Audeen W. Fentiman
Jeffery A. Henkel
Ronald J. Veley
Human beings are continuously exposed to background radiation from such sources as cosmic rays from space, radon from the soil, and naturally occurring radioactive materials within their bodies. It is not possible to protect people from sources of background radiation distributed throughout the environment. However, when a source of radiation is concentrated and confined in a small area, the radiation dose people receive from that source can be limited by the use of carefully planned structures and procedures. The three factors that determine the size of the radiation dose received are time, distance, and shielding.
This fact sheet explains how these three factors affect the dose, with emphasis on a discussion of shielding, including shielding for low-level radioactive waste. First, a brief review of ionizing radiation and its effects is presented.
When radioactive atoms decay, they release their extra energy in the form of ionizing radiation. Three types of ionizing radiation are alpha particles, beta particles, and gamma rays. They are called ionizing radiation because as they move through matter, they "knock" electrons out of their orbits and form ions. The ionizing radiation uses some of its energy each time it creates an ion. Eventually, the radiation uses all of its energy and can no longer cause damage. (For a more detailed explanation of ionizing radiation, see the fact sheet entitled "What is Ionizing Radiation?"--RER-21.)
Ionizing radiation can affect living things by altering the cells that make up the living organism. Cells are made up of molecules. Cell damage is caused by interaction of radiation with these molecules, forming ions.
Radiation effects on a cell are random. That is, the same type and amount of radiation could strike the same cell many times and have a different effect, including no effect, each time. However, in general, the more radiation that strikes the cell, the greater the chance of an effect occurring. If radiation does not reach the living organism, it has no effect on that organism.
Methods of protecting people from radiation focus on reducing the amount of radiation that reaches human beings. Three factors that determine how much radiation reaches a person are time, distance, and shielding.
The dose of radiation a person receives depends on how long the person is near the radiation source. The shorter the time spent near the source, the smaller the dose. Radiation protection procedures are designed to keep the time people spend near a source of radiation as short as possible.
Similarly, the radiation dose a person receives depends on how close the person is to the source. The greater the distance between the person and the source of radiation, the smaller the dose. In fact, the dose decreases with the square of the distance. A person ten feet from the radiation source receives only one one-hundredth as much radiation as a person one foot from the source.
One way to minimize the amount of radiation that reaches people is to put some material, called shielding, between the radiation source and people. When the radiation strikes the shielding, it begins to create ions in the shield. Each time an ion is created, the radiation uses some of its energy. If the shield is thick enough, the radiation will use up its energy before it gets through the shield.
Any material provides some shielding. Common shielding materials are steel (iron), concrete, lead, and soil. Scientists measure the shielding ability of a material by determining the thickness of the material required to absorb half of the radiation from a given source. This thickness of the material is called the half-thickness. Radiation that has passed through one half-thickness will be reduced by half again if it passes through another half-thickness. The half-thickness depends on both the characteristics of the shielding material and type and energy of the radiation being emitted. Some common shielding materials and their half-thicknesses for high energy gamma rays are shown in Table 1.
| Table 1. Shielding Materials and Their Half-Thicknesses for Gamma Radiation | |
|---|---|
| Lead | 0.7 inches |
| Iron | 1.3 inches |
| Concrete | 4.7 inches |
The three types of ionizing radiation commonly released from low-level radioactive waste are alpha particles, beta particles, and gamma rays. Alpha particles consist of two protons and two neutrons and are positively charged. They can be stopped, or shielded, by a sheet of paper or the outer layer of skin. Beta particles are high-speed electrons. More penetrating than alpha particles, they can pass through an inch of water or human flesh. Beta particles can be effectively shielded with a sheet of aluminum 1/25 of an inch thick. Gamma rays can pass through the human body like x-rays. Dense materials such as concrete and lead can provide shielding from gamma radiation. Shielding for alpha, beta, and gamma radiation is illustrated in Figure 1.
Figure 1. Types of Radiation and Shielding
Low-level radioactive waste is shielded by various methods at the generator site, during transportation, and at the disposal facility.
At the generator site, low-level radioactive waste is typically stored in sealed containers designed to contain the waste and provide shielding. Common containers include steel and polyethylene drums. Additional shielding at the generator site may be provided by the building in which the waste is stored. The kinds of shipping containers used for transportation of low-level radioactive waste depend upon the type and concentration of radionuclides in the waste. Regulations governing the transportation of low-level waste set limits on the level of radiation allowed at the surface of the container. Limits are also set on the radiation level allowed at the outer surface of the truck. Additional shielding, usually in the form of steel or lead sheets, is attached to the inside walls of the truck, if needed, to meet the vehicle outer surface radiation requirements.
Shielding ionizing radiation at low-level radioactive waste disposal facilities is accomplished in various ways, depending on the type of facility. Waste containers, concrete walled buildings, and mounds of soil over the buildings are typically used for shielding at such a facility.
If you would like to read more about protection from radiation, some of the references and other fact sheets listed below may be helpful.
Eric J. Hall, Radiation and Life, Second Edition, Pergamon Press, New York, 1984.
Raymond L. Murray, Understanding Radioactive Waste, Battelle Press, Columbus, Ohio, 1989.
RER-20 What is radioactive material, and how does it decay?
RER-21 What is ionizing radiation?
Dr. Audeen W. Fentiman is an Assistant Professor in Nuclear Engineering at The Ohio State University. Jeffery A. Henkel is a Graduate Research Associate in Nuclear Engineering. Ronald J. Veley is a Graduate Research Associate, Ohio State University Extension.
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Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Keith L. Smith, Director, Ohio State University Extension.
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