The Basics of Nuclear Power
Abstract
The source of nuclear power is the energy released during reactions involving atomic nuclei. These reactions, which involve the splitting of atoms and the reconfiguring of atomic nuclei, release enormous amounts of energy when mass is converted into energy. This energy emerges from the reactions as kinetic energy carried by fast moving particles, normally neutrons. In the core of a power plant, this energy is converted into heat that can be used to raise steam and drive a steam turbine. Atoms that undergo fission reactions with the release of energy are called fissile species. There are also nuclear reactions during which a new fissile species is generated, such as the conversion of uranium-238 into plutonium after capture of a neutron. A further type of nuclear reaction, fusion, involves the fusing of two small nuclei to produce a larger one, also with the release of energy.
Keywords
Nuclear fission; nuclear fusion; fissile isotope; fertile isotope; controlled nuclear reaction; chain reaction; uranium-235; moderator; control rod; nuclear island
Nuclear power reactors release energy in the form of heat from reactions involving the nuclei of atoms. In this context, the reactor is conceptually identical to the boiler in a coal-fired power station, which also releases heat energy from a fuel. In both cases that heat energy is then captured within the boiler walls (in the nuclear plant the boiler is called a steam generator) where water is converted to high-pressure, high-temperature steam and this steam is used to drive a steam turbine generator.
The steam generated by a nuclear power plant never reaches the same temperature and pressure as that in a modern coal-fired boiler; in consequence of this, the steam turbines in nuclear power plants are often much larger than in the fossil fuel plants. There are also design differences resulting from the need to keep all the radioactivity within the confines of the reactor chamber. Nevertheless, apart from the difference in energy source, the layouts of the two types of power station are broadly similar.
The part of a nuclear power plant containing the nuclear reactor and other elements that are exposed to radiation is called the nuclear island. This is normally housed within a large concrete vessel called the containment, a safety-vessel designed to withstand explosions and external impacts. The primary steam generators for a nuclear plant are normally within the containment. However, much of the steam cycle, including the steam turbine generators lie outside the containment. Taken together, these components are called the conventional island. The actual boundary between the nuclear island and the conventional island will depend upon the specific reactor design.
Nuclear Reactions
The nuclear reactions that generate energy in a nuclear reactor involve the reconfiguring of atomic nuclei. The forces binding atomic nuclei together are incredibly strong and so the amount of energy involved in changing nuclear configurations during a nuclear reaction is large.
The reactions that take place require a rearrangement of the building blocks of atomic nuclei, the protons and neutrons, known collectively as nucleons. In effect, one element is transmuted into another. These processes can be considered to be reactions that create more stable nuclear configurations from less stable ones in the same way as chemical reactions usually create more stable molecules from less stable ones. As with chemical reactions, some of these processes are more or less spontaneous, others initially require an enormous amount of energy to push them to a conclusion, though once started they become self-sustaining. The net result in each case is that the mass of the product nuclei is slightly smaller than that of the starting nuclei. (More precisely, the protons and neutrons in the product nuclei are bound together more strongly than they were in their starting nuclei.) This small mass loss emerges from the reactions as energy which, by Einstein’s equation relating the two, leads to a very large amount of energy being released.
All the nuclear power stations operating today generate electricity by utilizing energy released when the nuclei of a large atom such as uranium are split into smaller components, a process called nuclear fission. This reaction can occur spontaneously in nature and can also be triggered relatively easily in certain atomic species—called fissile isotopes—such as uranium-235 in a nuclear reactor. The amount of energy released by uranium fission is enormous. One kilogram of naturally occurring uranium could, in theory, release around 140 GWh of energy. (140 GWh represents the output of a 1000-MW coal-fired plant operating a full power for nearly 6 days.)
There is another type of nuclear reaction, nuclear fusion, which involves the reverse of a fission reaction. In this case small atoms are encouraged to fuse at extraordinarily high temperatures to form larger atoms. Like nuclear fission, fusion releases massive amounts of energy. However, fusion reactions will only take place under extreme conditions and do not occur spontaneously under any conditions found naturally upon the Earth. The fusion of hydrogen atoms is the main source of energy within the Sun and it is a similar reaction that forms the basis for research into nuclear fusion for power generation.
The reason why both fission and fusion can release energy lies in the relative stability of different elements. It turns out that atomic species in the middle of the periodic table of elements, elements such as barium and krypton (these are typical products of uranium fission) are generally more stable than either lighter elements such as hydrogen or heavier elements such as uranium. The nuclei of these more stable atoms are bound together more strongly, and their nuclear components, the protons and the neutrons, require more energy to separate them than those in lighter or heavier nuclei. When less stable nuclei are converted into more stable ones, energy is released. Conversely, energy must be absorbed to turn these more stable nuclei back into less stable ones.
Nuclear Fission
Many large and even some small atoms undergo nuclear fission reactions naturally. One of the isotopes of carbon (isotopes are atoms of a single element with different numbers of neutrons) called carbon-14 behaves in this way. Carbon-14 exists at a constant concentration in natural sources of carbon. Thus living entities that constantly exchange their carbon with the biosphere maintain this constant concentration. However, when they die, the carbon-14 is no longer renewed and it gradually decays. Measuring the residual concentration gives a good estimate of the time since the organism died. It is this property that allows archeologists to use carbon-14 to date ancient artefacts and remains.
Carbon-14 is unstable because of the relative number of protons and neutrons that nucleus contains. Atoms of the most common form of carbon, carbon-12 contain six protons—the positively charged nucleons that confer charge on the nucleus—and six neutrons. (In nuclear reactions this is written 
to show the numbers of protons and neutrons) A second stable isotope, carbon-13 contains six protons and seven neutrons. Carbon-14, in contrast, contains six protons and eight neutrons. For light elements such as carbon a ratio of protons to neutrons of around 1.1 appears to be the most stable configuration. The ratio for carbon-14, 1.33, is well above this and the nucleus is not stable.
Other atoms can be induced to undergo fission by bombarding them with subatomic particles. One of the isotopes of uranium, the element most widely used in nuclear reactors, behaves in this manner (see Fig. 3.1). Naturally occurring uranium is composed primarily of two slightly different isotopes called uranium-235 and uranium-238 (the numbers refer to the sum of protons and neutrons each atom contains).
Most uranium is uranium-238, but 0.7% is uranium-235. Uranium-235 is naturally radioactive with a half-life of 703.8 million years. It decays by releasing an alpha particle1 to produce thorium. However, it is also fissile which means it is capable of undergoing a fission reaction on absorption of a neutron. The likelihood of reaction depends on the speed of the neutron and it is much more likely to happen with a slow neutron.
When an atom of uranium-235 is struck by a slow neutron, it has a high chance of undergoing a nuclear fission reaction. The most frequent products of this reaction are an atom of krypton, an atom of barium, three more neutrons, and a significant quantity of energy.
In theory each of the three neutrons produced during this reaction could strike another atom of uranium-235, causing three additional fission reactions. However, this also depends on the quantity of uranium present. If a piece of uranium is too small, then most of the neutrons will escape into the surroundings without ever meeting another uranium-235 nucleus. As the size of the piece of uranium increases, so the likelihood of a neutron from a fission reaction being absorbed by another uranium-235 atom increases. The critical point arrives when the piece of uranium is large enough so that the probability of one of the neutrons from a single fission reaction striking another uranium-235 atom, stimulating a second reaction reaches one. This size is known as the critical mass. Once the critical mass is exceeded and the probability of further fission reactions exceeds one, even by a tiny amount, the result is a rapidly accelerating series of individual fission reactions called a chain reaction. The chain reaction releases an enormous amount of energy in a very short space of time and forms the basis for the atomic bomb.
In fact a critical mass of natural uranium will probably not explode because, as already noted above, the uranium-235 atoms have a high probability of undergoing a fission reaction only when struck by slow moving neutrons. However the neutrons created during the fission process are fast neutrons, too fast to readily induce further fission reactions to take place. They need to be slowed down first. This is crucial to the development of nuclear power.
Controlled Nuclear Reaction
In order for uranium fission is to be harnessed in a power station, the nuclear chain reaction must first be tamed. The chain reaction is explosive and dangerous. However it can be managed by carrying away the energy released by the fission reactions, by controlling the number of neutrons within the reactor core, and then by slowing the remaining neutrons so that they can initiate more fission reactions.
An accelerating chain reaction will take place when each fission reaction causes more than one further identical reaction. If the fission of a single uranium-235 atom causes only one identical reaction to take place, the reaction will carry on indefinitely—or at least until the supply of uranium-235 has been used up—without accelerating. But if each fission reaction leads to an average of less than one further reaction, the process will eventually die away naturally. If the number of reactions resulting from each fission reaction can be controlled then, in principle, the nuclear chain reaction can be tamed. A reactor in which each nuclear reaction produces one further nuclear reaction is described as critical. Once the product of each nuclear reaction is more than one additional reaction, the reactor is described as supercritical. Operation must be controlled so that the reactor is just—but barely—supercritical.
To achieve this, the reactor core must contain more than the critical mass of uranium. Otherwise even with the ability to control the number of neutrons, the chain reaction would soon cease because the mass of uranium would rapidly fall below the critical level. Since there is more uranium than is needed to cause a chain reaction, there must also be a way of absorbing and removing neutrons inside the reactor in order to stop a runaway reaction taking place. Finally there must also be a way of slowing the fast neutrons generated by the fission of uranium-235.
Although uranium-235 forms only a small part of natural uranium, it is possible to build a reactor that uses natural uranium for fuel. However, it is easier to build a reactor that uses fuel containing more than the natural amount of uranium-235. Uranium enrichment plants are used to produce reactor fuel that typically has around 3.0–5.0% uranium-235 instead of the 0.7% found naturally. Enriched uranium makes it easier to start and control a sustained nuclear fission reaction.
The uranium is loaded into the reactor core in the form of pellets inside special rods, as described in Chapter 2. In addition to the uranium, the reactor also contains rods made of boron. Boron is capable of absorbing the neutrons generated during the nuclear reaction of uranium-235. If a sufficiently large amount of boron is included within the reactor core, it will absorb and remove the neutrons generated during the fission reaction, stopping the chain reaction from proceeding by keeping the reactor subcritical. The boron rods are moveable and by moving the rods in and out of the reactor core, the number (or flux) of neutrons and hence nuclear process can be controlled.
One further crucial component is needed to make the reactor work, something to slow down the fast neutrons. The neutrons from each uranium-235 fission move too fast to easily stimulate further reactions but they can be slowed by adding a material called a moderator. Water makes a good moderator and is used in most operating reactors. Graphite also functions well as a moderator and has been used in some reactor designs.
When a uranium fission reaction takes place, the energy it releases emerges as kinetic energy. In other words the products of the fission process carry the energy away as energy of motion; they move extremely fast. (Fast neutrons initially travel at around 3% of the speed of light.) Much of the energy from the fission of uranium-235 is carried away by the three fast neutrons produced during the reaction. These neutrons will dissipate their energy in collision with atoms and molecules within the reactor core. In many reactors this energy is absorbed by the moderator, water. So while the neutrons are slowed, the water within the core becomes hotter. By cycling the water from the reactor core through a heat exchanger, this heat can be extracted and used to raise steam and generate electricity. Extracting the heat also helps maintain the reactor in a stable condition by preventing overheating.
The operation of a nuclear fission reactor is therefore a careful balancing act. As a consequence, a reactor always has the potential to generate a runaway chain reaction. Modern reactor design relies heavily on fail-safe systems to try to ensure that there is no possibility of this happening in the event of a component or operational failure.
Breeder Reactions
Uranium-235 most easily undergoes fission with slow (sometimes called thermal) neutrons. However, fast neutrons such as those it produces during fission can cause a reaction with uranium-238. In this case the result is not fission. Instead the uranium-238 atom captures the neutron to form uranium-239. This is unstable and rapidly decays, losing a beta particle (an electron) to form neptunium-239; neptunium is also unstable and loses a further beta particle to create plutonium-239.
Plutonium is a fissile material like uranium-235 and will undergo a nuclear reaction when it absorbs a neutron. As a consequence of the reaction shown above, a certain amount of plutonium is generated in all nuclear reactors during their operation. A significant part of this undergoes fission, in the same way as uranium-235, releasing energy for power production. However, some remains in the spent fuel. This can be isolated if the fuel is reprocessed.
Since plutonium can be produced from uranium-238, and plutonium is a fissile material, it is possible to build a reactor that uses plutonium as its fuel, so long as enough of the fuel can be derived from uranium. If the reactor core of this plutonium reactor also contains uranium-238, then depending on the number of neutrons being produced in the core, it is possible to generate plutonium at the same time as producing power. This is the principle of the breeder reactor. In operating examples of this type of reactor uranium-238 is contained within a blanket that surrounds the core, where it can be irradiated with fast neutrons escaping the core. With careful design this type of reactor can produce more plutonium in the blanket surrounding the reactor that is used in the core—hence the name breeder.
Breeder reactors (sometimes called nuclear fast reactors because they usually exploit fast neutrons) use a coolant that is not a very efficient moderator and therefore does not slow down the neutrons significantly. This is typically liquid sodium although other moderators are possible. This coolant carries away the heat from the reactor but does not slow the neutrons in the core. These fast neutrons can still generate enough fission reactions in plutonium to form a sustainable nuclear reaction. Meanwhile the core of the reactor is surrounded with a blanket of uranium-238 and neutrons escaping the core react with uranium in this blanket to produce more plutonium. Eventually the uranium in the blanket is processed to isolate the plutonium, which can be used as further fuel for the reactor, and to produce yet more plutonium.
Another type of breeder reactor is based on the use of thorium. Thorium-232, the naturally occurring form, is of no use as a nuclear fuel because it is not fissile. However, if it is exposed to neutrons it reacts to form proactinium-233 which then decays in a second reaction to produce uranium-233.
Uranium-233 is a good fissile material, like uranium-235. One advantage of thorium over uranium-238 for breeder reactors is that it will react with slow neutrons and so can be introduced into conventional reactors in order to generate uranium-233.
There are, therefore, three fissile isotopes that can be used in nuclear reactors: uranium-233, uranium-235, and plutonium-239.
Fusion
The alternative energy-yielding nuclear reaction to fission is fusion. Fusion is the process that generates energy in the Sun and stars. In the Sun, hydrogen atoms combine to produce deuterium (heavy hydrogen) atoms and then deuterium and hydrogen atoms fuse to produce helium with the release energy. The reaction takes place at 10–15 million degrees Celsius and at enormous pressure.
The conditions in the Sun cannot be easily recreated on Earth, although fusion of the type taking place within the Sun has been achieved in laboratories. However for the purposes of electricity generation, another fusion reaction offers more potential because it takes place under more benign conditions than those in the Sun. This is the reaction between two isotopes of hydrogen, deuterium, and tritium, shown schematically in Fig. 3.2. Deuterium 
is found naturally in small quantities in water while tritium 
is made from lithium. These two will react to produce helium and energy.
The reaction between deuterium and tritium will only take place at 100 million degrees Celsius (but at much lower pressure than in the Sun). At this temperature all the atoms separate into a sea of nuclei and electrons, a state called a plasma. Since the constituents of a plasma are all charged, either positively or negatively, they can be controlled and contained using a magnetic field. This is crucial since there is no material that can withstand temperatures this severe. The most promising magnetic field for containing a plasma is toroidal and this has formed the basis for most fusion research. There is an alternative method of containing a fusion plasma called inertial confinement. This relies on generating extreme conditions within a small charge of tritium and deuterium, in essence creating a tiny sun in which the fusion takes place too fast for the particles to escape. Both systems of confinement are being developed for power generation.
Nuclear Power Plant Conventional Island Components
The reactor in a nuclear power station forms the heart of the nuclear island of the plant. However the plant cannot produce electric power without its conventional island too. The conventional island of a nuclear power plant encompasses the components that make up the steam cycle of the plant. These include the steam generator, the steam turbines, the condenser, and system pumps. There will also be a range of other ancillary equipment including the control systems and the transformer substation that steps-up the voltage from the plant generators to that required by the grid.
The two most important components in the steam cycle are the steam generator and the steam turbines. Depending on the power plant design, the steam generator may be within the reactor vessel—as in a boiling water reactor—or it may be external as in a pressurized water reactor. In either case the thermal conditions in commercial nuclear power plants are such that the steam will be produced at relatively low pressure and temperature compared to that in most fossil fuel plants. Typically the steam pressure will be less than 80 atmospheres and around 300°C. This limits the overall thermodynamic efficiency that can be achieved in the steam turbine to around 30–33%.
Commercial nuclear power plants are usually large and consequently produce a very large amount of energy in the form of hot steam at these mild steam conditions. Typical operating plants have capacities of around 1000 MW and modern advanced designs can be up to 1500 MW or more. In order to extract the maximum amount of energy under these mild steam conditions requires that the turbines must be able to accommodate a large steam flow and this in turn means that steam turbines must be extremely large. In addition, the relatively low steam temperature means that the steam from the steam generator in a nuclear power plant is “wet”; wet steam contains small droplets of water.
Wet steam is a problem for steam turbines as it leads to water drop erosion of high-speed steam turbine blades. The higher the turbine blade tip speed, the greater the erosion. In order to minimize this, most nuclear power plants use half-speed turbine generators, so that for a 50-Hz system the steam turbine generator operates at 1500 rev/m instead of 3000 rev/m. The lower rotational speed reduces the turbine blade tip speed. The generator in such a plant must then be a four pole machine to achieve the necessary frequency.
In order to accommodate the large steam flow, there will usually be one high-pressure or high-pressure/intermediate-pressure turbine that takes the flow directly from the steam generator. The steam exiting this turbine will then be dried before passing into four or more low-pressure steam turbines. These will be extremely large turbines, often with last stage turbine blades of up to 1 m in length. All the turbines will be mounted on a single shaft driving the 1500-rev/m generator. The steam exiting the low-pressure turbines is then condensed, usually using water, before returning to the steam generator.
Nuclear power plant efficiency could be increased if the plants could provide hotter, higher pressure steam. Research into nuclear configurations that can generate supercritical steam2 at conditions similar to those found in modern coal-fired power stations is underway. This could increase steam cycle efficiency in a nuclear power station to 45% or more.