chapter-12 nuclear heat energy.ppt

1、PROFESSIONAL(SPECIAL) ENGLISH COURSENUCLEAR ENERGY,Chapter 12 Nuclear Heat Energy （核热能）,Most of the energy released in fission or fusion appears as kinetic energy of a few high-speed particles. As these pass through matter, they slow down by multiple collisions and impart (给予) thermal energy to the

2、medium. It is the purpose of this chapter to discuss the means by which this energy is transferred (转移) to a cooling agent (介质) and transported (传送) to devices that convert mechanical energy into electrical energy. Methods for dealing with the large amounts of waste heat generated will be considered

3、.,12.1 METHODS OF HEAT TRANSMISSION (热传递的方式),We learned in basic science that heat, as one form of energy, is transmitted by three methods: conduction (传导), convection (对流, 传递), and radiation (辐射, 发光).,接上页(P124第二段第二行),The physical processes for the methods are different: In conduction, molecular mot

4、ion in a substance at a point where the temperature is high causes motion of adjacent (邻近的) molecules, and a flow of energy toward a region of low temperature takes place. The rate of flow is proportional to the slope (倾斜, 斜坡) of the temperature, i.e., the temperature gradient (温度梯度).,接上页(P124第二段第七行

5、),In convection, molecules of a cooling agent such as air or water strike a heated surface, gain energy, and return to raise the temperature of the coolant. The rate of heat removal is proportional to the difference between the surface temperature and that of the surrounding medium, and also depende

6、nt on the amount of circulation of the coolant in the vicinity (附近) of the surface.,接上页,In radiation, molecules of a heated object emit and receive electromagnetic radiations, with a net transfer of energy that depends on the temperatures of the body and the adjacent (邻近的) regions, . For reactors, t

7、his mode of heat transfer is generally of less importance than are the other two.,12.2 HEAT GENERATION AND REMOVAL (热能的产生和排除),The transfer of heat by conduction in a flat plate (insulated (绝缘的) on its edges) is reviewed. If the plate has a thickness x and cross-sectional area A, and the temperature

8、difference between its faces is T, the rate of heat flow through the plate, Q, is given by the relation,P125第三段第五行,If the surface is maintained at a temperature Ts by the flow of coolant, the center of the rod must be at some higher temperature T0. As expected, the temperature difference is large if

9、 the rate of heat generation per unit volume q or the rate of heat generation per unit length q1=R2q is large. We can show that,P125 倒数第二行,Convective cooling depends on many factors such as the fluid speed (流动速度), the size and shape of the flow passage, and the thermal properties of the coolant, as

10、well as on the area exposed and the temperature difference between surface and coolant Ts-Tc. Experimental measurements yield the “heat transfer coefficient (传热系数)” h, appearing in a working formula for the rate of heat transfer Q across the surface S, Q=hS(Ts-Tc),P126 第二段,As coolant flows along the

11、 many channels surrounding fuel pins in a reactor, it absorbs thermal energy and rises in temperature. Since it is the reactor power that is being extracted (引出), we may apply the principle of conservation of energy. If the coolant of specific heat (比热) c enters the reactor at temperature Tc (in) an

12、d leaves at Tc(out), with a mass flow rate M, then the reactor thermal power P is P=cMTc(out)- Tc(in) =cMT.,P128 第二段第五行,In the design of a reactor, a great deal of attention is given to the determination of which channels have the highest coolant temperature and at which points on the fuel pins “hot

13、 spots” occur. Ultimately, the power of the reactor is limited by conditions at these channels and points. The mechanism of heat transfer from metal surfaces to water is quite sensitive to the temperature difference.,接上页(P128 倒数第四行),As the latter increases, ordinary convection gives way to nucleate

14、boiling (泡核沸腾), in which bubbles form (汽泡) at points on the surface, and eventually film boiling (薄膜状沸腾) can occur, in which a blanket of vapor reduces heat transfer and permits hazardous melting. A parameter (参数) called “departure from nucleate boiling ratio (偏离泡核沸腾比)”(DNBR) is used to indicate how

15、 close the heat flux is to the critical value. For example, a DNBR of 1.3 implies a safety margin of 30%.,12.3 STEAM GENERATION AND ELECTRICAL POWER PRODUCTION (蒸汽的产生与电力的生产),Thermal energy in the circulating reactor coolant is transferred to a working fluid (工作流体) such as steam, by means of a heat e

16、xchanger or steam generator. In simplest construction, this device consists of a vessel partly filled with water, through which many tubes containing heated water from the reactor pass, as in Figure 12.6.,接上页(P131 第三行),Steam is evolved (散发) and flows to a turbine (涡轮), while the water returns to the

17、 reactor. The conversion of thermal energy of steam into mechanical energy of rotation (旋转) of a turbine and then to electrical energy from a generator is achieved by conventional (常规的, 传统的) means.,接上页(P131 第六行),Steam at high pressure and temperature strikes the blades (齿片) of the turbine, which dri

18、ves the generator. The exhaust steam (废汽) is passed through a heat exchanger that serves as condenser (冷凝器), and the condensate (冷凝水) is returned to the steam generator as feed water (给水). Cooling water for the condenser is pumped from a nearby body of water or cooling tower (冷却塔), as discussed in S

19、ection 12.4.,12.4 WASTE HEAT REJECTION (废热的排放),The generation of electric power by consumption of any fuel is accompanied (伴随) by the release of large amounts of waste heat (废热). For any conversion process the thermal efficiency, e, the ratio of work done to thermal energy supplied, is limited by th

20、e temperatures at which the system operates. According to the second law of thermodynamics (热力学第二定律), an ideal cycle has the highest efficiency value, e=1-T1/T2.,P133 第一行,The overall efficiency of the plant is lower than this because of heat losses in piping (管道系统), pumps, and other equipment. The e

21、fficiency of a typical nuclear power plant (核电站) is only around 0.33. Thus twice as much energy is wasted as is converted into useful electrical energy. Fossil fuel plants (化石燃料电厂, 如煤电厂) can operate at higher steam temperatures, giving overall efficiencies of around 0.40.,P133 第二段,A nuclear plant op

22、erating at electrical power 1000 MWe would have a thermal power of 1000/0.33=3030 MWt and must reject a waste power of P=2030 MWt. We can calculate the condenser cooling water mass flow rate M required to limit the temperature rise to a typical figure of T=12. Using a specific heat of c =4.18103J/kg

23、-,P133 倒数第三行,This corresponds to a flow of 925 million gallons (加仑) per day. Smaller power plants in past years were able to use the “run of the river, ” i.e., to take water from a stream, pass it through the condenser, and discharge heated water down stream. Stream flows of the order of a billion g

24、allons a day are rare, and the larger power plants must dissipate (驱散) heat by utilizing a large lake or cooling towers.,P134 第二段第七行末,When the thermal discharge goes into a lake, the ecological effects (生态效应) are frequently called “thermal pollution”, especially when plants and animals are damaged b

25、y the high temperatures. Other effects are the death of aquatic (水生的) animals by striking screens (过滤网), or passing through the system, or being poisoned by chemicals used to control the growth of undesirable algae (海藻).,P134 第三段,Many nuclear plants have had to adopt the cooling tower for disposal o

26、f waste heat into the atmosphere, in fact, the hyperboloid (双曲面) shape (see Fig. 12.10) is so common that many people mistake it for the reactor. A cooling tower is basically a large heat exchanger with air flow provided by natural convection (自然对流) or by blowers (鼓风机), .,12.5 SUMMARY,The principal modes by which fission energy is transferred in a reactor are conduction and convection. The radial temperature distribution in a fuel pellet is approximately parabolic (抛物线的). The rate of heat transfer from fuel surface to coolant by convection is directly proportional