The energy produced from nuclear power plants results in the produc-tion of over 20 percent of the total electric power produced in the United States. Worldwide, electric production from nuclear power is also approximately 20 percent. Some countries rely heavily on this source of energy because power from nuclear plants accounts for over 33 percent of the electrical power in Japan and over 77 percent in France. Nuclear power, therefore, is an extremely important source of energy because it is second only to coal as a fuel energy source for the production of electricity (see Chap. 1).
The nuclear steam generator produces steam and therefore is a boiler. The furnace for burning conventional fossil fuels is replaced by a reactor, which contains a core of nuclear fuel.
The heart of the reactor is the core. The core contains uranium fuel, which, as a result of the fissioning of the uranium, generates heat to produce steam. The core is encased in a pressure vessel, which in turn is enclosed by shielding and a reactor building, all of which con-tain the radiation emitted from the nuclear reaction.
In commercial reactors for utility applications, the core consists of a number of fuel elements, each fuel element containing fuel rods that encapsulate the uranium dioxide (UO2) fuel pellets. These fuel ele-ments are arranged so that a self-sustaining nuclear chain reaction takes place. The fuel utilizes the fissionable isotope of uranium 235U, and in most reactor designs it is enriched to approximately 3 percent, with the remaining 97 percent consisting of the uranium isotope 238U.
238U is a fertile isotope of uranium that absorbs neutrons, and it even-tually transforms into another element, plutonium, which can be used as a nuclear fuel, usually mixed with uranium. There are also some reactor designs that use only natural uranium, i.e., uranium that has not been enriched in the isotope 235U.
Ch02_Lammers_1418466 10/8/04 11:26 AM Page 128
Boilers
When a neutron strikes the nucleus of the fissionable isotope
235U, the nucleus splits and releases a large amount of heat together with the release of additional neutrons that maintain the fissioning process; thus there is a chain reaction. The liberated neutrons travel at a high rate of speed. Since slow-moving neutrons are more effec-tive in splitting nuclei of 235U than are fast-moving neutrons, the neutron velocity must be slowed. This is accomplished by the use of a moderator.
The moderator does the slowing down. Various materials are used as moderators, such as graphite, ordinary water, or heavy water (water that contains heavy hydrogen instead of ordinary hydrogen).
The moderator can slow speeding neutrons without absorbing them.
In commercial light-water reactors, ordinary water serves as both the moderator and the coolant.
The control rods contain materials that absorb neutrons readily.
They are arranged so that they may be inserted or withdrawn from within the fuel core as required to control the chain reaction. When the control rods are inserted into the reactor core, they absorb neu-trons so that the chain reaction is slowed or stopped. As the rods are withdrawn, the neutrons become active again, and the nuclear chain reaction starts up again. Thus the control rods are used to raise or lower the power output of the reactor.
Another component of a reactor is the coolant. The function of the coolant is to remove the heat developed in the core, which can be used to produce steam to generate electricity. The coolant may be ordinary water, heavy water, a gas, or a liquid such as liquid sodium.
In one power reactor system, called a pressurized-water reactor (PWR) system, water is used as both the moderator and the coolant. The water is kept under pressure in the reactor vessel and the primary system.
From the reactor vessel, the water is pumped to a heat exchanger (steam generator), which converts the water to steam in a secondary piping system. The steam is then used to power a turbine generator.
The schematic shown in Fig. 2.57 shows a PWR system that is fueled by slightly enriched uranium in the form of uranium oxide pellets held in zirconium-alloy tubes in the core. Water is pumped through the core to transfer heat to the steam generator. The coolant water is kept under pressure in the primary system through the core to prevent boiling, and it transfers its heat to the water in the steam generator (the secondary system) to make the steam.
In another power reactor system, called a boiling-water reactor (BWR) system, water is again used as both moderator and coolant, but here the water is allowed to boil within the reactor vessel. The steam thus generated then passes directly to the turbine generator.
The schematic shown in Fig. 2.58 shows a BWR system that also uses a reactor fueled by slightly enriched uranium in the form of
130 Chapter Two
Figure 2.57 Pressurized-water reactor (PWR) nuclear power plant. (American Nuclear Society.)
uranium oxide pellets held in zirconium-alloy tubes in the core.
Since there is no secondary system with this design, the turbine portion of the plant is designed to handle any radioactivity carried by the steam, and this requires special shielding and containment structures.
Figure 2.59 shows the arrangement of a liquid-metal fast breeder reactor (LMFBR) nuclear plant design that pumps molten sodium in the primary loop through the reactor core containing the fuel. This sodium in the primary loop collects the heat and transfers it to a sec-ondary liquid sodium loop in the heat exchanger, from which it is pumped to the steam generator, where steam is generated and used to power the turbine generator.
In addition to producing electricity, this type of reactor also pro-duces more fissionable material than it consumes, which results in the name breeder reactor. When irradiated, certain nonfissionable materials may be transformed into material that is fissionable. An LMFBR begins operation with a core of fissionable 235U surrounded by nonfissionable 238U. During operation, the 238U is bombarded by high-velocity neutrons and transmuted to fissionable plutonium-239
Ch02_Lammers_1418466 10/8/04 11:26 AM Page 130
Boilers
(239Pu). The plutonium is extracted periodically and fabricated into a new fuel. This design uses fast neutrons as compared with the slow neutrons that resulted from the moderator of the PWR and BWR designs.
Another type of nuclear system is shown in Fig. 2.60 and uses helium gas as a coolant. This system is called a high-temperature gas-cooled reactor (HTGR). The HTGR shown is a type of reactor that is fueled by uranium carbide particles distributed in graphite in the core. Helium gas is used as a coolant to transfer the heat from the core to the steam generator. Steam is generated in this secondary cycle and is used to drive a turbine generator in a con-ventional turbine cycle.
Figure 2.61 shows still another design that uses heavy water as both the coolant and moderator, and this design is called the CANDU pressurized heavy water reactor (PHWR). In this design, the calan-dria, or reactor vessel, is a cylindrical tank filled with a heavy water (deuterium oxide) moderator at low temperature and pressure.
Hundreds of pressure tubes (fuel channels) penetrate the calandria, and fuel bundles containing natural uranium fuel are inserted in the
Figure 2.58 Boiling-water reactor (BWR) nuclear power plant. (American Nuclear Society.)
pressure tubes. Pressurized heavy water coolant is pumped past the uranium fuel, and the heat of fission is transferred to the coolant.
The coolant flows to the steam generators, where it gives up its heat to ordinary light water to produce steam that drives a conventional turbine generator.
A by-product of nuclear energy is the release of radioactivity dur-ing the fission process. The nuclear plant is designed to prevent the release of radioactivity by having a series of barriers that prevent its release. Figure 2.62 shows the various barriers. The fuel rods contain the fission products. If these rods were to leak, the series of additional barriers, the primary system including the pressure ves-sel, and the design of the reactor building prevent the release of radioactive products. Monitors within the reactor system will alert the operators when levels become too high. If this occurs, the nuclear system is shut down, and the leaking fuel assembly is iden-tified and replaced.
In the United States, no new nuclear power plants have been placed into operation for approximately 30 years, yet nuclear power is critical to the nation’s electric generation because it produces about 20 percent of the electricity. It is predicted that without new nuclear
132 Chapter Two
Figure 2.59 Liquid-metal fast breeder reactor (LMFBR) nuclear power plant. (American Nuclear Society.)
Ch02_Lammers_1418466 10/8/04 11:26 AM Page 132
Boilers
Figure 2.60 High-temperature gas-cooled reactor (HTGR) nuclear power plant.
(American Nuclear Society.)
Figure 2.61 CANDU pressurized heavy water reactor nuclear power plant. (American Nuclear Society.)
134 Chapter Two
plant construction, this 20 percent level of production will decrease to 14 percent by 2020. This loss of power would have to be filled by fossil fuel–fired power plants.
Even with more attention provided to electricity from renewable energy sources such as wind, solar, and biomass, growth from these sources may increase from 3 percent to 6 percent by the year 2020. At the same time, though, there is expected to be a near-equivalent decline in hydropower because no new large dams are expected to be built. This then leaves increased power demands being supported by fossil fuel or nuclear power plants.
Nuclear power is receiving more favorable attention than it has in the past. This attention can be attributable to several factors:
■ The nuclear power plants in the United States are reliable and pro-duce low-cost electricity. Not only do they propro-duce approximately 20 percent of the electrical power in the United States, but they also achieve this with high availability, over 90 percent.
■ Nuclear fuel prices are stable as compared with highly fluctuating natural gas prices, and this fuel comes primarily from stable allies such as Canada and Australia.
Figure 2.62 Barriers against radioactive release in a nuclear power plant. Shield building: reinforced concrete structure
≥3 ft. Steel containment: essentially leaktight shell of steel plate. Pressure vessel: height ≤75 ft; diameter 20 ft; walls 9 in thick. Fuel rods: zirconium alloy about 12 ft long; diame-ter 1⁄2in. Fuel pellets: dense ceramic pellets in which most of the fission products remain bound. (American Nuclear Society.)
Ch02_Lammers_1418466 10/8/04 11:26 AM Page 134
Boilers
Figure 2.63Comparison of fossil fuel–fired power plant with nuclear power plant systems. (a) Fossil fuel–fired power plant. (b) Nuclear power plant, pressurized-water reactor (PWR). (c) Nuclear power plant, boiling-water reactor (BWR) (d) Nuclear power plant, high- temperature gas-cooled reactor (HTGR) and gas-cooled fast reactor (GCFR). (e) Nuclear power plant, liquid metal fast breeder reactor (LMFBR). (American Nuclear Society.)
■ Nuclear power plants do not have the emission concerns of fossil fuel–fired power plants, which have to deal with CO2, NOx, SO2, particulates, and other emissions.
■ Most communities near current nuclear power plants support these plants because they provide high-paying jobs, clean air, and property tax contributions to local schools and city services.
As with any project, there are problems that have to be overcome to ensure any expansion in nuclear plant construction. The disposal of spent fuel is a primary concern and requires that the disposal site in Nevada be operational to handle this radioactive spent fuel. In addi-tion, accident indemnification in the case of a nuclear accident must be extended, and the process of commissioning a new plant must be man-aged properly to prevent unscheduled delays and the resulting costly overruns of project costs.
Figure 2.63 shows a schematic comparison of a fossil fuel–fired power plant with various nuclear power plant system designs and the plant efficiencies for each type of design. Note that because of losses in the plant cycle, the power plant efficiency is 40 percent or less.
Questions and Problems 2.1 What is a boiler?
2.2 What are the requirements of a good boiler?
2.3 Define a boiler setting.
2.4 What is meant by heat being transmitted by radiation? By conduction? By convection?
2.5 Describe the process of boiling. What is the saturation temperature?
2.6 What methods are used for the circulation of water and steam through a boiler?
2.7 What is a fire-tube boiler? Describe its operational features.
2.8 In today’s power plants, what type of fire-tube boiler is the most common, and how is it fired?
2.9 If a fire-tube boiler is a four-pass design, describe the features of such a design.
2.10 What is a water-tube boiler?
136 Chapter Two
Ch02_Lammers_1418466 10/8/04 11:26 AM Page 136
Boilers
2.11 What are the advantages and disadvantages of package boilers?
2.12 What is a high-temperature-water (HTW) boiler? What prevents the water from boiling? For what type of application does this type of design provide advantages?
2.13 Provide some major comparisons between fire-tube and water-tube boilers, and provide the predominant advantages and disadvantages of each.
2.14 Identify the major purposes of the steam drum.
2.15 Why is steam-water separation equipment so important to good boiler operation?
2.16 Describe how a cyclone steam separator works.
2.17 What is a superheater? What advantage does it serve in the overall power plant operation? Where is the superheater located in the boiler?
2.18 What is a radiant type of superheater? A convection type superheater?
2.19 What methods are used to provide a constant steam temperature over a specified load range?
2.20 What is a reheater, and when is it used?
2.21 What two primary areas have the greatest influence on the superheater design other than flue gas temperature and flow?
2.22 What are the three basic methods for maintaining a constant steam temper-ature? Which of these is the predominant method? Describe its operation.
2.23 What is an economizer? What is the advantage of using an economizer? If an air preheater is part of the overall boiler design, is the economizer located before or after the air heater with regard to the flue gas flow? Why?
2.24 Describe the various types of air preheaters and their purpose in boiler design. What are the methods used in air heater designs to minimize cold-end corrosion problems?
2.25 In reference to the designs of superheaters, economizers, and air heaters, what is meant by the terms parallel flow and counterflow? What are the advantages of each design?
2.26 What effect, if any, does the dew point have on the corrosion of economizers and air heaters? Does sulfur in the fuel change the dew point? Why is this important?
2.27 When is a steam coil air heater required? Where is it located?
2.28 Describe the purpose of the furnace portion of the boiler, and define the more important design features of it.
2.29 How are boilers supported to allow for expansion and contraction during operation?
2.30 Why are waterwall furnaces so important to the boiler design as com-pared with older refractory boiler designs?
2.31 What is meant by membrane furnace wall construction? What is its advantage?
2.32 How would you classify industrial and utility boiler designs?
2.33 For coal firing, what are the three primary methods for the combustion of coal? Describe the major characteristics of each, including their advan-tages and disadvanadvan-tages.
2.34 What is the difference between a pressurized and a balanced-draft boiler design?
2.35 Describe the fluidized bed combustion process and how it compares with pulverized-coal and stoker firing.
2.36 Name the major advantages of fluidized bed combustion.
2.37 What are the two types of fluidized bed boilers? Describe their general characteristics.
2.38 For a CFB boiler design, what component is the most distinguishing design feature, and what is its purpose?
2.39 How are the emissions of SO2, NOx, and particulates controlled in a fluidized bed boiler?
2.40 What is used for the bed material in a fluidized bed boiler? Why?
2.41 Why is a combined cycle system important? Describe such a system that uses a gas turbine generator. Define its advantages and disadvantages.
2.42 In a nuclear power plant, briefly describe the fission process that releases heat for the generation of steam.
2.43 Describe and develop a simple schematic sketch of a pressurized water reactor (PWR) system and a boiling water reactor (BWR) system.
2.44 What systems are part of a nuclear power plant and are designed to pre-vent the release of radioactivity?
138 Chapter Two
Ch02_Lammers_1418466 10/8/04 11:26 AM Page 138
Boilers