Nuclear Physics

An atom can be visualized as being composed of neutrons and protons within its nucleus, with electrons moving in stable orbits around that nucleus. Normally, chemical reactions involve interactions between orbiting electrons, with relatively small energy changes. However, the phenomenon of radioactivity involves changes in the nuclei of atoms in which a nucleus spontaneously emits a particle and changes into another type of nucleus. Other types of nuclear reactions are brought about by the bombardment of nuclei by particles such as neutrons, protons, alpha particles, or gamma rays. Compared to chemical reactions, nuclear reactions can involve very large amounts of energy.

The number of protons in the nucleus of an atom determines the element and its atomic number. Atoms containing the same number of protons but varying numbers of neutrons are called isotopes of that element. An element may have several isotopes. Examples are the two isotopes of uranium found in nature (uranium-235 and uranium-238), which each have 92 protons. However, uranium-235 contains 143 neutrons (92+143=235) while uranium-238 has 146 neutrons. The numbers 235 and 238 indicate the mass number of these isotopes, which is the sum of the protons and neutrons in the nucleus of each. At the other end of the atomic scale is hydrogen. Its most common state is one proton and no neutrons; however, another isotope of hydrogen called deuterium has one proton and one neutron and comprises about 0.015 percent of all hydrogen. The third isotope (called tritium) has one proton and two neutrons.

Many isotopes are stable, meaning they undergo no spontaneous nuclear changes.' Isotopes that are not stable may gain stability during radioactive decay by releasing nuclear energy, mostly in emitted particles and/or emitted electromagnetic radiation. Such unstable isotopes are called radionuclides, and the process is referred to as radioactivity."

Some nuclear emissions can cause ionization in the atoms they strike by causing orbital electrons to be separated from their atoms.' Essentially five types of ionizing nuclear emissions and one type of ionizing radiation from orbital electrons can be initiated by nuclear reactions. They are alpha particles, beta particles, protons, neutrons, and gamma and x-rays.

Alpha particles: These are nuclear emissions consisting of particles composed of two neutrons and two protons, which makes them essentially the nuclei of helium atoms. Alpha particles are the largest particles emitted during radioactive dacay. Compared to other types of radiation, alpha particles have great ability to ionize atoms, because they transfer more of their energy to each of the atoms they meet than do other types of emissions. Consequently, alpha particles are brought to rest rapidly, penetrating less than 0.013 centimeters in soft tissues; low energy alpha particles penetrate only 0.0008 centimeters. Because of the inability of alpha particles to penetrate deeply into tissue, alphaemitting radionuclides are generally hazardous to humans only if they decay while inside the body."

Beta particles: These are electrons emitted during radioactive decay of nuclides.10 High energy beta particles can penetrate 1.5 centimeters in soft tissue, and low energy beta particles can penetrate 0.025 centimeters. Because of these short ranges, beta-emitting nuclides are of concern mainly when they decay within the body. However, beta penetration can be deep enough to constitute some danger from decay of nuclides on the ground or on skin."

Protons: These are particles of positive electricity, with charges equal in magnitude to electrons, but with 1,836 times the mass of electrons. When emitted from nuclei, protons can ionize atoms."2 Protons are not emitted during the spontaneous radioactive decay of atoms, but rather they must be given energy through a nuclear collision with a high energy photon (such as a gamma ray) before they can be expelled from the nucleus.13

Neutrons: These are uncharged particles, normally constituting parts of atomic nuclei, each with a mass about equal to one proton. Because they have no charge, they do not transfer energy to atoms by electrical attraction and replusion as do alpha, beta, or proton emissions, Thus, there is little impediment to neutron passage through atoms, and they are deeply penetrating. Neutrons can react with atomic nuclei, causing them to emit gamma rays or protons that in turn ionize other atoms, but most damage from neutrons occurs during collisions with atoms that recoil from the momentum, become charged, and then penetrate tissue as directly ionizing particles." However, neutrons are rarely encountered in nature, because very few radioactive materials spontaneously emit them.16

Gamma rays and X-rays: These are essentially identical forms of electromagnetic radiation differing only in their energy levels and in their places of origin within the structure of the atom." Gamma rays are emitted from the nucleus during nuclear changes, and X-rays radiate from the orbital electrons as the electrons change energy levels. Energy from a nuclear reaction can be transferred to an orbital electron, which in turn can emit X-rays.18 Gamma and x-rays are very penetrating. In general, the greater the energy of the ray the greater the penetration into tissue. The energy of gamma rays varies over a wide range, so penetration also varies widely. However, gamma radiation that strikes a human can always be considered to carry at least some of its initial energy entirely through the body.19

The travel distance of emitted particles depends upon their energy, usually measured in millions of electron volts (Mev), and upon the medium through which they are traveling.20 Distances vary greatly by particle. In air, for example, the approximate path lengths are: three quarters of a centimeter per Mev for an alpha particle, and 2.5 meters per Mev for a beta particle." The energy emitted by radionuclides varies from 0.019 Mev for tritium emissions to 10.54 Mev for alpha emissions from thorium.22

There is no simple relationship between energy and travel distance. For example, the neutron, which has the same atomic mass as a proton, travels a greater distance than does a proton, because the neutron has no charge.23 Travel paths are reduced when particles travel in denser media. 24

The decay time, called half-life, is the time required for one half the atoms in a sample of a radioactive element to decay.25 Radionuclides with short half-lives decay quickly to stability. For example, tritium whose half-life is 12.26 years, decays to less than onethousandth of its original number in ten half-lives (or 122.6 years). Emission rates and decay rates of radionuclides vary considerably; it takes 1.28 billion years for half of a quantity of potassium-40 nuclides to decay to the stable isotope potassium-39; the same number of oxygen-19 nuclides would take 29 seconds to decay to the stable nuclide oxygen-18.26

Some elements decay to another unstable radionuclide rather than to a stable isotope. An example is the complicated uranium-238 decay series, shown in Figure A-1, in which uranium decays 14 times before it finally comes to rest as the stable lead-206 isotope.

A common measure of radioactivity is the curie, defined as the quantity of any radioactive nuclide which undergoes 3.7 x 10 10 disintegrations per sec

ond." Thus, a curie measures only the number of disintegrations, not the kind of radiation emitted, nor the energy emitted. For example, potassium-40, the main contributor to natural radioactivity in seawater, (see Table IV-2 of this report) emits a beta particle of relatively high maximum energy (1.35 Mev) and a gamma ray of high energy (1.46 Mev). In contrast, the radionuclide tritium, a hydrogen isotope that contributed most of the radioactivity to the sea from the nuclear weapons tests (see Table II-2 of this report) emits only a weak beta particle (0.019 Mev). 28 Thus, a curie of potassium-40 represents a far greater potential for physiological damage than does a curie of tritium.

There are three sources of ionizing radiation on Earth. Cosmic radiation consists of both the charged particles arriving from outer space and the secondary particles generated from the interaction of the original particles with the atmosphere.29 Other radiation results from the decay of radionuclides that are naturally present in the Earth's components. These two sources together are called natural radiation. The third source of ionizing radiation results from the activities of human beings, including nuclear weaponry, nuclear power, medical procedures, and consumer products.30

Radiation from nuclear fission and the resulting radionuclides is by far the largest source of radiation hazard." So far nuclear fission has been the source of energy for all nuclear power plants. Of all the isotopes known, only uranium-233, uranium-235, and plutonium-239 are fissionable.32 Of these, only uranium235 is found in nature; the other two are anthropogenically produced in nuclear reactors. When an atom of one of these three isotopes is struck by a neutron, the neutron may be absorbed into the nucleus, causing the fissile isotope to break into two roughly equal parts, releasing energy and usually two or three neutrons. The energy at first is in the form of kinetic energy of the fission fragments,34 but this is converted into heat as the fragments slow down. A typical fission reaction is:35

235U + n = 95Mo + 139La + 2n + 205 Mev energy 95 42 57

(In other words: uranium-235 absorbs a neutron and yields the elements molybdenum and lanthanum, plus two neutrons and 205 million electron volts of energy.)

Fissile atoms can be caused to undergo fission by a single neutron, fast or slow, with kinetic energy that may be no greater than a small fraction of an electron volt.

If one of the neutrons released during fission can be used to induce another atom to fission, the process can be maintained. The amount of fissile material required to sustain a chain reaction is called the criti

cal mass. This amount depends on the concentration of the fissile material and its surroundings, which may or may not be reflective of neutrons. Devices used to control the rate of the fission process after critical mass is reached are called nuclear reactors. If two subcritical masses are combined very quickly so that an uncontrolled chain reaction occurs, the violent explosion of an atomic bomb occurs.37

In nuclear reactors, the neutrons given off during fission must be slowed down because at first they are too energetic and the probability of causing fission is much greater for slow neutrons. The fissile material is therefore surrounded by a light material such as hydrogen or carbon with which neutrons collide and thereby lose energy. The rate of fission in a reactor is controlled by control rods, made of materials such as boron or cadmium, which have extremely large probabilities of absorbing neutrons. The rods are gradually pulled out of the reactor to allow fission to proceed, and adjusted to produce the desired rate of fission.38

Health Physics

Ionizing radiation can injure humans by causing changes in the chemical reactivity of cellular components. Molecules can be damaged so they cannot function normally, or the products of molecular disintegration can tend to clog and poison the cell. If only a relatively few atoms in cell are ionized, it may recover from the damage without difficulty. But if a relatively large number of ionizations occur, the cell may be unable to carry on its activities and die. Injury to chromosomes can occur when cells are irradiated, and it has been proved by experiments with insects, mammals, and plants that X-rays and the nuclear radiations (alpha, beta, gamma, neutron) cause mutations. There is every reason to believe that cosmic rays have similar genetic effects.39

The degree of injury caused by radiation depends in part on the type of particle or ray,40 and the amount of energy in the emission." The relative localization or dispersion of the effects of ionizing radiation affects the body's ability to repair damage; the massive local damage done by non-penetrating particles is generally more harmful than is the damage caused by rays that spread the same amount of energy through larger volumes of the body. Alpha particles cause damage that is essentially non-repairable.42 The degree of damage also depends on the type of tissue irradiated, with rapidly dividing cells being especially sensitive."

The basic unit of radiation dose is the rad. One rad equals 100 ergs of energy deposited per gram of absorption material." The basic health radiation unit is the rem, which is an abbreviation of "rad equivalent in man." 45 A rem is that dose of ionizing radiation, measured in rads, which produces in humans a biological

effect equivalent to that produced by one rad of X-rays or gamma rays." For example, for equal amounts of energy transferred to tissue, neutrons cause 10 times the damage of X-rays, and aipna particles cause 20 times the damage of X-rays. Therefore, one rad of neutrons represents 10 rems, and one rad of alpha radiation represents 20 rems. X-rays, gamma rays, and beta particles all cause about equal damage per unit energy, so for these radiations one rad equals one rem."

Organisms living on Earth receive ionizing radiation from a variety of sources, and the amount of natural radioactivity to which humans are subjected is varied. The intensity of cosmic radiation depends on altitude due to absorption of the rays by the atmosphere, and on latitude, because the Earth's magnetic field deflects cosmic rays away from the equator. Inhabited areas of the Earth receive cosmic radiation varying from 35 millirems per year to 300 millirems per year. Seattle receives about 50 millirems and Denver about 90 millirems per year from cosmic radiation."

Natural radiation from the radionuclides in the Earth's crust also is highly variable from one region to another. In general, natural radionuclides are concentrated in granite rocks. Limestone and sandstones are low in radioactivity, but certain shales are very radioactive, especially those containing organic matter. The average dose at a height of about one meter above limestone is about 20 millirems per year, while for granite areas the corresponding figure is 150 millirems per year."

In some places, natural radioactivity is much higher than average. In India, a population of over 100,000 people live in an area that gives them an average dose of 1,300 millirems per year. In the Northern Nile Delta, people in several villages receive doses of 300 to 400 millirems per year. About seven million people in France live in areas where the rocks are principally granite, which exposes them to doses of 180 to 350 millirems per year.90

So far it has not been possible to establish any connection between the level of background radiation and differences in biological disorders. Differences in other health-related factors between various areas of the world make conclusions about the effects of background radioactivity difficult; however, no differences have so far been detected in genetic anomalies or the incidence of cancer between various peoples of the world who live in areas where natural background radiation levels differ by a factor of 10. This gives some justification for thinking that small amounts of anthropogenic radiation are unlikely to cause detectable harm to a human population."

In the last century, human exposure to natural radiation has increased due to such technological developments as air travel and the use of naturally radioactive goods-phosphate fertilizers, natural gas, coal, and oil. Additional exposure occurs from radiationemitting consumer products, medical uses of radiation, the nuclear fuel cycle, and nuclear explosions."2 The total exposure to US citizens each year is summarized in Table A-1.

What constitutes a safe level of radiation for humans? The potential effects of ionizing radiation have been a concern to scientists for several decades. The International Commission on Radiological Protection (ICRP), formed in 1928, and the National Council on Radiation Protection and Management (NCRP), a US. organization formed in 1929 as the Advisory Committee on X-Ray and Radiation Protection, are the oldest scientific organizations with responsibility for the health effects of the radiation. Since their beginnings, the accepted "safe level" of radiation dose has steadily decreased from 0.1 roentgen/day in 1934 to 15 rem/year in 1950, and to 5 rem/year today." (See Table A-2.) Recommendations by research groups to advisory bodies have become more conservative as knowledge of radiation effects and the desire to avoid those effects have increased. Radiation protection guidelines have become dependent upon public value judgments and a concept that some risk exists at all levels of exposure."4

The present system of ICRP dose limits, summarized in Table A-2, incorporates objectives which aim to ensure that: 1) no practice shall be adopted unless its introduction causes a positive net benefit (that is the combined effects of the costs, risks, and benefits of procedures utilizing radiation must be more favorable than those of alternative procedures that do not use radiation); 2) all exposures shall be kept "as low as reasonably achievable," with economic and social factors being taken into account, (known as the ALARA principle for as low as reasonably achievable); and 3) the dose equivalent to individuals shall not exceed the limits recommended by the ICRP."

In setting a tolerance level of 5 rem per year, the ICRP has not defined an acceptable level of risk, but rather taken the position that this level is the maximum allowable and should be rarely approached and never exceeded. Together with the ALARA principle, the 5 rem/year limit results in individuals working around radioactive materials receiving only about onetenth of that level, or 500 mrem per year. Furthermore, the ICRP limits result in the general public receiving no more than 50 mrem per year beyond natural doses.