Pebble bed reactor

The technology in various forms is under development by MIT, the South African power utility Eskom, the Dutch company Romaha B.V., and Adams Atomic Engines, a U.S. Company.

Basic design

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. This carries 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, usually it is 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 they can be designed to be inherently safe. As they get hotter, the fissionables get farther apart, and the reactor's criticality falls. 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 pebble, and the pebbles are designed so that a containment failure releases at most a 0.5mm sphere of radioactive material.

A large advantage of the pebble bed reactor over a conventional light-water reactor is that they operate at higher temperatures. The reactor can directly heat fluids for low pressure gas turbines. The high temperatures permit systems to get more mchanical energy from the same amount of thermal energy. They therefore use 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reactor can be simpler because it does not need to operate well with the varying neutron profiles caused by partially-withdrawn control rods.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 that are inserted through tubes for maintenance, or "absorbers" 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). This makes it adaptable to vehicle power. 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. One of the projects in progress is to develop pebbles and reactors that use the decayed Plutonium from surplus or expired nuclear explosives.

Stationary designs and History

In most PBMR stationary 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 pebbles 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. Teh cocnept was enabled by the realization that engineered forms of silicon carbide and pyrolytic carbon were quite strong, even at temperatures as high as 2000C. 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 15MW demonstration reactor (the AVR) was built in Germany in the 1960s. It ran successfully for 21 years, until the project was ended in the wake of the Chernobyl disaster.

The German prototype used carbon dioxide coolant. This is heavier than air, and just by placing the reactor below grade, it became effectively fire-proof even if the reactor is breached. However, the carbon of the coolant became radioactive over time, and had to be retired and replaced. This was a significant life-cycle cost.

Eskom is South Africa is 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 protoype 110MW pebble-bed modular reactor for Koeberg, South Africa. Eskom also has approval for a pebble-bed fuel production plant in Pendilaba. The Uranium is to be imported from Russia through the S. African port of Durban.

The modular concept allows a small reactor to be mass-produced, reducing the life-cycle costs of safety-certification and design qualification. Sites that require larger generation capcity, simply install more reactors. The cooling system to cool the turbine's exhaust must be adapted to the site. The module is 165mWe. 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. This permits the turbine's pressurization to be decoupeld from the generator speed. Utility generators must be synchronized to the power grid. The prototype test of the closed-cycle helium system: 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. Helium-cooled reactors need fire-prevention features to keep the graphite of the pebbles from burning. Luckily, these are not hard.

Eskom's reactor's design can be throttled in real time to meet peak electric power loads. Conventional light-water reactors can only provide base power, because they require so long to change steam production rates. The modular design also supports the speculation that it is to power peak loads. South Africa lacks fossil fuels for the gas turbines that normally power peak loads, but it exports Uranium and Thorium.

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

An inherently safe modular reactor that can provide peaking-power and fresh water would be a genuninely useful addition to the market, and a valuable export item. If the trial is successsful, Eskom said 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$ 600 million)/yr, and could employ about 57,000 people. The program's total cost is about US$ 1 thousand million, and the developers estimate that about 30 plants will need to be produced to break even.

The environmental group Earthlife Africa filed a court challenge 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 Capetown. Earthlife africa also opposes the Pendilaba 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 8MW (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 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 heat exchanger (the largest part of most light-water reactors- the big cooling tower) is just 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 partical pressure 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 deisgned for air should work well almost without changes. Though AAE's design might require a larger secondary heat exchanger, this might not be a practical problem with a sea-water-cooled heat exchanger, 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 pabble-bed reactor.

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

Safety Features

The continuous refueling means that there is no excess reactivity in the core.

When a pebble-bed reactor gets hotter, the expansion of the pebbles moves the fissionable materials apart. This reduces the reactor's criticality. 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, 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 reactor in England (not a PBR) had a fire from 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 criticality excursion took the graphite above its annealing temperature, releasing accumulated Wigner energy, and causing a fire.

Most pebble-bed reactors contain five levels of containment. First, most reactor systems are enclosed in a containment. The reactor itself is in a vessel that can be sealed.

The design of the pebbles is crucial to the reactor's simplicity and safety, because they include no less than three of the five containments contain the poisonous fuel and wastes. The pebbles are the size of tennis balls. Each weight 210g, and has 9g or Uranium. It takes 380,000 to fuel a reactor of 120 MW. 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 6cm hollow sphere of pyrolytic graphite. The sphere is one containment layer. The hollow contains several thousand small "seeds" with further containment layers. Each seed surrounds a sand-grain-sized 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.

Pyrolytic graphite is the main structural material in these pebbles. It melts at 3000C, 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 oriented parallel layers of crystalline carbon sheets. 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.

The one disadvantage of pyrolytic graphite is that it can burn in air. Of course, all pebble-bed reactors are cooled by inert gasses that prevent fire. However, all pebble designs also have at least one layer of silicon carbide as a fire break. The fissionables are also stable oxides of Uranium, Plutonium or Thorium. These cannot burn, have a higher melting point than the metals, and 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.

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

The surrounding layer of porous pyrolytic graphite absorbs the radioactive gasses (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.

External links

• Eskom's PBMR subsidiary
• Romawa- Diesel replacement
• Adam's Atomic Engines- Diesel replacement, non-airbreathing engines
• http://www.scienceinafrica.co.za/2003/june/pbmr.htm
• http://www.eia.doe.gov/emeu/cabs/safr_nuke.html
• http://www.nirs.org/intl/SThomasPMBRpaper.htm

Anti-PBMR news and links

• http://www.eskom.co.za/pressOffice/intheNews/viewEskomNews.asp?Id=1913
• http://www.mindfully.org/Nucs/2003/PMBR-South-Africa23sep03.htm
• http://m1.mny.co.za/mnsd.nsf/0/C2256D050044B0A142256D400026EB16?OpenDocument
• http://m1.mny.co.za/mnsd.nsf/0/C2256D050044B0A1C2256D6E003B7D8B?OpenDocument
• http://writersclub.iafrica.com/scott_smith/ws_articles/185070.htm
• http://www.anti-atom.de/akwsudaf.htm

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