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1 Overview of Nuclear Reactor Systems and Fundamentals j1 1 Overview of Nuclear Reactor Systems and Fundamentals “Someday man will harness the rise and fall of the tides, imprison the power of the sun, and release atomic power.” —Thomas Alva Edison 1.1 Introduction There is no doubt that energy has been driving and will drive the technological prog- ress of the human civilization. It is a very vital component for the economic devel- opment and growth, and thus our modern way of life. Energy has also been tied to the national security concerns. It has been projected that the world energy demand will almost double by the year 2040 (based on 2010 energy usage), which must be met by utilizing the energy sources other than the fossil fuels such as coal and oil. Fossil fuel power generation contributes to significant greenhouse gas emissions into the atmosphere and influences the climate change trend. Although several research and development programs (e.g., carbon sequestration and ultrasupercrit- ical steam turbine programs) have been initiated to make the fossil power genera- tion much cleaner, they alone will not be enough to fend off the bigger problem. Therefore, many countries worldwide have recognized the importance of clean (i.e., emission-free) nuclear energy, and there are proven technologies that are more than ready for deployment. The use of nuclear energy for the power genera- tion varies widely in different parts of the world. The United States produces about 19% (2005 estimate) of its total energy from nuclear sources, whereas France pro- duces 79% and Brazil and India rely on the nuclear energy for only about 2.5% and 2.8% of their energy needs, respectively. Japan, South Korea, Switzerland, and Ukraine produce 30%, 35%, 48%, and 40%, respectively, of their energy require- ments from the nuclear sources. It is important to note that the fast growing econo- mies like China, India, and Brazil produce relatively less electricity from the nuclear sources. Hence, there are tremendous opportunities for nuclear energy growth in these emerging economies as well as many other countries. Nuclear reactors have been built for the primary purpose of electricity production, although they are used for desalination and radioisotope production. An Introduction to Nuclear Materials: Fundamentals and Applications, First Edition. K. Linga Murty and Indrajit Charit. # 2013 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2013 by Wiley-VCH Verlag GmbH & Co. KGaA. 2j 1 Overview of Nuclear Reactor Systems and Fundamentals There are now about 440 nuclear power reactors worldwide generating almost 16% of the world electricity needs; among them, 104 nuclear reactors are in the United States. Since the first radioactive chain reaction that was successfully initiated at the University of Chicago research reactor in the 1940s, the field has seen an impressive growth until Three Mile Island and Chernobyl accidents happened. Following these incidents, the public confi- dence in the nuclear power dwindled, and the nuclear power industry saw a long stagnation. However, the US government’s decision to increase energy security and diversity by encouraging nuclear energy generation (as laid out in the US government’s Advanced Energy Initiative in 2005) has rekindled much hope for the revival of the nuclear power industry in the United States, and as a matter of fact, the Nuclear Renaissance has already begun – the US Nuclear Regulatory Commission (NRC) approved an early site permit application for the Clinton Power Station in Illinois (Exelon Power Corporation) in March 2007. As the scope of the nuclear energy is expanded, the role of materials is at the front and center. Recent (2011) accidents in Japan due to earthquakes and tsunami are now pointing toward further safeguards and development of more resistant materials. Thus, this book is devoted to addressing various important fundamental and application aspects of materials that are used in nuclear reactors. 1.2 Types of Nuclear Energy Nuclear energy can be derived from many forms such as nuclear fission energy, fusion energy, and radioisotopic energy. 1.2.1 Nuclear Fission Energy The essence of nuclear fission energy is that the heat produced by the splitting of heavy radioactive atoms (nuclear fission) during the chain reaction is used to gener- ate steam (or other process fluid) that helps rotate the steam turbine generator, thus producing electricity. Nuclear fission energy is the most common mode of produc- ing the bulk of the nuclear energy. 1.2.2 Nuclear Fusion Energy A huge amount of energy (much higher than fission) can be produced using the nuclear fusion reaction (deuterium–tritium reaction). There is currently no com- mercial fusion reactors and is not envisioned to be set up for many years. A proto- type fusion reactor known as ITER (International Thermonuclear Experimental Reactor) is being built in France and scheduled to produce the first plasma by 2018. 1.5 Interactions of Neutrons with Matter j3 1.2.3 Radioisotopic Energy 238 210 Either radioactive isotopes (e.g., Pu, Po) or radioactive fission products (e.g., 85 90 Kr, Sr) can produce decay heat that can be utilized to produce electric power. These types of power sources are mainly used in remote space applications. 1.3 Neutron Classification Chadwick discovered neutron in 1932. Generally, neutrons are generated during radioactive chain reactions in a power reactor. Neutron is subatomic particle pres- ent in almost all nuclides (except normal hydrogen isotope or protium) with a mass of 1.67 Â 10À27 kg and has no electrical charge. Neutrons are classified based on their kinetic energies. Although there is no clear boundary between the categories, the following limits can be used as a useful guideline: Cold neutrons (<0.003 eV), slow (thermal) neutrons (0.003–0.4 eV), slow (epithermal) neutrons (0.4–100 eV), intermediate neutrons (100 eV–200 keV), fast neutrons (200 keV–10 MeV), high-energy (relativistic) neutrons (>10 MeV). Note:1eV¼ 1.6 Â 10À19 J. Generally, thermal neutrons are associated with a kinetic energy of 0.025 eV that translates into a neutron speed of 2200 m sÀ1! 1.4 Neutron Sources Various radiation types are produced in a nuclear environment. It could be alpha particles, beta particles, gamma rays, or neutrons. In this book, we are primarily concerned with the radiation damage and effects caused by neutrons. There could be several sources of neutrons, including alpha particle-induced fission, spontane- ous fission, neutron-induced fission, accelerator-based sources, spallation neutron source, photoneutron source, and nuclear fusion. 1.5 Interactions of Neutrons with Matter Collision of neutrons with atom nuclei may lead to different scenarios – scattering of the neutrons and recoil of nuclei with conservation of momentum (elastic scat- tering) or loss of kinetic energy of the neutron resulting in gamma radiation (inelastic scattering). The capture of neutrons may result in the formation of new 4j 1 Overview of Nuclear Reactor Systems and Fundamentals nuclei (transmutation), or may lead to the fragmentation of the nucleus (fission) or the emission of other nuclear particles from the nucleus. We shall discuss some of the effects in more detail in Chapter 3. a) Elastic Scattering Elastic scattering refers to a neutron–nucleus event in which the kinetic energy and momentum are conserved. b) Inelastic Scattering This interaction refers to neutron–nuclide interaction event when the kinetic energy is not conserved, while momentum is conserved. c) Transmutation When a nuclide captures neutrons, one result could be the start of a sequence of events that could lead to the formation of new nuclide. The true examples of this type of reaction are (n, a), (n, p), (n, bþ), (n, bÀ), and (n, f ). Reactions like (n, c) and (n, 2n) do not result in new elements, but only produce isotopes of the origi- nal nuclide. d) Fission Fission is a case of (n, f ) reaction, a special case of transmutation reaction. Ura- nium is the most important nuclear fuel. The natural uranium contains about 0.7% U235, 99.3% U238, and a trace amount of U234. Here, we discuss the neu- tron-induced nuclear fission, which is perhaps the most significant nuclear reaction. When a slow (thermal) neutron gets absorbed by a U235 atom, it leads to the formation of an unstable radionuclide U236, which acts like an unstable oscillating droplet, immediately followed by the creation of two smaller atoms known as fission fragments (not necessarily of equal mass). About 2.5 neutrons on average are also released per fission reaction of U235. An average energy of 193.5 MeV is liberated. A bulk of the energy (160 MeV or 83%) is carried out by the fission fragments, while the rest by the emitted neutrons, gamma rays, and eventual radioactive decay of fission products. Fission fragments rarely move more than 0.0127 mm from the fission point and most of the kinetic energy is transformed to heat in the process. As all of these newly formed parti- cles (mostly fission fragments) collide with the atoms in the surroundings, the kinetic energy is converted to heat. The fission reaction of U235 can occur in 30 different ways leading to the possibility of 60 different kinds of fission frag- ments. A generally accepted equation for a fission reaction is given below: 235 1 92 142 1 ; : U92 þ n0 ! Kr36 þ Ba56 þ 2n0 þ Energy ð1 1Þ which represents the fission of one U235 atom by a thermal neutron resulting into the fission products (Kr and Ba) with an average release of two neutrons and an average amount of energy (see above). It is clear from the atomic masses of the reactant and products, that a small amount of mass is converted into an equivalent energy following Einstein’s famous equation E ¼ mc2.
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