4.7
NUCLEAR POWER
Purpose
This chapter introduces nuclear power.
Summary
Nuclear power plants, like coal-fired power plants, operate by producing superheated steam to drive electrical turbines. Like coal plants, nuclear plants benefit by operating at extremely high temperatures. The higher the temperature that these plants can operate at, the more efficient they become. A low temperature plant might only return 25 percent of its heat energy as electricity. A larger, hotter plant might return 50 to 60 percent of its heat energy as electricity.
The primary difference between nuclear and coal plants is the way that they generate heat. Nuclear power plants generate heat through nuclear fission. Nuclear fission breaks protons and neutrons free from the nucleus of the nuclear fuel. This isn’t a combustion process, so no carbon dioxide (CO2) is produced. However, nuclear power produces a different type of pollution—radioactive waste.
When concentrated, uranium, the fuel used in most nuclear reactors, is highly toxic. The fuel can also be very difficult to obtain. Although there is a lot of uranium in the Earth’s crust, it is seldom found in large deposits. There are a limited number of areas where sufficient quantities can be found to make its extraction economically feasible. Another worry is that the refining process is often identical to the process needed to make fuel for nuclear weapons. As a result, fears of nuclear weapon proliferation are closely linked to the construction of nuclear reactors.
Key Topics
• Nuclear power doesn’t release CO2 emissions, but it does cause other environmental problems.
• The primary fuel for nuclear power, uranium, has to be mined and purified before it is used.
• Nuclear fuel is not particularly common. Techniques to refine nuclear fuel are often government secrets.
• Nuclear plants, like coal plants, have an economy of scale. They need to be built large and no one wants one built in his or her backyard. As a result, nuclear plants are usually built far from consumer regions. That distance places a lot of strain on the electrical grid.
Nuclear power plants use steam turbines to generate electricity. This technology is identical to the technology used to generate electricity in large coal-fired power plants. Both heat water until it becomes pressurized steam. That steam is used to power a spinning turbine. Because of this, like coal plants, nuclear power plants produce more electricity per unit of fuel when they can operate at high temperatures. Also, they are most efficient when they operate full time without letting the water cool down overnight.
The primary difference between the two types of power plants is the fuel that they consume. Coal power plants produce heat through combustion. Combustion (burning) creates CO2 and releases any particulate matter trapped in the coal. In contrast, nuclear power plants produce heat through nuclear fission. Nuclear fission involves splitting large atoms (like uranium) into two separate atoms.
Nuclear Fission
Nuclear fission is a natural process that occurs every day. It occurs when the nucleus of an atom splits into smaller particles. Certain elements, like uranium and plutonium, are naturally radioactive. In these elements, the nucleus is unstable enough that it splits spontaneously. This is called spontaneous fission or radioactive decay. Humans can speed up this process so that it happens very quickly rather than over a long period of time. When fission is sped up, it is called induced fission.
It is easiest to induce fission in an element that is already breaking down. That is why induced fission utilizes elements that are already radioactive like uranium and plutonium. When fission occurs, an alpha particle consisting of two protons and two neutrons breaks off from the nucleus of an atom. This also causes a pair of electrons to spin off from the nucleus (beta particles) and a release of electromagnetic energy (gamma radiation).
Almost any atomic nucleus can be broken apart by fission if enough energy is used. However, some atomic nuclei are especially susceptible to fission and require much less energy to break apart than others. These substances are the key to creating a nuclear chain reaction. In a chain reaction, the amount of energy released by fission will be sufficient to cause another nearby nucleus to split apart as well. If the nearby nuclei are stable, the first fission won’t be enough to break another nucleus apart.
The difference between radiation, a nuclear reactor, and a nuclear bomb is the speed of the fission reaction. Nuclear radiation is a slow breakdown of atomic nuclei. There isn’t enough energy being released to create a chain reaction. A nuclear reactor involves a faster reaction, where the energy being released from fission triggers additional nuclear reactions at a measured rate. A nuclear bomb explodes every atom in the fuel extremely quickly in a fast chain reaction.
The Nuclear Cycle
The life cycle of nuclear material is typically called the nuclear cycle. The first stages in the process are exploration and mining. These stages remove rocks containing nuclear fuel from the ground which are brought to a processing plant. At the processing plant, uranium is separated from other rocks and milled by grinding it up into a powder-like material called yellowcake. Uranium is then converted into a gas by combining it with fluoride to form uranium hexafluoride (UF6). Then, uranium is ready to be enriched. The enrichment process brings fuel to a desired ratio of fissionable material (Uranium-235) to nonfissionable uranium (Uranium-238). The most common way to separate the uranium is to spin the uranium hexafluoride gas in a centrifuge. The heavier Uranium-238 will move to the outside of the centrifuge allowing the Uranium-235 to be collected from the inside.
For a sustained nuclear reaction, there has to be a sufficient quantity of fissionable material in a fuel. Most uranium that is found in nature is not the right type to sustain a nuclear reaction. Nuclear enrichment is the process of getting the right amount of fissionable material into nuclear fuel. One type of uranium, an isotope called Uranium-235, is very easy to fission. An atom of U-235 produces more energy when the nucleus splits apart than is required to break the nucleus. Other types of uranium are not as easy to break apart. As a result, getting the right amount of U-235 into nuclear fuel is the key to nuclear fission.
When uranium or any other nuclear fuel is removed from the ground, it won’t have the proper amount of fissionable material present. It is important that there is neither too much nor too little Uranium-235 in nuclear fuel. Otherwise, either a reaction won’t happen or it will happen too fast. Processing raw uranium until it has the proper consistency is one of the most important steps in nuclear power. For example, a nuclear power plant might want its fuel to contain about 4 percent Uranium-235 for a moderate reaction. In comparison, a nuclear weapon might require 90 percent purity or higher.
After it is separated, the uranium is then converted back into a solid form (uranium dioxide, UO2) in a fabrication plant and shaped into its final form. A nuclear power plant will partially control the speed of the nuclear reaction by the shape of the fuel. A nuclear weapon might want the fuel shaped into a ball to maximize the speed of a chain reaction. A nuclear power plant might want the fuel shaped into a flat sheet. That way, a lot of the energy will escape from the top and the bottom of the sheet. This will reduce the speed of the nuclear chain reaction. The speed of the fission process is fine-tuned by the use of graphite control rods. These rods can be used to slow down reactions. When they are inserted into the reactor chamber, graphite rods soak up radiation before it triggers fission in another atom.
After the nuclear fuel is used, it must be removed from the reactor. At this point, the fuel, control rods, and water used to cool the reaction and power the steam turbines are all radioactive. Those materials will be undergoing nuclear fission as well. However, this rate will be much slower than the rate at which the nuclear reactor was operating. The fuel may be reprocessed to remove any remaining Uranium-235 that is still unused. After that, it will be necessary to get rid of the wastes.
There are two choices with any toxic waste. The waste can either be diluted or concentrated. If diluted, a low level of toxic waste will be spread out over a wide area. If concentrated, the waste can become concentrated into something unbelievably toxic. Most areas have regulations to limit release of any toxic substances into the biosphere, so diluting the toxic waste is not a viable solution. As a result, the nuclear waste gets concentrated and placed in a storage facility that is hopefully far away from anywhere it can do any harm.
Problems with Nuclear Power
From an engineering standpoint, nuclear power plants present many of the same challenges as coal plants. Both types of plants need to be built in an extremely large scale and placed far away from consumers. Adopting nuclear power as a primary power source means making a substantial commitment to a certain type of power grid—one that involves long-distance transmission of power. This type of power grid is inherently less reliable than a power grid where generation capacity is located close to consumers.
Compared to coal, nuclear power also doesn’t address the problem of importing fuel from other countries. Uranium has to be mined from somewhere. This makes the nuclear power industry just as fuel dependent as any fossil-fuel-fired power plant. For example, North America has some uranium reserves, but these are much less extensive than its vast coal reserves. Other areas, like Europe, have to import nearly all of their nuclear fuel. As a result, nuclear power is not necessarily the best approach for a country to be energy independent.
Another problem is the concern of nuclear weapon proliferation. The enrichment process used to create nuclear fuel is nearly identical to the process used to create nuclear weapons. Ensuring there is a ready supply of nuclear fuel to supply civilian use without making it available for military use is a problem that challenges public policy.
From a pollution standpoint, the good thing about nuclear power is that it doesn’t produce CO2, sulfur dioxide, or nitrogen oxide pollution. However, none of those types of pollution are likely to kill people outright. Fossil fuel pollution might cause climate change, kill off wildlife, and destroy the environment, but it doesn’t present the immediate health threat of nuclear waste. Nuclear waste is deadly when concentrated. Storing concentrated nuclear waste is very problematic because of how dangerous it is.