Energy. on this world and elsewhere. Instructor: Gordon D. Cates Office: Physics 106a, Phone: (434)

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Cates Office: Physics 106a, Phone: (434) 924-4792

edu Course web site available at www.phys.

1 Energy on this world and elsewhere Instructor: Gordon D. Cates Office: Physics 106a, Phone: (434) Course web site available at click on classes and find Physics or at November 14,

2 Announcements Homework is due Today. Midterm, Thursday, November 21. You are allowed a 1-page cheat sheet, 8.5x11 or equivalent. ONE SIDE ONLY, and it must be HAND WRITTEN. 2

3 Readings (post Feynman and Energy and Technolgy) Muller, Energy for Future Presidents - Preface and Intro - Section I (all), Chapters Section II (all), pre-chapter material, and Chapters Section III, chapters 8-10 Richter, Beyond Smoke and Mirrors - Intro - Chapter 1. - Part I (all), Climate, Chapters Part II, Energy, Chapters 6-10 Class Notes through Chapter 4, Lecture notes, other review materials, Chapter 5 only if I hand it to you on Tuesday (or before). 3

4 Nuclear Power 4

5 Burning nuclear fuel When nuclear fuel is burned, nuclear reactions take place instead of chemical reactions. Because nuclear forces are so strong, the potential energy stored (per unit mass) in nuclei is much greater than the potential energy stored chemical bonds. The primary nuclear reaction that releases the energy is something called fission. The two fuels that are most commonly used are uranium-235, plutonium

6 Energy Density Coal 29 x 106 J/kg Gasoline: 44 x 106 J/kg Natural Uranium: 580 x 109 J/kg Pure U-235: 82 x 1012 J/kg Pure U-235 has an energy density almost 3 million times that of coal. 6

7 Natural uranium occurs as two Isotopes 7

8 Isotopic makeup of uranium 0.7% 99.3% U-238 U-235 The vast majority of natural uranium is U-238, which for all practical purposes, is not fissionable. 8

9 So what are isotopes? 9

10 Isotopes So when we speak of isotopes, we are generally referring to a specific element. 10

11 Two isotopes of helium 4 He 3 He Proton Neutron Electron Both isotopes have two protons, as they must to be the same element. One isotope has two neutrons, the other isotope has one neutron. 11

12 The number of protons in the nucleus determines the element associated with the atom 1 proton: 2 protons: 3 protons: 4 protons: 5 protons: 6 protons: hydrogen helium lithium beryllium boron carbon Why is it that the number of protons determines the number of electrons? Because atoms are usually neutral, so the number of protons also determines the number of electrons, and the number of electrons determines the atom s chemical properties. 12

13 The number of protons in the nucleus determines what element the atom is This atom has two protons and two neutrons in its nucleus, making it an isotope of helium (He-4). This atom has three protons and four neutrons in its nucleus, making it an isotope of of lithium (Li-7) 13

14 The number of protons in the nucleus determines what element the atom is 14

15 Isotopes If two nuclei have the same number of protons but different numbers of neutrons, we say that they are two isotopes of a particular element. A nucleus is always a particular isotope of an element. A nucleus is uniquely identified when you identify both the element and the mass number, the total number of neutrons plus protons. Two nuclei with the same mass number but different numbers of protons are not the same isotope, they are two particular isotopes of two different elements. 15

16 Identifying a particular isotope The mass number, equal to the total number of protons plus the total number of neutrons. 235 U 92 Sometimes one would also just refer to this as U-235 The atomic number, which is equal to the number of protons. This number is sometimes left out since it is essentially redundant with he chemical symbol The chemical symbol (here U for uranium) identifying the element. 16

17 The two isotopes of natural uranium # neutrons = = U # neutrons = = U 17

18 Only one isotope of uranium is easily fissionable U 99.28% 99.27% of all uranium is U-238 When hit by a neutron it will sometimes undergo fission, but most of the time the neutron is just absorbed U 0.72% 0.63% of all uranium is U-235 When hit by a neutron it will almost always undergo fission. 18

19 So how does fission work? 19

20 Fission reactions in uranium 20

Uranium-235 can undergo fission Here the liquid drop model illustrates how the addition of a neutron can make a nucleus unstable While both isotopes of uranium can undergo fission,

21 Uranium-235 can undergo fission Here the liquid drop model illustrates how the addition of a neutron can make a nucleus unstable While both isotopes of uranium can undergo fission, only 235 U will undergo fission after absorbing a slow neutron. Note that after fissioning, the resulting nuclei are in a size range where a lower neutron to proton ratio is favored. 21

22 Uranium-235 undergoing a chain reaction In a very real sense, a chain reaction is the same thing as the phenomena of burning when considered in the context of chemistry. 22

23 Mousetrap chain reaction Notice that on average, every mousetrap must release more than one ball or the chain reaction will not grow in size. 23

24 Nuclear reactors Pellets of uranium oxide are combined into what are called fuel rods. The fuel rods are arranged into a matrix. The space between the fuel rods is filled with a moderator that slows down the neutrons after they emerge from a fission reaction. Control rods, that are very good at absorbing neutrons, are inserted and withdrawn to control the rate at which reactions take place. 24

25 file:///users/gordon/gordon's%20files/ Energy/energy_2013/lectures/ Reactor6.webarchive 25

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Energy. on this world and elsewhere. Instructor: Gordon D. Cates Office: Physics 106a, Phone: (434)

Energy. on this world and elsewhere. Instructor: Gordon D. Cates Office: Physics 106a, Phone: (434) Energy on this world and elsewhere Instructor: Gordon D. Cates Office: Physics 106a, Phone: (434) 924-4792 email: cates@virginia.edu Course web site available at www.phys.virginia.edu, click on classes