to resist aircraft crashes and earthquakes. The reactor itself is 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 can be sealed, as well.

The design of the pebbles (called "TRISO" fuel) is crucial to the reactor's simplicity and safety, because they include no less than four of the seven containments. The pebbles are the size of tennis balls. Each weighs 210 g, and has 9 g of uranium. It takes 380,000 to fuel a reactor of 120 MWe. The pebbles are constructed of ceramics that are known not to melt at the maximum equilibrium temperature of the reactor. The ceramics also act as a renewable 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.

Each pebble is a 60 mm hollow sphere of pyrolytic graphite. The sphere is one containment layer. The hollow contains fifteen thousand small "seeds" with further containment layers. Each seed surrounds a sand-grain-sized (0.5 mm) kernel of fissionables. Breaking the fissionables into pebbles, and pebbles into seeds assures that the maximum release by a cascade of containment failures will be small—at most the fissionables in one seed.

Each seed, from the inside out, consists of the fission fuel in the form of metal oxides or carbides (a containment), low density porous pyrolytic carbon, high density nonporous pyrolytic carbon (another containment), a wrapping of silicon carbide (another containment), and another wrapping of pyrolytic carbon (another containment).

Pyrolytic graphite is the main structural material in these pebbles. It melts at 3000 °C, more than twice the design temperature of most reactors. It slows neutrons very ably, 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.

Some authorities believe that pyrolytic graphite can burn in air, and cite the famous accidents at Windscale and Chernobyl—both graphite-moderated reactors. Others insist that it cannot. Of course, all pebble-bed reactors are cooled by inert gases that prevent fire. However, all pebble designs also have at least one layer of silicon carbide that serve as a fire break, as well as a seal.

The fissionables are also stable oxides or carbides of uranium, plutonium or thorium. These cannot burn, and have a higher melting point than the metals. 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.

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 3 orders of magnitude less radioactive gas than American, which is not surprising in view of the Germans' longer operational experience.

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 American coatings.

The silicon carbide coating is precipitated from a mixture of hydrogen and methyltrichlorosilane. Again, the German process is continuous, while the American is batch-oriented. The more porous German pyrolitic 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.

Criticism

Several critics of pebble bed reactors have claimed that encasing the fuel in potentially flammable graphite poses a hazard. The reactor's use of an inert gas as a coolant possibly alleviates this issue.

Additionally, current 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 by caused by an external factor, as the design does not suffer from the same steam-explosion vulnerability in water-cooled reactors.

Since the fuel is contained in graphite pebbles, the actual volume of radioactive waste is greater, although the waste tends to be less hazardous. Defects in the production of pebbles may also cause problems.

The strength of silicon carbide is known from its use in abrasion and compression, but public debators may misrate this material by quoting not its strength against expansion and shear force but its hardness. Crystalline materials such as diamond can be cut with a sharp blow, and since the disintegration products such as Xenon have a limited absorbance in carbon, after some point the nugget accumulates a large amount of gas and may rupture. The system needs to be actively purged of oxygen, and the points at which the fuel pellets inserted, and more importantly, are removed, are an opportunity for disastrous introduction of air, and need to be specifically and sturdily enclosed. The containment structure must be capable of serving as a secondary tank capable of containing the hot reaction products that would result from a breach of the primary container, should oxygen enter the system. High temperature for extended periods may produce gas reactions that may have not been well studied because of the expense of maintaining such conditions over long periods of time.

Critics also often point out an accident in Germany in 1986, which involved a jammed pebble. This accident released radiation into the surrounding area, and led to a shutdown of the research program by the West German government.