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Breeder reactor The reactors which are designed so that breeding will take place is known as breeder reactor. Breeder reactors are capable of producing more fissile material than they consume during the fission chain reaction (by converting fertile U-238 to Pu-239, or Th-232 to U-233). Thus, a uranium breeder reactor, once running, can be re-fueled with natural or even depleted uranium, and a thorium breeder reactor can be re-fueled with thorium; however, an initial stock of fissile material is required.
Types of breeder reactors: Liquid-metal cooled fast breeder- reactor (LMFBR) Gas-cooled fast breeder-reactor (GCFR) Molten Salt Breeder reactor Light Water Breeder Reactor
Liquid-metal cooledfast breeder reactorThe first experimental breederreactor was a small plutonium-fueled, mercury-cooled device,operating at a power level of 25 kW,cooled with a mixture of sodium andpotassium, was placed in operationin 1 95 1 at the Argonne NationalLaboratory in Idaho.
This reactor, the Experimental Breeder Reactor-I (EBR-I), produced steam in a secondary loop that drove a turbine generator. The system produced 200 kW of electricity, the worlds first nuclear- generated electricity-and it came from LMFBR . Since these early experiments, dozens of LMFBRs have been constructed around the world.
Sodium has been universally chosen as the coolant for the modern LMFBR. With an atomic weight of 23, sodium does not appreciably slow down neutrons by elastic scattering, although, it does moderate neutrons to some extent by inelastic scattering. Sodium is also an excellent heat transfer agent, so that an LMFBR can be operated at high power density.
Its melting point, 980oC is much higher than room temperature, so the entire coolant system must be kept heated at all times to prevent the sodium from solidifying. This is accomplished by winding a spiral of insulated heating wire called tracing along coolant piping, valves, and so forth.
Unfortunately, sodium absorbs neutrons, even fast neutrons, leading to the formation of the beta-gamma emitter 24Na, with a half-life of 15 hours. Therefore, sodium that passes through the reactor core becomes radioactive. LMFBR plants operate on the steam cycle-that is, the heat from the reactor is ultimately utilized to produce steam in steam generators.
Gas cooled fast breeder-reactor . It is a helium-cooled reactor fueled with a mixture of plutonium and uranium. The core of the GCFR is similar to that of an LMFBR, with mixed PUO2 and UO2 pellets in stainless steel pins, except that the pins are not as close together as they are in the LMFBR. Also, the pins in the GCFR have a roughened outer surface to enhance heat transfer to the passing coolant.
Molten salt breeder-reactor (MSBR) This is a thermal breeder that operates on the 233 U-thorium cycle. It is recalled that 233U is the only fissile isotope capable of breeding in a thermal reactor. The MSBR concept is a unique design among reactors in that the fuel, fertile material, and coolant are mixed together in one homogeneous fluid. This is composed of various fluoride salts that, at an elevated temperature, melt to become a clear, nonviscous fluid.
Advantages of MSBR: Because of the low vapor pressure of the molten salts, the MSBR operates at just a little above atmospheric pressure and thus no expensive pressure vessel i s required. High temperatures are possible with the molten salts, the MSBR can produce superheated steam at 24 MPa and 540 C, which leads to a very high overall plant efficiency of about 44% .
Light water breeder reactor Even when a special effort is made in the design of the LWBR to reduce neutron losses, its overall breeding gain will be very small-too small to make the reactor a net producer of 233U for other reactors of this type. To see whether breeding can actually be achieved in a light-water reactor,the U.S. Department of Energy developed an LWB R core that was installed in the government-owned pressurized water reactor at Shippingport, Pennsylvania.
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