The student will:
- trace the energy transfers that occur in a nuclear power reactor from the binding energy of the nuclei to the electricity that leaves the plant.
- define breeder reactor.
- list some medical uses of nuclear energy.
- control rod
- Geiger counter
Perhaps the two better known applications of nuclear energy are nuclear weapons and electricity generation. Nuclear energy, however, has many other applications. Radioactivity has huge applications in scientific research, several fields of medicine both in terms of imaging and in terms of treatment, industrial processes, some very useful appliances, and even in agriculture.
A nuclear reactor is a device in which a nuclear chain reaction is carried out at a controlled rate. When the controlled chain reaction is a fission reaction, the reactor is called a fission reactor. Fission reactors are used primarily for the production of electricity, although there are a few fission reactors used for military purposes and for research. The great majority of electrical generating systems all follow a reasonably simple design, as seen in the image below. The electricity is produced by spinning a coil of wire inside a magnetic field. When the loop is spun, electric current is produced. The direction of the electric current is in one end of the loop and out the other end. The machine built to accomplish this task is called an electric generator.
Another machine usually involved in the production of electric current is a turbine. Although actual turbines can get very complicated, the basic idea is simple. A turbine is a pipe with many fan blades attached to an axle that runs through the pipe. When a fluid (air, steam, water) is forced through the pipe, it spins the fan blades, which in turn spin the axle. To generate electricity, the axle of a turbine is attached to the loop of wire in a generator. If a fluid is forced through the turbine, the fan blades turn the turbine axle, which turns the loop of wire inside the generator, thus generating electricity. A steam turbine is illustrated in the image below.
The essential difference in various kinds of electrical generating systems is the method used to spin the turbine. For a wind generator, the turbine is a windmill. In a geothermal generator, steam from a geyser is forced through the turbine. In hydroelectric generating plants, water falling over a dam passes through the turbine and spins it. In fossil fuel (coal, oil, natural gas) generating plants, the fossil fuel is burned to boil water into steam that passes through the turbine and makes it spin. In a fission reactor generating plant, a fission reaction is used to boil the water into the steam that passes through the turbine. Once the steam is generated by the fission reaction, a nuclear power plant is essentially the same as a fossil fuel plant.
Naturally occurring uranium is composed almost entirely of two uranium isotopes. It contains more than 99% uranium-238 and less than 1% uranium-235. It is the uranium-235, however, that is fissionable (will undergo fission). In order for uranium to be used as fuel in a fission reactor, the percentage of uranium-235 must be increased, usually to about 3%. Uranium in which the U-235 content is more than 1% is called enriched uranium. Somehow, the two isotopes must be separated so that enriched uranium is available for use as fuel. Separating the isotope by chemical means (chemical reactions) is not successful because the isotopes have exactly the same chemistry. The only essential difference between U-238 and U-235 is their atomic masses; as a result, the two isotopes are separated by a physical means that takes advantage of the difference in mass.
Once the supply of U-235 is acquired, it is placed in a series of long cylindrical tubes called fuel rods. These fuel cylinders are bundled together with control rods (see diagram below) made of neutron-absorbing material. In the United States, the amount of U-235 in all the fuel rods taken together is adequate to carry on a chain reaction but is less than the critical mass. The amount of heat generated by the chain reaction is controlled by the rate at which the nuclear reaction occurs. The rate of the nuclear reaction is dependent on how many neutrons are emitted by one U-235 nuclear disintegration and how many strike a new U-235 nucleus to cause another disintegration. The purpose of the control rods is to absorb some of the neutrons and thus stop them from causing further disintegrations. The control rods can be raised or lowered into the fuel rod bundle. When the control rods are lowered all the way into the fuel rod bundle, they absorb so many neutrons that the chain reaction essentially stops. When more heat is desired, the control rods are raised so that the chain reaction speeds up and more heat is generated. The control rods are operated in a fail-safe system so that power is necessary to hold them up. During a power failure, gravity will pull the control rods down to shut off the system.
U-235 nuclei can capture neutrons and disintegrate more efficiently if the neutrons are moving slower than the speed at which they are released. Fission reactors use a moderator surrounding the fuel rods to slow down the neutrons. Water is not only a good coolant but also a good moderator, so a common type of fission reactor has the fuel core submerged in a huge pool of water. This type of reaction is called a light water reactor or LWR. All public electricity generating fission reactors in the United States are LWRs.
You can follow the operation of an electricity-generating fission reactor in the image above. The reactor core is submerged in a pool of water. The heat from the fission reaction heats the water, which is pumped into a heat exchange container. There the heated water boils the water in the heat exchanger. The produced steam is forced through a turbine that spins a generator and produces electricity. After the water passes through the turbine, it is condensed back to liquid water and pumped back to the heat exchanger.
U-235 is the only naturally occurring fissile isotope, and it constitutes less than 1% of naturally occurring uranium. A fissile substance is a substance capable of sustaining a chain reaction of nuclear fission. It has been projected that the world's supply of U-235 will be exhausted in less than 200 years. It is possible, however, to convert U-238 to a fissionable isotope that will function as a fuel for nuclear reactors. The fissionable isotope is plutonium-239 and is produced by the following series of reactions:
The final product from this series of reactions is plutonium-239, which has a half-life of 24,000 years and is another nuclear reactor fuel. This series of reactions can be made to occur inside an operating nuclear reactor by replacing some of the control rods with rods of U-238. As the nuclear decay process proceeds inside the reactor, it produces more fuel than it uses. It would take about 20 such breeder reactors to produce enough fuel to operate one addition reactor. The use of breeder reactors would extend the fuel supply a hundred fold. The problem with breeder reactors, however, is that plutonium is an extremely deadly poison. Furthermore, unlike ordinary fission reactors, it is possible for out-of-control breeder reactors to explode. None of the civilian nuclear power plants in the U.S. are breeder reactors.
A variety of methods have been developed to detect nuclear radiation. One of the most commonly used instruments for detecting radiation is the Geiger counter.
A Geiger counter.
The detecting component of the Geiger counter is the Geiger-Muller tube. This tube is a cylinder filled with an inert gas, and it has a window in one end made of porous material that will not allow the inert gas to escape but will allow radiation particles to enter. A conducting wire extends into the center of the tube and is electrically insulated from the tube where it passes through the wall. Electric current, however, does not flow because the inert gap does not conduct electricity and the circuit is not complete. When a radiation particle enters the tube through the window, the particle creates a line of ionized gas particles along its path through the tube. The line of ions does conduct electric current, so an electric current will flow along the ionized path. The ions only exist for a very short period because the ions of inert gas will quickly regain the lost electrons and become atoms again, which do not conduct. The result is a very short burst of electric current whenever a radiation particle passes through the tube. The control box provides the electric potential for the tube and also provides some means for demonstrating the burst of current. Some machines simply make a clicking sound for each burst of current, while others may provide a dial or a digital meter.
Other methods used for the detection of nuclear radiation include: 1) scintillation counters – a screen coated with a material that gives off a small flash of light when struck by a particle, 2) cloud chambers (see Figure below) – a chamber of supersaturated gas that produce a condensation trail along the path of a radiation particle, and 3) bubble chambers – a chamber of superheated liquid that produces a trail of bubbles along the path of a radiation particle.
Cloud chamber shows vapor trails produced by sub-atomic particles.
Cloud chambers and bubble chambers have an additional value because the vapor trails or bubble trails left by the nuclear radiation particle are long-lasting enough to be photographed and can therefore be studied in great detail.
In the early 1900s, the use of alpha particles for bombarding low atomic number elements became a common practice. Researchers found that the alpha particles were absorbed by the nuclei, causing a proton to be ejected. This was the first artificially caused transmutation of one element into another. In order to continue these bombardments with alpha particles or protons, the speed of the bombarding particle had to be increased. Several machines were devised to accelerate the particles to the required speeds.
The cyclotron (illustrated below) was developed by Ernest Lawrence in 1930 and used to accelerate charged particles so they would have sufficient energy to enter the nuclei of target atoms. A cyclotron consists of two hollow half cylinders called “dees” because of their D-shapes.
A different particle accelerator is the linear accelerator, which is a long series of tubes that are connected to a source of high frequency alternating voltage. As the charged particles leave each tube, the charge on the tubes are altered so the particle is repelled from the tube it is leaving and attracted to the tube it is approaching. In this way, the particle is accelerated between every pair of tubes.
A number of the elements listed in the periodic table are not found in nature. These elements include all elements with atomic numbers greater than 92, technetium (#43), and promethium (#61). The transuranium elements (those with atomic numbers greater than 92) are all man-made elements, many of which were produced in the cyclotron in the radiation laboratory at the University of California at Berkeley under the direction of Glenn Seaborg (Figure below).
The field of nuclear medicine has expanded greatly in the last twenty years. A great deal of the expansion has come in the area of imaging. Radioiodine (I-131) therapy involves the imaging and treatment of the thyroid gland. The gland uses iodine in the process of its normal function, which includes producing hormones that regulate metabolism. Any iodine in food that enters the bloodstream is usually removed by, and concentrated in the thyroid gland. In some individuals, this gland becomes overactive and produces too much of these hormones. The treatment for this problem uses radioactive iodine (I-131), which is produced for this purpose in research fission reactors or by neutron bombardment of other nuclei. When a patient suffering from an overactive thyroid swallows a small pill containing radioactive iodine, the I-131 is absorbed into the bloodstream and follows the same process to be concentrated in the thyroid. The concentrated emissions of nuclear radiation in the thyroid destroy some of the gland’s cells and control the problem of the overactive thyroid.
Smaller doses of I-131 (too small to kill cells) are also used for purposes of imaging the thyroid. Once the iodine is concentrated in the thyroid, the patient lies down on a sheet of film, and the radiation from the I-131 makes a picture of the thyroid on the film. The half-life of iodine-131 is approximately 8 days, so after a few weeks, virtually all of the radioactive iodine is out of the patient’s system.
Positron emission tomography (PET) scan is a type of nuclear medicine imaging. Depending on the area of the body being imaged, a radioactive isotope is either injected into a vein, swallowed by mouth, or inhaled as a gas. When the radioisotope is collected in the appropriate area of the body, the gamma ray emissions are detected by a PET scanner (often called a gamma camera), which works together with a computer to generate special pictures providing details on both the structure and function of various organs. PET scans are used to:
- detect cancer
- determine the amount of cancer spread
- assess the effectiveness of treatment plans
- determine blood flow to the heart muscle
- determine the effects of a heart attack
- evaluate brain abnormalities such as tumors and memory disorders
- map brain and heart function
External beam therapy (EBT) is a method of delivering a high energy beam of radiation to the precise location of a patient’s tumor. These beams can destroy cancer cells and, with careful planning, not kill surrounding cells. The concept is to have several beams of radiation, each of which is sub-lethal, enter the body from different directions. The only place in the body where the beam would be lethal is at the point where all the beams intersect. Before the EBT process, the patient is mapped three-dimensionally using CT scans and X-rays. The patient receives small tattoos to allow the therapist to line up the beams exactly. Alignment lasers are used to precisely locate the target. The radiation beam is usually generated with a linear accelerator. EBT is used to treat the following diseases as well as others:
- breast cancer
- colorectal cancer
- head and neck cancer
- lung cancer
- prostate cancer
There are two basic types of nuclear weapons: fission bombs using supercritical masses of either U-235 or Pu-239, and fusion bombs using heavy isotopes of hydrogen. The fission bombs were called atomic bombs (a misnomer since the energy comes from the nucleus), and fusion bombs are called thermonuclear bombs. Fission bombs use two or more subcritical masses of fissile materials separated by enough distance that they don’t become critical. The materials are surrounded by conventional explosives. The conventional explosives are detonated to drive the subcritical masses of fissile material toward the center of the bomb. When these masses are slammed together, they form a supercritical mass, and a nuclear explosion ensues. Hydrogen bombs (fusion) are detonated by using a small fission explosion to compress and heat a mass of deuterium or deuterium and tritium to the point that a fusion reaction ignites. An image of a nuclear explosion (as part of a nuclear test) is shown in Figure below.
Castle-Romeo nuclear explosion.
- The fission of U-235 or Pu-239 is used in nuclear reactors.
- The critical mass is the amount of fissile material that will maintain a chain reaction.
- Nuclear radiation also has many medical uses.
Further Reading / Supplementary Links
For more details about nuclear power versus other sources of power, you can read the following report.
Nuclear Power VS. Other Sources of Power, Neil M. Cabreza, Department of Nuclear Engineering, University of California, Berkeley, NE-161 Report.
The following web site contains a history of the discovery (creation) of the transuranium elements and includes some of the reactions used to produce them.
- What is the primary physical difference between a nuclear electricity generating plant and a coal-burning electricity generating plant?
- What do the control rods in a nuclear reactor do and how do they do it?
- What is a breeder reactor?
- Name two types of particle accelerators.
- In the medical use of radioactivity, what does EBT stand for?
- Is it possible for a nuclear explosion to occur in a nuclear reactor? Why or why not?
All images, unless otherwise stated, are created by the CK-12 Foundation and are under the Creative Commons license CC-BY-NC-SA.