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  1. 1. PEBBLE BED MODULAR REACTOR Prepared by Sandeepreddy
  2. 2. INTRODUCTION The development of the nuclear power industry has been nearly stagnant in the past few decades . In fact there have been no new nuclear power plant construction in the United States since the late 1970s . Nuclear technology's lack of popularity is not difficult to understand since the fear of it has been promoted by the entertainment industry, news media, and extremists. Also, the lack of understanding of nuclear science. The accidents at Three Mile Island and their effects were dangerous and, in the latter case, lethal.
  3. 3. HISTORY The history of gas-cooled reactors began in November of 1943 with the graphite-moderated, air-cooled, 3.5-MW, X-10 reactor in Oak Ridge, Tennessee. Gas-cooled reactors use graphite as a moderator and a circulation of gas as a coolant. Development of the more advanced HTGRs began in the 1950s to improve upon the performance of the GCRs. HTGRs use helium as a gas coolant to increase operating temperatures. Initial HTGRs were the Dragon reactor in the U.K., developed in 1959. D.r Rudolf Schulten considered "father" of the pebble bed concept .
  4. 4. The Pebble Bed Modular Reactor is being developed for commercial use, Approval for the design will need to be granted by the South African government, which may happen late-2002. Almost in parallel in January of 1998, and without prior knowledge of the PBMR effort in South Africa, a group of MIT students began their ambitious effort of developing a conceptual design of a reactor, that is now known as the Modular Pebble Bed Reactor. The MPBR design is very similar to the PBMR and will theoretically generate 110 MWe or 250 MWt.
  5. 5. Fuel Pebbles The most unique feature of the PBMR are the 370,000 fuel pebbles or spheres that produce the nuclear reaction. An illustration of the fuel spheres is given in Figure .  The spheres are the triple coated type.  Each sphere has 60-mm diameter(billiard ball size).  Sphere is coated with a 5-mm thick graphite layer. The graphite can withstand temperatures of 2,800 degrees Celsius which is much higher than the maximum 1,600 degrees Celsius that the reaction can produce. Within this graphite layer are approximately 15,000 coated particles that are embedded in a graphite mix.
  6. 6. Figure : Pebble Bed TRISO Fuel Sphere Cross Section
  7. 7. REACTOR The nuclear reaction takes place within the "reactor vessel" which is a vertical steel pressure enclosure that is 6 meters in diameter and 20 meters high. This lining is drilled with vertical holes for insertion of the control rods. Illustrated by the red and blue pebbled granules, the inner reactor core portion consists of two zones and is 3.7 meters in diameter and 9.0 meters high. The blue, or inner zone, contains approximately 185,000 graphite spheres and the red, outer zone, contains approximately 370,000 fuel spheres. The graphite spheres serve as a moderator for the nuclear reaction.
  8. 8. Figure: Pebble Bed Modular Reactor Cross Section
  9. 9. THE GENERATOR AND COMPRESSORS As it illustrates, helium enters the reactor at 500 degrees Celsius and at a pressure of about 8.4 MPa. It leaves the reactor at about 900 degrees Celsius and drives the high pressure turbine. After the high pressure turbine, the helium flows through the low pressure turbine . While still hot, the helium leaves the low pressure turbine and drives the power turbine to produce the electricity through the generator. The coolers increase the efficiency. The helium has also been cooled back down to 500 degrees Celsius and the cycle repeats itself as it travels back to the reactor. This process is called the gas turbine Cycle. The advantage of this process is its high efficiency of thermal energy transfer to electrical energy.
  10. 10. Figure : Pebble Bed Modular Reactor Schematic Diagram
  11. 11. THE PBMR FACTORY The PBMR's size might seem more like a disadvantage. The figure illustrates both above and below ground components of the factory. About half of the factory will be above ground and the other half below. The dimensions of the PBMR factory will be about 59 m long x 36 m wide x 57 m high. The main support structures for the reactor are the helium inventory control systems and the fuel handling and storage systems.
  12. 12. Figure : PBMR Reactor and Support Structures
  13. 13. CONTAINMENT Most pebble-bed reactors contain 1.Most reactor systems are enclosed in a containment building designed to resist aircraft crashes and earthquakes. 2.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. 3.The reactor vessel is usually sealed. 4.Each pebble, within the vessel, is a 60 millimetres hollow sphere of pyrolytic graphite.
  14. 14. CURRENT DESIGNS CHINA China has licensed the German technology and is actively developing a pebble bed reactor for power generation. 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. If PBMRs are successful, there may be a substantial number of reactors deployed. This may be the largest planned nuclear power deployment in history.
  15. 15. ADVANTAGES  Safe  Cost competitive  Proliferation resistant DISADVANTAGES  No containment building  Fuel pebble risky  Large amount of waste
  16. 16. CONCLUSIONS In conclusion, the concepts and ideas being developed through the MIT pebble bed reactor effort await the next step which is a more detailed conceptual design to allow for the demonstration of its economic viability, particularly the modularity approach, which offers potentially large advantages in terms of shorter construction times, lower costs of power and more reliable power net output compared to ultra large power stations.