|The illustration shows an 8-pack configuration of the PBMR, with one module (bottom left) completed and the others in various stages of construction. The PBMR has been configured into 2, 4 and 8-pack layouts to maximise the sharing of support systems.|
The concept allows for additional modules to be added in accordance with
demand and to be configured to the size required by the communities they serve.
It can operate in isolation anywhere, provided that there is sufficient water
for cooling. Dry cooling, although more expensive, is an option that would
provide even more freedom of location.
|The power conversion unit of the Pebble Bed Modular Reactor. The reactor unit is on the left, the turbo compressor units are in the centre and the generator, power turbine and recuperator on the right.|
The fundamental concept of the design is aimed at achieving a plant that has no physical process that could cause a radiation hazard beyond the site boundary. This is achieved in the PBMR because the integrated heat loss from the reactor vessel exceeds the decay heat production in the post-accident condition. In addition, the peak temperature reached in the core during the transient is below the demonstrated fuel degradation point, and far below the temperature at which the physical structure is affected. This will preclude any prospect of a core melt accident such as happened at the Chernobyl nuclear reactor in Russia in 1986.
"We're trying to change the nuclear culture," says Phumzile Tshelane, senior manager Strategic Development of the PBMR Company. "If the promise of the PBMR materializes, it could dramatically boost the prospects of nuclear energy on a global scale, fulfilling at last the dream of a non-polluting power source that is safe, competitive and perhaps even popular."
The PBMR concept is based on the philosophy that new reactors should be small. A commercial PBMR module would be sized to produce about 165 MW, which is about five times smaller than the capacity of the conventional reactors at Koeberg. The reactor consists of a vertical steel pressure vessel lined with graphite bricks. It uses silicon carbide coated particles of enriched uranium oxide encased in graphite to form a fuel sphere or pebble, each containing about 15 000 uranium dioxide particles. Helium is used as the coolant and energy transfer medium.
|The fuel spheres or pebbles are 60 mm in diameter, which is slightly smaller than a tennis ball. The PBMR fuel is based on a proven high-quality German fuel design consisting of coated uranium particles contained in a molded graphite sphere.|
If the demonstration plant achieves its targets, it is expected to have a number of world-wide sales opportunities, no less so because of its built-in safety characteristics. "Its passive safety features require no human intervention," says Tshelane. "If a fault occurs during reactor operations, the system, at worst, will come to a standstill and merely dissipate heat on a decreasing curve without any core failure or release of radioactivity to the environment. In fact, the PBMR's inherent safety is fundamental to the cost reduction achieved over other nuclear designs."
The PBMR could also provide a mitigation strategy for greenhouse gas reductions, since nuclear power generation produces no carbon dioxide emissions, smoke or any other gases. France's carbon dioxide emissions from electricity generation fell by 80 percent between 1980 and 1987 as its nuclear capacity increased, and Germany's nuclear power programme has saved the emission of over two billion tons of carbon dioxide from fossil fuels since it began in 1961.
The South African economy could benefit on an unprecedented scale from the PBMR. If as few as 10 modules per year are exported, the project could contribute up to R8 billion to the local Gross Domestic Product (GDP) and R10 billion per year in exports. In addition, about 57 000 direct and indirect jobs could be created.
The project recently achieved a major engineering milestone with the successful starting up of a test rig of the PBMR power conversion system. The test rig represents the first closed-cycle, multi-shaft gas turbine in the world. The model was designed and built by the Faculty of Engineering at Potchefstroom University near Johannesburg, with technical input from the PBMR project team.
|The test rig which was built by - and successfully tested at - the engineering department of the University of Potchefstroom. The main pressure vessel of the test rig is 17,5 metres long and weighs 12 tons. The test rig represents the first closed-cycle, multi-shaft gas turbine in the world.|
The South African Nuclear Energy Corporation (NECSA), which is under contract from PBMR (Pty) Ltd to develop the fuel manufacturing capability, is in the mean time making good progress with the development of the exacting production techniques required for the manufacture of complete fuel spheres. Uranium was recently loaded in the coater at the fuel laboratories for the first time.
For more information visit the PBMR Website at www.PBMR.co.za or phone the PBMR communication department at (012) 6779400.
The website provides an outline of the technology behind the PBMR and also addresses some of the issues associated with nuclear energy and its safety.
Copyright 2002, Science in Africa, Science magazine for Africa CC. All Rights Reserved