10.4: Applications of Nuclear Changes

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Skills to Develop

Define and give examples of fission and fusion.
Classify nuclear reactions as fission or fusion.
List some medical uses of nuclear energy.

Introduction

Nuclei that are larger than iron-56 may undergo nuclear reactions in which they break up into two or more smaller nuclei. These reactions are called fission reactions.

Conversely, nuclei hat are smaller than iron-56 become larger nuclei in order to be more stable. These nuclei undergo a nuclear reaction in which smaller nuclei join together to form a larger nucleus. Such nuclear reactions are called fusion reactions.

Fission and Chain Reactions

CK12 Screenshot 10-4-2.png In both fission and fusion, large amounts of energy are given off in the form of heat, light, and gamma radiation. Nuclear fission was discovered in the late 1930's when \(\ce{U}\)-235 nuclides were bombarded with neutrons and were observed to split into two smaller-mass nuclei.

\[\ce{_0^1n} + \ce{_{92}^{235}U} \rightarrow \ce{_{56}^{141}Ba} + \ce{_{36}^{92}Kr} + 3 \ce{_0^1n}\]

The products shown are only one of many sets of products from the disintegration of a \(\ce{U}\)-235 nucleus. Over 35 different elements have been observed in the fission products of \(\ce{U}\)-235.

When a neutron strikes a \(\ce{U}\)-235 nucleus and the nucleus captures a neutron, it undergoes fission producing two lighter nuclei and three free neutrons. The production of the free neutrons makes it possible to have a self-sustaining fission process - a nuclear chain reaction. It at least one of the neutrons goes on to cause another \(\ce{U}\)-235 disintegration, the fission will be self-sustaining.

Fission Reactors

Fission reactions can be used in the production of electricity if we control the rate at which the fission occurs. The great majority of all electrical generating systems (whether coal burning power plants, hydroelectric plants, or nuclear power plants) all follow a reasonably simple design. The electricity is produced by spinning a coil of wire inside a magnetic field. 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. When a fluid is forced through the turbine, the fan blades turn, the turbine axle turns, and the loop of wire inside the generator turns, thus generating electricity.

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 and the heat is used to boil water into steam and then the steam passes through the turbine and makes it spin. In a fission reactor generating plant, a fission reaction is used to boil the water into steam and the steam passes through the turbine to make it spin. 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 totally 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 \(\ce{U}\)-235 content is more than \(1\%\) is called enriched uranium.)

Once the supply of \(\ce{U}\)-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 made of neutron-absorbing material. The amount of \(\ce{U}\)-235 in all the fuel rods taken together is adequate to carry on a chain reaction but is less than the critical mass. (In the United States, all public nuclear power plants contain less than a critical mass of \(\ce{U}\)-235 and therefore, could never produce a nuclear explosion.) 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 \(\ce{U}\)-235 nuclear disintegration and strike a new \(\ce{U}\)-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 they catch fewer neutrons, and 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 into the shut off position.

\(\ce{U}\)-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.

You can follow the operation of an electricity-generating fission reactor in the figure. The reactor core is submerged in a pool of water. The heat from the fission reaction heats the water and the water is pumped into a heat exchanger container where the heated water boils the water in the heat exchanger. The steam from there is forced through a turbine which 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.

In the United States, heavy opposition to the use of nuclear energy was mounted in the late 1960's and early 1970's. Every environmentalist organization in the US opposed the use of nuclear energy and the constant pressure from environmentalist groups brought increased public fear and therefore, opposition. This is not true today; at least one environmental leader has published a paper in favor of nuclear powered electricity generation.

CK12 Screenshot 10-4-3.png

In 1979, a reactor core meltdown at Pennsylvania's Three Mile Island nuclear power plant reminded the entire country of the dangers of nuclear radiation. The concrete containment structure (six feet thick walls of reinforced concrete), however, did what it was designed to do - prevent radiation from escaping into the environment. Although the reactor was shut down for years, there were no injuries or deaths among nuclear workers or nearby residents. Three Mile Island was the only serious accident in the entire history of 103 civilian power plants operating for 40 years in the United States. There has never been a single injury or death due to radiation in any public nuclear power plant in the U.S. The accident at Three Mile Island did, however, frighten the public so that there has not been a nuclear power plant built in the U.S. since the accident.

The 103 nuclear power plants operating in the U.S. deliver approximately \(19.4\%\) of American electricity with zero greenhouse gas emission. There are 600 coal-burning electric plants in the US delivering \(48.5\%\) of American electricity and producing 2 billion tons of \(\ce{CO_2}\) annually, accounting for \(40\%\) of U.S. \(\ce{CO_2}\) emissions and \(10\%\) of global emissions. These coal burning plants also produce \(64\%\) of the sulfur dioxide emissions, \(26\%\) of the nitrous oxide emissions, and \(33\%\) of mercury emissions.

Fusion

Nuclear reactions, in which two or more lighter-mass nuclei join together to form a single nucleus, are called fusion reactions or nuclear fusions. Of particular interest are fusion reactions in which hydrogen nuclei combine to form helium. Hydrogen nuclei are positively charged and repel each other. The closer the particles come, the greater is the force of repulsion. In order for fusion reactions to occur, the hydrogen nuclei must have extremely high kinetic energies so the velocities can overcome the forces of repulsion. These kinetic energies only occur at extreme temperatures such as those that occur in the cores of the sun and other stars. Nuclear fusion is the power source for the stars where the necessary temperature to ignite the fusion reaction is provided by massive gravitational pressure. In stars more massive than our sun, fusion reactions involving carbon and nitrogen are possible. These reactions produce more energy than hydrogen fusion reactions.

Intensive research is now being conducted to develop fusion reactors for electricity generation. The two major problems slowing up the development is finding a practical means for generating the intense temperature needed and developing a container that won't melt under the conditions of a fusion reaction. Electricity-producing fusion reactors are still a distant dream.

Uses of Nuclear Radiation

It is unfortunate that when the topics of radioactivity and nuclear energy come up, most thoughts probably go to weapons of war. The second thought might be about the possibility of nuclear energy contributing to the solution of the energy crisis. Nuclear energy, however, has many applications beyond bombs and the generation of electricity. 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. CK12 Screenshot 10-4-4.png

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. This section will focus on nuclear medicine involving the types of nuclear radiation introduced in this chapter. The x-ray imaging systems will not be covered.

Radioiodine \(\left( \ce{I}-131 \right)\) Therapy involves imaging and treatment of the thyroid gland. The thyroid gland is a gland in the neck that produces two hormones that regulate metabolism. In some individuals, this gland becomes overactive and produces too much of these hormones. The treatment for this problem uses radioactive iodine \(\left( \ce{I}-131 \right)\) which is produced for this purpose in research fission reactors or by neutron bombardment of other nuclei.

The thyroid gland uses iodine in the process of its normal function. Any iodine in food that enters the bloodstream is usually removed by, and concentrated in, the thyroid gland. When a patient suffering from an overactive thyroid swallows a small pill containing radioactive iodine, the \(\ce{I}\)-131 is absorbed into the bloodstream just like non-radioactive iodine 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 \(\ce{I}\)-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 lays down on a sheet of film and the radiation from the \(\ce{I}\)-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. During that time, they are advised that they will set off radiation detectors in airports and will need to get special permission to fly on commercial flights.

Positron Emission Tomography or 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 three-dimensionally mapped 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

Lesson Summary

Naturally radioactive elements exist in the earth and are either alpha or beta emitters.
Artificial transmutation of elements can be accomplished by bombarding the nuclei of some elements with alpha or subatomic particles.
Nuclear fission refers to the splitting of atomic nuclei.
Nuclear fusion refers to the joining together to two more smaller nuclei to form a single nucleus
The fission of \(\ce{U}\)-235 or \(\ce{Pu}\)-239 is used in nuclear reactors.
Nuclear radiation also has many medical uses.

Vocabulary

Chain reaction: A multi-stage nuclear reaction that sustains itself in a series of fissions in which the release of neutrons from the splitting of one atom leads to the splitting of others.
Critical mass: The smallest mass of a fissionable material that will sustain a nuclear chain reaction at a constant level.
Fission: A nuclear reaction in which a heavy nucleus splits into two or more smaller fragments, releasing large amounts of energy.
Fusion: A nuclear reaction in which nuclei combine to form more massive nuclei with the simultaneous release of energy.
Control rods: Control rods are made of chemical elements capable of absorbing many neutrons and are used to control the rate of a fission chain reaction in a nuclear reactor.