Nuclear Energy & Society

Transcription

1 Nuclear Energy & Society By Ilan Lipper & Jon Stone Introduction Only 30 years ago, nuclear energy was an exotic, futuristic technology, the subject of experimentation and far fetched ideas. Today, nuclear energy is America's second largest source of electric power after coal. More than 110 nuclear energy plants supply more electricity than oil, natural gas or hydropower. Since 1973, they have saved American consumers approximately $44 billion, compared to the other fuels that would have been used to make electricity. Since our electricity system is interconnected, practically every American gets some electricity from nuclear energy. In addition to the economic benefits achieved through the use of nuclear energy, there are environmental benefits as well. There are, however, various drawbacks caused by the production of electricity through nuclear power. Although there are various risks involved when using nuclear energy as a source of power, we argue that the benefits greatly outweigh any potential problems that may arise. Nuclear Reactors and Their Fuel Cycles

2 The use of nuclear reactors to generate electricity continues to increase all over the world. By December of 1979, about 128,000 million watts were being generated by 249 reactors operating in 22 countries. around the United States Nuclear reactors Before we can truly understand how a nuclear reactor works, we must first examine the processes that occur in its core. In order for a reactor to work there needs to be at least one free neutron per fission. Nuclear reactors are fueled by uranium or plutonium in a solid form. They are ceramic pellets approximately the size of the end of your finger. These pellets are placed into 12 foot long, vertical tubes, which are bundled together and placed underwater inside the reactor. When the plant starts up, neutrons are let loose to strike the uranium atoms or the plutonium atoms. When the neutrons hit either of these types of atoms in pellets, the atoms split to release neutrons of their own, along with heat. On average 235U and 239Pu yield two free neutrons. Initial fissioning of 235U produces neutron energies of 2 Mev. To convert to more everyday units, this is equal to approximately 3.2 x 10-11J. These neutrons must be slowed down in order

3 to increase the fission probability in the core of the reactor. The way in which these neutrons slow down is by hitting something that has approximately its own mass. Water is effective at slowing down neutrons. Once the neutrons slow down, they go back into Uranium and fission probability increases considerably. Heat is then transferred from the core of the reactor to the water and then induces steam. Sometimes a neutron and proton will combine and produce a deuteron and therefore that neutron is now lost. Companies use heavy water in order to alleviate this dilemma. Some neutrons are captured directly by 235U or 238U and gamma rays are emitted. Some neutrons simply escape form the core altogether. These are considered fast neutrons. These fast neutrons have the ability to produce a fission reaction with 238U to produce heat and more neutrons. 239U could also be produced if 238U were to capture a slow neutron. The product rapidly decays into 239Pu. 239Pu has a greater fission probability than 235U, hence as 239Pu builds up, it fissions and contributes fuel (neutrons) to the reactor. Control rods absorbs neutrons in order to keep the number of neutrons, and therefore, the reactions are controlled. They are usually made of boron steel or graphite, since they are high neutron absorption material. Pressurized Water Reactor System Pressurized water reactors and boiling water reactors are the two major types of generators that the US. uses to produce electricity. Pressurized water reactors consist of a single fuel element assembly of up to 200 zircaloy cadded fuel 'pins'. These 'pins' are immersed in a large steel pressure vessel containing ordinary 'light' water. The light water serves as both a coolant and moderator. Light water has a higher neutron-absorbing capacity than heavy water (D2O). This causes it to increase the percentage of 235U in the core. Uranium dioxide is a source of fuel for this reactor. The pressure vessel consists of control rods that pass through the lid, the light water under pressure, and the reactor core. The water attains a temperature of approximately 270 C without boiling, due to a pressure of about 13.8 to 17.2 MPa. This pressure is maintained through a pressurizer. The 'light' water passes in a closed circuit to a heat exchanger. This causes

4 the water in the heat exchanger to heat up and convert to steam. This steam drives one or more turbine generators, is condensed, and pumped back to the steam generator. Another stream of water from a lake, river, or cooling tower, is used to condense the steam. It is necessary to shut down the reactor completely, remove the lid, and replace an appropriate portion of the fuel pin assembly, in order to refuel it, which occurs every 12 to 18 months. (see above figure) A potential danger exists with the possibility of a rupture of the cooling system tubing. If this were to occur there would be no way of preventing the reactor from overheating. Due to this danger, reactors are surrounded by a double-walled pressure containment building and contain a number of emergency core-cooling systems. A more efficient way of removing heat is allowing water to boil. The boiling water reactor allows the coolant within the reactor core to boil. The steam generated is then separated, dried, and passed directly to the turbine generators. After going through the generators, the steam is condensed and passed back into the reactor core. Like the pressurized water reactor, the boiling water reactor fuel is 235U, enriched as uranium dioxide. In addition, the steam collection also occurs on top of the reactor. One other thing the boiling water reactor has in common with the pressurized water reactor is that it must be shut down for refueling. (see above figure) As far as safety is concerned, the entire reactor is housed within a primary containment chamber which incorporates, underneath, a large ring-shaped tunnel somewhat filled with water. If any water or steam were to escape, it enters this tunnel, and condenses. In addition to this tunnel, there are several emergency systems in place.

5 Pressurized Water Reactor. This is a Life-like The Benefits of Using Nuclear Energy Powering Our Economy: Since the oil embargo of 1973, Americans have used energy more wisely and more efficiently. During this time, our population has grown from 211 million to almost 280 million, our economy has grown about 50 percent, but our use of energy has grown only 10 percent. But our economic growth, however, has been fueled largely by electric power. Between 1973 and 1990, our GDP, which is the measurement of a nation's wealth, grew by about 50 percent. In the same period, electricity use grew by 58 percent. From this information, we can conclude that in order to meet the needs of our strong economy and our growing population, we must have reliable supplies of electric power. The nation's nuclear power plants produced 674 billion kilowatt-hours of electricity in This was more electricity than the entire country consumed in the early 1950s. Worldwide, there are 442 nuclear power plants at work, contributing about 19 percent of the world's electricity supply.

6 Reduction of Dependence on Oil: At the time of the 1973 oil embargo, oil accounted for about 17 percent of US. electric supply; nuclear energy was about 5 percent. In 1990, however, oil represented only about 4 percent of U.S. electric supply, while nuclear energy accounted for about 21 percent. Consequently, the U.S. imports 20 million barrels less of oil each year. For example, since l973, nuclear energy has displaced 4.3 billion barrels of imported oil and reduced our trade deficit by $12 billion. This decrease in our trade deficit causes a direct increase of our Gross National Product, which is also measure of a nations wealth. Protecting Our Environment: Nuclear energy plants produce electricity through the fission of uranium, not the burning of fuels. Consequently, nuclear power plants do not pollute the air with nitrogen oxides, sulfur oxides, dust or greenhouse gases like carbon dioxide. America's nuclear energy plants reduce electric utility emissions of greenhouse gases by 20 percent, or 128 trillion tons per year. Without our nuclear power plants, electric utility emissions of nitrogen oxides would be 2 million tons per year higher. Emissions of sulfur dioxide would be 5 million tons a year higher. Thus, nuclear energy has drastically cut our dependence on foreign imported oil. In France for example, from 1980 to 1986, SO2 and NOX emissions in the electric power sector were reduced by 71% and 60% respectively, causing reductions of 56% and 9% respectively, in total SO2 and NOX emissions in France (Trudeau 160). Nuclear energy also offers an alleviation of the global carbon dioxide (CO2) problem that the world can do without. About 1,600 million tons of CO2 annual emissions would have resulted if

7 16 percent of the world's electricity now generated by nuclear power were to have been generated using coal. This is a significant amount. In fact, it is 8 percent of CO2 now emitted annually from the burning of fossil fuels. Another important benefit that nuclear generated energy has on our environment is that the wastes produced are completely isolated from the environment. Would we have produced the electricity with coal instead of nuclear energy, at least 90,000 tons of toxic heavy metals would have been released, in addition to tremendous amounts of CO2, SO2, and NOx. Some of these toxic heavy metals include arsenic, cadmium, lead, and mercury. Although the radioactive wastes produced by nuclear energy may be dangerous for thousands of years, part of the waste caused by the burning of coal remains dangerous forever. The environmental benefits of nuclear energy can he seen clearly in France. In the 1980s, because of concerns over imported oil, France more than tripled its nuclear energy production. During that same period, total pollution from the French electric power system dropped by percent. Worldwide Benefits: More than 400 nuclear power plants are operating in 25 countries around the world today, supplying almost 17 percent of the world's electricity. In most countries, nuclear energy plays an even larger role as a source of electricity than in the United States. Many of these nations are building new nuclear energy plants to meet the needs of their growing populations and expanding economies. About 83 new nuclear energy plants are currently being built around the world.

8 The Drawbacks to using Nuclear Energy Despite the fact that nuclear energy offers great benefits as an alternative source of electric power, nuclear energy as a whole, is still a controversial issue in many countries. The reasons for this center round the issues of safety, waste, and nuclear weapons. Nuclear Safety National and international anxiety about nuclear power stems directly from a fear of release of radioactive material and its consequences on people and the environment. The problem, however, is that there is a huge information gap between specialists on the exposures from nuclear power and the public. When one looks at the 1991 report by the United Nations Scientific Committee on the Effects of Atomic Radiation, (UNSCEAR) one would see that the routine generation of nuclear electricity releases only negligible amounts of radioactive materials to the environment. "The average dose any individual in the world receives each year from all of the activities in the peaceful nuclear fuel cycle is less than 0.1 percent of the inevitable exposures he or she receives from natural radiation sources, such as cosmic rays and radon emitting building materials" ( Trudeau 59).

9 One has to accept that electricity production can't be totally free of risk. The accident at Chernobyl, in the former USSR, was undoubtedly the most severe radioactive accident the world has experienced since the arrival of nuclear energy as an alternative source of electric power. Although there 31 deaths can be attributed to the Chernobyl accident, there are many misgivings about the true nature of the accident. For example, the people who died, including the nuclear operators and the figherfighters, received very high doses, unlike the surrounding areas that were relatively safe from exposure to high radiation levels. "Contrary to some erroneous reports, no accurate health effects from the incident have been found in the population in the Ukraine and Byelorussia. Elsewhere in Europe, countermeasures taken in many countries immediately after the accident effectively reduced the levels of exposure to the public" (Trudeau 159). One can also see from UNSCEAR data that outside of the Soviet Union, the Chernobyl accident has emitted a dose that is a fraction of what the population receives every year from natural radiation found. One positive result from the tragic Chernobyl accident is that there is now increased awareness and commitment of the nuclear community to international cooperation in the field of safety. "Through the efforts of utilities and governments, of the IAEA and others, an international nuclear safety regime is emerging, which includes a wide range of arrangements for improving operational safety and emergency preparedness and response to accidents" (Trudeau 159). The United States has also had a serious accident concerning the production of nuclear energy. "An accident with potential for a core meltdown occurred in the PWR at the Three Mile Island Nuclear Station Unit 2 near Harrisburg, Pennsylvania, on March 28, 1979" ( Glasstone 105). The three Mile Island accident appears to have resulted from a combination of design deficiencies, inadequate procedures, and operator errors. "The consequences will be far reaching" (Glasstone). Like the Chernobyl accident, some good has come from the accident at Three Mile Island. After the accident, the Electric Power Research Institute established a Nuclear Safety Analysis Center to review and analyze information relative to the safety of nuclear power plants. The fact of the matter is that nuclear power plants are safer today than ever before, and they will be unquestionably safer tomorrow than today. Nuclear Waste Another drawback that is often associated with the use of nuclear energy is that of nuclear waste. There is a huge misunderstanding that the waste created by nuclear energy is more "dangerous" than that of other means of producing electricity. The truth of the matter is that radioactive waste from nuclear energy may be dangerous for thousands of years, while wastes resulting from the burning of coal, remains dangerous forever. The reason for this is because the toxicity of these stable elements does not decrease over time as does the toxicity of radioactive materials. Other interesting facts concerning nuclear waste include the reduction in emissions of SO2 and NOx in countries using nuclear power is revealing. "In France, for example, during the period from 1980 to 1986, SO2 and NOX emissions in the electric power sector were reduced by 71 percent and 60 percent, respectively, making a major contribution to reductions of 56 percent and 9 percent, respectively, in total SO2 and NOX emissions in France" (Trudeou p.160). These

10 tremendous reductions were made possible by a fourfold increase in nuclear electricity generation. Nuclear Weapons A major drawback to the peaceful use of civilian nuclear power for the production of electricity is that it has allowed for the production of nuclear weapons. While there is no question that nuclear energy has various benefits, the fact that nations can create nuclear weapons of mass destruction t is particularly disturbing. Atomic weapons are created through the splitting of the atom and detonated through the process of fission, while hydrogen bombs are detonated through the process of fusion. Hydrogen bombs are 1000 times more explosive than atomic bombs, thus nations with hydrogen bomb technology can destroy nations within minutes. This thought has led to intense debate over the issue of nuclear energy as an alternative source for energy. Conclusion Overall, nuclear energy has proven to be most beneficial to our society. As a result of this technology, the United States has decreased its dependency on foreign-imported oil. In fact, the United States saves about 12 billion dollars each year through the lack of oil it imports from other nations. Nuclear energy has also proven to be a protector of the environment because of the lack of CO2, greenhouse gasses, and other gases it emits into the atmosphere. There are, however, some major drawbacks to using nuclear energy. These drawbacks include the actual safety of using nuclear energy, the waste it produces, and the atomic weapons that nuclear energy promotes. Overall, however, we believe that the use of nuclear energy greatly outweighs any other source of energy.

11 Key Terms NOX: NO2 or NO3 Nuclear reactor: Designed to harness the energy liberated in the fusion of certain atomic nuclei in order to generate electricity. Nuclear fission: The breakdown of an atomic nucleus of an element of relatively high atomic number into two ore more nuclei of lower atomic number, with conversion of part to its mass into energy Neutrons: A neutral particle with approximately the same mass as a proton. neither positive or negative. Zircaloy: The typical fuel cladding in a reactor. Pressuizer: Maintains the pressure by either heating or cooling an appropriate quantity of water Control rods: Used to absorb neutrons to keep the number of neutrons under control. Coolant and Moderator: The moderator slows neutrons down. example- graphite. Turbine: A hydraulic motor in which a vaned wheel or runner is made to revolve by the impingement of a free jet of fluid or by the passage of fluid which completely fills the motor. Bibliography In-Fei Liu, Paul, Energy and the Environment, Van Nostrand Reinhold, New York, Oppenheimer, Robert, The Open Mind, Simon and Schuster, New York, Pentreath, R.J., Nuclear Power, Man and the Environment, Taylor and Francis LTD, London, Trudeau, Pierre, Energy for a Habitable World, Crane Russak, New York, Energy and the Environment How Nuclear Power Plants Work Nuclear Energy Institute