Nuclear Energy I ask you to look both ways. For the road to a knowledge of the stars leads through the atom; and important knowledge of the atom has been reached through the stars. ~ Sir Arthur Eddington (1882 - 1944) CHAPTER 11 On December 2, 1942, the world’s first nuclear reactor went critical on the floor of an abandoned squash court at the University of Chicago. At 3:26 p.m. that afternoon, scientists achieved the first self-sustaining nuclear chain reaction inside what was known as Chicago Pile-1, initiating the atomic age. This opened the possibility of providing power from the energy locked safely inside the atom (Figure 11-1). To promote nuclear energy, electric companies assured the public of a source that provided power so cheap that there would be no need to even meter it. This optimism and excitement was soon tarnished as the hazards, environmental costs, and the dangers associated with the accidental release of radiation became apparent. Today, the same optimism exists toward the future development of controlled fusion - that all our energy needs will be satisfied and this source of clean, renewable energy will substitute for all other forms of energy. Atoms The entire physical world is made of atoms. The word atom is derived from the Greek word atomos which means “indivisible.” It was coined by ancient Greek philosophers, who thought of the atom as the smallest possible constituent of a substance. We have since learned that atoms are not indivisible, but are rather made of even smaller particles. Different elements have atoms that contain different numbers of these smaller particles. Today, the idea of the atom has been modified to specify the smallest particle of a chemical element that still exhibits all the chemical properties unique to that element. The atoms of these elements react with one another and combine in different ways, forming a virtually unlimited number of chemical compounds. When two or more atoms combine, they form a molecule. For example, two atoms of hydrogen combine with one atom of oxygen to form a molecule of water. In other instances, atoms of one element can change their entity and form new elements. In these reactions, called nuclear reactions, the elementary particles within the nucleus of the atom are rearranged. Nuclear reactors and our sun rely on such reactions to produce energy. Figure 11-1 This 12-foot bronze sculpture by Henry Moore marks the site at the University of Chicago where the first sustained nuclear reaction was achieved Science has promised us truth: an understanding of such relationships as our minds can grasp. It has never promised us either peace or happiness. ~ Gustav Lebon (1841-1931) Did You Know That ...? Atoms: The Facts • A row of 100 million atoms of hydrogen would only be about one centimeter long. • Eight of the elements known to us make up 98.5 % of the earth’s crust by weight: oxygen makes up 46.6%, silicon 27.7%, and aluminum 8%, with Fe, Ca, Na, K, and Mg composing the rest. • Hydrogen is the most abundant element in the universe. • Of 92 naturally occurring elements, 76 are solids, 11 are gases, and 5 are liquids at room temperature. • Nearly all of the mass of an atom is contained in the nucleus, which has a density of around 100 million tons per cubic centimeter. electron neutron proton Figure 11-2 Every atom consists of a nucleus surrounded by electrons. Inside the atom You can picture the nucleus of an atom as a minuscule cluster of particles surrounded by a whirling cloud of much tinier particles, the electrons. The nucleus itself is composed of neutrons and protons, collectively known as nucleons. Protons are positively charged, whereas neutrons, as the name implies, carry no charge. It is the number of protons that gives the element its chemical identity; in other words, no two different elements have the same number of protons (Figure 11-2). Although the nucleus contains most of the mass of the atom (about 99%), the volume it occupies is very small, leaving a relatively large empty space between the nucleus and the electrons. Hydrogen, the lightest of all elements, has only a single proton. Helium has two. Carbon, the basis of all life and the main constituent of fossil fuels, contains 6 protons. Oxygen has 8. The Uranium atom, with 92 protons, is the heaviest of all natural elements that make up our planet. Besides these natural elements, other elements have been discovered, but only in a laboratory environment. Every atom is designated by its atomic number (P), which represents its number of protons and by its atomic mass (A), which is equal to the total number of neutrons (N) and protons ( A X) . Almost all uranium naturally P present in the universe have 92 protons (and also 92 electrons)and 146 neutrons (P = 92, N = 146, and A = 238) and is represented as(238U)or simply 92 238 U. The nucleus of a small fraction, however, has three less neutrons (but still 92 protons and 92 electrons); as it will be seen, this form of uranium called uranium-235(235U) is very unstable and plays an important role in construction of both nuclear reactors and nuclear bombs(Figure 11-3). Different atoms make up everything in the universe. A list of these elements along with their atomic masses and numbers are summarized in the Periodic Table shown on the next page. Binding Energy The binding energy of a nucleus is a measure of how tightly its protons and neutrons are held together by nuclear forces. The binding energy per nucleon, the energy required to remove a neutron or a proton from a Atomic Mass 235 92 Atomic Number U 143 Neutron Number Figure 11-3 Uranium-235 244 Chapter 11 - Nuclear energy Extra mass per nucleon nucleus, varies with mass number and is shown in Figure 11-4. The figure implies that when a heavy nucleus (such as that of uranium) splits or two light nuclei (such as the nuclei of deuterium and tritium) coalesce, more stable nuclei form and energy is released. The former example describes fission, while the latter represents the basic mechanism that occurs in fusion reactions. Nuclear versus Chemical Reactions There is a fundamental difference between chemical reactions (such as the burning of coal or natural gas) and nuclear reactions. Consider the chemical reaction: CH4 + 2O2 CO2 + 2H2O + DE Carbon dioxide + Water vapor + Heat 0 .0 8 0 .0 6 0 .0 4 0 .0 2 0 -0 .0 2 0 Hydrogen Deuterium Tritium Lithium Uranium 40 80 120 160 200 240 Atomic mass number Methane + Oxygen Figure 11-4 Energy can be liberated -when the nuclei of © Energy and the Environment Toossi light elements combine (fuse) to form heavier elements, or the nuclei of heavy elements are split to form lighter elements (fission). The energy release per unit mass of fuel from fusion is much more than that of fission. Note that after the reaction is completed, we still have one atom of carbon, four atoms of oxygen, and four atoms of hydrogen. The number of molecules, however, is not the same in a chemical reaction since atoms are rearranged to form new molecules. In this reaction, one molecule (or one mole) of methane reacts with two molecules (or two moles) of oxygen, and results in one molecule (or one mole) of carbon dioxide and two molecules (or two moles) of water. The total mass of reactants (methane and oxygen in our example) and products (in this case carbon dioxide and water vapor) also remain the same. Chemical energy appears because of the reconfiguration of atoms and the lower energies in newly created bonds, as compared to that in bonds which have been broken. In chemical reactions, atoms are conserved - the total number of atoms of each element remains constant. Question: W hy do elements making up a chemical compound 245 combine in definite proportions? Answer: John Dalton, an English scientist, pointed out that all elements are made of atoms that look like billiard balls and that these atoms must bind with each other in full or not bind at all. Although the description of atoms as rigid balls is not quite right, the explanation was a great step forward in understanding the nature of atoms. In a nuclear reaction, atoms lose their identity. For example, when uranium-235 is hit with a neutron, two lighter elements such as krypton and barium are formed and two or three neutrons are released. In this case, the total number of elementary particles (electrons, protons, and neutrons) is conserved. The mass of the product is slightly smaller than the mass of the reactants. Energy is released because the configuration of elementary particles forming the nucleus of the fragments has a lower binding energy (mass) than that of the nucleus of the atom undergoing fission. In nuclear reactions, the total number of nucleons is conserved - the total number of neutrons and protons remains constant. Another notable distinction between chemical and nuclear reactions is the huge difference in the amount of energy each releases. This is because the binding energy that holds a nucleus together is far greater than the energy that holds electrons to a nucleus. For example, the fission of one kilogram of uranium provides as much energy as 2.4 million kilograms of the best quality coal. Even though nuclear fuel contains only 3-5% fissionable material, the tremendous energy released from a nuclear reaction is apparent. Question: Throughout history, alchemists have tried to transform copper, mercury, and other metals into gold, only to experience disappointment. To what do you attribute the cause of their failures? Answer: Alchemists mistakenly tried to perform this task by means of chemical reactions. As we now know, the process requires the deconstruction of the nucleus and rearrangement of the elementary particles (protons and neutrons) into new elements. This is only possible using nuclear reactions that were unknown to them at the time! Isotopes To make atoms electrically neutral, all that is required is that the number of protons and electrons be equal. The number of neutrons, however, can change, so it is possible for two atoms to have a different number of neutrons for the same number of protons (different A, but same P). These different atoms of the same element are known as isotopes. Uranium and hydrogen, the heaviest and lightest of all the naturally-occurring elements and the fuel of choice for nuclear fission and fusion reactions, are both 246 Chapter 11 - Nuclear energy found naturally in three isotopes. Figure 11-5 shows the three isotopes of hydrogen. 234 92 U 235 92 U 238 U 92 (0.006%) (0.714%) (99.28%) 1 1H 2 H 1 3 H 1 (99.80%) (0.15%) (0.05%) H D T The numbers in parenthesis give the relative abundance of the natural occurrence of each isotope. Since different isotopes are only different in the number of neutral charges they contain, they are chemically identical. As we will see, these isotopes may have significantly different nuclear characteristics. R adioactive materials are those isotopes which have unstable atomic nuclei and may undergo spontaneous decay, forming lighter isotopes (called daughters) and releasing highly energetic ionizing radiation in the process. For example, the isotope of uranium-238 decays into several isotopes before it becomes stable lead (Figure 11-6). All elements with atomic masses heavier than bismuth are naturally radioactive. It is, however, possible to artificially produce isotopes of lighter elements. Half-life is an important quantity in measuring the effect of harmful radiation from a radioactive source. Half-life Isotopes have an interesting property; they decay to form other more stable isotopes. The rate of decay is expressed by the isotopes’ half-lives. A half-life is the time in which the concentration of an isotope decays to one half of its original concentration. The rate of emission from a radiation source reduces to 1/2 in one half-life, to ¼ in two half-lives, to 1/8 in three half-lives, and to 1/2n in n half-lives. Half-lives of some of the isotopes of interest to us are uranium-235 (about 0.7 billion years), plutonium-239 (24,360 years), cesium-137 (30 years), and strontium-90 (28.8 years). Iodine-131 has a half-life of only 8.04 days. Example 11-1: A radiation leak released cesium-137 radionuclide. How long would it take until the radioactivity of cesium drops to 1/1000 of its initial value? The half-life of 137Cs is 30 years. (Hint: 210 = 1024) Solution: About ten half-lives or 300 years. In Chapter 1, we derived the equation for the doubling-time of a quantity that grows exponentially with time. Examples given were population growth, interest accumulating on a savings account, and energy consumption. The same equation can be shown to apply for the radioactive decay of isotopes, except decay rate (negative growth rate) and half-life are used instead of growth rate and doubling time. Following the approach used in equation 1-1, we can write: Figure 11-5 There are three isotopes of hydrogen that differ only in the number of neutrons they possess. The hydrogen isotope has only one proton, the deuterium isotope has one proton and one neutron, and the tritium isotope has one proton and two neutrons. Figure 11-6 Decay chain of isotope of uranium-238 247 Digging Deeper ... An Even Smaller World! R ecent experiments with large particle accelerators have indicated that both protons and neutrons are composed of even smaller particles called quarks that have fractional charges and held (glued) together by gluons. Some believe that quarks and electrons are themselves made up of tiniest particles called strings. If the string theory proved to be valid then all matters can be thought of a collection of these strings. T1/2 = 70 Percentage Rate of Decay (11-1) Example 11-2: Using data from the previous example, calculate the rate of decay for cesium-137? Solution: Using equation 11-1, rate of decay is 70/30 = 2.33% per year. Fission Ba-143 HEAT neutron U-235 Kr-90 neutrons Figure 11-7 The fission process. When U-235 is bombarded by a neutron, it breaks into smaller fragments and 2 or 3 neutrons releasing a huge amount of energy. Fission is the splitting of an atomic nucleus. Very heavy nuclei, like isotopes of uranium and plutonium, are easiest to split. The process involves bombardment of these atoms by small sub-atomic particles like neutrons, which splits them into two fission products; two to three neutrons and excess energy in the form of gamma rays are also produced. These neutrons collide with more uranium atoms, thus initiating a selfsustaining chain reaction that results in more fissions and the continuous release of enormous amount of energy (Figure 11-7). Any substance capable of sustaining a fission chain reaction is known as a fissile material. The materials most suitable for fission are 235U and 239Pu. These radioactive isotopes are fissionable by what are termed as thermal neutrons. The isotopes 238U (and 232Th) are fissile if bombarded by fast neutrons. The physics of nuclear energy is simple. When a neutron collides with an atom of the isotope uranium-235, a highly excited atom of uranium-236 is formed, which immediately fissions into two lighter nuclei. Fission reactions can produce any combination of lighter nuclei, as long as the number of protons and neutrons in the products add up to the fissioning nuclei. Among possible reactions are: 235 U + n 236U 143Ba + 90Kr + 3 n + energy (g ray) 235 U + n 236U 140Cs + 93Ru + 3 n + energy (g ray) 235 U + n 236U 134Xe + 100Sr + 2 n + energy (g ray) The profound feature of fission is that the mass of the products (fission fragments and neutrons) is less than the mass of a 236U atom. The mass difference (or the mass defect) appears as the kinetic energy of the fission fragments in an amount determined according to Einstein’s famous E = mc2 formula, where m is the mass defect and c is the speed of light in a vacuum (300,000 kilometers per second).1 1 Speed of light is usually denoted by c, from latin “celeritas” meaning rapidity of motion or action. 248 Chapter 11 - Nuclear energy Carbon Dating Digging Deeper ... A rcheologists use a simple approach to determine the age of fossilized objects buried underground a long time ago. A technique known as carbon dating uses the ratio of an isotope of carbon, carbon-14, to that of common and stable carbon, carbon-12, to estimate the age of a fossil that was buried many decades or centuries before. All living organisms naturally contain an isotope of carbon,14C, which is produced when the earth’s upper atmosphere is bombarded by cosmic radiation that breaks nitrogen down into unstable carbon-14. Because C-14 is chemically identical to stable C-12, they attach to complex organic molecules through photosynthesis in plants, and enter the food chain. Since all living matters undergo a continual carbon exchange process through inspiration or respiration of carbon dioxide, they contain the same ratio of 14C/12C (1.28x10-12). When a tree is cut down or an animal dies, the carbon exchange process stops, and C-14 starts to decay, whereas stable C-12 remains largely unchanged. The decrease in the ratio of 14C/12C can therefore be used as a measure of the age of a fossil or an artifact. Example: A group of British scientists have studied the charcoal remains from Stonehenge and found it to contain the carbon isotope ratio of 8x10-13. Estimate the possible age of the Stonehenge site. Solution: Assuming the charcoal is leftover from social activities during the Stonehenge era, we can write: 14 12 Substituting for the carbon ratio (14C/12C = 8x10-13), we get t = 3,800 years. It must be noted that the accuracy of the carbon dating technique decreases with less sensitive instruments and with the age of the sample. With a half-life of 5,700 years, an initial C-14 population will decay to 1/16 of its original value in 22,800 years – about the limit of detection possible with current instruments. For estimating the age of older fossils and rocks, scientists use other radioactive isotopes (See table below). Parent Isotope Daughter Isotope C = 1.28 x 10 -12 x 2 -t/5700 C Half-Life Carbon-14 Aluminum-26 Uranium-235 Potassium-40 Nitrogen-14 Magnesium-26 Lead-207 Argon-40 5,760 years 700,000 years 700 million years 1.25 billion years Question: In the example of nuclear fission of uranium-235 given above, the mass appears to be the same on both sides of the reaction (235+1 = 143+90+3 = 134+100+2 = 236). Thus, it seems that no mass is converted into energy. In this case, where does the energy come from? Answer: The statement is not entirely correct. Actually, the mass of a nucleus is more than the sum of the individual masses of its protons and neutrons, and contains the extra mass equal to the binding energy that holds the protons and neutrons of the nucleus together. Question: Carbon has two stable isotopes, 12C and 13C. The isotope 14 C is unstable and decays to 14N, with a neutron changing into a proton. Describe the reaction and how it can be used in dating fossils and other archeological artifacts.2 Answer: Both 14C and 14N have the same atomic mass. The difference 2 R adioactive dating is not limited to biological systems. A similar technique called pottasium-argon dating can be used to determine the age of volcanic rocks that contain potassium. W hen volcanic liquids --called lava-- was solidified into a rock, it contained an isotope of potassium K-40 which decays into argon gas. The technique works by melting the rock and measuring the concentration of the trapped argon gas to estimate the rock’s age. 249 is in the number of protons and neutrons in their nuclei. 14C with eight neutrons and six protons is less stable than tightly bound 14N which has seven protons and seven neutrons. The half life of the decay is 5730 years which is used in determining the age of fossils and other biological organisms (See box “Carbon Dating”). Example 11-3: W hat is the energy equivalent of one gram of graphite, that is, the energy available if all its mass were annihilated? Solution: The energy release is calculated as E = mc2 = (0.001 kg) (3x108 m/s)2 = 9x1013 J, or the energy-equivalent of 10,000 tons of TNT! Nuclear Fuel Most nuclear reactors use uranium as fuel. Uranium is a common element on earth, formed during the earlier stages of its formation. At the time our planet was formed 4.5 billion years ago, about 75% of all uranium was in the form of U-238 and the remaining 25% was in the form of isotope U-235. As uranium decayed (T1/2 = 700 million years), the ratio of U-235 to U-238 dropped considerably. Today only 0.7 % of uranium found in nature is U-235. The non-fissile isotope U-238 makes up the rest. Uranium is scattered throughout the earth’s crust in most rocks and soils, as well as in many rivers and in seawater. Uranium is not an unlimited resource and, like fossil fuel, has only a limited lifetime. With 24% of the world’s total uranium resources, Australia is the leading producer. Other countries with vast uranium reserves are Kazakhstan (16%), Canada (10%), and South Africa (7%). The United States, with 340,000 tons, has about 5% of the earth’s uranium reserves. Eighty-five percent (85%) of all US uranium reserves are in New Mexico and Wyoming. Processing Burnup Fuel rods Spent fuel Fuel fabrication Pu-239 Depleted uranium 3%U-235 Storage Reprocessing Wastes Enrichment 0.7%U-235 Virtification Storage Conversion U3O8 Tailings Mining Disposal Figure 11-8 Nuclear fuel cycle from extraction to disposal. From extraction to disposal, uranium fuel undergoes many changes. Various stages of the nuclear fuel cycle are shown in Figures 11-8 and 11-9 and consist of: 1. Mining and milling - Like coal and other minerals, uranium must be mined and hauled away to a mill where uranium ore is crushed and ground to a fine slurry that is poured into an acid, which dissolves the uranium, but not the rest of the crushed rock. The acid solution is dried into a yellow powder called yellow cake. The leftover rock is known as tailing. 2. Conversion - The yellow cake is then shipped to a conversion plant where it is purified and chemically converted to uranium hexafluoride (UF6) gas. 3. Enrichment - The flow stream has very little uranium-235, the fuel for nuclear fission. During the enrichment process, about 85% of uranium-238 is removed from the flue gas to raise the concentration Figure 11-9 Uranium from extraction to storage. 250 Chapter 11 - Nuclear energy of U-235 isotopes from the 0.7% which is naturally present in the ore to the 3-5% required for use as nuclear fuel.3 The enrichment is carried by gaseous diffusion or gaseous centrifuge. In gaseous diffusion, the gas is pumped through filters with holes large enough to allow uranium-235 atoms to pass through but not the slightly bigger atoms of uranium-238. In a gaseous centrifuge plant, gas is sent through hundreds of centrifuges that spin uranium hexafluoride gas at very high speeds, separating the lighter uranium-235 hexafluoride molecules from the heavier uranium-238 ones. Fabrication – Enriched UF6 gas is sent to a fuel fabrication plant where it is converted to uranium dioxide (UO2) powder and pressed into small ceramic pellets that are then stacked inside long zirconium or stainless steel tubes to form fuel rods. Finally, fuel rods are bundled together to make the fuel assemblies used in nuclear reactors. Some U-235 remains in the tail and is called depleted uranium. Because of its high density, depleted uranium has been used in yacht keels, antitank missiles, and artillery shells. Burn-up – A reactor core contains several hundred fuel assemblies where the fission process takes place and fuel is “burned,” producing heat in a process called a chain reaction. Fission products are plutonium and various fission-fragments. Spent Fuel Storage – Over time, the uranium fuel becomes depleted and the concentration of fission fragments begins to build up in the reactor. As more and more neutrons are absorbed, a point is reached where old fuel is no longer efficient and must be replaced with new fuel. The spent fuel and other fission products remain highly radioactive and continue to release heat and radiation long after they are removed from the reactor core. Typically, spent fuel is unloaded into a storage water pond immediately adjacent to the reactor. This allows the fuel to cool and the short-lived isotopes to decay and radiation levels to decrease by as much as 90%(Figure 11-10). Spent fuel is held in such pools from several months to many years before it is sent out to be reprocessed or dried (vitrified) and deposited in storage facilities. Each year, over 10,000 tons of spent fuel is generated by the world’s current 441 operating nuclear plants, of which less than one-third is reprocessed for recycling as mixed-oxide (MOX) fuel.4 The remainder is placed into interim storage facilities. Reprocessing - The fissionable part of the fuel rod assembly is only a few percentages. About 95-97% of the uranium is still intact after fuel is burned. Plutonium makes up 1% of its mass and the rest are other radioactive elements. During reprocessing, uranium and plutonium 4. 5. 6. Figure 11-10 Storage pond for spent fuel at a reprocessing plant 7. E nrichment is a physical process which relies on the small mass difference between atoms of two isotopes. No chemical methods can separate uranium-isotopes, as all isotopes are c hemically identical. The two main enrichment processes are diffusion and centrifuge. In the diffusion technique, the mass differences between different isotopes result in different rates of diffusion through a membrane. In centrifuge, a similar separation occurs as the uranium substrate is spun at a very high speed. Today, most uranium enrichment plants use centrifuges to separate lighter uranium-235 hexafluoride from heavier uranium-238 hexafluoride. Successive operations allow production of more and more highly concentrated uranium-235. 4 I n the last few years many European countries have been using mixed uranium with plutonium extracted from their surplus stockpiles of nuclear weapons as a nuclear fuel (called MOX for mixed oxide fuel). The United States has not been using MOX to fuel its nuclear reactors yet, but is planning to build a MOX facility at a DoE site at Savannah River, South Carolina. The project is a part of the disarmament process and is aimed at reducing the nuclear threat by minimizing the number of nuclear weapons and disposing of nuclear-grade plutonium which has accumulated over many years of weapons manufacturing and reactor operation. Critics of the plan warn that opening the large stockpiles of plutonium to commercial sites make power plants more attractive to thieves and terrorists, because the plutonium can be separated from the MOX fuel rather easily and used for constructing an atomic bomb. By 2010 it is expected t hat MOX fuels will power some 15-20% of the world’s power reactors. 3 251 T F YI ... Units Commonly Used by Nuclear Scientists he mass of nucleons is usually represented in atomic mass units. 1 amu = 1/12 of the mass of atom 12C = 1.66x10-27 kg • mass of proton = 1.00728 amu • mass of neutron = 1.00866 amu • mass of electron = 5.486x10-4 amu he energy of nucleons is usually expressed in electron volts (eV or Mev) 1 eV = energy that an electron would gain if it were accelerated through an electrical potential difference of 1 volt (1 eV = 1.6x10-19 J and 1 MeV =106 eV) • Energy equivalent of 1 amu = 931.5 MeV T are separated out from the rest, returned to the conversion plant, and blended with additional enriched uranium to build new fuel rods. During WWII, nuclear reactors were primarily designed for manufacturing large quantities of plutonium. The existing stockpile of plutonium is sufficient to fuel many breeder reactors. All US reprocessing plants were shut down at the end of the twentieth century and no new reprocessing plants are believed to be necessary in the foreseeable future. 8. Vitrification – High-level liquid waste is dried and stored in special containment vessels called casks. Casks are currently made of stainless steel alloy, but ceramic might be a more favorable material. Ceramics do not rust and have good resistance to radiation and heat. 9. Disposal – The canisters are stored in an underground permanent repository. Example 11-4: How much U-238 must be removed from the uranium oxide (UO2) ore to enrich the U-235 concentration to 3.5%? Solution: Initially, only 0.7% of the uranium oxide is U-235, while the remaining 99.3% is U-238. If X is the fraction of U-238 to be removed from the mixture, then its concentration has reduced to 99.3(1-X) percent and we have: 0.7 [U - 235] = = 3.5% or X = 0.8 [U - 238] 99.3(1-X) That means 80% of the U-238 must be removed and discarded. This is a major source of radioactive waste. Example 11-5: How much energy is released from the fission of 1 kg of natural uranium enriched to 3.5% U-235? Assume that each atom of uranium releases 200 MeV when undergoing the fission process. Solution: Assuming natural uranium is mainly U-238, then the number of nuclei in one kg of uranium is N = (1000 g)(6.02x1023 atoms/mole)/(238 g/mole) = 2.5x1024 Assuming that only 3% of these nuclei participate in fission reaction, the total energy release is E = (0.03x2.5x1024)(200x106)(1.6x10-19) = 2.4x1012 J = 667 MWh 252 Chapter 11 - Nuclear energy Plutonium Plutonium is only slightly heavier than uranium, but unlike uranium that is abundant in mines, there is no plutonium left in nature. This is because, compared to uranium, plutonium has a very short life and has therefore decayed to lighter, more stable elements. Plutonium, however, can be manufactured in nuclear reactors. When uranium-238 – which comprises the bulk of nuclear fuel – is collided with neutrons it turns to highly radioactive uranium-239 with a half-life of only 23 minutes. The resulting isotope is called neptunium-239, which is also radioactive and decays further to the isotope plutonium-239. Plutonium is not only the fuel for breeder reactors but also the ideal fuel for making nuclear bombs. Plutonium is also highly toxic and causes death within a few hours or days if ingested. In bulk quantities, plutonium is not very dangerous, but if vaporized or aerosolized it can cause great damage and must therefore be carefully safeguarded from falling into the hands of terrorist organizations. Nuclear Reactors As of January 1, 2006, there are 441 nuclear power reactors in the world producing 368,000 MWe, or one-sixth of the world’s electricity (Table 11-1). The United States has the greatest number of nuclear reactors, but the fraction of electricity they generate is small compared to that in some European countries. For example, while nuclear energy accounts for 20% of US and 50% of Europe electricity. In total, there are 31 countries that use nuclear power to meet some of their electricity demands.5 Most nuclear reactors were constructed during the 1960s and early 1970s. Following the accidents at Three Mile Island (TMI) in 1978 and Chernobyl in1986 and the mounting volume of nuclear waste, public confidence in the use of nuclear reactors has declined. In the United States, the last nuclear plant went online in 1996, and in many other countries the number of new facilities being constructed has declined considerably. In contrast, in a rush to join the club of nuclear nations, many countries are building new nuclear reactors (Figure 11-11). In light of continued concerns regarding the storage and disposal of nuclear waste, even with the addition of these reactors, it is unlikely that overall nuclear power generation will increase in the near future, and some even predict that it may decline (Figure 11-12). However, the increasing dependence of the industrial world on imported oil, the volatility in prices of crude oil and natural gas, and the concern over environmental issues may tilt the balance in favor of nuclear energy sometime in the future. A nuclear power plant produces electricity in almost exactly the same way that a conventional (fossil fuel) power plant does. We discussed the basic components of a power generating station in Chapter 5. In a conventional thermal power plant, fuel (coal or oil) is burned to create heat, which boils water in a boiler (steam generator) to create steam. The steam is expanded in a turbine; the turbine runs a generator, which in turn 5 Table 11-1. Nuclear Generation of Electricity by Country* Country Reactors in Operation 103 59 17 31 55 9 1 441 Total GWe 98.2 63.0 20.3 21.7 47.6 6.6 1.2 368 % Electricity 20 78 31 16 29 2 70 16 U. S. France Germany Russia Japan China Lithuania World *Source: IAEA General Conference, Nuclear Technology Review, July 2006. Figure 11-11 Nuclear power reactors under construction as of January 2003. Source: IAEA, Data Series 2, “Power Reactor Information System”, January 2003. Figure 11-12 The history and predicted consumption of various forms of energy. It appears only demand for nuclear energy would be in decline. I nternational Atomic Energy Agency (http://www.iaea.org/programmes/a2). 253 Figure 11-13 A typical nuclear reactor has several primary components. Inside the reactor “core” are the fuel rods, the control rods, the moderator, and the coolant. Outside the core are the turbines, the heat exchanger, and the main cooling system. generates electricity. A nuclear power plant uses the same basic principles, except that the boiler is replaced by a reactor core where nuclear fuel undergoes fission processes to produce heat (Figure 11-13). Reactor Core The reactor core is the heart of any nuclear power generation facility. Inside the reactor core, fission takes place and provides energy to produce the steam required by the power plant. A reactor vessel made of several inches of carbon steel surrounds and protects the reactor core. The pressure vessel itself is housed in a containment structure made of several feet of thick concrete reinforced with thick steel bars. The reactor core consists of four main components. a. Fuel Assembly Nuclear fuel consists of pellets (usually about 1 cm in diameter and 1.5 cm long, about the size of your fingertip) of ceramic uranium oxide arranged in long tubes in the fuel rods, which are grouped into bundles (fuel assemblies) in the reactor core (Figure 11-14). The bundles are submerged in a coolant inside a pressure vessel. Depending on the type, nuclear reactors have a number of fuel assemblies that must be replaced occasionally as they deplete and fission products buildup. b. Moderator A fast neutron is capable of causing fission in either U-235 or U-238. In lieu of the much larger concentration of U-238, it is unlikely that a fast neutron will hit a U-235 atom and cause fission. For a neutron to be captured by U-235, it must be sufficiently slowed down. Under these conditions neutrons have more times to be pulled by the nuclear forces imparted by the uranium nuclei and there is a higher probability to be captured. The moderator is the material that slows down the fast neutrons to thermal neutrons (neutrons with an average kinetic energy similar to molecules of gas of the same temperature). Without a moderator, the fission chain reaction cannot be sustained. With ordinary water as a moderator, uranium-235 must be enriched to about 3%. It happens that natural uranium (at 0.7% concentration) will also work if instead of ordinary water either Figure 11-14 Fuel Assembly- Fuel rods are made of zirconium alloys grouped into fuel assemblies which are then placed in the reactor core. Inside each fuel rod are hundreds of pellets of uranium fuel stacked end to end. The rods are each about 3.5 meters long and about a centimeter in diameter. Zirconium is a hard, corrosion-resistant metal and is permeable to neutrons. 254 Chapter 11 - Nuclear energy graphite or heavy water (D2O) is used as a moderator. The Canadian deuterium reactor called (CANDU) is of this type. c. Control Rods It was previously explained that a fission reaction splits a uranium atom into two smaller fragments and two or three neutrons. To sustain a steady and controlled chain reaction, one of these neutrons is needed to bombard the next uranium atom, causing the next fission and continuing the chain reaction. This means that in order to prevent a possible run away reaction, the concentration of neutrons in the reactor must be controlled. Control rods are responsible for removing the extra neutrons and control the power output from the reactor. To control the rate of reaction, the control rods, made of neutron-absorbing material alloys such as cadmium, hafnium, or boron, slide up and down between the fuel rods; these materials absorb neutrons, but do not cause fission. To produce more heat (power up the reactor), the rods are raised and more of the uranium bundles are exposed; to create less heat, the rods are lowered. To shut the reactor down in the case of an accident or to change the fuel, the rods are lowered all the way. d. Coolant To carry away the heat of the reaction, coolants are needed. Water is usually the material of choice. Some gas-cooled reactors use carbon dioxide or helium gas, while breeder reactors use sodium, potassium, bismuth, or other liquid metals. Classification of Nuclear Reactors In addition to commercial reactors used for the generation of electricity, nuclear reactors are used in research, in ships, and in submarines. Depending on the type of coolants used, commercial fission power reactors can be divided into water, gas, or sodium cooled reactors. Depending on whether ordinary or heavy water is used, water reactors can be classified as light water or heavy water reactors. Table 11-2 gives summary of all nuclear reactors currently in operation throughout the world. Besides their intended use for generating electricity, nuclear plants have been used for the propulsion of large surface naval vessels and submarines. The United States, the United Kingdom, Russia, and France all have nuclear-powered submarines. The Soviet government built and operated the first nuclear-powered icebreaker in the Arctic. Other countries have constructed nuclear cargo ships, but high operating costs and restrictive port regulations have prevented their commercialization. In addition to the stated applications, a variety of small nuclear reactors at power levels around one megawatt have been built for education, training, and research. Light Water Reactors (LWR) Light water reactors (LWR) typically use enriched uranium as nuclear fuel, ordinary water as the moderator, and may be of either pressurized or boiling water types. A pressurized-water reactor (PWR) operates at a Table 11-2. Nuclear Reactors in Operation (2008) Type PWR BWR HWR HTGR Breeder PBMR Total Number 268 106 40 23 4 0 441 % 61 24 9 5 1 0 100 Source: World Nuclear Association (http:// www.world-nuclear.org) 255 pressure high enough so that water never boils. Boiling-water reactors (BWR), operate at a somewhat lower pressure, allowing the water to boil within the core. The steam produced in the reactor is piped directly to the turbine before it is condensed and pumped back to the reactor. Since the steam becomes radioactive, it is necessary to shield the entire steam loop, including the turbine, from the outside by placing it inside the containment building. All US commercial nuclear plants (including the power reactors at Three Mile Island) are light-water reactors, using water as both the coolant and the moderator. About two-thirds of these are pressurized water reactors; the remaining third are boiling water reactors. Heavy Water Reactors (HWR) Heavy water reactors use heavy water (D2O) instead of ordinary (light) water (H2O) as the moderator. The major advantage of heavy water reactors is that they can use natural uranium instead of enriched uranium to operate. Because deuterium is a more stable isotope of hydrogen, it has less affinity to absorb neutrons6, and there is a higher probability of collision with uranium-236 and a higher rate of conversion to plutonium. Since no enrichment facilities are needed, these reactors are considered dangerous from a proliferation perspective. Worldwide, there are 40 reactors of this type, mainly in Europe and Russia. There are no commercial heavy water reactors in the United States. High Temperature Gas-Cooled Reactors (HTGR) These reactors operate with natural or enriched uranium, use graphite as the moderator, and employ helium as the coolant. Helium is much less corrosive than steam, and is thus less likely to cause a leak in the pipes. The hot helium gas can directly drive a gas turbine or be used through a heat exchanger to heat water to steam that drives a steam turbine. Because of the high operating temperature (750oC) common with this type of reactor, efficiency is higher - around 40% compared to the 33% achieved by the water-cooled reactors. Fast Breeder Reactors (FBR) Like fossil fuel, uranium reserves are limited.7, 8 It is possible to extend the supply of nuclear fuel if non-fissionable component of nuclear fuel is converted into fissile materials. By doing so, more fissile plutonium nuclei is produced than consume, hence the term “breeding”. The breeder system uses uranium-238 as its fuel. It can however, transform into fissile plutonium-239 through collision with fast neutrons (Figure 11-15). The sequence of reactions is: 238 HEAT neutron U-238 Pu-239 neutrons Figure 11-15 In fast breeder reactors the fuel is Pu-239 which is made by bombarding non-fissionable U-238 with fast neutrons U+n 239 U 239 N 239 Pu 6 7 8 I n other words, the neutron cross-section of heavy water is significantly smaller than in light water. World Nuclear Association website (http://www.world-nuclear.org). The estimate is of course based on low cost availability of uranium fuel and may be extended if the market can bear higher electricity prices. 256 Chapter 11 - Nuclear energy Uranium-238 + Neutron Neptunium-239 Plutonium-239 Because neutrons are not slowed down, no moderator must be used, which rules out water as a coolant. A suitable coolant is a liquid metal like sodium which has excellent heat transfer properties and a very high boiling point, which provide significantly better cooling and allow operation at essentially atmospheric pressure eliminating need for a heavy pressure vessel.9 The main concern with these reactors is that, in the presence of water or even moisture in the air, liquid metals become highly explosive. Therefore, breeder reactors require special care to assure the safety of reactor operation. The first large-scale plant of this type, called Super Phoenix, went into operation in France in 1984. From the beginning, the plant was the focus of anti-nuclear and environmental groups, who eventually succeeded in closing down the plant in 1998. Currently, the only reactor of this type, BN-600, is operating in Russia producing 600 MWe. India has started construction of a 500 MWe prototype reactor, scheduled to be operational by 2010. Many factors including the availability of sufficient uranium fuel for the near future, safety concerns, the fear of terrorism, and nuclear proliferations have greatly dampened the enthusiasm for these reactors and many countries have stopped work on fast breeder technology all together. Pebble Bed Modular Reactors (PBMR) The reactor core is made of thousands of billiard-ball sized pebbles, each consisting of several thousand enriched uranium (up to 10%) microspheres (kernels) the size of small sand grains (0.9-mm). Each microsphere is coated with several layers of graphite that act as moderator and a silicon carbide outer shell. Unlike today’s nuclear reactors that boil water to drive steam turbines, pebble bed reactors use an inert gas like helium to drive gas turbines directly. Helium coolant enters the reactor and heated to about 900oC before it enters the turbines to generate electricity. After the fission is completed, the coating acts as a casket isolating and retaining the radiation long enough for the uranium to decay to reasonably safe limits (Figure 11-16). Shutdown will be done similar to other reactors, by inserting the control rods. These reactor are small, modular, flexible in design, and can be competitive with fossil fuels. Proponents of this type of reactor claim that this design is intrinsically safe and that there is no possibility of catastrophic nuclear accidents or fire; even if all helium coolants were lost, the temperature would never rise above 1,500°C (well below the 3,000°C needed to melt the uranium). Furthermore, the silicon carbide and graphite coating holds up better than the zirconium-alloy jackets of conventional fuel rods. So even in the worst case scenario where all cooling water has stopped, all contaminations stay within the pebbles walls and reactor automatically 9 Figure 11-16 Pebble Bed Modular Reactor M atzke, H., “Development status of metallic, dispersion and non-oxide advanced and alternative fuels for power and research reactors,” IAEA-TECDOC-1374, September 2003. 257 shuts off. The opponents agree that, although the reactor is inherently safer than the current water reactors, it may still pose a danger similar to the Chernobyl nuclear accident (to be discussed later in this chapter). A crack in the reactor could allow air into the reactor core and cause the graphite to burn. No commercial reactors of this type are operating anywhere yet. China operates an experimental PBMR with a larger 200 MWe “demonstration” reactor planned. South Africa is planning to construct several commercial plants to eventually produce 4,000 - 5,000 MWe of electric power, but because of substantial financial risks, its fate is uncertain.10 No PBMR is operating in the United States. Retiring of Nuclear Reactor Nuclear reactors are usually designed to last about 40 years, after which embrittlement and corrosion of the reactor vessels could pose a danger. By the year 2033, all 103 nuclear reactors currently operating in the United States will have reached the end of their original 40-year license period and must be shut down or must be upgraded for new licenses.11 Nuclear reactors cannot be simply shut down and demolished as most non-nuclear facilities are. The building, equipment, and the leftover fuel are radioactive and will continue to generate heat and emit radiation long after the reactor has stopped operation. Most often following the reactor close down, the plant enters a storage mode during which it is basically left alone but guarded for many years or decades to allow the intense short-lived radioactive material to decay and become less active. Another approach is to entomb the reactor under heavy protective layers of lead and concrete and leave it there until they are permanently disposed. The final phase of retirement is the dismantling or decommissioning of the plant by tearing it up using special robots and moving and storing its critical parts in a permanent storage site. Nuclear waste disposal will be discussed in Chapter 12 in more detail. Reactor Safety One of the most frightening aspects of nuclear power is the possibility of something going wrong with disastrous consequences. In one scenario, a malfunction of the control rods or a loss of coolant results in a “runaway” reaction and an uncontrolled release of energy that melts the fuel rods and causes them to fuse. The fused rods would then burn through the reactor vessel and the containment floor. Such a meltdown would cause leakage of dangerous radioactive materials into the environment. Two of the most famous (or infamous) nuclear accidents occurred at the Three Mile Island (TMI) reactor no. 2 in the United States and the Chernobyl reactor no. 4 in Ukraine. 10 11 Pebble Bed Modularized Reactor, Centurion, Republic of South Africa (http://www.pbmr.co.za/). Farber, D., and Weeks, J., “A Graceful Exit? Decommissioning Nuclear Power Reactors,” E nvironment, 43, No. 6, July/August 2001. 258 Chapter 11 - Nuclear energy Three Mile Island Although various safety features make serious nuclear accidents unlikely, an accident did occur on March 28, 1979 at the Three Mile Island Nuclear Power Station near Harrisburg, Pennsylvania (Figure 11-17). The incident was triggered by a sudden pressure drop in the core because a relief valve failed to close. Failing to notice the valve malfunction, operators turned off the emergency cooling pumps that went into operation automatically when the water started draining. This and a number of additional operator errors eventually led to the loss of some coolant, which ultimately resulted in severe core damage and the release of large volume of fission products into the containment structure. The reactor suffered a partial meltdown of the uranium fuel rods. To prevent overpressure, some of the radioactive gases was vented out. Fortunately, majority of the radioactive release stayed within the containment, and very little radiation escaped outside the reactor. Although human exposure to radiation was relatively minor, the economic cost and the psychological stress on the public was significant. The TMI accident is considered the worst nuclear disaster in US history. Under mounting pressure from the public, Congress enacted legislations for more stringent regulations for design, construction, and operation of nuclear reactors. In addition, the nuclear industry formed its own watchdog, the Institute of Nuclear Power Operation (INPO) that oversees safety standards and assures that power plants follow these standards and maintain active and continuous training of nuclear operators. Following the Chernobyl accident, the World Association of Nuclear Operators (WANO) was formed to fulfill a similar role to that of INPO on a global basis. Chernobyl On April 26, 1986, about 7 years after the TMI incident, the controversy was set ablaze again, this time at the Chernobyl nuclear power plant located about 130 km north of Kiev, the capital of Ukraine. One of the four nuclear reactors exploded and released more than 50 tons of radioactive material - 10 times that of bomb dropped on Hiroshima - into the environment. The scientific consensus is that the accident was the product of a flawed reactor design coupled with inadequate training and serious mistakes made by the operators of the plant.12 Unlike the TMI, the Chernobyl accident was not a result of loss of coolant, but followed a sequence of events that led to a fire to the carbon moderator. The effects of the disaster at Chernobyl were widespread. The immediate One fundamental difference between the Chernobyl reactor and the water-cooled reactors operated in the United States and elsewhere is the role that steam plays during an accidental loss of coolant. In water-cooled reactors, steam may accumulate to form pockets known as voids. With excess steam, more voids are created and water becomes less effective as a moderator; as a result, the chain reaction is not sustained and less power is produced which tends to shut down the reactor. When moderator and coolant are kept separate (as was the c ase in Chernobyl), any loss of coolant reduces the cooling capacity without affecting the moderator and the rate that neutrons are released. As the reactor heated, the rate of the fission c hain reaction increased, until the cooling water turned to steam and exploded. This in turn, accelerated the chain reactions and increased power output. More power means additional steam, less cooling, less neutron absorption, and still more power. With a lack of proper safety precautions the process may continue to dangerous levels. Water-cooled nuclear reactors are t herefore inherently safer because they do not face this risk. In addition, the lack of containment structures in the Chernobyl plant, similar to those present in the American, European, a nd Japanese nuclear plants, resulted in substantial releases of radionuclide and added to the severity of the accident. 12 Figure 11-17 Three Mile Island Nuclear Power Plant Photo Courtesy: Nuclear Regulatory Commission. 259 casualties were among the clean-up crew, firefighters, and pilots who died from acute radiation exposure. Environmental damages included the destruction of vast areas of agricultural and farmland and poisoning of major surface and underground water reservoirs. Large areas of Ukraine, Byelorussia, and Russia were contaminated, resulting in the relocation of roughly 200,000 people from within 30 kilometers of the plant. Inhabitants of surrounding areas could not drink water or eat food produced locally. In Europe, many plants and animals were also contaminated and had to be destroyed. The economic damage from the accident is estimated at more than 13 billion dollars (Figure 11-18). Initially, prevailing winds carried the radioactivity northwest from the plant across Byelorussia and into Poland, Hungary, and Sweden. Radiation levels in many parts of Europe rose well above normal before wind carried and scattered the radionuclide a long way away from the reactor site. Long-term effects on public health have been difficult to determine and are subject to considerable controversy, but many believe that the effects will be felt for many years in terms of widespread birth defects and various forms of cancer. Some believe that in Russia alone, 10,000 people will die as a direct result of radiation exposure from the Chernobyl accident over a 70 year period, the average lifespan of humans at the present. Worldwide the number is estimated at 25,000. Most, however, argue that the health problems caused directly by radiation are not easy to measure; many are caused by poor nutrition, lower health care conditions, and the anxiety and stress produced by fear of radiation exposure. What is clear, however, is that the rate of cancer and other deadly diseases among the more than half a million workers who participated in the Chernobyl cleanup has been significantly higher. Environmental and health consequences of radiation exposure are discussed in Chapter 12. Question: A common fear among many ordinary people is that a reactor meltdown may result in an explosion similar to those of nuclear bombs. Could nuclear reactors detonate in this fashion? Answer: No! Explosions of this type are mostly a myth. The fuels used in the construction of nuclear weapons are plutonium and highly enriched uranium (90% or higher). Commercial nuclear reactors use low-grade fuels containing only 3-5% fissionable uranium-235. The remainder is uranium-238, which acts to absorb the neutrons and prevent chain reactions of the type necessary for nuclear bombs. The China Syndrome In 1979, the release of the movie “The China Syndrome” had, and continues to have, a stalwart effect on the public view of nuclear power plants (Figure 11-19). The themes discussed in the film remain relevant, as the issues it raised over the safety of nuclear power are still a source of debate. The title refers to a worst-case scenario of a US reactor core meltdown where the contents of the reactor would have enough heat to Figure 11-18 Chernobyl nuclear power plant after the nuclear explosion. Figure 11-19 An advertising poster featuring the movie “The China Syndrome.” 260 Chapter 11 - Nuclear energy melt a hole through the earth, all the way to China.13 Although scientists agree that this scenario is highly unlikely, the idea of a nuclear accident does give nuclear power an ominous image that is difficult to refute.14 Perhaps the most effective aspect in the film’s power of persuasion was its unintentional release date. The film debuted in March 1979, less than two weeks before the Three-Mile Island accident. The real-life headlines were so similar to the movie’s plot, says China Syndrome executive producer Bruce Gilbert, that he thought “someone had seen the picture and sabotaged the plant.” Together, the film and the parallel crisis sparked a move to pull the plug on the nuclear-power industry. In the following months, several power plants were shut down as safety precautions while plans to open others were scrapped. The latest global public opinion poll conducted by IAEA shows that only 28% of the population consider nuclear power safe. 25% consider it to be dangerous and all existing plants should be shut down.15 Nuclear Power and the Environment The major environmental concern with nuclear plants is the disposal of nuclear waste, which will be discussed in the next chapter. Putting this problem aside, after solar and wind, nuclear plants are among the cleanest of all available electricity-generating alternatives. Because no carbon is present in the fuel, it does not generate carbon dioxide or other green house gases. In fact, compared to coal power plants that produce around 1000 grams of CO2 per kilowatt-hour of electricity generated and natural gas that produce around 500 grams of CO2 per kilowatt-hour of electricity generated, nuclear plants release only 5 grams per kilowatt-hour electricity. Furthermore, nuclear generating stations occupy much less land space than solar or wind power plants of similar capacity. In smaller countries such as France and Belgium, there is a great incentive to prefer nuclear power over fossils and even renewable resources such as solar and wind. Economics of Nuclear Power Relative to other power generating plants, nuclear reactors require large initial capital cost. The operating cost is usually low, and over the life of the plant, nuclear can compete with alternative sources of energy for base load power. If the savings in the cost of pollution abatement and healthrelated issues is included, nuclear power can turn out to be even cheaper than fossil power. Nuclear Power and Public Opinion The debate over nuclear energy dates back over 50 years. 1950s marked the golden era of nuclear energy where it was supposed to produce unlimited 13 14 15 A hole initiated in the United States extended all the way through the earth would not end up in China, but somewhere in the Atlantic Ocean. Sauter, M., Entertainment Weekly, March 20 1998 Issue. “Global Public Opinion on Nuclear Issues and the IAEA,” Nuclear Technology Review 2006, pp.7, IAEA GC(50)/INF/3. 261 amount of energy, too cheap to be metered. Although the dream of “almost free” electricity never materialized, the oil embargo of 1970s provided the new ammunition to nuclear enthusiasts and nuclear energy became once more the cornerstone of US energy policy. The nuclear accidents at TMI in 1979, and Chernobyl in 1986 along with release of the movie “China Syndrome” dampened the public enthusiasm and nuclear power became a “dirty” name, seemingly nobody wanted to be a part of it. The public reaction forced the government to ban construction of new nuclear reactors that is still in effect. The new surge in the price of petroleum, desire for a greater independency from foreign oil, and the rising awareness on threat of CO2 emission and global warming has, however, renewed the debate once again. The new polls suggest that over 50% of the population want to keep the nuclear option open and part of the overall US energy policy. Similar trends can be seen in Europe and Asian countries. Fusion Right after WWII, nuclear technology was perceived to offer a future of cheap, plentiful energy that would replace increasingly scarce fossil fuels. In hope for the transition of nuclear power from wartime to peaceful uses, the general public and even environmentalists embraced this technology, as they foresaw a major reduction in air pollution and reduced strip mining. Nuclear plants do not generate harmful emissions often associated with burning fossil fuels (See Chapter 8), and there appear to be no significant adverse effects to water, land, habitat, species, and air resources. With the passing decades, however, reservations about nuclear energy began to grow as greater attention was focused on the issues of nuclear safety, weapons proliferation, high construction costs, and problems associated with waste disposal took away much of the glory of nuclear plants. At the start of the twenty-first century, some European countries began to reduce their dependence on nuclear energy. Currently, the US is generating only 20% of its electricity needs by nuclear power; this share is expected to decline even further, as there have been no new orders for nuclear plants since 1978. Except for in Asia, where new nuclear power plants are still being constructed, the growth for nuclear power is expected to be limited. Fortunately, as demand for nuclear fission reduces, a new technology called “fusion” is rekindling an interest in nuclear energy. Fusion is the same process that powers our sun (Figure 11-20). The enormous amount of energy released by the sun in the form of radiation is a result of the conversion of some of its mass to energy, as predicted by Einstein’s equation E = mc2. Unlike in fission reactions, atoms are not split apart, but instead are fused together, hence the name nuclear fusion. In the case of our sun (and all other stars), massive gravitational forces compress and heat deuterium and tritium gases to form plasma. Plasma is sometimes Figure 11-20 Fusion is responsible for energy released by suns and stars. Solid Liquid Gas Plasma Figure 11-21 Four states of matter 262 Chapter 11 - Nuclear energy referred to as the fourth state of matter (Figure 11-21) and is formed when gases are heated to a temperature that is high enough to cause ionization. This process knocks electrons out of the atom, causing it to be left with a positive charge. As the plasma is heated further, eventually a point will be reached where the positively charged atomic nuclei overcome their mutual electrostatic (Coulomb) repulsion and fuse together. In the process, helium, neutrons, and a tremendous amount of energy are produced. Researchers inspired by such reactions are working feverishly to replicate these processes on earth. If successful, fusion reactors have the potential of providing unlimited energy with fewer disadvantages than fission reactors. Physics of Fusion The thermonuclear fusion in the sun involves a series of reactions starting with the fusion of two atoms of hydrogen nuclei (protons) to produce deuterium and a positron (positive electron). The deuterium subsequently interacts with a proton to produce a helium-3 isotope. Finally, two He-3 atoms are fused together to produce a He-4 nucleus (alpha particle). The reactions involved in the solar thermonuclear process are far too slow to be useful for producing energy here on earth. A better approach would be to use deuterium or a mixture of deuterium and tritium. Two specific reactions of particular interest are: D + D 100 million degrees 3He + 1n + 3.3 MeV D + T 45 million degrees 4He + 1n + 17.6 MeV Unfortunately, these isotopes will not fuse to each other at ordinary temperatures and pressures because they are all positively charged. The attractive forces exist only over very short distances; the repulsive forces between the positive nuclei and negative electrons are too strong to allow these nuclei to bring them close enough unless their average speeds (temperature) are raised. The D-D reaction has an ignition temperature of 100 million degrees, much higher than the ignition temperature of 45 million degrees needed for a D-T reaction. If normal hydrogen is used as the fuel for fusion, the required temperature would be even greater. The D-T reaction is currently the reaction of choice (Figure 11-22). Deuterium is naturally present in seawater, but at a very low concentration– one atom for every 7,000 atoms of hydrogen. Tritium is also very rare and must be manufactured. One possible source of tritium is lithium, which is plentiful both in the earth’s crust and in seawater. Indeed, one gallon of sea water contains enough hydrogen isotopes for fusion to equal the energy which would be released by burning 300 gallons of gasoline. Large amounts of tritium can be obtained if an isotope of lithium (Li-6) is bombarded by neutrons in a reactor. 4 n + 6Li He + 3T neutron + lithium helium + tritium 1 Deuterium Energetic Neutron Tritium Fusion Reaction Helium Nucleus Figure 11-22 In a thermonuclear reaction, an atom of deuterium and an atom of tritium fuse to produce an atom of helium. A neutron and a great amount of energy in the form of gamma rays are produced. A temperature of 45 million degrees is required to initiate the ignition. 263 Fusion Dilemma The basic challenges facing the commercial development of fusion reactors are (a) to achieve ignition with a net gain in the energy, beyond the break-even point; and (b) to find a method that supports fusion chain reaction in a sustained manner. The break-even point occurs when the amount of energy released by the nuclear reaction exceeds the amount of energy required to raise the temperature (and pressure) of the reactants to the level needed to initiate fusion (namely, Q of 1). These requirements can be reduced to three tasks of acquiring: 1. Very high temperatures (In excess of 100 million degrees). This is needed to overcome the repulsive forces of the positively charged nuclei. 2. Very high compressions. This increases the collision frequency and the rate of reaction. 3. Long residence times. This is needed to keep the reactants in proximity long enough to sustain the reaction; i.e. to increase energy production at a rate greater than that needed to initiate the reaction. In 1991, the Princeton Tokomak was the first fusion reactor to reach this critical break-even point for about two seconds, opening the door for further research toward development of commercial nuclear fusion reactors. The current world record for a sustained high-temperature, high-pressure plasma is held by JT-60, at 24 seconds. To make the reaction sustainable, we need to find a way to allow this reaction to proceed at a much larger scale and in a continuous way. Obviously, no vessel exists that can tolerate the extreme temperatures and pressures required for the fusion reaction to take place. Three approaches have been investigated: a) gravitational confinement, b) inertial confinement, and c) magnetic confinement. Gravitational confinement is what happens in the sun, where gases are compressed and heated by gravitational forces. Gravitational confinement cannot be duplicated on earth. Inertial confinement mimics a short-lived microminiature star; a spherical capsule containing a deuterium-tritium (D-T) mixture is heated and compressed by an array of powerful lasers directed toward the pellet for a few nanoseconds. In the third approach, magnetic confinement, hot ionized gases are confined in a toroidal path that is held away from the reactor walls by strong magnetic fields. Fusion: The Facts FYI... • Each day, our sun loses 5x1016 kg of its mass and produces 3.6x1031 joules of energy. • The deuterium contained in one liter of sea water can generate as much power as 2.5 barrels of petrol. • Relative to a fission reaction, fusion produces little radioactive material. 264 Chapter 11 - Nuclear energy Inertial confinement was first used in the design of the Shiva16 reactor at Lawrence Livermore National Laboratory (LLNL) in California. The original lasers were later replaced by NOVA lasers which were ten times more powerful (Figure 11-23). Very strong laser pulses strike and uniformly heat the deuterium-tritium pellet (made of substances such as lithium deuteride, the active ingredient of a hydrogen bomb) placed inside a vacuum sphere. The laser beams blast and incinerate the pellet and compress the gas to very high temperatures and pressures that are required to initiate fusion.17 The next generation high powered lasers are being constructed to deliver at least 60 times more energy than any previous laser system. The National Ignition Facility (NIF) -- scheduled to be completed in 2010 – plans to deliver two million joules (2 MJ) of ultraviolet laser energy by focusing its 192 giant lasers to about 4 billionths of a second (producing 500 trillion watts of power) on a tiny 0.5-millimeter-diameter capsule in the center of its target chamber, creating conditions similar to those that exist only in the cores of stars and giant planets and inside nuclear weapons.18 The preliminary results seem to indicate that this approach will probably not be as successful as the magnetic confinement approach used in a number of other facilities throughout the world ( JT-60 in Japan, Tokomak Fusion Test Reactor (TFTR) in Princeton (Figure 11-24), and EU’s Joint European Torus ( JET) located in England). Magnetic confinement is a process where hot hydrogen plasma is circulated inside a huge coil of wire (magnetic bottle) that is cooled until it becomes a superconductors. If a current flows through this coil, it creates a magnetic field that confines the plasma within the torus shaped cavity. The plasma is continuously heated by the magnetic field as it circulates through the coil. Once it reaches the fusion temperature, deuterium and tritium fuse to form helium and neutron. The excess energy is carried by neutrons and deposited in a lithium jacket and produce electricity. The 500 MW International Thermonuclear Experimental Reactor (ITER, “the way” in Latin) being based on Tokomak design is a joint, €10 B ($16 B) cooperative effort between the European Union, Japan, the US, Russia, China, India, and South Korea. ITER will be built in Cadarache, France, for operation in or near 2016 (Figure 11-25). The initial goal is to sustain a deuterium-tritium chain reaction and generate ten times as much energy as was put into it, to a Q of 10. Furthermore, research will be conducted to test critical issues of fusion safety (such as finding materials that can withstand the high-energy neutrons created by the ITER, coolants, etc.) that must be resolved before the full-scale 16 17 17 Figure 11-23 Fusion inertial research facilities in Livermore, California (NOVA). Figure 11-24 The Tokomak Fusion Test Reactor (TFTR) in Princeton, New Jersey. The design was originally proposed in the early 1950’s by Igor Tamm and Andrei Sakharov of Moscow University. Electric current produces a magnetic field spiraling around a torus or donut where the plasma forms a continuous circuit. Figure 11-25 The ITER device. Source: ITER, http://www.iter.org. S hiva often called the Destroyer or The Lord of the Dance is the Hindu God of many hands and is attributed with great power and physical prowess. M aniscalco, J. A. , “Inertial Confinement Fusion,” Ann. Rev. Energy, 5; 33-60, 1980. Yamanaka, C., “Inertial confinement fusion: The quest for ignition and energy gain using indirect drive,” Nucl. Fusion 39 825 (1999). 265 commercial reactors can be built. Cold Fusion When people talk about fusion, they are mostly referring to “hot fusion”. There has been much talk about the possibility of “cold fusion” in the last couple of decades. In 1989, Stanley Pons of the University of Utah, and Martin Fleischmann from the University of Southampton in the United Kingdom, claimed that they had achieved nuclear fusion in a simple photochemical cell.19 Cold fusion takes advantage of an elementary particle called a muon. Muons have negative charges, and when they collide the nucleus they cancel positive charges of protons, thus eliminating the need for extremely hot temperatures to overcome the repulsive forces of hydrogen nuclei.20 These experiments generated enormous publicity around the world, but were later denounced by leading experts who could not verify and reproduce the results. Although there are still some researchers who believe that cold fusion is possible, no credible experimental proof has yet to be presented! Future of Fusion Because of a wide range of technical difficulties that must be overcome, it is difficult to predict when fusion will become practical. As with many other research efforts, economics will have a major impact on the degree of progress that can be achieved. Because of its complexity and cost, fusion research requires not only cooperation between private industries, utility companies, and the government, but also a high degree of international collaboration. The optimistic view is that commercial fusion power plants can be constructed just before the earth’s fossil fuels are depleted, somewhere around the years 2045-2060. Summary Nuclear power has been shown to be an economical, clean source of energy with no emission of carbon dioxide, acid rain, and other chemical pollution. Public concern about radiation associated with nuclear fuel production, waste disposal, nuclear proliferation, and now terrorism has, however, made nuclear fission controversial. Furthermore, the nuclear accidents in Three Mile Island and Chernobyl and the resulting fallout of radio nuclides heightened the fear that what happened by accident could also happen by design. According to the latest poll carried out on behalf of the European Commission21, only 7% of the public is “totally in favor” of nuclear power. 19 20 21 E ditorial (April, 2004c): “DOE Warms to Cold Fusion”. Physics today. R afelski, H. E., et al., “ C old fusion: muon-catalysed fusion,” J. Phys. 9: At. Mol. Opt. Phys. 2 4 (1991) 1469-1516. Special Eurobarometerm Radioactive Waste, commissioned by DG TREN, European Commission, September 2006. 266 Chapter 11 - Nuclear energy Many believe the problem is not of a technical nature, but of a social nature instead. The public has lost confidence in the government and political and economic influence by various lobbyist groups. Unless concrete measures devoid of political influence are taken to assure the public of nuclear safety, for many, the nuclear option may remain one of the last choices to meet our energy needs. Unlike fission, the physics of fusion make it inherently safer. A fusion reactor cannot go through a meltdown, and the waste generated by fusion is less radioactive and has a shorter half-life, therefore it is easier to dispose. The fusion fuel, deuterium, is found in ordinary seawater and is cheap and abundant. However, existing technical problems make the reality of fusion technology a distant dream. Cold fusion, if proven possible, eliminates much of the technological difficulties associated with hot fusion and does so at a very low cost. This would level the playing field and provide the less developed and poorer countries with an inexpensive source of energy. Because of its high costs, hot fusion will potentially be available only to rich and developed countries, giving these countries the power to monopolize the energy industry. Additional Information Books 1. Bodansky, Nuclear Energy Principles, Practices, and Prospects, Second Ed., Springer, 2004. 2. Seaborg, G., T., Peaceful Uses of Nuclear Energy, University Press of the Pacific, 2005. Periodicals 1. International Journal of Nuclear Engineering and Design, Direct Science Elsevier Publishing Company, devoted to the Thermal, Mechanical, Material and Structural Aspects of Nuclear Fission. 2. Journal of Fusion Energy, Springer Netherlands. It features articles pertinent to development of thermonuclear fusion. Government Agencies and Websites 1. Federation of American Scientists (http://www.fas.org/nuke/intro/ nuke/index.html). 2. International Atomic Energy Agency (http://www.iaea.org). 3. DoE Office of Nuclear Energy, Science & Technology (http://www. ne.doe.gov). Non-Government Organizations and Websites 1. American Nuclear Society, (http://www.ans.org). 2. World Association of Nuclear Operator (WANO) (http://www.wano. org.uk). 267 Exercises I. Consult the periodic table of elements and answer the following questions: 1. Consult the periodic table to determine the atomic numbers of the following elements: a. Helium b. Iodine c. Chlorine d. Lead e. Barium 2. The atomic structures of gold and mercury are 01 97 represented as (179 Au) and (280 Hg) . For each of these atoms, determine: a. The atomic number b. The atomic mass c. The number of protons d. The number of neutrons II. Select the term that best fits the definition given: ______1. Nuclear reaction involving splitting atoms. ______2. Nuclear reaction in which two light atoms are joined together. ______3. The elementary particle that differs in number between different isotopes of an atom. ______4. The elementary particle in number equal to the number of protons in a nucleus. ______5. Sequence of reactions that release neutrons that cause additional atoms to fission. ______6. Material used to slow down the neutrons and improve their chance of capture by nuclear fuel. ______7. Component of the reactor core responsible for absorbing the excess neutrons and prevent the runaway reactions. ______8. The process of concentrating uranium-235 to make it suitable for nuclear fuel. ______9. The type of reactor that uses uranium-238 as its fuel. ______10. The type of reactor that uses graphite as the moderator and helium as the coolant. 268 III. Essay Questions 1. What are the differences between chemical and nuclear reactions? In each case, which of the following quantities are conserved? a. Number of nucleons b. Number of electrons c. Number of atoms d. Number of molecules 2. An atom of the element iron can be represented as 56 Fe. Determine the number of neutrons, the 26 number of protons, the atomic mass and the atomic number for iron. 3. What is a half-life? How many half-lives does it take for the activity of a radioactive isotope to drop by a million times? 4. What does NIMBY stand for? 5. Explain the fuel cycle, from mining to storage. 6. What were the causes of the TMI and Chernobyl accidents? What are the differences in design of the two reactors? 7. What is a chain reaction? How can a chain reaction be stopped? What will happen if chain reaction continues indefinitely? 8. What is the magnetic confinement? Inertial confinement? 9. A radioactive substance with a half-life of 5 years contains 6,000 radioactive atoms. How many atoms in the sample are expected to remain radioactive after 20 years? 10. A quantity of radioactive substance has lost 15/16 (94%) of its radioactivity after one minute. What is its half-life? 11. If there is a nuclear power plant near you, - What is it called? - When did it start operating? - What type is it? - What is its electric generating capacity? Chapter 11 - Nuclear energy 12. Name three advantages and three disadvantages of nuclear power plant. 13. What is fusion? How does fusion release energy? W here does fusion occur in nature? What are the basic fusion reactions? 14. What are the main obstacles to development of fusion reactors? 15. Name three approaches proposed as possible alternatives to storing nuclear wastes. What are advantages and disadvantages of each? IV. Multiple Choice Questions 1. The mass of a nucleus is_________ the total mass of its separate constituent particles. a. Equal to b. More than c. Less than 2. The nucleus of ________is fissile, while that of _________is fertile. a. 235U, 238U b. 235U, 239Pu c. 238U, 235U d. 239U, 235U e. 238U, 239Pu 3. Two isotopes of the same element can have the a. Same number of electrons but a different number of protons b. Same number of neutrons but a different number of protons c. Same number of protons but a different number of neutrons d. Same atomic mass but a different atomic number e. Same atomic mass and same atomic number 4. The energy released in a nuclear reaction (fission or fusion) is associated with a. A decrease in mass b. An increase in total mass c. An increase in total binding energy d. No change in total mass e. None of the above 5. The reason that alchemists could not produce gold is that a. They did not use the right chemicals b. They did not know how the chemical reacted c. They did not know about the structure of atom d. They did not have a nuclear reactor e. Both c and d 6. Energy released from a nuclear reaction is the result of a. Reconfiguration in the molecular bonds b. Change in the number of electrons c. Change in the number of atoms d. Change in the binding energy of the nucleus e. All of the above 7. The breeder reactor may eventually substitute light and heavy water reactors because a. It is safer than the BWR and PWR b. We need to use 235U instead of 238U c. We need to use 238U instead of just 235U d. It has a higher efficiency e. It is easier to construct 8. The known supplies of uranium (U-238) in the United State are estimated to be a. 400,000 tons b. 1 million tons c. 4 million tons d. 1 billion tons e. Practically unlimited 9. The largest reserves of uranium lies in a. The United States b. Kazakhstan c. Russia d. South Africa e. Australia 10. Nuclear fission can be best described as a. The direct conversion of nuclear energy to electrical energy b. A chemical reaction involving uranium atoms c. A process where uranium atoms ionize to produce energy d. The splitting atoms accompanied by release of 269 a large amount of energy e. The splitting atoms by electron bombardment 11. In the equation E = mc2, E represents a. The potential energy of an object of mass m b. The kinetic energy of an object of mass m c. The total energy of an object of mass m d. The internal energy of an object of mass m e. None of the above 12. Nuclear chain reactions can be controlled by a. Controlling the number of electrons released by the fission process b. Controlling the number of protons released by the fission process c. Controlling the number of neutrons released by the fission process d. Controlling the number of photons released by the fission process e. Controlling the electromagnetic radiation released by the fission process 13. How long would it take for the strength of a 10,000-curie radioactive source to drop to only 10 curies? a. 33 years b. 75 years c. 150 years d. 300 years e. 10,000 years 14. Which of the following statements applies to nuclear reactors? a. A neutron smashes the nucleus of a uranium atom with no neutrons produced. b. A neutron smashes the nucleus of a uranium atom with 2-3 additional neutrons released on average. Each neutron goes on to smash additional nuclei. c. A neutron smashes the nucleus of a uranium atom with 2-3 additional neutrons released on average. All except one neutron is captured. d. Is essentially the same as a nuclear bomb. e. Differs from a nuclear bomb only in the amount of energy that it releases. 15. Which of the following statements is correct? a. Both moderator and control rods absorb excess 270 b. c. d. e. neutrons. Both moderator and control rods slow the neutrons down. The control rods slow the neutrons down, whereas the moderator captures the excess neutrons. Depending on the reactor type either moderator or control rods are used. The moderator slows the neutrons down so they can be captured by the fuel rods. 16. Which one of the following is a principal disadvantage of nuclear power? a. Dangerous waste can be produced. b. Greenhouse gases are not produced. c. Greenhouse gases are produced. d. It is not safe to operate. e. The cost is much higher than a fossil fuel reactor. 17. A suitable fuel for a light water reactor is a. Natural uranium containing 0.7% 235U b. Uranium enriched to contain at least 3% 235U c. Uranium enriched to contain at least 90% 235U d. Uranium enriched to contain 99% 235U e. Pure 235U 18. A suitable fuel for a heavy water reactor is a. Natural uranium containing 0.7% 235U b. Uranium enriched to contain at least 3% 235U c. Uranium enriched to contain at least 90% 235U d. Uranium enriched to contain 99% 235U e. Pure 235U 19. A suitable fuel for a fast breeder reactor is a. U-235 b. U-238 c. Pu-239 reprocessed from U-238 d. Deuterium e. Hydrogen 20. A suitable coolant for most fission reactors is a. Ordinary water b. Heavy water c. Graphite d. Sodium e. Helium Chapter 11 - Nuclear energy 21. A suitable moderator for most fission reactors is a. Water or graphite b. Liquid sodium or argon c. Water or liquid sodium d. Carbon dioxide or helium e. Boron or cadmium 22. “Chain reaction” is a common occurence in a. Nuclear reactors and weapons b. Rumors and legends c. Spread of diseases like cancer and smallpox d. Population growth e. All of the above 23. Most European nuclear reactors use _________ as a moderator. a. Graphite b. Ordinary water c. Heavy water d. Sodium e. Helium 24. Which one of these statements is true about the moderator and the control rods in a nuclear power station? a. The moderator and control rods slow the neutrons down. b. The moderator and control rods absorb the neutrons. c. The moderator slows the neutrons down, while the control rods absorb the neutrons. d. The control rods slow the neutrons down, while the moderator absorbs the neutrons. e. None of the above. 25. The reactors at the Three Mile Island plant are a. Boiling light water reactors b. High temperature gas reactors c. Pressurized heavy water reactors d. Pressurized light water reactors e. Fast breeder reactors 26. The reactors at the Chernobyl plant are a. Water-cooled with water moderators b. Water-cooled with graphite moderators c. Gas-cooled with water moderators d. Gas-cooled with graphite moderators e. Graphite cooled with water moderators 27. The principal cause of the explosion in the nuclear accident at Chernobyl was a. An uncontrolled chain reaction in the fissile material b. A chemical reaction between elements released by the overheated reactor c. Excess pressure of steam in the reactor d. A massive electrical short-circuit e. All of the above 28. Which organization is in charge of inspecting and ensuring safeguard of nuclear power plants around the world? a. US Atomic Energy Commission b. International Atomic Energy Commission c. Defense Nuclear Agency d. Department of Energy e. Department of Defense. 29. Fusion reaction requires that plasma be heated to very high temperatures because a. Enough energy is needed to split the atoms b. Combining particles are positively charged c. Combining particles are negatively charged d. Combining particles have opposite charges e. The higher the temperature of the plasma, the higher is the fusion efficiency 30. Most nuclear fission reactors use fuel rods enriched to a. About 0.7% uranium–235 b. 3% or greater in uranium–235 c. 90% or greater uranium-235 d. 3% or greater in uranium–238 e. 3% or greater in plutonium–239 31. The main advantage of plutonium breeder reactors over conventional hot water reactors is a. Reduction of the operation cost of the reactor b. Reduction of thermal pollution c. Extension of the lifetime of uranium reserves d. Reduction of the amount of plutonium produced e. Reduction of the threat of nuclear weapons proliferation 271 32. The energy released in a nuclear fission reaction is proportional to a. The mass which disappears b. The mass of the neutron c. The mass of the proton d. The mass of 235U e. The number of neutrons emitted 33. What is the fuel used in most present-day nuclear power plants? a. Coal b. Uranium-238 c. Uranium-235 d. Platinum-196 e. Plutonium-239 34. Most nuclear reactors in operation today are of this type. a. Coal b. Fission c. Fast breeder d. Hot fusion e. Cold fusion 35. The coolant most suitable in a fast breeder reactor is a. Water b. Graphite c. Liquid sodium d. Carbon dioxide e. Helium V. True or False? 1. All the atoms in a material have the same number of neutrons in their nuclei. 2. The only difference between various isotopes of an atom is in the number of neutrons in their nuclei. 3. The nuclei of a normal hydrogen atom is called a proton. 4. All the atoms in a material have the same number of electrons. 5. Most instances of cancer in the United States are caused by man-made radiation. 272 6. All nuclear materials remain highly toxic for thousands of years. 7. For an equal radiation dose, man-made radiation is more toxic to humans than naturally occurring radiation. 8. About half of all electricity in the world is generated by nuclear reactors. 9. Heavy water is ordinary water that has been enriched by the addition of deuterium. 10. The function of a control rod is to slow down fast neutrons to thermal neutrons. 11. The easiest way to shut down a reactor is to remove the fuel rods. 12. In case of a rapid temperature rise in the core and the need for rapid reactor shutdown, all the control rods are lowered immediately. 13. Nuclear power plants can explode like a nuclear bomb. 14. On average, people receive more radiation from nuclear power plants than from building materials and radon gas in homes. 15. Most high-temperature, gas-cooled reactors use helium as a coolant. 16. The only commercial pebble bed reactors operating today is in the United States. 17. Breeder reactors use uranium-238 instead of uranium-235 for fuel. 18. The first cold fusion experiment was successfully concluded in 1989. 19. It is easier to achieve fusion by fusing atoms of D-T than D-D. 20. The mass of a nucleus is less than the sum of the Chapter 11 - Nuclear energy masses of its elementary constituents. VI. Fill-in the Blanks 1. Isotopes have the same number of ______ but a different number of _______. 2. The mass of an atom is ______ than the sum of the individual masses of its protons and neutrons. The extra mass is a result of the _________ that holds the protons and neutrons of the nucleus together. 3. As a result of isotopic decay, unstable carbon-14 changes to stable _________________. 4. It is possible to extend the remaining supply of uranium by converting U-238 to Pu-239 and using it as the fuel for ________ reactors. 5. Isotopes of an element that can be split through fission are called _________. 6. The four main components of conventional nuclear reactors are the fuel rod, _________, moderator, and __________. 7. _________ has the greatest number of nuclear reactors; ______________ and Lithuania are fulfilling the greatest percentage of their electricity need using nuclear energy. 8. The process in which high-temperature liquid waste is dried is called ____________. 9. Uranium ore is commonly referred to as _______________. 10. When the temperature in the core rises, _____ rods are lowered and the energy output decreases because fewer ________ are available to sustain chain reactions. 11. The principal causes of major nuclear accidents are the production of an excessive number of _______________ and the loss of coolants. 12. All US commercial reactors are of the ______ _______ water reactor type. 13. A breeder reactor produces more ______ than it uses. 14. The most serious nuclear accident that has ever happened was in Chernobyl in the Republic of _________. 15. PWRs and BWRs are both _________ water reactors. 16. The agency responsible for granting operating licenses and oversee a safe operation of nuclear reactors in the United States is ____________. 17. Fusion reactors are not expected to be commercially available before ____________. 18. Compared to natural gas power plants, nuclear plants produce ________ amount of greenhouse gas emissions. 19. Uranium separated from rocks and concentrated by a series of chemical processes is called the ______________. 20. The fuel made of recycled uranium and plutonium oxides from spent fuel is called ___________. VII. Project - Half-Life of Pennies In this experiment you will use a statistical approach to gain a better understanding of the concept of half-life. For this experiment you need 100 pennies (or other coins), a large tray, a pencil, and a piece of paper. Procedure: 1. Lay all pennies heads up in the tray. 2. Shake the tray for a few seconds; remove all the coins which have turned tails up. Count the number of the remaining pennies in the tray (those which stayed heads up). Enter the data in a table similar to Table 1. 3. Shake the tray some more; remove all the coins that have turned tails up. Enter the number of heads in the table. 4. Repeat this procedure until no pennies are left on the tray. 273 5. Now repeat the whole experiment two more times. Data reduction: a. On graph paper, plot the number of pennies remaining in the tray versus the trial #. Make the y-axis represent the number of heads and x-axis the trial #. b. On the same graph, repeat the procedure for the second and third attempts. Use different symbols or colors to distinguish between different attempts. c. Take the average number of heads for each trial and enter it in Table 4. Plot this graph superimposed on top of the earlier graphs. Now answer these questions: 1. How reproducible were your data? 2. How does this experiment illustrate the behavior of radioactive isotopes? Which set of results represents the best model of half-life? 3. How many trials does it take for the population of pennies to reduce to 50? To 25? 4. What is the rate of decay of pennies in the tray? 5. Compare your results with those of three other classmates. What do you conclude? Table 1. Half-Life of Pennies 1 Trial # 0 1 2 3 ..... 2 3 No of Heads No of Heads No of Heads 274