1 CHAPTER 3 Nuclear Processes and Neutron Physics prepared by Dr. Guy Marleau École Polytechnique de Montréal Summary: In this section, we first describe the nucleus, including its composition and the fundamental forces that affect its behaviour. Then, after introducing the concepts of radioactivity and nuclear decay, we discuss the various interactions between radiation and matter that can take place in and around a nuclear reactor. We finish with a presentation of neutron physics for fission reactors. Table of Contents 1 Introduction .............................................................................................................................3 1.1 Overview......................................................................................................................... 3 1.2 Learning Outcomes......................................................................................................... 4 2 Structure of the Nucleus..........................................................................................................4 2.1 Fundamental Interactions and Elementary Particles...................................................... 4 2.2 Protons, Neutrons, and the Nuclear Force..................................................................... 6 2.3 Nuclear Chart.................................................................................................................. 7 2.4 Nuclear Mass and the Liquid Drop Model.................................................................... 10 2.5 Excitation Energy and Advanced Nuclear Models ........................................................ 12 2.6 Nuclear Fission and Fusion ........................................................................................... 15 3 Nuclear Reactions..................................................................................................................16 3.1 Radioactivity and Nuclear Decay .................................................................................. 16 3.2 Nuclear Reactions......................................................................................................... 20 3.3 Interactions of Charged Particles with Matter.............................................................. 22 3.4 Interactions of Photons with Matter ............................................................................ 24 3.5 Interactions of Neutrons with Matter .......................................................................... 30 4 Fission and Nuclear Chain Reaction.......................................................................................34 4.1 Fission Cross Sections ................................................................................................... 35 4.2 Fission Products and the Fission Process...................................................................... 36 4.3 Nuclear Chain Reaction and Nuclear Fission Reactors ................................................. 40 4.4 Reactor Physics and the Transport Equation ................................................................ 42 5 Bibliography ...........................................................................................................................44 6 Summary of Relationship to Other Chapters.........................................................................45 7 Problems................................................................................................................................46 8 Acknowledgments .................................................................................................................46 ©UNENE, all rights reserved. For educational use only, no assumed liability. Nuclear Physics – June 2015 2 The Essential CANDU List of Figures Figure 1 Nuclear chart, where N represents the number of neutrons and Z the number of protons in a nucleus. The points in red and green represent respectively stable and unstable nuclei. The nuclear stability curve is also illustrated. ..............................................8 Figure 2 Average binding energy per nucleon (MeV) as a function of the mass number (number of nucleons in the nucleus)......................................................................................................9 Figure 3 Saxon-Wood potential seen by nucleons inside the uranium-235 nucleus. ...................13 Figure 4 Shell model and magic numbers......................................................................................14 Figure 5 Decay chain for uranium-238 (fission excluded). The type of reaction (β or α-decay) can be identified by the changes in N and Z between the initial and final nuclides.............20 Figure 6 Microscopic cross section of the interaction of a high-energy photon with a lead atom.24 Figure 7 Photoelectric effect..........................................................................................................26 Figure 8 Compton effect. ...............................................................................................................28 Figure 9 Photonuclear cross sections for deuterium, beryllium, and lead....................................29 Figure 10 Cross sections for hydrogen (left) and deuterium (right) as functions of energy..........32 235 238 Figure 11 Cross sections for 92 U (left) and 92 U (right) as functions of energy. ........................33 Figure 12 Effect of temperature on U238 absorption cross section around the 6.64-eV resonance...............................................................................................................................34 235 238 Figure 13 Capture and fission cross sections of 92 U (left) and 92 U (right) as functions of energy.....................................................................................................................................36 235 Figure 14 Relative fission yield for 92 U as a function of mass number........................................37 235 Figure 15 Fission spectrum for 92 U ..............................................................................................39 List of Tables Table 1 Properties of the four fundamental forces .........................................................................5 Table 2 Properties of leptons and quarks ........................................................................................6 Table 3 Main properties of protons and neutrons. .........................................................................6 Table 4 Energy states in the shell model, where g is the degeneracy level of a state...................14 Table 5 Comparison of the energy, U, required for a fission reaction in a compound nucleus with the energy, Q, available after a neutron collision with different isotopes. ...................35 Table 6 Average numbers of neutrons produced by the fission of common heavy nuclides........37 Table 7 Distribution of kinetic energy among the particles produced by a fission reaction.........38 1 2 Table 8 Maximum values of the elastic scattering and absorption cross sections for 1H , 1 D , 12 and 6 C in the energy range 0.1 eV < E <0.1 MeV. ...............................................................41 ©UNENE, all rights reserved. For educational use only, no assumed liability. Nuclear Physics – June 2015 Nuclear Processes and Neutron Physics 3 1 Introduction Energy production in an operating nuclear reactor is mainly the result of fission reactions initiated by neutrons. Following these reactions, radiation is produced in the form of alpha particles, electrons ( -particles), photons, and neutrons, as well as a large number of unstable nuclei (including actinides and fission products) that may decay, thereby producing additional radiation and energy.ߚ The neutrons generated directly in the fission reaction as well as those resulting from fission product decay are a key factor in maintaining and controlling the chain reaction as discussed in Chapters 4 and 5. Radiation also has an impact on the properties of the various materials in the core as well as on living cells. It is therefore important to under- stand how radiation interacts with matter if one needs, for example, to evaluate the radio- toxicity of burned fuel (Chapter 19) or to determine the kind of barrier that must be put in place to protect the public, the environment, or the personnel in a nuclear power plant (Chapter 12). Moreover, the energy produced by these decays contributes to heating the fuel, whether it is still in the reactor or in the spent fuel pools. The aim of this chapter is to provide the reader with an overview of the nuclear processes that take place in and around a reactor. 1.1 Overview Several very important processes take place simultaneously in a nuclear reactor, including neutron slowdown as a result of collisions with nuclei, nuclear fission, and decay of radioactive nuclei with radiation emissions that subsequently lose energy or are absorbed. To understand how all these physical processes take place, it is important to possess a general knowledge of the physics of the nucleus, including its composition and the fundamental forces that affect its behaviour. Although one rarely uses a description of the nucleus in terms of elementary particles and their interaction in power-reactor applications, such a description provides a practical background for a study of nuclear structure. In fact, the nucleus is such a complex object that it can be described only through simplified models that provide useful information. For example,