Energy and the Environment

Energy and the Environment Fall 2014 Instructor: Xiaodong Chu Email：chuxd@sdu.edu.cn Office Tel.: 81696127, 13573122659

Nuclear Power Plants: Nuclear Energy • The nucleus of an atom (原子核)is made up of protons(质子) and neutrons(中子) • Each is about 2000 times the mass of the electron (电子), and thus constitutes the vast majority of the mass of a neutral atom (中性原子)(equal number of protons and electrons) • Proton has positive charge; mass = 1.007276 a.m.u. Neutron has no charge; mass = 1.008665 a.m.u. • Proton by itself will last forever • Neutron by itself will “decay” (衰变)with a half-life of 10.4 min • Size of nucleus is about 0.00001 times size of atom • Atom is then mostly empty space

Nuclear Power Plants: Nuclear Energy • If like charges repel, and the nucleus is full of protons (positive charges), why doesn’t it fly apart? • Repulsion(斥力) is from electromagnetic force • At close scales, another force takes over: the strong nuclear force (强相互作用力) • The strong force operates between quarks(夸克) : the building blocks of both protons and neutrons • It’s a short-range force only: confined to nuclear sizes • This binding overpowers the charge repulsion

proton Poof! electron neutron neutrino Nuclear Power Plants: Nuclear Energy • A neutron, which is heavier than a proton, can (and will!) decide to switch to the lower-energy state of the proton • Charge is conserved, so produces an electron too • And produces an anti-neutrino (反微中子), a chargeless, nearly massless cousin to the electron

Nuclear Power Plants: Nuclear Energy • Another force, called the weak nuclear force(弱相互作用力) , mediates these changes • Does this mean the neutron is made from an electron and proton? • No. But it will do you little harm to think of it this way • Mass-energy conservation: • Mass of neutron is 1.008665 a.m.u. • Mass of proton plus electron is 1.007276 + 0.000548 = 1.007824 • Difference is 0.000841 a.m.u. (more than the electron mass) • In kg: 1.410-30 kg = 1.2610-13 J = 0.783 MeV via E = mc2 • 1 a.m.u. = 1.660510-27 kg • 1 eV = 1.60210-19 J • Excess energy goes into kinetic energy of particles

Nuclear Power Plants: Nuclear Energy • A nucleus has a definite number of protons (Z), a definite number of neutrons (N), and a definite total number of nucleons(核子) : A = Z + N • For example, the most common isotope (同位素) of carbon has 6 protons and 6 neutrons (denoted 12C; 98.9% abundance(丰度)) • Z = 6; N = 6; A = 12 • Another stable isotope of carbon has 6 protons and 7 neutrons (denoted 13C; 1.1% abundance) • Z = 6; N = 7; A = 13 • An unstable isotope of carbon has 6 protons and 8 neutrons (denoted 14C; half-life is 5730 years) • Decays via beta decay to 14N • Isotopes of an element have same Z, differing N

Nuclear Power Plants: Nuclear Energy • A fully annotated nucleon symbol has the total nucleon number, A, the proton number, Z, and the neutron number, N positioned around the symbol • In that A = Z + N • Examples: • Carbon-12: • Carbon-14: • Uranium-235: • Uranium-238: • Plutonium-239:

Nuclear Power Plants: Nuclear Energy • Any time a nucleus spontaneously emits a particle… • Electron through beta (-) decay • Increase Z by 1; decrease N by 1; A remains the same • Positron(正电子) (anti-electron) through beta (+) decay • Decrease Z by 1; increase N by 1; A remains the same • Alpha () particle (4He nucleus) • Decrease Z by 2; decrease N by 2; decrease A by 4 • Gamma () ray (high-energy photon of light) • Z,N, A unchanged (stays the same nucleus, just loses energy) • We say it underwent a radioactive transformation (放射性蜕变) • Certain isotopes of nuclei are radioactively unstable • They will eventually change by a radioactive particle emission (粒子发射) • , ,  emission constitutes a minor change to the nucleus • Not as dramatic as splitting the entire nucleus in two large parts

+   Chart of the Nuclides 3 2 1 Z 0

Nuclear Power Plants: Nuclear Energy • Have a Geiger counter that clicks whenever it detects a gamma ray, beta decay particle, or alpha particle. • Not 100% efficient at detection, but representative of rate • Have two sources: • 14C with half life of 5730 years • About 4000 - decays per second in this sample • Corresponds to 25 ng, or 1015 particles • 90Sr with half-life of 28.9 years • About 200 - decays per second in this sample • Contains about 40 pg (270 billion nuclei; was 450 billion in 1987) • Produced in nuclear reactor

Nuclear Power Plants: Nuclear Energy

Nuclear Power Plants: Nuclear Energy In 1939, just at the beginning of World War II, a nuclear reaction was discovered that released much more energy per atom than radioactivity, and had the potential to be used for both explosions and power production This was the splitting of the atom, or nuclear fission

Nuclear Power Plants: Nuclear Energy Nuclear fission occurs when the repelling electrical forces within a nucleus overpower the attracting nuclear strong forces.

Nuclear Power Plants: Nuclear Energy The splitting of atomic nuclei is called nuclear fission. Nuclear fission involves the balance between the nuclear strong forces and the electrical forces within the nucleus. In all known nuclei, the nuclear strong forces dominate Nuclear deformation leads to fission when repelling electrical forces dominate over attracting nuclear forces.

Nuclear Power Plants: Nuclear Energy In a typical example of nuclear fission, one neutron starts the fission of the uranium atom and three more neutrons are produced when the uranium fissions

Nuclear Power Plants: Nuclear Energy Note that one neutron starts the fission of the uranium atom, and, in the example shown, three more neutrons are produced • Most nuclear fission reactions produce two or three neutrons • These neutrons can, in turn, cause the fissioning of two or three other nuclei, releasing from four to nine more neutrons • If each of these succeeds in splitting an atom, the next step will produce between 8 and 27 neutrons, and so on Chain Reaction

Nuclear Power Plants: Nuclear Energy A chain reaction is a self-sustaining reaction A reaction event stimulates additional reaction events to keep the process going.

Nuclear Power Plants: Nuclear Energy Chain reactions do not occur in uranium ore deposits Fission occurs mainly for the rare isotope U-235 Only 0.7% or 1 part in 140 of uranium is U-235; the prevalent isotope, U-238, absorbs neutrons but does not undergo fission A chain reaction stops as the U-238 absorbs neutrons

Nuclear Power Plants: Nuclear Energy If a chain reaction occurred in a chunk of pure U-235 the size of a baseball, an enormous explosion would likely result In a smaller chunk of pure U-235, however, no explosion would occur • A neutron ejected by a fission event travels a certain average distance before encountering another uranium nucleus • If the piece of uranium is too small, a neutron is likely to escape through the surface before it “finds” another nucleus • Fewer than one neutron per fission will be available to trigger more fission, and the chain reaction will die out

Nuclear Power Plants: Nuclear Energy An exaggerated view of a chain reaction is shown here • In a small piece of pure U-235, the chain reaction dies out

Nuclear Power Plants: Nuclear Energy An exaggerated view of a chain reaction is shown here • In a small piece of pure U-235, the chain reaction dies out • In a larger piece, a chain reaction builds up

Nuclear Power Plants: Nuclear Energy The critical mass is the amount of mass for which each fission event produces, on the average, one additional fission event A subcritical mass is one in which the chain reaction dies out. A supercritical mass is one in which the chain reaction builds up explosively Critical Mass

Nuclear Power Plants: Nuclear Energy Two pieces of pure U-235 are stable if each of them is subcritical If the pieces are joined together and the combined mass is supercritical, we have a nuclear fission bomb

Nuclear Power Plants: Nuclear Energy Each piece is subcritical because a neutron is likely to escape When the pieces are combined, there is less chance that a neutron will escape The combination may be supercritical

Nuclear Power Plants: Nuclear Energy A simplified diagram of a uranium fission bomb is shown here

Nuclear Power Plants: Nuclear Energy Building a uranium fission bomb is not a formidable task. The difficulty is separating enough U-235 from the more abundant U-238 It took more than two years to extract enough U-235 from uranium ore to make the bomb that was detonated over Hiroshima in 1945 Uranium isotope separation is still a difficult, expensive process today

Nuclear Power Plants: Nuclear Energy • There are only three known nuclides (arrangements of protons and neutrons) that undergo fission when introduced to a slow (thermal) neutron: • 233U: hardly used (hard to get/make) • 235U: primary fuel for reactors • 239Pu: popular in bombs • Others may split if smacked (撞击) hard enough by a neutron (or other energetic particle)

Nuclear Power Plants: Nuclear Energy Bottom line: at thermal energies (arrow), 235U is 1000 times more likely to undergo fission than 238U even when smacked hard

Nuclear Power Plants: Nuclear Energy

Nuclear Power Plants: Nuclear Energy Pu-239, like U-235, will undergo fission when it captures a neutron.

Nuclear Power Plants: Nuclear Energy When a neutron is absorbed by a U-238 nucleus, no fission results. The nucleus that is created, U-239, emits a beta particle instead and becomes an isotope of the element neptunium This isotope, Np-239, soon emits a beta particle and becomes an isotope of plutonium This isotope, Pu-239, like U-235, will undergo fission when it captures a neutron

Nuclear Power Plants: Nuclear Energy The half-life of neptunium-239 is only 2.3 days, while the half-life of plutonium-239 is about 24,000 years Plutonium can be separated from uranium by ordinary chemical methods It is relatively easy to separate plutonium from uranium

Nuclear Power Plants: Nuclear Energy During fission, the total mass of the fission fragments (including the ejected neutrons) is less than the mass of the fissioning nucleus

Nuclear Power Plants: Nuclear Energy The key to understanding why a great deal of energy is released in nuclear reactions is the equivalence of mass and energy Mass is like a super storage battery It stores energy that can be released if and when the mass decreases

Nuclear Power Plants: Nuclear Energy If you stacked up 235 bricks, the mass of the stack would be equal to the sum of the masses of the bricks Is the mass of a U-235 nucleus equal to the sum of the masses of the 235 nucleons that make it up? Mass Energy

Nuclear Power Plants: Nuclear Energy Consider the work that would be required to separate all the nucleons from a nucleus Recall that work is equal to the product of force and distance Imagine that you can reach into a U-235 nucleus and, pulling with a force, remove one nucleon That would require considerable work Then keep repeating the process until you end up with 235 nucleons, stationary and well separated

Nuclear Power Plants: Nuclear Energy You started with one stationary nucleus containing 235 particles and ended with 235 separate stationary particles Work is required to pull a nucleon from an atomic nucleus This work goes into mass energy The separated nucleons have a total mass greater than the mass of the original nucleus The extra mass, multiplied by the square of the speed of light, is exactly equal to your energy input: ∆E = ∆mc2

Nuclear Power Plants: Nuclear Energy Binding Energy One way to interpret this mass change is that a nucleon inside a nucleus has less mass than its rest mass outside the nucleus How much less depends on which nucleus The mass difference is related to the “binding energy” of the nucleus

Nuclear Power Plants: Nuclear Energy For uranium, the mass difference is about 0.7%, or 7 parts in a thousand The 0.7% reduced nucleon mass in uranium indicates the binding energy of the nucleus

Nuclear Power Plants: Nuclear Energy A graph of the nuclear masses for the elements from hydrogen through uranium shows how nuclear mass increases with increasing atomic number The slope curves slightly because there are proportionally more neutrons in the more massive atoms

Nuclear Power Plants: Nuclear Energy A more important graph plots nuclear mass per nucleon from hydrogen through uranium This graph indicates the different average effective masses of nucleons in atomic nuclei Nuclear Mass per Nucleon

Nuclear Power Plants: Nuclear Energy The lowest point of the graph occurs at the element iron This means that pulling apart an iron nucleus would take more work per nucleon than pulling apart any other nucleus Iron holds its nucleons more tightly than any other nucleus does Beyond iron, the average effective mass of nucleons increases Nuclear Mass per Nucleon

Nuclear Power Plants: Nuclear Energy The binding energy curve is obtained by dividing the total nuclear binding energy by the number of nucleons The fact that there is a peak in the binding energy curve in the region of stability near iron means that either the breakup of heavier nuclei (fission) or the combining of lighter nuclei (fusion) will yield nuclei which are more tightly bound (less mass per nucleon) The higher the binding energy per nucleon, the more stable is the nucleus

Nuclear Power Plants: Nuclear Energy If a uranium nucleus splits in two, the masses of the fission fragments lie about halfway between uranium and hydrogen The mass per nucleon in the fission fragments is less than the mass per nucleon in the uranium nucleus

Nuclear Power Plants: Nuclear Energy When this decrease in mass is multiplied by the speed of light squared, it is equal to the energy yielded by each uranium nucleus that undergoes fission The missing mass is equivalent to the energy released

Nuclear Power Plants: Nuclear Energy The mass-per-nucleon graph is an energy valley that starts at hydrogen, drops to the lowest point (iron), and then rises gradually to uranium Iron is at the bottom of the energy valley, which is the place with the greatest binding energy per nucleon

Nuclear Power Plants: Nuclear Energy Any nuclear transformation that moves nuclei toward iron releases energy Heavier nuclei move toward iron by dividing—nuclear fission A drawback is that the fission fragments are radioactive because of their greater-than-normal number of neutrons A more promising source of energy is to be found when lighter-than-iron nuclei move toward iron by combining

Nuclear Power Plants: Radioactivity • Radioactivity is the spontaneous decay of certain nuclei, usually the less stable isotopes of theelements, both natural and man-made, which is accompanied by the release of very energeticradiation • After the emission of radiation, an isotope of the element (or even a new element) isformed, which is usually more stable than the original element • It is important to note that radiationemanates directly from the nucleus, not the atom as a whole • In radioactive decay, there are three types of radiation: α, β, and γ • Only the latter is a formof electromagnetic radiation; the two former are emissions of very high energy particles • All threeare called ionizing radiation (电离辐射) because they create ions as their energy is absorbed by matter through which they travel

Nuclear Power Plants: Radioactivity • In alpha radiation, a whole nucleus of a helium atom, containing two protons and two neutrons,is emitted • The daughterisotope moves two elements backward from the parent element in the periodic table • Because α particles are relatively heavy, their penetration depth into matter is very small, on theorder of a millimeter, which a sheet of paper or a layer of dead skin on a person can stop

Nuclear Power Plants: Radioactivity • In beta radiation, an electron is emitted • This electron stems from the nucleus itself: a neutron converts into a proton with the emission of an electron • A parent isotope converts into a daughter isotope, which is an element forward in the periodic table because a proton has been added to the nucleus • Because 90Sr is one of the products of uranium fission, this isotope is a major source of radiation from the spent fuel of a nuclear reactor • The emitted electron is relatively light; therefore it can penetrate deeper into matter, on the order of centimeters, to shield against which, a plate of metal, such as lead, is necessary

Energy and the Environment