9. Inherently Safe Reactor Designs

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Unformatted text preview: y developed for jet engines such as the Boeing 747. High-strength high-temperature steel vessels are used. New technology has also been become available in the recuperators from the fossil fuel power production field. Today's recuperators are five times smaller than the designs of a few years ago. Highly efficient plate-fin heat exchange equipment is used. The design consists of two pressure vessels, both located underground in a concrete containment structure. The first vessel houses the reactor system. The second vessel houses the power conversion system including the gas turbine, two compressors, and a generator. The helium gas in the core is heated to 1,562 degrees F. The heated helium flows to the power generator unit generating electricity at an overall thermal efficiency of 48 percent, compared with the value of 28-30 percent in water cooled reactors. The reactor possesses passive inherently safe features in that it can shut itself down and cool itself down in the case of n emergency. Its cooling towers are one-sixth the size of those of conventional power plants, which reduces the overall cost. The cooling towers can be air-cooled rather than water cooled, which suggests that the plants can be located where water resources are scarce. Compared with light water reactors, a reduction of 50 percent in the amounts of nuclear waste and thermal discharge is claimed. The control and monitoring system are based using digital programmable equipment. Panels based on traditional equipment are used for controlling the safety systems. Operation at high temperatures open the possibility for process heat applications such as the extraction and production of chemical products and mineral fertilizers, coal gas saturation, production of synthetic natural gas from coal, and ferrous and nonferrous metallurgy, as well as district heating. The expected completion date for the 4 modules first Russian design is 2005. THE PEBBLE BED MODULAR REACTOR: PBMR The PBMR is being pursued jointly by the Exelon corporation in the USA and South Africa's state owned utility Eskom. This concept is a high temperature helium cooled reactor with unit sizes of 110 MWe, These small sizes can be factory built before assembly at a site. The pressure vessel of the PBMR is shown in Fig. 18. This design is the dependence on fuel in the form of pebbles 6 cm in diameter. About 400,000 of these pebbles lie within a graphite lined vessel that is 20 m high and 20 m in diameter as shown in Fig. 19. Each pebble contains about 15,000 fuel particles where the fuel is enclosed in layers of pyrolitic graphite and silicon carbide. The use of graphite as fuel element cladding, moderator, core structural material, and reflector, provides the reactor with a high degree of thermal inertia. A core melt situation would be practically unlikely, since a large difference exists between the normal average operating temperature of 1,095 degrees C, and the maximum tolerable fuel temperature of 2,800 degrees C. Helium at a temperature of about 500 degrees C is pumped in at the top of the reactor, and withdrawn after sufficient burnup from the bottom of the reactor. The coolant gas extracts heat from the fuel pebbles at a temperature of 900 degrees C. The gas is diverted to three turbines. The first two turbines drive compressors, and the third drives an electrical generator from which electrical power is produced. Upon exit from the compressors or generator, the gas is at 530 degrees C. It passes through recuperators where it loses excess energy and leaves at 140 degrees C. A water cooler takes it further to about 30 degrees C. The gas is then repressurized in a turbo-compressor. It then moves back to the regenerator heat-exchanger, where it picks up the residual energy before being fed to the reactor. Fig. 18: The Reactor Pressure Vessel of the Pebble Bed Modular Reactor. Refueling is done online, eliminating refueling outages. The PBMR would shut down every few years for maintenance of other mechanical parts of the plant. The staff would be constantly taking pebbles out of the bottom, checking their burnup, eliminating any leakers, and then reloading them back from the top, or adding fresh pebbles to replace the discarded ones.. The spent fuel pebbles are passed pneumatically to large storage tanks at the base of the plant. This storage space can hold all spent fuel throughout the plant's life. These tanks can hold the fuel for 40 to 50 years after shutdown. About 2.5 million walls are normally used over the 40 years design life of a typical reactor. The silicon carbide coating on the fuel particles can isolate the fission products, at least in theory for a million years. For permanent storage, these pebbles are easier to store than fuel rods from PWRs. The PBMR is expected to have an overall thermal efficiency of bout 40-42 percent. It operates at a low power density of less than 4.5 MW/m3, compared with a value of 100 for a PWR. The economic advantage of the PBMR is that it can al...
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This note was uploaded on 06/16/2010 for the course NPRE 402 taught by Professor Ragheb during the Spring '08 term at University of Illinois at Urbana–Champaign.

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