XVIII. Nuclear Chemistry Nuclear Reactions involve energies that are millions of times greater than those found in ordinary chemical reactions. Energies of these magnitudes often are the result of the conversion of mass into energy. Fission Fission is the process in which a neutron strikes the nucleus of an atom of a heavy element like uranium and is absorbed. This absorbed neutron makes the heavy atom nucleus so unstable that it splits apart into two lighter weight atoms. The fission products are new elements different than the original uranium. They are also radioactive. Several neutrons and a great deal of heat are released during the process. The additional neutrons can go on to split other heavy atoms making more neutrons and so on. This is known as a Chain Reaction. By controlling the speed of the chain reaction, one can control the amount of heat produced. The amount of energy produced in the fission can be determined from Einstein's equation: E = mc 2. The first self-sustaining nuclear chain reaction was attained by a team of scientists lead by Enrico Fermi on December 2, 1942, at the University of Chicago. This was part of a large scale effort by the government to develop atomic military weapons during WW II, called the Manhattan Project. After the war, scientists also began to develop nuclear power reactors for peaceful uses. One use was development of nuclear power reactors to produce electricity. Today more and more of our electricity is being produced by nuclear power plants. Uranium is the key element from which power derives its energy. When prepared in its purest metallic form, uranium is a silvery white solid, much heavier than lead. It is composed of two different atomic isotopes--a heavy variety called U-238, and a lighter and rare species known as U-235. It is the latter that is used in electrical power production. However, it constitutes only 0.7 % of natural uranium, the rest (99.3 %) being U-238. No amount of natural uranium, with only 0.7 % U-235, could ever be used in a reactor of American design to economically produce power. This is because fission of U-238 does not release enough neutrons to continue a chain reaction, whereas fission of U-235 does. If, however the natural uranium is processed (enriched) to produce a higher percentage of U-235, then the possibility of producing power becomes a reality. Neither natural uranium nor the high enrichment weapons-grade uranium (greater than 93 % U-235) are used in reactors that constitute the principle designs of US nuclear power plants. Instead, uranium, enriched or fortified so that it contains approximately 2-3 % of U-235, forms the basis of nuclear fuel for US power reactors. This low percentage of U-235 in the reactor's fuel makes it physically impossible to have an accident configuration of the uranium fuel, or core as it is commonly called, is such as to further negate the possibility of an explosion. One cubic foot of U-235 contains the energy equivalent of 1.7 million tons of coal or 7.2 million barrels of oil. Summary of Fission: Splitting of an atomic nucleus into smaller fragments and a large amount of energy. Example: 1 0 92 n 235U 141Ba 36 Kr 301 n Energy 92 56 Fusion Fusion is the process in which light nuclei like hydrogen combine to form heavier nuclei. Fusion is the exact opposite of fission. The energy of the sun comes from fusion. Here hydrogen atoms combine to form helium. Energy released during the process, since the mass of the helium atom is less than the sum of the masses of the hydrogen atoms, and Einstein's equation applies. Thus, as with fission, some mass has been converted to energy. Since light atoms are very stable, a considerable amount of energy is needed to make the nuclei come together. The fusion reactor occurs in nature only at temperatures found in the stars, of many millions of degrees. Only at these temperatures can enough energy be present to cause fusion to occur. No material on the earth will remain solid at these temperatures, so fusion reactions produced by man are held together by magnetic fields rather than steel vessels, such as with fission. Controlled fusion as an energy source is only in the very early stages of development. However, since the elements that undergo the reaction are commonplace (found in ordinary water), the potential for unlimited energy is great. One gallon of sea water contains enough hydrogen isotopes for fusion to equal energy released by burning 300 gallons of gasoline. Summary of Fusion: Combining two or more light nuclei to form one with more mass and energy. Example: Review Book pg. 180: 24 – 33. Uses of Radioisotopes Radioisotopes are used for basically three reasons. 1) Based on Chemical Reactivity The chemical behavior of a radioactive isotope is the same as that of a stable isotope of the same element. As a result, the radioactive isotope can be used to trace the course of a reaction. The reaction continues in a normal fashion, but the path of the substituted radioactive material can be traced using radioactivity detection devices and methods. Many organic reaction mechanisms, including those in living systems are studied using carbon-14 as the tracer. (Tracer: used to study biological systems and reactions in medicine, research or industry.) 2) Based on Radioactivity Isotopes with a very short half-lives are administered to patients for diagnostic purposes. Tumors in various organs can be located and levels of activity of those organs can be monitored by administering and tracing radioactive substances known to concentrate in those organs. Technetium-99 is used to determine the location of brain tumors. Iodine-131 is used for the diagnosis of thyroid disturbances. Radium-226 and Cobalt-60 are used in cancer therapy. Intense beams of gamma radiation can be used to irradiate foods to kill bacteria. By killing the bacteria present, the food last longer without spoiling and causes fewer bacterial infections in those who consume it. Cobalt-60 and Cesium-137 are two of the sources of gamma radiation currently being used to destroy anthrax bacilli. 3) Based on Half-Life Radiochemical dating is a method of determining the age of fossils and rocks. The half-life of a radioisotope is a constant factor. It is not affected by temperature, pressure or any external factor. The ratio of Uranium-238 to Lead-206 in a mineral can be used to determine the age of the mineral. As long as an organism is living, the ratio of Carbon-14 to Carbon-12 is constant and remains constant. As soon as the organism dies, the carbon-14 lost through radioactive decay is not replaced. Thus, the ratio of the carbon-14 present in a fossil to that in the atmosphere can be used to determine the age of the fossil. Irradiation is a process used as treatment to kill living tissues and the preservation of foods by killing bacteria, molds, insect eggs and yeast. Review Book pg. 184: 48 – 57. 4 0 4 01H 2 He 2 1 e Energy OR 2 1 2 4 H 1 H 2 He Energy Questions Answer the following questions on your own paper. 1) Compare and contrast fission and fusion reactions (In other words: Name at least two similarities between fusion and fission. Then name at least two differences between them). Discuss the general process of both. What are the advantages of each? What are the disadvantages of each? Which is more practical use of energy? Which process is used in the sun? 2) Nuclear fusion produces enormous energy (The Sun and other stars are fueled by fusion.) Fusion fuel is abundant and inexpensive. (The ocean is full of hydrogen.) Fusion fuel and products are not radioactive. But, all working nuclear reactors in the world use fission reaction which have many risks. What makes fusion reactors difficult to make? 3) How many neutrons are produced as a result of a fission reaction. 4) The exposure to and uses of radioactive isotopes is a controversial topic. a) List at least two benefits of radioactivity to humans. b) List at least two risks of radioactivity to humans. 5) Which kind of isotopes are more dangerous in nuclear waste, isotopes with short half-lives or isotopes with long half-lives? Explain. 6) Half-Life Review Book pg. 183: 34 – 47. 7) Transmutation Review Book pg. 179: 16 – 23. 8) Nuclear Particles Review Book pg. 178: 1 – 15.