Electric Power For The People

Antoine Henri Becquerel (1852-1908) was clever. He had to be to win a Nobel Prize. How did he do it? He wrapped heavy sheets of opaque, black paper around light-sensitive photographic plates and placed them in opaque envelopes. He then placed the envelopes in a desk drawer with lumps of uranium salts sitting on the envelopes, or with metal coins or a metal Maltese cross sitting between the envelope and the uranium salts. For one trial he left the envelope undisturbed in the dark drawer for several days; in another, for 5 hours. After each trial he removed the photographic plates, developed them, and found them either fogged from the lumps of uranium, or with silhouettes of the coins or Maltese cross. He found that all uranium compounds fogged his light-sensitive plates. Obviously invisible rays emanating from the uranium salts (pitchblende) were affecting his plates. Becquerel had discovered natural radiation; spontaneous, penetrating, natural radioactivity. For that discovery, he was awarded half the Nobel Prize in Physics for 1903. The other half went to Pierre and Marie Curie for their work on Becquerel radiation [1].

But "natural" is the relevant term. Radiation is all around us, and has been for the millions of years of our evolution and life on earth. Indeed, our exist-

Our Precarious Habitat. . . It's In Your Hands, Fourth Edition. By Melvin A. Benarde Copyright © 2007 John Wiley & Sons, Inc.

ence has occurred in concert with the continuous and unrelenting cosmic radiation arriving from the sun and other energy sources in our galaxy. But its density is affected by the earth's magnetic field, which makes it greater nearer the poles than at the equator. The natural doses people receive increases with latitude and longitude.

The earth's crust contains components that are naturally radioactive. Uranium is dispersed throughout rocks, and soil, as are thorium and potassium- 40. They emit gamma rays, which irradiate our bodies. Building materials, bricks and cinderblock that originated in the earth, are radioactive so that we are irradiated both in and outdoors. Radon is a naturally occurring radiative gas that results from the decay of uranium-238, in rocks and soil. When radon, as radon-222, enters a building, its concentration increases. Since radioactive materials occur everywhere, it is inevitable that they will be present in our food and water. Potassium-40 is a major source of internal radiation. Foods such as shellfish and Brazil nuts concentrate radioactive particles so that those who consume fair quantities can receive a dose of radiation significantly above average. And our bones contain radioactive potassium, while our body tissues contain radioactive carbon. Radiation is a fact of life that cannot be eliminated or undone. We can only consider radiation as an integral part of our environment.

Because radiation can be precisely measured and controlled, a wide variety of artificial sources of radiation have been developed to improve our quality of life. So, for example, radiation is used in nuclear medicine to diagnose and treat disease, and in dentistry to locate and identify a range of dental defects. The most widely used radioactive substance in nuclear medicine is technetium-99, given to millions of people annually as "tracers," permitting radiologists to see how internal organs are functioning. The procedures are pain-free, and avoid the need for surgery. Radio tracers are also used in basic medical, chemical, and biological research. Food irradiation, as we have seen, keeps food safe by destroying harmful bacteria. Radiation is used in agriculture to protect crops from pests, and is used to ensure the structural integrity of planes, trains, bridges, and pipelines. And not to be overlooked are smoke detectors, TV sets, and nuclear and coal-fired power plants. Natural sources of radiation contribute far more radiation than do all others (Table 6.1). Natural radiation contributes 2.4 millisieverts (mSv; 1 Sv = 100 rems), or 86% of the total; artificial radiation, with 0.407 mSv, or 14.5%, is a meager contributor. Clearly, if there is concern about radiation, it must be with our natural background, which contributes 6 times more radiation than that contributed synthetically (by humans). If not for medical uses, man-made would approach nil. Nevertheless, we know that extremely high radiation doses can cause sickness and death. But we also know that large populations have been subjected to exposures from nuclear weapons testing, nuclear reactor accidents, and occupational exposures, as well as unusually high naturally occurring radiation. How have these people fared? We shall consider each.

Radiation is categorized according to the effects produced in tissues as ionizing and nonionizing. Ionizing includes cosmic rays, X rays, and radiation from

TABLE 6.1. Doses of Radiation from Natural and Artificial Sources

Source

Dose (mSv)

Natural

Cosmic

0.4

Gamma rays

0.5

Internal

0.3

Radon

1.2

2.4

Artificial

Medical

0.4

Atmospheric nuclear testing

0.005

Chernobyl

0.002

Nuclear Power

0.0002

0.4072

Total (round) mSv

2.8

radioactive substances. Atoms of different types are known as elements. Some of the 100+, such as uranium, radium, and thorium, are unstable; that is, their nuclei are overloaded with neutrons. The nucleus of each atom contains a specific number of protons and neutrons and as such is either stable or unstable. Unstable atoms that want to become stable and must shed neutrons to do so. In the process of shedding neutrons they emit invisible and highly energetic particles or rays. This emittance is known as ionization or ionizing radiation. In the process of ionizing, an atom can be stripped of an electron, which can alter the chemical composition of living tissue.

Three types of ionizing radiation are alpha and beta particles and gamma rays. Alpha particles are the most energetic and the most massive, some 7000 times that of the beta particle, but despite their energy, they can travel only a few inches in air, losing their energy as soon as they collide with any matter. Ergo, they have weak penetrating power. As shown in Figure 6.1, alpha particles can be stopped by a sheet of paper, or the outer layer of human skin. Beta particles are smaller, high-speed electrons ejected from the nucleus of radioactive atoms, which can penetrate water, or paper, but are stopped by aluminum foil, an inch of wood, and glass, but can penetrate the top layer of human skin. Heavy exposure to beta particles can cause skin burns, and can be hazardous if inhaled or ingested.

Gamma rays are electromagnetic waves emitted from the nucleus of some radioactive atoms, and have more energy and penetrating power than do alpha and beta particles, traveling at the speed of light. This combination of high energy and high penetrating ability makes gamma rays useful in cancer treatment as a means of killing harmful tumor cells. Gamma rays can be shielded by dense barriers of concrete, steel, or lead. Gamma rays and X rays

Gamma

Concrete

Concrete

Gamma

Figure 6.1. Types of ionizing radiation and their penetrating powers. The three main types of ionizing radiation are alpha (a), beta (p), and gamma (y) rays. Alpha particles are the most energetic but can travel only a few inches in the air. They lose their energy as soon as they collide with matter. Beta particles can pass through paper, but are stopped by wood, glass, or foil. Gamma rays travel at the speed of light and are highly penetrating. A wall of concrete, lead, or steel stops gamma rays. (Courtesy of the U.S. Department of Energy.)

are essentially similar. However, gamma rays are also most dangerous because of their ability to penetrate large thicknesses of matter.

The radioactivity of an unstable element decreases with time, ultimately becoming nonradioactive. This process of decay is referred to as half-life, and each radioactive element has its own unique fingerprint or half-life. The halflife is the time it takes for 50% of the element's activity to dissipate or decay away. This information allows us to know exactly how much of a radioactive material still remains. Table 6.2, lists a sampling of half-lives for a dozen radioactive elements, while Figure 6.2 diagrams the activity of two radioactive elements (polonium and radium) versus time. Take note that the time needed for the activity to decrease by a factor of one-half (0.5) is always the same. The figure shows that the time taken for activity to drop by 50% of this value is constant throughout the entire decay process. In one half-life, the activity of each sample decreases by a factor of 2. During the next half-life by a second factor or 2, to one-fourth its initial value. After each additional half-life the activity remaining would be |th, ^th, and 32 nd that of their original values. It takes about 7 half-lives for the original activity to fall below 1% of its initial value. So, for example, strontium-90, a radionuclide always found in fallout

TABLE 6.2. Half-Lives of Radioactive Elements

Isotope

Half-Life

Nuclide

Thorium-223

0.9 second

222fh 90 th

Nitrogen-16

8 second

176N

Fluorine-17

66 second

197Fl

Bromine-85

3 minues

35Br

Iodine-131

8 days

15331Ra

Sodium-22

2.6 years

22Na

Strontium-90

38Sr

Cesium-137

30 years

8525Cs

Radium-226

1620 years

"iRa

Carbon-14

5,630 years

14C

Plutonium-239

24,000 years

19445Pu

Uranium-238

4.5 x 106 years

19426U

O 140 280 420 560 Polonium

Days

1600 5200 4S00 6400 Radium

O 140 280 420 560 Polonium

Days

1600 5200 4S00 6400 Radium

Years

Figure 6.2. Radiation half-life decay rate. Continued activity indicates that nuclei not yet decayed.

from nuclear weapons testing, with a half-life of 30 years, will, after 6 half-lives, 180 years, still be around, albeit in small amounts— h th of its original level, but it will not be zero.

The cell is the basic unit of biological tissue, and its nucleus is its control center. About 80% of the cell consists of water; the remaining 20% consists of complex biological and chemical compounds. When ionizing radiation passes through cellular tissue, it produces charged water molecules, which break up into highly reactive free radicals (OH)—reactive oxygen species— that can disrupt proteins, damaging the large DNA molecules by destroying individual bases, particularly thymine, by breaking single- and double-stranded DNA. The chromosomes that carry DNA are also at risk. DNA damage can lead to cancer, birth defects, and death. However, cells have repair mechanisms that can reverse these damaging effects, allowing the body to tolerate low-dose radiation.

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