Opening the book…
Introduction to Radioactivity
In section 2.7 you learned about elemental isotopes and that atoms are defined by the number of protons held within the nucleus. Atomic theory in the nineteenth century presumed that nuclei had fixed compositions. But in 1896, the French scientist Henri Becquerel found that a uranium compound placed near a photographic plate made an image on the plate, even if the compound was wrapped in black cloth. He reasoned that the uranium compound was emitting some kind of radiation that passed through the cloth to expose the photographic plate. Further investigations showed that the radiation was a combination of particles and electromagnetic rays, with its ultimate source being the atomic nucleus. These emanations were ultimately called, collectively, radioactivity.
There are three main forms of radioactive emissions. The first is called an alpha particle, which is symbolized by the Greek letter α. An alpha particle is composed of two protons and two neutrons and is the same as a helium nucleus. (We often use 24He to represent an alpha particle.) It has a 2+ charge. When a radioactive atom emits an alpha particle, the original atom’s atomic number decreases by two (because of the loss of two protons), and its mass number decreases by four (because of the loss of four nuclear particles). We can represent the emission of an alpha particle with a chemical equation—for example, the alpha-particle emission of uranium-235 is as follows:
\[ \Large\boldsymbol{ {}^{235}_{92}\mathrm{U} \quad \rightarrow \quad {}^{4}_{2}\mathrm{He} \quad + \quad {}^{231}_{90}\mathrm{Th} } \]
Rather than calling this equation a chemical equation, we call it a nuclear equation to emphasize that the change occurs in an atomic nucleus. How do we know that a product of this reaction is Thorium-231? We use the law of conservation of matter, which says that matter cannot be created or destroyed. This means we must have the same number of protons and neutrons on both sides of the nuclear equation. If our uranium nucleus loses 2 protons, there are 90 protons remaining, identifying the element as thorium. Moreover, if we lose four nuclear particles of the original 235, there are 231 remaining. Thus we use subtraction to identify the isotope of the Th atom—in this case, Thorium-231.
Chemists often use the names parent isotope and daughter isotope to represent the original atom and the product other than the alpha particle. In the previous example, Uranium-235 is the parent isotope and Thorium-231 is the daughter isotope. When one element changes into another in this manner, it undergoes radioactive decay.
The second major type of radioactive emission is called a beta particle, symbolized by the Greek letter β. A beta particle is an electron ejected from the nucleus (not from the shells of electrons about the nucleus) and has a -1 charge. We can also represent a beta particle as -10e. The net effect of beta particle emission on a nucleus is that a neutron is converted to a proton. The overall mass number stays the same, but because the number of protons increases by one, the atomic number goes up by one. Carbon-14 decays by emitting a beta particle:
\[ \Large\boldsymbol{ {}^{14}_{6}\mathrm{C} \quad \rightarrow \quad {}^{0}_{-1}\mathrm{e} \quad + \quad {}^{14}_{7}\mathrm{N} } \]
Again, the sum of the atomic numbers is the same on both sides of the equation, as is the sum of the mass numbers. (Note that the electron is assigned an “atomic number” of –1, equal to its charge.)
The third major type of radioactive emission is not a particle but rather a very energetic form of electromagnetic radiation called gamma rays, symbolized by the Greek letter γ. Gamma rays themselves do not carry an overall electrical charge, but they may knock electrons out of atoms in a sample of matter and make it electrically charged (for which gamma rays are termed ionizing radiation). For example, in the radioactive decay of radon-222, both alpha and gamma radiation are emitted, with the latter having an energy of 8.2 × 10−14 J per nucleus decayed:
\[ \Large\boldsymbol{ {}^{222}_{86}\mathrm{Rn} \quad \rightarrow \quad {}^{4}_{2}\mathrm{He} \quad + \quad {}^{218}_{84}\mathrm{Po} \quad + \quad {}^{0}_{0}\gamma } \]
This may not seem like much energy, but if 1 mol of Rn atoms were to decay, the gamma ray energy would be 4.9 × 107 kJ! That is enough energy to power an average American home for an entire year.
| Characteristic | Alpha Particles | Beta Particles | Gamma Rays |
|---|---|---|---|
| Symbols |  |
 |
 |
| Identity | helium nucleus | electron | electromagnetic radiation |
| Charge | +2 | -1 | none |
| Mass Number | 4 | 0 | 0 |
| Penetrating Power | minimal (will not penetrate skin) | short (will penetrate skin and some tissues slightly) | deep (will penetrate tissues deeply) |
Nuclear Fission Reactions and Energy
Occasionally, an atomic nucleus breaks apart into smaller pieces in a radioactive process called spontaneous fission. Typically, the daughter isotopes produced by fission are a varied mix of products, rather than a specific isotope as with alpha and beta particle emission. Often, fission produces excess neutrons that will sometimes be captured by other nuclei, possibly inducing additional radioactive events. Uranium-235 undergoes spontaneous fission to a small extent. One typical reaction is
\[ \Large\boldsymbol{ {}^{235}_{92}\mathrm{U} \quad \rightarrow \quad {}^{139}_{56}\mathrm{Ba} \quad + \quad {}^{94}_{36}\mathrm{Kr} \quad + \quad 2\,{}^{1}_{0}\mathrm{n} } \]
where 01n is a neutron. As with any nuclear process, the sums of the atomic numbers and mass numbers must be the same on both sides of the equation. Spontaneous fission is found only in large nuclei. The smallest nucleus that exhibits spontaneous fission is lead-208. Fission is the radioactive process used in nuclear power plants and one type of nuclear bomb.
When a critical mass of a fissile isotope is achieved, the resulting flux of neutrons can lead to a self-sustaining reaction. Natural uranium is a mixture of isotopes, and includes a small proportion (0.720%) of the fissile isotope, U-235. When the nucleus of a U-235 atom is split by a neutron, it discharges two or three additional neutrons and releases energy in the form of heat (Fig 2.21). The additional neutrons released can react with other U-235 particles and form a sustaining chain reaction. To sustain such a reaction, a critical amount (3-5%) of U-235 is required in the source material. Thus, naturally occurring uranium cannot be used for nuclear fission. The uranium must first be enriched for the U-235 isotope through a complicated and difficult process. Compared to energy grade uranium which is enriched to 3-5% of U-235, bomb grade uranium is highly enriched (>90% U-235).
Figure \(\PageIndex{1}\): Nuclear Fission Reaction of U-235. The addition of a neutron to U-235 results in the formation of an unstable U-236 atom that splits into Kr-92 and Ba-141 and releases 3 neutrons. This figure was created by Fastfission
A variety of techniques can be used to control the flow of neutrons from such a reaction, which allows nuclear fission reactions to be maintained at safe levels. With regards to energy production, if the neutron flow in a reactor is carefully regulated so that only enough heat is released to boil water, then the resulting steam can be used to produce electricity. Thus a nuclear reactor is similar in many respects to the conventional power plants, which burn coal or natural gas to generate electricity; the only difference is the source of the heat that converts water to steam. Many levels of control are required, along with a fail-safe design, because otherwise the chain reaction can accelerate so rapidly that it releases enough heat to melt or vaporize the fuel and the container, a situation that can release enough radiation to contaminate the surrounding area. Light-water reactors are the most common type of fission reactor, and are used extensively to produce electricity in countries such as Japan, Israel, South Korea, Taiwan, and France—countries that lack large reserves of fossil fuels. The essential components of a light-water reactor are depicted in Figure 2.22. Approximately 11% of the world’s energy is generated by nuclear power, with more than a dozen countries obtaining more than 30% of their energy from nuclear sources.
Figure \(\PageIndex{2}\): A Light-Water Nuclear Fission Reactor for the Production of Electric Power. The fuel rods are made of a corrosion-resistant alloy that encases the partially enriched uranium fuel; controlled fission of 235U in the fuel produces heat. Water surrounds the fuel rods and moderates the kinetic energy of the neutrons, slowing them to increase the probability that they will induce fission. Control rods that contain elements such as boron, cadmium, or hafnium, which are very effective at absorbing neutrons, are used to control the rate of the fission reaction. A heat exchanger is used to boil water in a secondary cooling system, creating steam to drive the turbine and produce electricity. The large hyperbolic cooling tower, which is the most visible portion of the facility, condenses the steam in the secondary cooling circuit; it is often located at some distance from the actual reactor.
All existing nuclear power plants have similar components, although different designs use different fuels and operating conditions. Fuel rods containing a fissile isotope in a structurally stabilized form (such as uranium oxide pellets encased in a corrosion-resistant zirconium alloy) are suspended in a cooling bath that transfers the heat generated by the fission reaction to a secondary cooling system. The heat is used to generate steam for the production of electricity. In addition, control rods are used to absorb neutrons and thereby control the rate of the nuclear chain reaction. Control rods are made of a substance that efficiently absorbs neutrons, such as boron, cadmium, or, in nuclear submarines, hafnium. Pulling the control rods out increases the neutron flux, allowing the reactor to generate more heat, whereas inserting the rods completely stops the reaction, a process called “scramming the reactor.”
All nuclear reactors require a powerful cooling system to absorb the heat generated in the reactor core and create steam that is used to drive a turbine that generates electricity. In 1979, an accident occurred when the main water pumps used for cooling at the nuclear power plant at Three Mile Island in Pennsylvania stopped running, which prevented the steam generators from removing heat. Eventually, the zirconium casing of the fuel rods ruptured, resulting in a meltdown of about half of the reactor core. Although there was no loss of life and only a small release of radioactivity, the accident produced sweeping changes in nuclear power plant operations. The US Nuclear Regulatory Commission tightened its oversight to improve safety.
The main disadvantage of nuclear fission reactors is that the spent fuel, which contains too little of the fissile isotope for power generation, is much more radioactive than the unused fuel due to the presence of many daughter nuclei with shorter half-lives than 235U. The decay of these daughter isotopes generates so much heat that the spent fuel rods must be stored in water for as long as 5 yr before they can be handled. Even then, the radiation levels are so high that the rods must be stored for many, many more years to allow the daughter isotopes to decay to nonhazardous levels. How to store these spent fuel rods for hundreds of years is a pressing issue that has not yet been successfully resolved. As a result, some people are convinced that nuclear power is not a viable option for providing our future energy needs, although a number of other countries continue to rely on nuclear reactors for a large fraction of their energy.
Uncontrolled nuclear fission reactions are relatively rare, but they have occurred at least 18 times in the past. One of the most recent occurred on April 25–26, 1986, at the Chernobyl nuclear power plant in the former Union of Soviet Socialist Republics (USSR; now in the Ukraine). During testing of the reactor’s turbine generator, a series of mechanical and operational failures caused a chain reaction that quickly went out of control, destroying the reactor core and igniting a fire that destroyed much of the facility and released a large amount of radioactivity. Thirty people were killed immediately, and the high levels of radiation in a 20 mile radius forced nearly 350,000 people to be resettled or evacuated. In addition, the accident caused a disruption to the Soviet economy that is estimated to have cost almost $13 billion. It is somewhat surprising, however, that the long-term health effects on the 600,000 people affected by the accident appear to be much less severe than originally anticipated. Initially, it was predicted that the accident would result in tens of thousands of premature deaths, but an exhaustive study almost 20 yr after the event suggests that 4000 people will die prematurely from radiation exposure due to the accident. Although significant, in fact it represents only about a 3% increase in the cancer rate among the 600,000 people most affected, of whom about a quarter would be expected to eventually die of cancer even if the accident had not occurred.
The most recent event resulted from the damaged Fukushima Dai-ichi plant after the March 11, 2011, earthquake and tsunami that devastated Japan. The plant used fresh water for cooling nuclear fuel rods to maintain controlled, sustainable nuclear fission. When the water supply was disrupted, so much heat was generated that a partial meltdown occurred. Radioactive iodine levels in contaminated seawater from the plant were over 4300 times the regulated safety limit. To put this in perspective, drinking one liter of fresh water with this level of contamination is the equivalent to receiving double the annual dose of radiation that is typical for a person. Dismantling the plant and decontaminating the site is estimated to require 30 years at a cost of approximately $12 billion.
Suggested Assignment:
Watch the Frontline Episode: Nuclear Aftershocks (Found here: Frontline Episode [www.pbs.org]Following the video, write a 500 word essay about whether or not you feel that nuclear energy should be used in the United States at the Indian Point Nuclear Power Plant. Be sure to include pros and cons within your argument.
This page titled 2.10: Focus on the Environment was last modified on Tue, 11 Aug 2026 18:40:40 GMT and is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Hadeel Abozenadah, Adam Bishop, Scott Bitner, Kaysha Lehman and Patricia Flatt.
The book could not be loaded just now.