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9. Inherently Safe Reactor Designs

In addition the fuel particles are enclosed in layers

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Unformatted text preview: ed in layers of pyrolitic graphite and silicon carbide that prevent radioactive releases even at high temperatures. Past designs used the Steam or Joule cycle as shown in Fig. 15. The newer designs take advantage of new developments in turbine technology such as magnetic bearing, and use the Brayton or Gas Turbine cycle shown in Fig. 15. With the use of Brayton gas turbine cycle, the helium coolant is enclosed in a single circuit moving from the compressor to the turbine. The possibility of its depressurization or leakage is minimized, and it is not reactive with graphite like steam would be. The designs can operate at higher temperatures and offer a high value of the thermal efficiency around 40 percent, compared with the 30 percent value for light water reactors. The high temperatures offer the possibility of process heat generation and use in industrial processes such as high temperature water electrolysis for the production of Hydrogen for future non fossil transportation fuel supplies. Fig. 15: Comparison of the Brayton cyle to the Steam Cycle on a Temperature Entropy Diagram. Table 2: Gas cooled Reactors Experience. Concept Power Characteristics (MWe) 15 Experimental Pebble Bed Reactor, operated for 21 years in AVR Germany. 300 Demonstration pebble bed reactor with steam turbine, THTR operated for 5 years in Germany. 330 Operated in the USA for 14 years. FSV (Fort Saint-Vrain) 30 Reached criticality in 1998 in Japan. HTTR 10 Reached criticality in 1999 in China. HTR-10 Russian gas cooled reactor designs. VG-400, VGM, VGM-P 80 German pebble bed reactor licensed for Siemens/Interatom in HTR-MODUL 1987. 100 German pebble bed reactor design by HRB/BBC. HTR-100 300 Helium Turbine design Russian-USA-French-Japan design. GT-MHR 110 Pebble bed South African design, to be licensed in the USA PBMR by Exelon Corp. Critical test facility in Switzerland. PROTEUS These designs build on a wealth of operational experience in the development of the gascooled reactor concept as shown in Table 2. The Brayton cycles designs build on new experience from the space and aeronautical field that did not exist a few years ago, such as the use of magnetic bearings as shown in Fig. 16. The advantage of magnetic bearings is the elimination of the dependence on liquid oil for lubrication. Fig. 16: Use of magnetic bearings in turbine design in advanced propulsion technology. THE GAS TURBINE MODULAR HELIUM REACTOR: GT-MHR Is based on an agreement between General Atomics in the USA and Russia. This concept pioneers the development of a next-generation modular nuclear reactor using the direct-cycle gas turbine, rather than the steam cycle. This approach operates at higher temperatures than light water reactors, increasing the overall thermal efficiency of the plant by 50 percent. This in turn reduces the cost of producing electricity to the range of 3 cents per kilowatt-hour, compared with the cost of 5 cents per kilowatt-hour in existing designs. The ability of handling higher temperatures is possible because of the use of ceramics such as silicon carbide. Helium as a coolant replaces water, which even though a splendid coolant, is corrosive. This limits the use of water to a temperature of 700 degrees C. Helium, in contrast, is inert and noncorrosive. It has no thermal limit, is in the gaseous form whether at room temperature or 3,000 degrees C, which is a tremendous advantage. Every time a coolant is passed through a heat exchanger before it can do its work, significant losses in efficiency occur. As shown in Fig. 17, the turbine is directly driven by the helium gas as it comes out of the reactor core. Since no heat exchange is needed, this improves the overall efficiency of the plant. The 300 MWe units will be able to burn either uranium fuel, or fuel made from weapons grade plutonium, thus converting weapons grade material into electricity. If plutonium is used, up to 95 percent of it is used during power production. A mixture of Uranium and Thorium can also be used. Fig. 17: The entire Gas Turbine Modular Helium Reactor power plant is contained in two pressure vessels, enclosed into a concrete containment structure below ground. This kind of development project is also advocated as a way for western nations to turn around the economic depression and cultural pessimism that are eroding scientific and technological capabilities threatening their futures. In addition it would help the former nations of the Soviet Union and Third world countries out of their current economic devastation. It has been even advocated as a contribution for preventing atomic scientists in these countries from getting involved in nuclear weapons projects in proliferation prone countries. Along this line, the U.S. has pledged $ 1.6 billion in terms of cleaning up problems in Russia's nuclear and weapons programs. The new technology of magnetic bearings virtually eliminates friction. It has also great properties in the dynamic dampening of rotating shafts; a technology that did not exist s few years ago. The gas turbines builds on new turbine technolog...
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