Nuclear fuel consists of a fissionable isotope, such as uranium-235, which must be present in sufficient quantity to provide a self-sustaining chain reaction. The above reaction can be abbreviated as . Two nuclei must collide for fusion to occur. While operating at low power during an unauthorized experiment with some of its safety devices shut off, one of the reactors at the plant became unstable. First artificial transmutation was done by Lord Rutherford in 1911. Usually, a radioactive element has a rate of decaying: half-life. Otherwise, the concentration of these fission products would increase and absorb more neutrons until the reactor could no longer operate. When fissionable material is in small pieces, the proportion of neutrons that escape through the relatively large surface area is great, and a chain reaction does not take place. The mass of a hydrogen atom ($_1^1\text{H}$) is 1.007825 amu; that of a tritium atom ($_1^3\text{H}$) is 3.01605 amu; and that of an α particle is 4.00150 amu. But as history has shown, failures of systems and safeguards can cause catastrophic accidents, including chemical explosions and nuclear meltdowns (damage to the reactor core from overheating). Since then, hundreds of different isotopes have been observed among the products of fissionable substances. A nuclear moderator is a substance that slows the neutrons to a speed that is low enough to cause fission. Nuclear fuel consists of a fissionable isotope, such as uranium-235, which must be present in sufficient quantity to provide a self-sustaining chain reaction. Both fusion and fission are nuclear reactions. Cite the function of each and explain why both are necessary. This decomposition is called fission, the breaking of a large nucleus into smaller pieces. Since then, fission has been observed in many other isotopes, including most actinide isotopes that have an odd number of neutrons. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. In the gaseous diffusion enrichment plant where U-235 fuel is prepared, UF6 (uranium hexafluoride) gas at low pressure moves through barriers that have holes just barely large enough for UF6 to pass through. The elements beyond element 92 (uranium) are called transuranium elements. Describe how the potential energy of uranium is converted into electrical energy in a nuclear power plant. The ultimate fate of the nuclear reactor as a significant source of energy in the United States probably rests on whether or not a politically and scientifically satisfactory technique for processing and storing the components of spent fuel rods can be developed. The principal source of energy in the sun is a net fusion reaction in which four hydrogen nuclei fuse and produce one helium nucleus and two positrons. Download for free at http://cnx.org/contents/85abf193-2bd...a7ac8df6@9.110). Consequently, hydrogen gas and radioactive gases (primarily krypton and xenon) were vented from the building. The principal source of energy in the sun is a net fusion reaction in which four hydrogen nuclei fuse and produce one helium nucleus and two positrons. It is possible to produce new atoms by bombarding other atoms with nuclei or high-speed particles. At the time of this writing, there are no self-sustaining fusion reactors operating in the world, although small-scale controlled fusion reactions have been run for very brief periods. The energy produced by a reactor fueled with enriched uranium results from the fission of uranium as well as from the fission of plutonium produced as the reactor operates. The basis for this process, Graham’s law, is described in the chapter on gases. For example, uranium-238 transmutes spontaneously into lead-206 through a series of steps. The temperature of the core climbed to at least 2200 °C, and the upper portion of the core began to melt. Textbook content produced by OpenStax College is licensed under a Creative Commons Attribution License 4.0 license. This occurs either through nuclear reactions (in which an outside particle reacts with a nucleus), or through radioactive decay (where no outside particle is needed). Two overlapping coolant loops are often used; this counteracts the transfer of radioactivity from the reactor to the primary coolant loop. A fissionable isotope must be present in large enough quantities to sustain a controlled chain reaction. It can be done in an artificial manner and also occurs naturally. High temperatures are required to give the nuclei enough kinetic energy to overcome the very strong repulsion resulting from their positive charges. Boron-10, for example, absorbs neutrons by a reaction that produces lithium-7 and alpha particles: When control rod assemblies are inserted into the fuel element in the reactor core, they absorb a larger fraction of the slow neutrons, thereby slowing the rate of the fission reaction and decreasing the power produced. A transmutation entails a change in the structure of atomic nuclei and hence may be induced by a nuclear reaction (q.v. In addition, an operating reactor is thermally very hot, and high pressures result from the circulation of water or another coolant through it. Similar fission reactions have been observed with other uranium isotopes, as well as with a variety of other isotopes such as those of plutonium. Within a week, cooling water circulation was restored and the core began to cool. A tremendous amount of energy is produced by the fission of heavy elements. The unstable nuclei and the transuranium isotopes give the spent fuel a dangerously high level of radioactivity. Useful fusion reactions require very high temperatures for their initiation—about 15,000,000 K or more. Some of the neutrons that are released during U-235 decay combine with U-238 nuclei to form uranium-239; this undergoes β decay to form neptunium-239, which in turn undergoes β decay to form plutonium-239 as illustrated in the preceding three equations. Other coolants include molten sodium, lead, a lead-bismuth mixture, or molten salts. Any nuclear reactor that produces power via the fission of uranium or plutonium by bombardment with neutrons must have at least five components: nuclear fuel consisting of fissionable material, a nuclear moderator, reactor coolant, control rods, and a shield and containment system. It has been determined that the nuclei of the heavy isotopes of hydrogen, a deuteron, $$^2_1$$ and a triton, $$^3_1$$, undergo fusion at extremely high temperatures (thermonuclear fusion). Chain reactions of fissionable materials can be controlled and sustained without an explosion in a nuclear reactor (Figure 7). In nuclear engineering, also nuclear reactors cause artificial transmutation by exposing elements to neutrons produced by fission. Neptunium-239 is also radioactive, with a half-life of 2.36 days, and it decays into plutonium-239. Heavier isotopes of plutonium—Pu-240, Pu-241, and Pu-242—are also produced when lighter plutonium nuclei capture neutrons. ). Prior to 1940, the heaviest-known element was uranium, whose atomic number is 92. If this occurs, we have a nuclear chain reaction (see Figure 4). A number of large projects are working to attain one of the biggest goals in science: getting hydrogen fuel to ignite and produce more energy than the amount supplied to achieve the extremely high temperatures and pressures that are required for fusion. $$\ce{^{206}_{82}Pb + ^{54}_{24}Cr ⟶ ^{257}_{106}Sg + 3 ^1_0n}$$, $$\ce{^{249}_{98}Cf + ^{18}_8O ⟶ ^{263}_{106}Sg + 4 ^1_0n}$$. They must first be slowed to be absorbed by the fuel and produce additional nuclear reactions. Modern nuclear reactors may contain as many as 10 million fuel pellets. For instance, when one mole of U-235 undergoes fission, the products weigh about 0.2 grams less than the reactants; this “lost” mass is converted into a very large amount of energy, about 1.8 × 1010 kJ per mole of U-235. The long-lived isotopes require thousands of years to decay to a safe level. At the time of this writing, there are no self-sustaining fusion reactors operating in the world, although small-scale controlled fusion reactions have been run for very brief periods. In a nuclear reactor used for the production of electricity, the energy released by fission reactions is trapped as thermal energy and used to boil water and produce steam. A control system. Stoichiometry of Chemical Reactions, 4.1 Writing and Balancing Chemical Equations, Chapter 6. Material that can sustain a nuclear fission chain reaction is said to be fissile or fissionable. The half-life is the amount of time it takes for a given isotope to lose half of its radioactivity. We will discuss these components in greater detail later in the section. The two general kinds of nuclear reactions are nuclear decay reactions and nuclear transmutation reactions. An atomic bomb (Figure 6) contains several pounds of fissionable material, $_{92}^{235}\text{U}$ or $_{94}^{239}\text{Pu}$, a source of neutrons, and an explosive device for compressing it quickly into a small volume. Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element. These neutrons may then cause the fission of other uranium-235 atoms, which in turn provide more neutrons that can cause fission of even more nuclei, and so on. An atomic bomb (Figure $$\PageIndex{6}$$) contains several pounds of fissionable material, $$\ce{^{235}_{92}U}$$ or $$\ce{^{239}_{94}Pu}$$, a source of neutrons, and an explosive device for compressing it quickly into a small volume. Early reactors used high-purity graphite as a moderator. Although zero discharge of radioactive material is desirable, the discharge of radioactive krypton and xenon, such as occurred at the Three Mile Island plant, is among the most tolerable. Natural transmutation by stellar nucleosynthesisin the past created most of the heavier chemical elements in the known Each fuel assembly consists of fuel rods that contain many thimble-sized, ceramic-encased, enriched uranium (usually UO2) fuel pellets. Even when shut down, the decay products are radioactive. Among the products of Meitner, Hahn, and Strassman’s fission reaction were barium, krypton, lanthanum, and cerium, all of which have nuclei that are more stable than uranium-235. Modern reactors in the US exclusively use heavy water ($_1^2\text{H}_2\text{O}$) or light water (ordinary H2O), whereas some reactors in other countries use other materials, such as carbon dioxide, beryllium, or graphite. Nuclear fission becomes self-sustaining when the number of neutrons produced by fission equals or exceeds the number of neutrons absorbed by splitting nuclei plus the number that escape into the surroundings. It can occur by the radioactive decay of a nucleus, or the reaction of a nucleus with another particle. In all accelerators, the particles move in a vacuum to avoid collisions with gas molecules. This process is repeated through hundreds of barriers, gradually increasing the concentration of 235UF6 to the level needed by the nuclear reactor. The first manmade nucleus was produced in Ernest Rutherford’s laboratory in 1919 by a transmutation reaction, the bombardment of one type of nuclei with other nuclei or with neutrons. All of the fission products absorb neutrons, and after a period of several months to a few years, depending on the reactor, the fission products must be removed by changing the fuel rods. $_7^{14}\text{N}\;+\;_2^4\text{He}{\longrightarrow}_8^{17}\text{O}\;+\;_1^1\text{H}$, $\begin{array}{r @{{}\longrightarrow{}} l} _{92}^{238}\text{U}\;+\;_0^1\text{n} & _{92}^{239}\text{U} \\[4em] _{92}^{239}\text{U} & _{93}^{239}\text{Np}\;+\;_{-1}^0\text{e}t_{1/2}\;\;\;\;\;\;\text{half-life} = 23.5\;\text{min} \\[1.125em] _{93}^{239}\text{Np} & _{94}^{239}\text{Pu}\;+\;_{-1}^0\text{e}t_{1/2}\;\;\;\;\;\;\text{half-life} = 2.36\;\text{days} \end{array}$, $\begin{array}{rcccl} & {\beta}^- & & {\beta}^- & \\ _{92}^{238}\text{U}\;+\;_0^1\text{n} {\longrightarrow} _{92}^{239}\text{U} & {\longrightarrow} & _{93}^{239}\text{Np} & {\longrightarrow} & _{94}^{239}\text{Pu} \end{array}$, $_5^{10}\text{B}\;+\;_0^1\text{n}{\longrightarrow}_3^7\text{Li}\;+\;_2^4\text{He}$, $\text{Zr(}s\text{)}\;+\;2\text{H}_2\text{O(}g\text{)}{\longrightarrow}\text{ZrO}_2\text{(}s\text{)}\;+\;2\text{H}_2\text{(}g\text{)}$, $4_1^1\text{H}{\longrightarrow}_2^4\text{He}\;+\;2_{+1}^0\text{e}$, $_1^2\text{H}\;+\;_1^3\text{H}{\longrightarrow}_2^4\text{He}\;+\;2_0^1\text{n}$, Figure 2 in Chapter 21.1 Nuclear Structure and Stability, http://glossary.periodni.com/glossary.php?en=control+rod, Creative Commons Attribution 4.0 International License, $_{94}^{239}\text{Pu}\;+\;_0^1\text{n}{\longrightarrow}_{95}^{240}\text{Am}\;+\;_{-1}^0\text{e}$, $_{94}^{239}\text{Pu}\;+\;_2^4\text{He}{\longrightarrow}_{96}^{242}\text{Cm}\;+\;_0^1\text{n}$, $_{96}^{242}\text{Cm}\;+\;_2^4\text{He}{\longrightarrow}_{97}^{243}\text{Bk}\;+\;2_0^1\text{n}$, $_{92}^{238}\text{U}\;+\;15_0^1\text{n}{\longrightarrow}_{99}^{253}Es\;+\;7_{-1}^0\text{e}$, $_{99}^{253}\text{Es}\;+\;_2^4\text{He}{\longrightarrow}_{101}^{256}\text{Md}\;+\;_0^1\text{n}$, $_{96}^{246}\text{Cm}\;+\;_6^{12}\text{C}{\longrightarrow}_{102}^{254}\text{No}\;+\;4_0^1\text{n}$, $_{98}^{249}\text{Cf}\;+\;_6^{12}\text{C}{\longrightarrow}_{104}^{257}\text{Rf}\;+\;4_0^1\text{n}$, $_{82}^{206}\text{Pb}\;+\;_{24}^{54}\text{Cr}{\longrightarrow}_{106}^{257}\text{Sg}\;+\;3_0^1\text{n} \\[1.25em] _{98}^{249}\text{Cf}\;+\;_8^{18}\text{O}{\longrightarrow}_{106}^{263}\text{Sg}\;+\;4_0^1\text{n}$, $_{83}^{209}\text{Bi}\;+\;_{26}^{58}\text{Fe}{\longrightarrow}_{109}^{266}\text{Mt}\;+\;_0^1\text{n}$, Describe the synthesis of transuranium nuclides, Explain nuclear fission and fusion processes, Relate the concepts of critical mass and nuclear chain reactions, Summarize basic requirements for nuclear fission and fusion reactors, The reactor vessel, a steel shell that is 3–20-centimeters thick and, with the moderator, absorbs much of the radiation produced by the reactor, A main shield of 1–3 meters of high-density concrete, A personnel shield of lighter materials that protects operators from γ rays and X-rays. An amount of material in which there is an increasing rate of fission is known as a supercritical mass. Thus, a reactor must withstand high temperatures and pressures, and must protect operating personnel from the radiation. The plant was closed for nearly 10 years during the cleanup process. a year ago. Because no solid materials are stable at such high temperatures, mechanical devices cannot contain the plasma in which fusion reactions occur. Because the reactor was not enclosed in a containment building, a large amount of radioactive material spewed out, and additional fission products were released, as the graphite (carbon) moderator of the core ignited and burned. If a radioisotope has a half-life of 14 days, half of its atoms will have decayed within 14 days. Modern reactors in the US exclusively use heavy water $$\ce{( ^2_1H2O)}$$ or light water (ordinary H2O), whereas some reactors in other countries use other materials, such as carbon dioxide, beryllium, or graphite. Any nuclear reactor that produces power via the fission of uranium or plutonium by bombardment with neutrons must have at least five components: nuclear fuel consisting of fissionable material, a nuclear moderator, reactor coolant, control rods, and a shield and containment system. It is possible to summarize these equations as: Heavier isotopes of plutonium—Pu-240, Pu-241, and Pu-242—are also produced when lighter plutonium nuclei capture neutrons. The function of this component is to protect workers from radiation produced by the nuclear reactions and to withstand the high pressures resulting from high-temperature reactions. Therefore, these nuclei tend to emit particles in order to become stable, and this process is named as the radioactive decay. The amount of a fissionable material that will support a self-sustaining chain reaction is a critical mass. Useful power is obtained if the fission process is carried out in a nuclear reactor. 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