Lesson 4 ReadingRadiation Exposures: Sources, Doses, and ProtectionAs we’ve discussed throughout this unit, ionizing radiation is highenergy radiation that has the ability to knock electrons out of atoms and molecules, creating charged particles called ions. Ionizing radiation can damage the molecules that compose the cells that make up human tissue and organs. Therefore, radiation can cause potential health problems. Some important forms of ionizing radiation are cosmic radiation, alpha and beta particles and gamma rays emitted by radionuclides, and the X-rays used for medical purposes.
In this lesson, we will focus on the pathways of exposure to ionizing radiation. We will also learn about protecting ourselves and society from receiving too much high-level radiation, especially from manmade sources such as spent nuclear fuel and other highly radioactive materials. Pathways of ExposureAs we’ve discussed earlier, radiation is all around us. In fact, it’s even in us. It’s literally impossible to avoid radiation. Some major pathways of exposure are shown in the figure “Radiation Exposure Pathways.” Some of the pathways illustrated by the figure include external exposure from cosmic radiation, inhalation of radon and its decay products in indoor air, external exposure from radionuclides in building materials, and ingestion of radionuclides in the food. Cosmic radiation reaches people directly. It also interacts with molecules in the atmosphere to produce radioactive carbon-14 and other radionuclides, which become part of the entire food chain on Earth. Everywhere, especially indoors, air contains radon that comes up naturally from rocks and soil underground. When trapped inside a building, the radon becomes more concentrated. Structural radiation refers to radiation from building materials, like bricks or granite, that naturally contain radioactive materials. Airborne or dissolved pollutants can reach people through the food chain (ingestion) or through direct inhalation. Sources and Nature of Doses
Radiation is radiation. In terms of its effects, there is no difference between natural and manmade radiation. It’s the dose and time period of exposure that count, not the source. In the United States, a person’s average radiation dose, from all sources, is about 360 millirem per year. This is called “background” radiation, some natural, some manmade. (For an explanation of millirem as a measurement of dose, see “Biological Effects of Ionizing Radiation,” Lesson 3 of this unit.) The illustration you see on this page shows the average American’s annual background radiation dose, by millirem and by percentage of total dose. Natural sources of ionizing radiation make up about 81 to 82 percent of the average annual radiation dose to the U.S. public. Of that amount, 11 percent comes from our own bodies. Of the remaining 18 percent, about 15 percent comes from medical procedures such as X-rays and nuclear medicine (imaging, diagnosis, and treatment with radionuclides). We receive about 3 percent of our annual dose from consumer products such as smoke detectors, television sets, and computer monitors. And an extremely small amount, less than 1 percent, comes from all other manmade sources, including nuclear power. People who smoke receive a much higher percentage of their personal annual average dose from consumer products than people who don’t smoke. As discussed in Lesson 3 of this unit, a cigarette smoker who smokes a pack and a half a day receives an extra 16,000 millirem per year to the lungs’ bronchial epithelia. Because of variations in natural background radiation, a person’s background radiation dose varies across the U.S, from about 200 millirem per year in Florida to 1700 millirem per year in northeastern Washington state. The higher dose in that part of Washington results from the fact that portions of the Spokane River valley have the highest radon levels in the country. Florida’s lower-than-average natural background dose is partly because the closer a person is to sea level, the lower the dose from cosmic radiation.
Ionizing Radiation from Spent Nuclear Fuel and High-Level Radioactive WasteAs we learned in Unit 1, to make electricity, nuclear reactors use fuel made of solid ceramic pellets of “enriched” uranium sealed in strong metal tubes. Most of the uranium left in our planet is uranium-238. Although radioactive, it does not fission easily. (To fission a radioactive atom is to split its nucleus by bombarding it with neutrons, thus releasing radiation and heat.) Uranium-235, on the other hand, can be made to fission much more readily than uranium-238 — if we can get enough of it to make the process practical.
Nuclear reactors require enough uranium-235 to sustain a controlled nuclear chain reaction (or criticality) to create the heat that turns water into steam. The steam then turns turbines that make the electricity. To consolidate enough fissionable uranium-235, special mechanical and chemical processes are used on the mined uranium ore, which contains mostly uranium-238. These processes concentrate the uranium-235 relative to uranium-238. In its natural state, uranium is a relatively soft metal and wouldn’t hold up well to the heat and pressure it experiences in a reactor. Therefore, it is turned into extremely strong, heat-resistant ceramic pellets. The complex process of increasing the amount of uranium-235 is called enriching the uranium. The tubes (or “fuel rods”) used to contain the enriched uranium pellets are made of a heat- and corrosion-resistant alloy called zircalloy, and are bundled together to form a nuclear fuel assembly. The uranium pellets are about the size of the tip of an adult’s little finger (from the last knuckle to the end of the finger). Although small, the pellets release a tremendous amount of energy when used in a nuclear reactor. For example, one pellet produces an amount of energy equivalent to burning almost one ton of coal. Radiation from Spent Nuclear FuelWith current technology, commercial nuclear fuel loses its efficiency after about three years inside the reactor. At that point it is considered “spent.” This really just means “used,” because there’s still a lot of energy left in the uranium pellets. Enough of the fissonable uranium-235 has been used up, however, that the fuel must be replaced with “fresh” nuclear fuel that has never been used in a reactor. A spent fuel assembly from the reactor of a nuclear power plant actually is much more radioactive than when it went into the reactor as fresh fuel. This is because of the many radioactive decay chains and their radioactive decay products that are produced during the fission process inside the reactor. Plutonium, for example, didn’t even exist in the nuclear fuel before it was placed in the reactor and bombarded with neutrons. Plutonium is a transuranic radionuclide. This means its atomic number is higher than that of the heaviest natural element, uranium. You can remember what “transuranic” means by remembering that transuranic radionuclides have an atomic number (i.e., the number of protons in their nuclei) that “transcends” the atomic number of uranium, which is 92. Plutonium has an atomic number of 94 and some time ago existed naturally in the Earth’s crust. At this point in the history of our planet, it’s essentially nonexistent in nature — because of Earth’s age and the half-life of plutonium. Plutonium only comes into existence during the fissioning of uranium atoms, which human beings have discovered how to do. Plutonium, once we’ve produced it, can be used to make an atomic bomb, or it can be used as a fuel itself in a nuclear reactor generating electricity. A number of other radionuclides also are produced during the fissioning of uranium-235. Because they themselves are radioactive, they decay into various radioactive decay products, and the many decay chains, or series, continue until all the various radionuclides decay to a stable nuclide. Counting all the radionuclides that occur in the natural decay chains, along with all the “artificial” radionuclides brought about by fissioning uranium atoms, scientists now know of the existence of several thousand different radionuclides. All of these brought forth from 92 natural elements! Because of the many nuclear transformations that occur in a nuclear reactor, the spent nuclear fuel will contain hundreds of thousands of curies of radioactivity. Spent nuclear fuel is much more radioactive than fresh nuclear fuel — at least until enough of its radionuclides decay to stability.
Protection from Radiation: Shielding, Time, Distance, IsolationThere are four key ways of protecting ourselves from too much radiation: shielding, time, distance, and isolation.
Protection Techniques and Spent Nuclear Fuel
When the radionuclides from spent nuclear fuel undergo radioactive decay, they release both alpha and beta particles and gamma rays. While the metal of the fuel rods making up the fuel assembly will stop the alpha and beta particles, it will not stop gamma rays. Therefore, we store spent fuel assemblies in deep pools of water at the power plant sites to protect workers from the ionizing gamma rays coming from the fuel assemblies. Remember, enough water, like enough concrete or lead, will reduce exposure from gamma rays (or X-rays). Once the spent nuclear fuel assemblies are under enough water, most of the gamma rays can’t reach people. And the alpha and beta particles are contained inside the sealed fuel rods. When handling a spent fuel assembly outside the pool, workers use both remote control machinery and shielding. Shielding is material that will block the gamma rays. When gamma rays are not a problem, but alpha and beta particles might be present in amounts greater than normal background, workers wear special protective suits. These suits and ventilation equipment prevent the workers from inhaling or ingesting radioactive particles. Shielding is one method of protecting workers and other people from sources of radiation. Radiation workers also know that the farther away they are from the source of radiation, the lower their potential dose. Reducing the amount of time they are exposed will also result in a lower dose. Radiation workers not only use shielding, distance, and time to protect themselves, but they also wear radiation monitors to measure the amount of radiation dose they receive during their work periods. Federal regulations set the maximum dose a nuclear worker can receive from work during any given year at 5,000 millirem (5 rem). If a female worker is, or becomes, pregnant, the limit is much lower. As Low As Reasonably Achievable (ALARA)The U.S. Department of Energy and its contractors, as well as the U.S. Navy and the nuclear industry, have a principle of keeping doses to nuclear workers even lower — As Low As Reasonably Achievable (ALARA). ALARA is a natioanl as well as an international principle. Application of the ALARA principle in practice brings actual worker doses very far below the maximum limit. The average worker received 160 millirem in 2003, for example, much less than some airline crews who fly long distance routes. Storing Commercial Spent Nuclear Fuel
When spent fuel is first removed from a reactor, it is placed in a special deep pool of water contained in a steel-lined concrete basin. The water cools the spent fuel and protects workers and the public from radiation. After it has cooled considerably, some commercial power plants move their spent fuel from the reactor pools to dry-storage containers made of steel and/or concrete to shield radiation. The containers are either placed upright on concrete pads, or stored horizontally in metal canisters in concrete bunkers. The concrete shields the radiation. During the first three months of storage, spent fuel loses approximately 50 percent of its radiation. After a year, it will have lost 80 percent, and in 10 years, it will have lost 90 percent. However, spent fuel contains some materials that emit radiation for many thousands of years, remaining potentially dangerous for an extremely long time. Because of these different rates of decay and the resultant continuing potential for danger, extreme caution must be exercised in the handling and storage of spent fuel. The longer the half-life of the radionuclide, the less its radioactivity. Yet even some very longlived radionuclides emit potentially dangerous amounts of radiation. Therefore, even after ten years, when 90 percent of the radiation is gone, workers dealing with spent nuclear fuel must use shielding and remote control equipment to handle a spent fuel assembly. To protect people and the environment, when spent fuel is transported offsite, workers enclose the spent fuel assemblies in a heavy shielded cask. If the U.S. Department of Energy receives a license to construct and, later, to operate an underground repository at Yucca Mountain, Nevada, much of the nation’s spent nuclear fuel will be disposed of in tunnels about 1,000 feet beneath the mountain’s surface. At the disposal facility, the spent fuel assemblies would be sealed inside specially designed disposal containers and moved into deep underground tunnels. Much like concrete, lead, or water, the large amount of volcanic rock between the spent fuel and the surface of the mountain would serve as shielding so that none of the gamma radiation from the fuel would be able to penetrate to the outside. At the same time, the alpha and beta particles would be contained for many thousands of years by natural and engineered features. The longer the fuel is kept isolated, the lower the potential exposure to the public becomes.
Government Spent Nuclear Fuel and High-Level Radioactive WasteIn addition to commercial spent nuclear fuel, an underground repository, if licensed and built, would also be used to contain and isolate government-owned spent nuclear fuel and highly radioactive materials from U.S. military programs. All nuclear reactors produce spent fuel. In the United States there are currently reactors at commercial power plants, at government and university research facilities, and on about 40 percent of the U.S. Navy’s submarines and ships. The federal government owns the spent fuel from government nuclear reactors as well as spent fuel from university research reactors. The government also owns the spent fuel from the Navy’s nuclear-powered ships and submarines. The Nuclear Waste Policy Act of 1982, as amended, requires that when an underground repository is built and operational, the U.S. Department of Energy will begin taking legal ownership of spent nuclear fuel from commercial nuclear power plants, as well. In addition, the U.S. government has loaned nuclear fuel to certain friendly foreign nations so they can do their own nuclear research. Those foreign governments are obligated to return the fuel to the United States for disposal. The United States requires ultimate control over the fuel it loans to other countries in order to prevent the plutonium in the spent fuel from being used to make nuclear weapons.
Government Spent Nuclear Fuel and High-Level Radioactive WasteIn addition to commercial spent nuclear fuel, an underground repository, if licensed and built, would also be used to contain and isolate government-owned spent nuclear fuel and highly radioactive materials from U.S. military programs. All nuclear reactors produce spent fuel. In the United States there are currently reactors at commercial power plants, at government and university research facilities, and on about 40 percent of the U.S. Navy’s submarines and ships. The federal government owns the spent fuel from government nuclear reactors as well as spent fuel from university research reactors. The government also owns the spent fuel from the Navy’s nuclear-powered ships and submarines. The Nuclear Waste Policy Act of 1982, as amended, requires that when an underground repository is built and operational, the U.S. Department of Energy will begin taking legal ownership of spent nuclear fuel from commercial nuclear power plants, as well. In addition, the U.S. government has loaned nuclear fuel to certain friendly foreign nations so they can do their own nuclear research. Those foreign governments are obligated to return the fuel to the United States for disposal. The United States requires ultimate control over the fuel it loans to other countries in order to prevent the plutonium in the spent fuel from being used to make nuclear weapons. Government High-Level Radioactive WasteUntil the late 1970s, the United States acquired materials for nuclear weapons by reprocessing spent nuclear fuel from government-owned nuclear reactors. Reprocessing is a method of chemically treating spent fuel to separate out uranium and plutonium. In addition to extracting plutonium useable in nuclear weapons, reprocessing produces a byproduct in the form of a highly radioactive sludge-like residue. Millions of gallons of this high-level radioactive waste are currently stored in tanks at U.S. Department of Energy sites in South Carolina, Idaho, and Washington State. In some cases, the waste has leaked into the nearby ground and even gotten into the groundwater. Environmental cleanup and decommissioning of the former weapons-production sites will require permanent disposal of all these materials. In the United States, all high-level radioactive waste is currently stored at government facilities. The government requires that all nuclear waste must be in solid form before it can be transported to another location. Therefore, the Department of Energy plans to solidify the material by a special process called vitrification, which turns the sludge into solid glass logs inside stainless steel containers. (Vitrification comes from the Latin word vitrum, meaning glass.) The containers are sealed shut and can then be shipped in specially shielded transportation casks to an underground geologic repository. Similar Radiation ConcernsMuch like spent nuclear fuel, high-level radioactive waste contains highly radioactive elements, such as plutonium, cesium, strontium, technetium, and neptunium. Some of these elements will remain radioactive for a few years, while others will be radioactive for millions of years. The shorter a radionuclide’s half-life, the more intense its radioactivity, but even radionuclides with extremely long half-lives can be highly radioactive. That is why scientists worldwide agree that the safest way to manage these materials is to dispose of them deep underground in a geologic repository. A Long-Term Environmental ProblemNuclear waste must be properly managed to minimize risk to the environment and to the health and safety of future generations. Since the mid-1940s, spent nuclear fuel and high-level radioactive waste have accumulated throughout the country. Currently, they are stored in temporary facilities at more than 120 sites in 39 states. These storage sites are located in a mixture of urban, suburban, and rural environments — most are located near important bodies of water. In the United States today, over 161 million people reside within 75 miles of temporarily stored nuclear waste. Current storage methods protect the public from harmful radiation and are considered safe for the near-term. However, modern aboveground storage structures are designed for temporary storage only. They will not withstand rain, wind, and other environmental factors for the tens of thousands of years during which the waste will be hazardous. Long-Term Protection: Permanent Disposal OptionsAfter analyzing many options, most scientists agree that disposal in an underground repository is the best long-term solution for safely managing highly radioactive wastes. This opinion is reflected in a 1990 report from the National Research Council of the National Academy of Sciences, which states that there is “a worldwide scientific consensus that deep geological disposal, the approach being followed by the United States, is the best option for disposing of highly radioactive waste.” In 2001, the National Research Council again reaffirmed that conclusion: “Geologic disposal remains the only scientifically and technically credible long-term solution available to meet the need for safety without reliance on active management.” * As long as nuclear waste remains in a solid form and is properly shielded, it will not harm people or contaminate the environment — and because of radioactive decay, over time it produces less and less radiation. The idea behind deep underground disposal, therefore, is to keep the waste as isolated as possible, for as long as possible, so that its radiation can diminish to safer and safer levels. Isolated in a deep underground repository, the waste would not be subject to many environmental factors that on the Earth’s surface would cause it to break down more rapidly into radioactive particles that could be dispersed by air or water into the accessible environment. Benefits, Risks, ResponsibilitiesRadiation is everywhere. We literally cannot avoid it. We receive it from the air, soil, and water in our environment. We are all exposed to cosmic radiation from outer space. Because all food contains some radioactivity, it’s even in our own bodies. We also receive radiation from such consumer products as television sets, computer monitors, smoke detectors, and fertilizers. Because scientists have learned so much about radionuclides and radiation in just over 100 years, modern industrialized society derives many benefits from it. For example, doctors and other health professionals use radiation to diagnose and treat more and more illnesses, including cancer. Another benefit for the United States, as an industrialized country, is that 20 percent of the nation’s electricity comes from nuclear power plants. Residents of some states get nearly 50 percent of their electricity from nuclear power. Because electric utilities buy and sell electricity to each other, nearly everyone in the country uses some nuclear power. However, too much exposure to high levels of ionizing radiation, particularly in a short time, can have negative health effects. So we have devised ways of limiting our exposures. People who work in nuclear facilities use radiation shielding for protection; they wear special badges that record how much radiation they receive at work; and their maximum exposure to radiation at work is strictly regulated. Spent nuclear fuel and high-level radioactive waste present a challenge society must address. But we also know that shielding, exposure time, and distance away from the source of radiation can limit the amount of dose over time, which is what matters most in protecting human health from the negative effects of radiation. The better the shielding, the farther away you are, and the shorter the exposure time, the lower the dose will be and the lower the chance that you will experience molecular and cellular damage. If the dose isn’t too high in too short a time, your body will usually just repair any damaged cells. The most effective protection of all is complete isolation of the radiation source. The U.S. Department of Energy bases its plans for a proposed repository at Yucca Mountain on all this knowledge about ionizing radiation, plus several other key facts about the radioactive materials that would go into it. Among these facts are the following:
* Board on Radioactive Waste Management, Rethinking High-Level Radioactive Waste Disposal, National Academy Press: Washington, D.C., 1990, p. vii. And: Disposition of High- Level Waste and Spent Nuclear Fuel: Continuing Societal and Technical Challenges, National Academy Press: Washington, D.C., 2001, p. 3.
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The “other” category of less than 1 percent of the average American’s exposure amounts to 3 millirem per year. This includes 0.1 millirem per year dose from the transportation of all radioactive materials, shipments to and from nuclear facilities, including universities and hospitals. Shipments of nuclear waste are included in this one-tenth of a millrem.
