原子能蒸汽供应系统翻译课程设计用.docx

1、原子能蒸汽供应系统翻译课程设计用Chapter 10 Nuclear Steam supply Systems10.1 Introduction Nuclear energy is one of the important fuels for todays electric power generation. Nuclear energy results from changes in the nucleus of atoms. As a nucleus splits, it releases a tremendous amount of heat. This nucleus splittin

2、g process is completely fissioned, it will create as much heat as the burning of 1500 short tons of coal. In 1982, approximately 12% of electric power produced in the United States was generated from nuclear power plants. The development of nuclear power progressed slowly in the nineteenth and early

3、 twentieth centuries. In 1911 the physicist Ernest Rutherford first discovered the existence of a subatomic particle, later referred to as the nucleus. Although Rutherford did not succeed in splitting a nucleus, he later showed the possibility of a fission process. In 1932, the physicist James Chadw

4、ick discovered the existence of a neutron. In 1938, two German chemists, Otto Hahn and Fritz Strassmann reported they had produced the element barium by bombarding uranium with neutrons. This experiment was later referred to as the first manmade fission reaction. This reaction had in fact split an u

5、ranium nucleus into two nearly equal fragments, one of which was a barium nucleus and another was a krypton nucleus. In this fission process two neutrons were also emitted. The mass of the two nuclei and two neutrons produced was somewhat less than that of an uranium nucleus and a neutron combined.

6、The reaction had therefore produced a significant amount of energy. In the same period Albert Einstein developed his famous relativity theory and related the matter to energy by the equation E=mc2. The equation states that the energy (E) in a substance equals the mass (m) of that substance multiplie

7、d by the speed of light squared (c2). The equation had been used by scientists to estimate the amount of energy released in a fission process. The first manmade chain fission process was not produced until December 2, 1942, when the physicist Enrico Fermi and his associates constructed an atomic pil

8、e, using 50 short tons of natural uranium embedded in 500 short tons of graphite. Cadmium rods were used to control the chain reaction. With the successful development of atomic bombs in 1945 and the first nuclear-powered vessel, the submarine Nautilus in 1954, the first full-scale nuclear power pla

9、nt began operations in 1956 at Calder Hall in northwestern England. Next year, the first large nuclear power station in the United States was completed in Shippingsport, Pennsylvania. The pressurized water reactor at this power station produced 60 MW of electricity. By 1960, nuclear power generating

10、 systems in the range of 150 to 200 MW were in commercial operation. At that time, these reactors were still in the demonstration phase. By the middle 1960s nuclear reactor systems were being ordered by utility companies on the basis of favorable economic comparisons with fossil-fuel power plants. T

11、hese nuclear reactor systems were in the range of 600 MW and were in commercial operation by 1970. By 1975, systems in the 1000MW range were in operation. The most recent nuclear reactor systems produce 1300 MW, which is the current technical limit on the size of a single reactor system. Present-day

12、 commercial nuclear reactors are of the “fission” type. Fission takes place when a fissionable nucleus (such as those of uranium) captures a free neutron. Capture upsets the internal force, which holds together the tiny particles called protons and neutrons in the nucleus. The nucleus splits into tw

13、o fission fragments. Besides the heat energy produced, fission releases an average of two or three neutrons and such nuclear radiation as gamma rays. The fission fragments give off beta rays. If one of the neutrons emitted is captured by another fissionable nucleus, a second fission takes place in t

14、he manner similar to the first. Another neutron may produce a third fission. When the fission becomes self-sustaining, the process is called a chain reaction. The device in which this chain reaction is generated to produce nuclear energy is called a nuclear reactor. 10.2 Nuclear reactors and their c

15、lassificationsNuclear reactors used for electric power generation consist of four main parts. They are (1) the fuel core, (2) the moderator and coolant, (3) the control rods, and (4) the reactor vessel. The fuel core contains the nuclear fuel and is the part of the reactor in which the fission takes

16、 place. The nuclear fuel may be either natural uranium or enriched uranium. The natural uranium contains 0.71% fissile U-235 and 99.28% fertile U-238 and fertile thorium Th-232. The enriched uranium is produced in a gaseous diffusion process and is expected to have a U-235 content up to 2 or 3%. Thi

17、s is three or four times the concentration in natural uranium. In fission process the fertile materials are converted to fissile. For instance, the U-238 becomes Pu-239 and Pu-241 and Th-232 becomes U-233. These converted fissile fuels are nuclear fuels themselves, which are then utilized in reactor

18、s.The nuclear fuel is generally contained in cylindrical rods surrounded by cladding materials. The fuel-rod cladding materials must be able not only to maintain the fuel rod in shape, but also to hold under the reactor conditions. These materials include aluminum, magnesium, zirconium, stainless st

19、eel, and graphite.The moderator is the substance used in nuclear reactor to reduce the energy of fast neutrons to thermal neutrons. Liquid and solid materials of small mass number and low neutron capture should be suitable. These include light water, heavy water, carbon, and beryllium.The reactor co

20、olant is used to remove heat from the reactor fuel core. The conditions for a good coolant include high specific heat, high thermal conductivity, and high boiling point at low pressure. The coolant should also have low power demand for pumping, low cost, and a high degree of stability in the reactor

21、 environment. For this purpose the coolants include light water, heavy water, air, carbon dioxide, helium, sodium, potassium, and some organic liquids.Control rods are long metal rods that contain such elements as boron, cadmium, or hafnium. These elements absorb fast neutrons and therefore help con

22、trol a chain reaction. The control rods are attached to an elevator-like mechanism just outside the nuclear reactor. The mechanism inserts the rods into the fuel core or withdraws them to slow down or speed up a chain reaction. Three types of control rods are used. These include (1) shim rods, (2) r

23、egulating rods, and (3) safety rods. Shim rods are used for making occasional coarse adjustment in neutron density, while regulating rods are used for fine adjustment. Safety rods are designed for use in emergency. The safety rods are made of boron steel and are capable of coming into the reactor co

24、re very rapidly and stopping the chain reaction.The reactor vessel is a tanklike structure that holds the reactor core and other internals. The walls of the vessel are designed for the high pressure and radiation environment. In most cased the vessel walls are lined with thick steel slabs to reduce

25、the flow of radiation from the core. As indicated in the last section, nuclear fission generates large amounts of neutrons and gamma rays. Both of them are very harmful. Because of these, biological shielding is required around the reactor vessel. This shield consists of concrete blocks, which may b

26、e up to 6 ft thick. According to their coolant and moderator, nuclear reactors can be classified into: pressurized water reactor (PWR), boiling water reactor (BWR), canadian deuterium-uranium reactor (CANDU), steam-generating heavy water reactor (SGHWR), high-temperature gas-cooled reactor (HTGR), a

27、dvanced gas-cooled reactor (AGR) and gas cooled reactor (GCR). The two principal types are the PWR and BWR. Both reactors use enriched uranium and light water as coolant as well as moderator. The difference between these two reactors is in the manner in which steam is being produced. The BWR generat

28、es steam within the reactor vessel. The steam is directly piped to a steam turbine and returned to the vessel after completing various processes. In the PWR, however, only hot water is produced. The hot water is then transferred to a separate heat exchanger in which the thermal energy of hot water i

29、s utilized to generate steam. The prominent reactors using the heavy water as moderator are the CANDU and SGHWR. Both reactors use enriched uranium (UO2) and have a separate heat exchanger for steam production. In the CANDU, the heavy water receives heat in the nuclear reactor and releases heat in t

30、he separate heat exchanger for steam production. Since the coolant heavy water flows through the tubes inside the reactor , there is no need for a pressure vessel like that in the PWR system. The SGHWR has a similar arrangement except that it used the light water as coolant in the reactor. While the

31、 heavy-water reactor does not have and efficiency as high as the PWR and BWR, it generally has considerably better overall utilization of the fission energy available in the nuclear fuel.The gas-cooled reactors frequently use graphite as moderator, CO2 as coolant and natural uranium as fuel. Since t

32、he fuel cladding is magnesium alloy (referred to as Magnox in England), the gas-cooled reactors (GCR) are often called Magnox reactors. The advanced gas-cooled reactor (AGR) is a result of continued development of the Magnox system. Again, the moderator is graphite and the coolant is CO2. The AGR is

33、 designed to raise the system conditions to the level comparable to those in the fossil-fuel power plant. Another kind of gas-cooled reactor is the high-temperature gas-cooled reactor (HTGR). Unlike the GCR and AGR, the HTCG uses helium as coolant which receives heat in the reactor and becomes a gas of high pressure and temperature. Next, the gas is