Chapter 2 Radiation Protection PDF

radiation radiation dose radioactive decay radioisotope radionuclides radon Radiation has diﬀerent types and sources. Some types of radiation produce damage in biologic tissue, whereas others do not. Some sources of radiation are considered natural because they are always present in the environment. However, other sources are created by humans for speciﬁc purposes and therefore are classiﬁed as manmade. This chapter presents an overview of the various types and sources of radiation, as well as the doses that are typically received from both natural and manmade sources. 95 Radiation Types of Radiation In the simplest terms, energy is the ability to do work, that is, to move an object against resistance. Energy in motion is called kinetic energy. Radiation is kinetic energy and exists in many forms. For objects with mass, kinetic energy is wri en as. However, photons, which don't have mass, have energy related to their frequency. Higher frequency photons carry more energy. Some examples of diﬀerent types of radiation are presented in Box 2.1. Box 2.1 Examples of Different Types of Radiation Example 1. Mechanical Vibrations of Materials Such mechanical vibrations can travel through the air or other materials to interact with structures in the human ear and produce the sensation we call sound. Ultrasound is the mechanical vibration of a material in which the rate of vibration does not stimulate the human ear sensors and therefore is beyond the range of human hearing. Example 2. the Electromagnetic Wave Radio waves, microwaves, visible light, and x-rays are all representatives of the electromagnetic wave. In these waves, electric and magnetic ﬁelds ﬂuctuate rapidly as they travel through space. A limited range of frequencies of this ﬂuctuation is interpreted by its interaction with the human system as visible light. Within this range, small variations in frequency—the number of cycles or wavelengths of a simple harmonic motion per unit of time—are interpreted as diﬀerent colors. However, frequencies both above and below the visible range exist and have many uses. Electromagnetic waves are also characterized by their wavelength, which is simply the physical distance between successive maximum values of oscillating wavelike electric and magnetic ﬁelds. At the beginning of the 20th century, leading scientists ﬁrst realized that electromagnetic radiation appears to have a dual nature, referred to as wave–particle duality. This means that this form of radiation travels or propagates through space in the form of a wave but can interact with 96 p p g g p ma er as a particle of energy called a photon. For this reason, x-rays may be described as both waves and particles. The Electromagnetic Spectrum The full range of frequencies and wavelengths of electromagnetic waves is known as the electromagnetic spectrum. Each grouping on this scale represents a type or category of radiation generated by varying electric and magnetic ﬁelds. Table 2.1 shows the electromagnetic spectrum in terms of frequency (given in units of her [Hz] i.e., cycles per second), wavelength (in meters), and energy (speciﬁed in electron volts [eV], a unit of energy equal to the quantity of kinetic energy an electron acquires as it moves through a potential diﬀerence of 1 volt). Each frequency within the spectrum has a characteristic wavelength and energy. Some of the practical uses of these diﬀerent frequency ranges are listed. Note that higher frequencies are associated with shorter wavelengths and higher energies; therefore as the wavelength ranges from largest to smallest, frequencies and energy cover the corresponding smallest to largest ranges. Precise frequency intervals a ributed to diﬀerent parts of the electromagnetic spectrum may vary in diﬀerent references, and there is substantial overlap of ranges (note that FM radio falls completely within the television range). Box 2.2 demonstrates the calculation of the wavelength and energy of electromagnetic radiation. All forms of electromagnetic radiation have one common characteristic: their velocity. It is equal to the speed of light. The speed of light (or any type of electromagnetic radiation) is 3 × 108 meters per second in empty space (a vacuum) and is slightly less in transparent materials such as glass or plastic. 97 TABLE 2.1 The Electromagnetic Spectrum* Use Frequency Wavelength Energy AM radio 0.54–1.6 MHz 0.6–0.2 km 2–7 neV FM radio 88–108 MHz 3.4–3 m 370–440 neV Television 54 MHz–0.8 GHz 5.6–0.4 m 220 neV–3.3 µeV Microwaves 0.1–100 GHz 3 m–3 mm 0.4 eV–0.4 meV Infrared 100 GHz–400 THz 3 mm–0.7 m 0.4 meV–1.6 eV Visible 400–700 THz 0.7–0.4 m 1.6–2.8 eV Ultraviolet 1–100 PHz 300–3 nm 4–400 eV X-rays 100 PHz–100 EHz 3 nm–3 am 0.4–400 keV Gamma rays 100 EHz–inﬁnity 3–0 am 400 keV–inﬁnity *Frequency (in units of hertz [Hz] or cycles per second), wavelength (in meters), and energy (in electron volts [eV]). Each member of the spectrum has a characteristic wavelength and frequency. Some of the uses of different frequency ranges are listed. Note that higher frequencies are associated with shorter wavelengths and higher energies. The values shown here are typical representations. See Appendix B for an explanation of the abbreviations (M, G, T, P, µ, etc.). Box 2.2 Calculation of the Wavelength and Energy of Electromagnetic Radiation The speed of light (c), wavelength (λ), and frequency (ν) are related by the following equation: where c = 3 × 108 m/sec. Therefore if the frequency of an electromagnetic wave is known, the wavelength may be calculated as follows: Example: Find the wavelength of a 0.5-MHz radio wave. 98 The energy (in electron volts, eV) of an electromagnetic wave may be calculated using the frequency (ν) and Planck's constant (h) as follows: where h = 4.14 × 10−15 eV-sec. Example: Find the energy of an x-ray having a wavelength of 1 picometer (1 pm = 10−12 m). Solution: The energy is given by the following relation: Ionizing and Nonionizing Radiation For our purposes in the study of radiation protection, the electromagnetic spectrum (Fig. 2.1) can be divided into two parts: 99 FIG 2.1 The electromagnetic spectrum. 1. Ionizing radiation 2. Nonionizing radiation Of the entire span of types of radiation included in the electromagnetic spectrum, only the following radiations are classiﬁed as ionizing radiations1: X-rays Gamma rays Ultraviolet radiation with energy greater than 10 eV Because they do not have suﬃcient kinetic energy to eject electrons from the atom, the following radiations are considered nonionizing: Ultraviolet radiation with energy less than 10 eV Visible light Infrared rays Microwaves Radio waves If electromagnetic radiation is of a high enough frequency, it can transfer suﬃcient energy to some orbital electrons to remove them from the atoms to which they were a ached. As mentioned in Chapter 1, this process, called ionization, is the foundation of the interactions of x-rays with human tissue. It makes them valuable for creating images but has the undesirable result of potentially producing some damage in the biologic material. 100 Particulate Radiation In addition to electromagnetic radiation, there is another category of ionizing radiation, called particulate radiation. This form of radiation includes the following: Alpha particles Beta particles Neutrons Protons All these are subatomic particles that are ejected from the nucleus of atoms at very high speeds. They possess suﬃcient kinetic energy to be capable of causing ionization by direct atomic collision. However, no ionization occurs when the subatomic particles are at rest. Alpha particles, also known as alpha rays, are emi ed from nuclei of very heavy elements such as uranium and plutonium during the process of radioactive decay. Radioactive decay is a naturally occurring process in which unstable nuclei relieve that instability by various types of nuclear spontaneous emissions, one of which is the emission of charged particles. Alpha particles contain two protons and two neutrons. They are simply helium nuclei, or helium atoms minus their electrons. Alpha particles have a large mass that is approximately four times the mass of a hydrogen atom and a positive charge twice that of an electron. This permits them to have the potential of transferring very substantial amounts of kinetic energy to orbital electrons of other atoms.2 Particulate radiations vary in their ability to penetrate ma er. Compared with beta particles, which are just fast electrons, alpha particles are much less penetrating. Because they lose energy quickly as they travel a short distance—for example, into the superﬁcial layers of the skin—they are considered virtually harmless as an external source of radiation. Several pieces of ordinary paper can signiﬁcantly a enuate them and therefore serve as a shield. However, as an internal source of radiation, the reverse is true. If emi ed from a radioisotope that was deposited in the body—for example, in the lungs—alpha particles will be absorbed in the relatively radiosensitive epithelial tissue and be extremely damaging to that tissue. The process is in a way analogous to what a bowling ball does to a set of pins. Beta particles, also known as beta rays, are identical to high-speed electrons except for their origin. Electrons originate in atomic shells outside of the nucleus, whereas beta particles, like alpha particles, are emi ed from within the nuclei of radioactive atoms. This process of beta 101 p decay occurs when a nucleus relieves instability by a neutron transforming itself into a combination of a proton and an energetic electron (called a beta particle). There is also emission of another particle called a neutrino, which has negligible mass and no electric charge but carries away any excess energy. Beta particles are 8000 times lighter than alpha particles and have only one unit of electrical charge (−1) as compared with the alpha's two units of electrical charge (+2). These a ributes mean that beta particles will not interact as strongly with their surroundings as alpha particles do. Therefore they are capable of penetrating biologic ma er to a greater depth than alpha particles with far less ionization along their paths. Not all high-speed electrons, however, are beta radiation. Alternative sources of high-speed electrons are produced in a radiation oncology treatment machine called a linear accelerator. These electrons are most often used to treat superﬁcial skin lesions in small areas or to deliver radiation boost treatments to breast tumors at tissue depths typically not exceeding 5 to 6 cm. Such very high-energy electrons require either millimeters of lead or multicentimeter-thick slabs of wood to absorb them. As previously stated, alpha rays can be absorbed by a few pieces of ordinary paper because they interact so readily with ma er and consequently lose their kinetic energy quite rapidly. Beta rays, however, with a noticeably lesser probability of interaction with atoms of ma er, will penetrate more deeply and therefore cannot be stopped by ordinary pieces of paper. For energies of less than 2 MeV, either a 1-cm-thick piece of wood or a 1-mm-thick lead shield would be suﬃcient for absorption. Protons are positively charged components of an atom. An isolated proton, which is simply identical to an ionized hydrogen atom, has a relatively small mass that exceeds the mass of an electron by a factor of 1800. The number of protons in the nucleus of an atom constitutes its atomic number, or “Z” number. The atomic number identiﬁes an element and determines its placement in the periodic table of elements (see Appendix C). Neutrons are the electrically neutral components of an atom and have approximately the same mass as a proton. If two atoms have the same number of protons but a diﬀerent number of neutrons in their nuclei, they are referred to as isotopes. If one of these combinations of Z protons and some number of neutrons leads to an unstable nucleus, then that combination is called a radioisotope. An Introduction to the Concept of Radiation Dose 102 In the remainder of this chapter, radiation doses that humans can receive and have received are described. To appreciate the relative magnitude of these exposures, it is necessary to discuss the quantities and units that are used to specify radiation dose. The purpose of this section is to provide a very brief introduction to this topic. The radiation quantity, absorbed dose (sometimes just shortened to “dose”), refers to the amount of kinetic energy per unit mass that has been absorbed in a material due to its interaction with ionizing radiation. It is usually measured in units of milligray (mGy). The amount of energy absorbed by human tissue is an important determinant of the extent of biologic harm that may occur. However, other factors must be considered when a empting to predict how much actual biologic harm might be caused by radiation dose. The equivalent dose (EqD) takes into account the type of ionizing radiation that was absorbed. In diagnostic radiology, this absorption is caused by x-rays. But exposure to radioisotopes in the environment or to radioactive materials released from nuclear reactors involves other types of ionizing radiation, such as neutrons, protons, electrons, etc. The equivalent dose provides an overall dose value that includes the diﬀerent degrees of tissue interaction that could be caused by the diﬀerent types of radiation. The most common unit of measure of equivalent dose is the millisievert (mSv). Another factor that plays a role in determining the degree of biologic damage that may be caused by ionizing radiation is the organ or organ systems irradiated. For example, irradiation of internal organs has very diﬀerent, and generally much more severe, consequences than irradiation of extremities. Therefore the contribution of radiation absorbed dose that aﬀects diﬀerent organs and organ systems, as well as the type of ionizing radiation that caused the dose, is considered in deriving the eﬀective dose (EfD). The eﬀective dose is intended to be the best estimate of overall harm that might be produced by a given dose of radiation in human tissue. It takes into account both the type of radiation and the part of the body irradiated. The unit of eﬀective dose is the same as the unit of equivalent dose, the millisievert (mSv). This implies that when a dose value is given in millisieverts, the radiation quantity that is being expressed needs to be identiﬁed. Biologic Damage Potential While penetrating body tissue ionizing radiation produces biologic damage primarily by ejecting electrons from the atoms comprising the 103 tissues. Destructive radiation interaction at the atomic level results in molecular change, and this in turn can cause cellular damage, leading to abnormal cell function or even entire loss of cell function. If excessive cellular damage occurs, the living organism will have a signiﬁcant possibility of exhibiting genetic or somatic changes such as the following: Mutations Cataracts Leukemia Changes in blood count are classic examples of organic damage that results from nonnegligible exposure to ionizing radiation. An EqD of 250 mSv delivered to the whole body may cause a substantial decrease within a few days in the number of lymphocytes or white blood cells that are the body's primary defense against disease. Table 2.2 provides some basic information on the known biologic eﬀects that result when radiation exposures of various EqDs are delivered to the whole body over a time period of less than a few hours (acute exposures). This emphasizes that the use of ionizing radiation should be limited whenever possible. TABLE 2.2 Radiation Equivalent Dose and Subsequent Biologic Effects Resulting From Acute Whole-Body Exposures* RADIATION EqD Subsequent Biologic Eﬀects Sv 0.25 Blood changes (e.g., measurable hematologic depression, substantial decreases within a few days in the number of lymphocytes or white blood cells that are the body's primary defense against disease) 1.5 Nausea, diarrhea 2.0 Erythema (diﬀuse redness over an area of skin after irradiation) 2.5 If dose is to gonads, temporary sterility 3.0 50% chance of death; lethal dose for 50% of population over 30 days (LD 50/30) 6.0 Death *Radiation exposures are delivered to the entire body over a time period of less than a few hours. Adapted from Radiologic health, unit 4, slide 17, Denver, Multi-Media Publishing (slide program). Sources of Radiation Human beings are continuously exposed to sources of ionizing radiation. Sources of ionizing radiation may be one of the following: 104 Natural Manmade (artiﬁcial) Table 2.3 summarizes current estimates of the radiation received on average by individuals in the United States per year. Note that the dose from natural background and the dose from manmade sources are both roughly 3 mSv. The total dose per year is approximately 6 mSv. Fig. 2.2 shows a pie chart that gives the percentage contributions of the various sources of background radiation. TABLE 2.3 Average Annual Radiation Equivalent Dose for Estimated Levels of Radiation Exposure for Humans Dose Category Type of Radiation mSv Natural Radon 2.0 Cosmic 0.3 Terrestrial and internally deposited radionuclides 0.7 Total 3.0 Medical imaging CT scanning 1.5 Radiography 0.6 Nuclear medicine 0.7 Interventional procedures 0.4 Total 3.2 Other manmade 0.1 ____________________________ Total annual EqD from all sources 6.3 CT, Computed tomography; EqD, equivalent dose. Adapted from Bushong SC: Radiologic Science for Technologists: Physics, Biology, and Protection, ed 10, St. Louis, 2013, Mosby. 105 FIG 2.2 Percentage contribution of each natural and manmade source to the total collective effective dose for the population of the United States, 2006. (From National Council on Radiation Protection and Measurements [NCRP]: Ionizing radiation exposure of the population of the United States, Report No. 160, Bethesda, 2009, NCRP.) Natural Radiation. Natural sources of ionizing radiation have always been a part of the human environment. They are a consequence of our planet's geology and its location relative to the sun and our solar system's location in the galaxy. Ionizing radiation from planetary and extraplanetary sources is called natural background radiation and has the following three components: Terrestrial radiation from radioactive materials in the crust of the earth Cosmic radiation from the sun (solar) and beyond the solar system (galactic) Internal radiation from radioactive atoms, also known as radionuclides, which make up a small percentage of the body's tissue If radiation from any of these natural sources grows larger because of accidental or deliberate human actions such as mining radioactive 106 elements, the sources are termed enhanced natural sources. Terrestrial radiation. Long-lived radioactive elements such as uranium-238, radium-226, and thorium-232 (all emi ers of densely ionizing radiations) are present in variable quantities in the crust of the earth. These sources of ionizing radiation are classiﬁed as terrestrial radiation. The quantity of terrestrial radiation present in any area depends on the composition of the soil or rocks in that geographic region. The most recently available data show that 37% of natural background radiation exposure comes primarily from the gaseous radionuclide, radon, and to a much lesser degree from the radionuclide thoron* (see Fig. 2.2). Both these gases emit alpha radiation. Radon initially does not cling to or interact with the atoms of other particles. Because of this property, it is sometimes referred to as a noble gas. It behaves as a free agent that ﬂoats around in the soil. As a consequence, the natural ﬂow of air can draw radon gas into the lower levels of homes through cracks or holes in the foundation, and then the gas may permeate upward as it decays and becomes solid particles.3,4 Geologic formations or soils containing granite, shale, phosphate, and pitchblende produce higher concentrations of radon than other commonly encountered materials. Radon is by far the largest contributor to background radiation. The average US resident receives approximately 2.0 mSv per year from indoor and outdoor levels of radon. Radon is the ﬁrst decay product of radium, a metallic chemical element, and is produced as radium decays in soil. It is a colorless, odorless, invisible, heavy radioactive gas that, along with its own decay products, polonium-218 (218Po) and 214Po (solid form), is always present to some degree in the air. Radon has a half-life of 3.825 days.4 To summarize, in homes, radon may gain access through the following areas (Fig. 2.3): 107 FIG 2.3 Radon gas can percolate up through soil and enter a home through holes or cracks in its framework, crawl spaces under the living areas, floor drains, sump pumps, and porous cement block foundations. 1, Spaces behind brick veneer on top of block foundation; 2, pores and cracks in concrete block foundation; 3, open top of block foundation walls; 4, floor to wall joints; 5, cracks in concrete floor; 6, exposed soil as in basement sump; 7, weeping drain tile draining into open sump; 8, mortar joints; 9, loose-fitting pipe wall penetration; 10, well water from some wells; 11, building materials such as stone. (From US Environmental Protection Agency, Washington, DC.) Crawl spaces under the living areas Floor drains Sump pumps Porous cement block foundations In many cases, a pressure gradient exists between a house and the soil on which it rests so that the house draws on the ground like a vacuum cleaner. Commonly used building materials such as bricks, concrete, and gypsum wallboard contain radon. These construction materials are classiﬁed as earth-based materials.2 Radon concentrations in a particular structure vary across days and seasons. In the cooler months, when homes and buildings are tightly closed, radon levels are usually higher. This is the best time to perform tests for radon.* High indoor concentrations of radon and radon decay products, which are actually solid particles that have a ached themselves to dust, have the potential to cause serious health hazards for humans.3 After being inhaled, 108 these airborne radioactive gases and decay products produce daughter radioactive isotopes that remain for lengthy periods in the epithelial tissue of the lungs. As these secondary isotopes decay, they give oﬀ alpha radiation that will injure lung tissues, thereby increasing the risk for lung cancer. The severity of this risk depends on the concentration of the radon and the length of time to which the person is exposed to the gas and solid particles.5 Smokers exposed to high radon levels face a higher risk of lung cancer than do nonsmokers. One reason for this may be that smokers have already been exposed to higher concentrations of radioactivity from the lead-210 (210Pb) and polonium-210 (210Po) isotopes contained in tobacco and tobacco smoke. The Environmental Protection Agency (EPA) considers radon to be the second leading cause of lung cancer in the United States. Radon is responsible for approximately 20,000 cancer deaths per year. The EPA recommends that action be taken to reduce elevated levels of radon to a concentration less than 4 picocuries* per liter (pCi/L) of air (the number of radioactive emissions per second that occur on average in 1 L of air). A radon air activity density of 4 pCi/L results in a yearly EqD to the lung of approximately 0.05 mSv.6 A presence of radon below this level is considered statistically safe by the EPA. The EPA estimates that 1 in every 15 homes in the United States exceeds the recommended action limit of 4 pCi/L.7 Hence, accurate radon testing and appropriate structural repair, if required, are essential to reducing the risk of lung cancer from radon. In actuality, radiation exposure to radon cannot be entirely eliminated, but with suitable structural correction, it can be signiﬁcantly reduced. Cosmic radiation. Cosmic rays are of extraterrestrial origin and result from nuclear interactions that have taken place in the sun and other stars. The amount of cosmic rays varies with altitude relative to the earth's surface. The greatest intensity occurs at high altitudes where there is less a enuation due to the low atmospheric density, whereas the lowest intensity occurs at sea level. The great reduction at sea level happens because the cosmic rays that are not deﬂected by the earth's magnetic ﬁeld must traverse the entire thickness and steadily increasing density of the earth's atmosphere before reaching the surface. The average US inhabitant received an EqD of approximately 0.3 mSv per year from extraterrestrial radiation (see Table 2.3). Cosmic radiations consist predominantly (about 90%†) of high-energy protons that have an estimated mean energy of 300 MeV. As a result of interactions with molecules in the earth's atmosphere, these protons may be accompanied by alpha particles, atomic nuclei, mesons*, gamma rays, 109 and high-energy electrons as they approach the earth's surface. These other forms of radiation are collectively referred to as secondary cosmic radiation. The gamma rays among them can be energetic enough to penetrate several meters of lead. Terrestrial and internal radiation. The tissues of the human body contain many naturally existing radionuclides that have been ingested in minute quantities from various foods or inhaled as particles in the air. The types of ionizing radiation released by these radionuclides may include the following: Alpha particles (helium nuclei) Beta particles (electrons) Gamma rays (similar to x-rays, but usually of higher energy, in the range of a million electron volts [MeV]) Some types of radioactive decay also aﬀect the distribution of electrons around the atom and result in the emission of x-rays. Examples of radioactive nuclides that exist in small quantities in the human body are as follows: Potassium-40 (40K) Carbon-14 (14C) Hydrogen-3 (3H; tritium) Strontium-90 (90Sr) Radionuclides in the soil and air also add to the human radiation dose burden. The average member of the general population received approximately 0.7 mSv per year from combined exposure to radiations from the earth's surface (terrestrial) and radiation within the human body. Radon (2.00 mSv), cosmic ray radiations (0.3 mSv), terrestrial, and internally deposited radionuclides (0.7 mSv) that comprise the natural background radiation in the United States result in an estimated average annual individual EqD of approximately 3.0 mSv (see Table 2.3). Manmade (Artificial) Radiation. Ionizing radiation created by humans for various uses is classiﬁed as manmade, or artiﬁcial, radiation. Sources of artiﬁcial ionizing radiation include the following: Consumer products containing radioactive material Air travel 110 Nuclear fuel for generation of power Atmospheric fallout from nuclear weapons testing Nuclear power plant accidents Nuclear power plant accidents as a consequence of natural disasters Medical radiation Manmade radiation contributes about 3.3 mSv to the average annual radiation exposure of the US population. Of this EqD, 0.6 mSv resulted from medical radiographic procedures, 0.7 mSv resulted from nuclear medicine imaging, 1.5 mSv resulted from computed tomography (CT) scanning, 0.4 mSv resulted from interventional procedures, and 0.1 mSv resulted from other manmade radiation sources (see Table 2.3). These ﬁgures represent an “average share” of dose to members of the population that would be true if the total medical radiation dose were shared equally among all individuals in the population. A qualiﬁed medical physicist can calculate an individual's actual medical radiation exposure from x-ray examinations if he or she is provided with the essential technical details (e.g., x-ray tube voltage used, exposure time, tube current [mA], patient dimensions, etc.) pertaining to the studies. Consumer products containing radioactive material. Consumer products containing radioactive material include the following: Airport surveillance systems Electron microscopes Ionization-type smoke detector alarms Industrial static eliminators These products contribute a very small fraction of the total average EqD to each member of the general population. As a result of technologic advances since the 1970s and strict regulations imposed within the United States by the Food and Drug Administration regarding such devices, the radiation exposure of the general public from consumer products may now be considered negligible. Air travel. Commercial airline ﬂights bring many humans to higher elevations and therefore in closer contact with high-energy extraterrestrial radiation (e.g., cosmic radiation) and consequently increase their exposure. A ﬂight on a typical commercial airliner results in an EqD rate of 0.005 to 0.01 mSv/hour. 111 Sunspots sometimes play a role in increasing radiation exposure during air travel. Sunspots are dark spots that every so often appear on the surface of the sun. They indicate regions of increased electromagnetic ﬁeld activity and are occasionally responsible for ejecting particulate radiation into space. This radiation normally constitutes a small fraction of our dose from cosmic radiation here on Earth. However, the solar contribution to the cosmic ray background increases substantially during periods of high sunspot activity and will contribute nonnegligible added radiation dose to airplane passengers and crew. If a person spends 10 hours ﬂying aboard a commercial aircraft during a period of normal sunspot activity, that individual will receive a radiation EqD that is about equal to the dose received from one chest x-ray examination. During a solar ﬂare, “a tremendous explosion on the surface of the sun,”8 however, this dose can be 10 to as much as 100 times higher. Awareness of these potentially large increases in radiation exposure at high altitudes is important information for pilots and airline crews and the general public. An increase in radiation exposure carries an immeasurably small health risk for those individuals who travel by air infrequently. However, for pilots, ﬂight a endants, and the general public who are “frequent ﬂyers,” the possibility exists that they “may unknowingly be exposed to excessively large doses of radiation.”9 With adequate knowledge, a person choosing air travel during periods of high sunspot activity and solar ﬂares can make an intelligent decision about whether the potential beneﬁt of the air travel outweighs any increased health risk. A commercial ﬂight crew's (pilot, ﬂight a endants, etc.) actual radiation exposure sometimes exceeds that of workers at nuclear power plants. The Federal Aviation Administration and other organizations maintain ongoing programs of monitoring and evaluation of radiation risks to maintain the safety of occupational exposed airline workers.9 Nuclear fuel for the generation of power. Nuclear power plants that produce nuclear fuel for the generation of power do not contribute signiﬁcantly to the annual EqD of the US population during their normal operating cycles. The nuclear fuel cycle, along with other manmade radiations, contributes only a very small portion of 0.1 mSv to the total average annual EqD for persons living in the United States. Atmospheric fallout from nuclear weapons testing. 112 An accurate estimate of the total annual EqD from fallout cannot be made because actual radiation measurements do not exist. The dose commitment, a dose that may ultimately be delivered from a given intake of radionuclide,10 may be estimated by using a series of approximations and simplistic models that are subject to considerable speculation. The actual radiation dose to the global population from atmospheric fallout from nuclear weapons testing is not received all at once. It is instead delivered over a period of years at changing dose rates. The changes in the dose rates depend on factors such as characteristics of the fallout ﬁeld and the elapsed time since the test occurred. No atmospheric nuclear testing has occurred since 1980. When spread over the inhabitants of the United States, fallout from nuclear weapons tests (Fig. 2.4) and other environmental sources along with other manmade radiation contributes only a small portion of 0.1 mSv to the EqD of each person. This annual EqD is still considered to have a negligible impact on the US population. FIG 2.4 The United States performed aboveground nuclear weapons tests before 1963. During the Priscilla Test, this atomic cloud resulted when a 37-kiloton testing device exploded from a balloon at the Nevada test site on June 24, 1957. The atomic cloud top, which contained manmade ionizing radiation, ascended approximately 43,000 feet. (From US Department of Energy, Nevada Operations Office, Las Vegas, Nevada.) Nuclear power plant accidents. 113 Although nuclear power beneﬁts humans by creating a needed supply of electricity, unfortunate accidents involving nuclear reactors can occur. This can lead to substantial unplanned radiation exposure for humans and the environment. Examples of two nuclear power plant accidents are addressed in the discussions that follow. Three Mile Island Unit 2. On March 28, 1979, the Three Mile Island Unit 2 (TMI-2) pressurized water reactor, situated on an island in the Susquehanna River located about 15 miles southeast of Harrisburg, Pennsylvania (Fig. 2.5A), underwent a loss of coolant that resulted in severe overheating of the radioactive reactor core at a temperature greater than 5000°F Consequently, signiﬁcant melting of the core occurred. The US Department of Energy estimated that about 40% of the material in the TMI-2 nuclear reactor core reached a molten state. Approximately 15% of the melted uranium dioxide fuel of the core actually ﬂowed through the undamaged portions of the core and se led on the bo om of the reactor vessel. This melted material in the nuclear reactor core and bo om of the reactor vessel formed crusts on its outside surfaces and in time cooled to resolidiﬁed debris (Fig. 2.5B). Although signiﬁcant melting of the core and ﬂowing of the molten radioactive material into intact portions of the reactor vessel occurred, fortunately no “melt-through” of the reactor vessel resulted. The accident did, however, result in the destruction of the reactor. 114 FIG 2.5 (A) Nuclear power stations, such as the one located on Three Mile Island (TMI) near Harrisburg, Pennsylvania, house nuclear reactors. The large round containment buildings holding the reactors retain radioactive liquids and gases even in a high-pressure environment. (B) TMI-2 end-state core conditions illustrating the damage to the radioactive nuclear reactor core after the loss of coolant accident on March 28, 1979. Some of the original core mass formed an upper layer of debris. A hard crust supports this material. Zones of previously molten material and standing fuel rod segments account for some of the core mass lying beneath the upper debris bed. The lower reactor vessel head contains some of the melted core material. Closed-circuit television, mechanical probing, and core-boring operations contributed to assessing the TMI-2 end-state core conditions. (A, From Pennsylvania State University Engineering Library. B, US Department of Energy, Nevada Operations Office, Las Vegas, Nevada.) Even though the potential existed for the release of signiﬁcant amounts of radioactive material, according to the General Public Utilities Nuclear Corporation, the quantity of radiation that actually escaped during the accident, which was approximately 15 curies of iodine-131 [131I]* was, as dispersed, not suﬃcient to cause health problems for persons occupationally exposed or for the 2 million people living within 50 miles of the plant. The average dose received by the exposed population living within a 50-mile radius of the TMI nuclear power station was determined to be 0.08 mSv, which is well below the average annual background radiation level. According to conventional methods of risk assessment, if 0.08 mGy is used as an upper limit dose of ionizing radiation, it can be predicted that no more than one additional case of fatal cancer may occur in this population as a result of radiation exposure from this accident.11 Therefore 115 detection of excess cancer deaths in this population as a consequence of the radiation dose it received is not expected. Additional malignant deaths in the population exposed to radiation during the entire time of the TMI incident can be evaluated in another way. This is by applying the average dose received by the population as the “population dose” for persons living within a 100-mile radius of the nuclear power plant at the time of the accident. During this time, these residents received an average radiation exposure of 15 microgray.* If this dose is used as the population dose, then no more than two additional resulting cancer deaths can be predicted in the exposed inhabitants as a consequence of radiation exposure.2 Beginning at the time of the accident and continuing through 1992, the University of Pi sburgh followed more than 32,000 people who lived within 5 miles of TMI and were exposed to the low-level radioactivity released by the accident. Researchers found no link between radiation released (primarily xenon and iodine radioisotopes) during the TMI accident and cancer deaths among persons residing in the area. During the 13-year study of the people who lived within 5 miles of TMI at the time of the accident, only a single death occurred as a consequence of thyroid cancer, and this death was not a ributed to radiation exposure.11 Because most radiation-induced cancers have a latent period of 15 years or more, continued monitoring of the health of the exposed residents is needed. Studies are expected to continue to obtain the necessary data to evaluate the mortality experience further. Since the TMI-2 nuclear power plant accident, more than 35 years have passed. There has been no signiﬁcant increase in cancer-related deaths reported among the population living near the TMI nuclear power station. Psychological stress at the time of the accident and shortly thereafter has been identiﬁed as the only detectable eﬀect.2,12 The Nuclear Regulatory Commission (NRC) reported that the TMI-2 pressurized water reactor is “permanently shut down and all of its fuel has been removed. The reactor coolant system is fully drained, the radioactive water decontaminated and evaporated. The accident's radioactive waste was shipped oﬀ-site to an appropriate disposal area, and the reactor fuel and core debris was shipped to the Department of Energy's Idaho Laboratory.”13 Long-term monitored storage of TMI-2 is expected to continue “until the operating license for the TMI-1 plant expires at which time both plants will be decommissioned.”13 “In 2009 the TMI-1 operating license was renewed, extending its life by 20 years to 2034.”14 116 Chernobyl. On April 26, 1986, an explosion at a nuclear power plant in Chernobyl (Fig. 2.6), located near Kiev in the Ukraine in the former Soviet Union, resulted in the release of a number of radioactive nuclides, including 46 megacuries of 131I, 136 megacuries of xenon radioisotopes, and 2.3 megacuries of cesium-137 (137Cs)†. This is far more than 1 million times the amount of radioactive material released at TMI or “30 to 40 times as much radioactivity as the Hiroshima and Nagasaki atomic bombs combined in 1945.”15 More than 200 people working at the Chernobyl plant received a whole-body EqD exceeding 1 Sv. More than 2 dozen workers died as a result of explosion-related injuries and the eﬀects of receiving doses greater than 4 Sv. The average EqD to the approximately quarter of a million individuals living within 200 miles of the reactor was 0.2 Sv. In some individuals, thyroid doses resulting from drinking milk contaminated with radioactive iodine actually exceeded several sieverts. Adverse health eﬀects from radiation exposure are expected to occur for many years as a consequence of the total collective EqD received by the aﬀected population, and “the number of people who could eventually die as a result of the Chernobyl accident is highly controversial.”16 117 FIG 2.6 (A) Nuclear power plant in Chernobyl, former Soviet Union, site of the 1986 radiation accident. (B) Aerial view of the four identical units of the Chernobyl nuclear power plant before the accident. Graphics point out each of the reactors. (C) Chernobyl nuclear power plant after the explosion of unit 4 on April 26, 1986. (A, Ken Graham Photography. B and C, US Department of Energy, Nevada Operations Office, Las Vegas, Nevada.) The ETHOS project. Beginning in 1996, a 3-year pilot research project called the ETHOS Project16 was launched in the Republic of Belarus. This project was 118 supported by the radiation project research program of the European Commission (DG XII). In the aftermath of the Chernobyl accident, the local citizens of the contaminated territories were empowered to make their own decisions to facilitate reconstruction of their overall quality of life. They were given the authority to manage their radiologic risk in the same way that the rural communities manage natural risk.17,18 The aim of the ETHOS Project was to rebuild acceptable living conditions by actively involving the local population in the reconstruction process. This process encompassed dealing with the aspects of daily living that had been changed or threatened as a consequence of radioactive contamination. One example was the establishment of guidelines for the amount of ash that is allowed to build up in wood stoves and ﬁreplaces before cleaning is recommended. The ash is residue that remains after burning wood from trees that have taken up radioactive materials from the soil. The ash contains radioactive materials and is more compact than the piles of wood from which it came. The elimination of use of wood from the surrounding forests would have posed an unreasonable economic hardship on the population and was unnecessary as long as appropriate guidelines were set. Through this program, local citizens have been engaging in cooperative problem-solving as they reconstruct their environment. Thyroid cancer, leukemia, and breast cancer as a result of the Chernobyl event. Thyroid cancer continues to be the main adverse health eﬀect of the 1986 Chernobyl nuclear power accident. Children and adolescents living in the Ukraine region of Russia, where the dose was heaviest after the disaster, continue to be the focus of the disease. More than 1700 cases of thyroid cancer were diagnosed between 1990 and 1998.19 Most of these cases are a ributed to the radiation dose delivered when 131I was taken up by the thyroid gland, although previous studies of atomic bomb survivors and Paciﬁc Island inhabitants exposed to fallout predicted only 10 or so extra cases of thyroid cancer.20 “The 2005 report prepared by the Chernobyl Forum, led by the International Atomic Energy Agency (IAEA) and the World Health Organization (WHO),* a ributed 56 direct deaths (47 accident workers, and 9 children with thyroid cancer), and estimated that there may be 4000 extra deaths due to cancer among the approximately 600,000 most highly exposed and 5000 among the 6 million living nearby.” 22,23 119 Since the time of the Chernobyl accident, there has also been an increase in the incidence of breast cancer directly a ributed to the radiation exposure.24,25 The WHO Expert Group revealed that “reports indicate a small increase in the incidence of pre-menopausal breast cancer in the most contaminated areas, which appear to be related to radiation dose.”26 However, follow-up epidemiologic studies are still necessary to conﬁrm these ﬁndings. Some early research indicated no other increases in the eﬀects that are generally associated with radiation exposure (leukemia, congenital abnormalities, or adverse pregnancy outcomes).27 For example, the WHO found no increase in leukemia incidence by 1993 in the population hit hardest by fallout from Chernobyl.28 Later studies began to show some of the expected eﬀects. It was reported that there has been about a 50% increase in leukemia cases in children and adults in the Gomel region since the Chernobyl disaster.29,30 Also reported in June 2001 at the Third International Conference on Health Eﬀects of the Chernobyl accident held in Kiev, the Russian liquidators who worked during 1986 and 1987 at the Chernobyl power station complex had a statistically signiﬁcant rise in the number of leukemia cases.31,32 The WHO reported that “recent investigations suggest a doubling of the incidence of leukaemia* among the most highly exposed Chernobyl liquidators.”26 Furthermore, this organization also revealed that “no such increase has been clearly demonstrated among children or adults in any of the contaminated areas.”28 More time will be required before all the implications of these ﬁndings are clearly understood. During the 6 months after the Chernobyl nuclear power plant disaster, a large concrete shelter known as the “sarcophagus” (Fig. 2.7) was constructed by the Soviets atop the remains of the reactor 4 building so that the other reactors could continue operating to provide nuclear power. Unfortunately, within 10 years after the shelter's construction the walls weakened, leaving the sarcophagus in danger of collapsing. Radiation leaks from the entombed reactor building also became apparent and caused great concern in the scientiﬁc community. During 1998 and 1999, some major repair work was carried out on the massive structure to strengthen the roof and structural pillars and stabilize the ventilation stack. In spite of the eﬀorts made to enhance the strength and stability of the sarcophagus, the integrity of the structure remained questionable. Furthermore, lethal radiation levels inside the shelter both complicated and limited opportunities for repair and maintenance. 120 FIG 2.7 The large concrete “sarcophagus,” encasing the remains of Chernobyl reactor unit 4. The structure is in danger of collapsing. (© Clive Shirley/Signum/Greenpeace.) Plans were made to cover the remains of Chernobyl reactor unit 4 and the concrete sarcophagus that entombs it with a weatherproof, massive steel vault (Fig. 2.8). Construction of this structure began in April 2012, with the now estimated completion date expected to be at the end of 2017.33 The arch-shaped steel vault with a 100-year designed lifetime, referred to as the New Safe Conﬁnement structure, is being built on site and, when completed, will be moved in place on rails and then slid over the collapsing sarcophagus. After this task has been accomplished, the concrete sarcophagus will be dismantled. The new shelter will provide protection so that highly radioactive fuel and damaged reactor remains can be be er conﬁned to protect the environment and the population more eﬀectively. 121 FIG 2.8 Sketch of the New Safe Confinement structure that is being built to cover Chernobyl reactor unit 4 and the concrete sarcophagus that entombs it. Nuclear power plant accidents as a consequence of natural disasters. Accidents can occur in nuclear power plants as a consequence of natural disasters. This devastation can result in widespread environmental and health eﬀects on the aﬀected population of the surrounding area. Fukushima Daiichi Nuclear Plant crisis. On March 11, 2011, a 9.0-magnitude earthquake that began approximately 96.6 kilometers (60 miles) oﬀ the northeast coast of Japan triggered a tsunami that slammed into the island's coast and bombarded it with 914- cm (30-foot)-high waves that actually traveled as far as 9.66 kilometers (6 miles) inland and devastated everything in their path within minutes.34 The Tokyo Electric Power Company's Fukushima Daiichi Nuclear Plant, housing six reactors, is located in the town of Naraha 93 miles southwest of the epicenter. As a consequence of the earthquake, the entire Japanese coastline dropped as much as 90 centimeters (3 feet). This left the shutdown nuclear power plant, in which the reactor cores had been automatically taken oﬄine by sensors, much more vulnerable to the seismic waves of the tsunami that were racing toward it. Even though the plant had survived the earthquake, its 549-centimeter (18-foot) protection walls were not high enough to stop 914-centimeter (30-foot) waves from ﬂooding the diesel generators cooling the nuclear reactor cores. Eventually even the backup ba eries that kept the pumps going failed. With the 122 reactors in lockdown and no power being generated to operate the cooling pumps, a critical situation was created as temperatures continued to rise in the reactors. This led to a signiﬁcant overheating of the fuel rods that resulted in the production of hydrogen gas, which eventually exploded. In desperation, an a empt was made to cool the reactors by dumping seawater on them. Unfortunately, this procedure did not work. Destruction of some reactors and severe damage to others occurred, leading to the release of a considerable amount of radiation (e.g., 137Cs) in the atmosphere and surrounding area.34 Because it is extremely diﬃcult to measure the amounts of radiation people received, the long-term eﬀects such as an increased incidence of cancer in the exposed population cannot be accurately determined.35 However, since the time of the earthquake and following tsunami that resulted in the Fukushima nuclear plant disaster, the WHO has estimated that the lifetime risk for development of some cancers may be somewhat higher “above baseline rates in certain age and sex groups that were in areas with the highest estimated doses.”36 Increased health risks are not expected to be observed beyond the borders of Japan.36 Areas of Fukushima Prefecture that were less aﬀected by radiation exposure from the accident are also not expected to demonstrate an increase in cancer risk as a consequence of the combined accident.36 Sometime after the nuclear power plant accident, some hospitals in Japan noticed strange-looking “black spots” on digital images. These spots were a ributed to radioactive particulate fallout from the accident. However, the black spots on the digital images did not have any eﬀect on the health of people but did possibly increase the fear of radiation exposure among the general population. In actuality, the spots proved to be somewhat of an annoyance during radiologic interpretation of digital images.37 Medical radiation. As mentioned earlier in this chapter, NCRP Report No. 160 was released on March 3, 2009. Data from this report are presented in Table 2.4. The previous report, NCRP Report No. 93 published in 1987, used data on medical usage from 1980 to 1982. The number of medical procedures involving the use of ionizing radiation had increased dramatically since the 1980s. Because of this trend, exposure of the US population from medical sources has also increased signiﬁcantly. 123 TABLE 2.4 Medical Radiation Exposure: 2006 Number of Percentage Collective Dose Percentage Per Capita Modalities Procedures (%) (Person-Sv) (%) (mSv) Computed tomography 67 million 16 440,000 49 1.50 Nuclear medicine 18 million 4 231,000 26 0.80 Radiography and 324 million 76 99,000 11 0.30 ﬂuoroscopy Interventional 17 million 4 129,000 14 0.40 Total ~426 million 899,000 ~3.0 Medical radiation exposure results from the use of diagnostic x-ray machines and radiopharmaceuticals in nuclear medicine. Diagnostic x-ray radiation (which includes CT scanning, interventional ﬂuoroscopy, and conventional radiography or ﬂuoroscopy) and nuclear medicine procedures are the two largest sources of artiﬁcial radiation, and they collectively accounted for 48% of the total collective EfD of the US population as of 2006 (see Fig. 2.2). The main reason for the increase is the enormously expanded use of CT. With the advent of multislice spiral (helical) computed tomography the use of this imaging modality in areas such as emergency medicine increased dramatically. In 1980 the CT usage resulted in a collective dose of 3700 person-sieverts. By 2006 that number had risen to 440,000 person-sieverts (see Table 2.4).38 Of course, modern CT oﬀers tremendous medical beneﬁt with regard to the diagnosis of disease and trauma, and the risk-to-beneﬁt ratio is still very small when CT examinations are ordered for appropriate reasons. Medical radiation accounts for approximately 3.2 mSv of the average annual individual EfD of ionizing radiation received (see Table 2.3). The total average annual EfD from manmade and natural radiation, including radon, is not associated with any measurable level of harm.38 Although the amount of natural background radiation remains fairly constant from year to year at 3.0 mSv, the frequency of exposure to manmade radiation in medical applications continues to increase rapidly among all age groups in the United States for a number of reasons. Among the main instigators of this are medicolegal considerations. Physicians, to protect themselves from often frivolous malpractice lawsuits, in general rely more and more on unneeded expensive sophisticated technology to assist them in making diagnoses for patient care rather than much less expensive and much lower radiation dose procedures such as basic x-ray projections that may well be just as informative. To reduce the possibility of genetic damage in future generations, this increase in frequency of radiation exposure in medicine must be counterbalanced by controlling 124 the amount of patient exposure in individual imaging procedures. This can best be accomplished by limiting the widespread substitution of unnecessary CT scans and/or repetitive CT scans by many emergency departments for convenience in place of using alternative, less costly diagnostic procedures. In other areas such as interventional procedures, radiation doses to the public can be kept in check through eﬃcient application of radiation protection measures (e.g., pulsed not continuous operation, lesser mA, last image hold and therefore less radiation beam on time) on the part of the radiographer, radiologist, and physicians using ﬂuoroscopy. Because of the large variety of radiologic equipment and diﬀerences in imaging procedures and in individual radiologist and radiographer technical skills, the patient dose for each examination varies according to the facility providing imaging services. The amount of radiation actually received by a patient from a diagnostic x-ray procedure may be indicated in terms such as the following: 1. Entrance skin exposure (ESE), which includes skin and glandular dose 2. Bone marrow dose 3. Gonadal dose In pregnant women, fetal dose also may be estimated. Table 2.5 provides some examples of patient ESEs (skin and glandular), bone marrow, and gonadal doses, and Table 2.6 provides some representative fetal doses for several diﬀerent radiologic examinations. 125 TABLE 2.5 Representative Entrance Skin Exposures, Bone Marrow Dose, and Gonadal Dose From Various Diagnostic X-Ray Procedures Exposure Factors Entrance Skin Dose Bone Marrow Dose Gonad Dose Examination (kVp/mAs) (mGyt)* (mGyt) (mGyt) Skull 76/50 2.0 0.10

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