Nuclear Energy 1
References ULLMANN'S Encyclopedia of Industrial Chemistry Kirk-Othmer Encyclopedia of Chemical Technology Industrial Inorganic Chemistry (by Karl Heinz Buchel, Hans-Heinrich Moretto, Dietmar Werner) Industrial chemistry-inorganic (by Dilwyn Morgan Samuel) Industrial inorganic chemistry (Werner Büchner, Hans-Heinrich Moretto, Peter Woditsch) 2
Nuclear Decay 3
Discoveries of Nuclear Reactions In 1914, Marsden and Rutehrford saw some thin tracks and spots among those due to a particles. They attributed them to protons and suggested the nuclear reaction: 14 N + 4 He 17 O + 1 H or 14 N (a, p) 17 O F. Joliot and I. Curie discovered the reaction 27 Al (a, n) 30 P (, b + or EC) 30 Si, half life of 30 P is 2.5 min a source & tracks with thin proton track a and thin proton spots on fluorescence screen 4
Nuclear Decay Most atoms are very stable. The strong nuclear force which bonds together the nucleons is stronger than any other force (over very small distances). It takes a HUGE amount of energy to overcome the strong nuclear force and get out that binding energy. However in some radioactive elements, atoms are inherently unstable because of size and will decay or release particles and energy, in the process turning into a different element. The process by which one element decays into another is called transmutation. 5
Decay, con t. This often occurs when an atom has an abundance of neutrons (over 1.5 time ), swelling the nucleus so that the strong nuclear force is overcome. The larger an atom, the higher the binding energy needed to be overcome. The early principles of radioactivity were worked out by Henri Becquerel and Pierre and Marie Curie. 6
Stability You can compare the stability of different nuclei by dividing the binding energy by the number of nucleons in the nucleus. Stability binding energy number. of nucleons Order the following atoms from most stable to least stable. 7
Radioactive Decay Unstable radioactive nuclei break apart, but they do not break into halves or large chunks. They only eject small particles. This process is called radioactive decay. These ejected particles make up nuclear radiation. 8
Types of Radiation There are three main types of decay : 1. Alpha Decay When a radioactive nucleus gives off an alpha particle Particles have a charge (+2), a large mass and a short range Not suitable for radiation treatment very dangerous if ingested Can be stopped by a thick sheet of paper, Al foil or a few cm of air 9
Alpha Decay Examples: Complete the decay reactions: a) b) Write the reaction if plutonium decays alpha 10
Mass Defect We can also calculate mass defects during alpha decay: ex) What is the energy released during the alpha decay of uranium-238? 11
Beta Decay 2. Beta Decay When a nucleus gives off a beta particle (electron): Can still be stopped by a thick piece of paper or a few metres of air In beta negative decay, a neutron is turned into a proton by the emission of a beta particle and an antineutrino 12
Beta Decay ex) carbon-14 13
Antineutrino ***In order to account for conservation of momentum and energy during decay, we must also introduce another particle, the antineutrino, that has no rest mass and no charge. 14
Full Decay Reaction The full beta negative reaction would look like this: *Note: conservation of mass is obeyed! All protons and neutrons are accounted for. *Note: conservation of charge is obeyed! A neutron turned into a proton and an electron, a net neutral charge. 15
Examples Write the beta-negative decay of: a) b) 16
Mass Defect Note that we can also calculate mass defects with a beta decay as well: ex) How much energy would be released when decays beta-negative? *note: the mass of the beta particle is so small, it can be ignored. 17
Positive Beta Decay A proton can also turn into a neutron through positive beta decay: The positron is the antiparticle of the electron also released is the neutrino, 18
The neutrino and antineutrino were originally proposed to account for some of the energy and momentum losses in a nuclear reaction. They were later experimentally proven to exist. They are identical in every way, except for their spins (a quality of subatomic particles we will study later). 19
Antimatter Annihilation The antineutrino is a type of antimatter. When matter and antimatter collide, there is a complete annihilation of the matter (actually a pure transfer to energy): The annihilation of an electron and positron (beta- and beta+ particle). NASA photo of annihilation in a solar flare. 20
Unique Examples Nuclei having too many neutrons for stability can change a neutron to a proton and an electron (-ive beta decay): Nuclei having too few neutrons for stability can change a proton to a neutron and a positron (+ive beta decay): 21
Gamma Decay 3. Gamma Radiation When a gamma ray is emitted, a high energy photon is emitted by the nucleus (i.e. a photon) This does not result in transmutation These are used in cancer treatment Can pass through 30 cm of steel and 1 m of aluminum Often occurs with alpha decay 22
Concept Check antineutrino 23
Review Alpha (α) Large, positively charged particles. Little penetrating power (about one sheet of paper) Beta (β) Smaller negatively charged particles. Penetrate as much as three mm of aluminum. Determined to be an electron. Gamma (γ ) EMR Very high penetrating ability (as much as several cm of lead) 24
Review Alpha Decay The strong nuclear forces in the atom that hold protons and neutrons in the nucleus, are unable to keep some nuclei intact. An alpha particle (helium nucleus) is emitted. Beta Decay The weak nuclear force is not always able to hold the quarks together that make up neutrons and protons. When a neutron decays it becomes a proton and an electron (beta particle) The beta particle is ejected from the nucleus, as well as an antiparticle, the electron s antineutrino, leaving a new element that contains one more proton but same mass. Positron Emission This is a case of the weak nuclear force being unable to hold the proton together, resulting in the emission of a neutron and a positron, or anti-electron, along with an electron neutrino. 25
Review Gamma Decay A photon emitted when an excited nucleus returns to ground state. Much like an electron can move energy levels so can the protons in the nucleus. **Remember: In all decay equations the mass and charge are conserved. (top and bottom numbers) 26
Decay Series When radioactive nuclei decay, they do so in a predictable order called the radioactive decay series. A diagram of the series is shown below: 27
Decay Series This diagram starts with uranium 238 and shows the decay series to the nonradioactive lead 206. There are downwards decays (where mass number and atomic numbers change) There are sideways decays (where only atomic numbers change) The downward decays indicate alpha decays. An alpha decay involves losing an alpha particle: a change of both mass number and atomic number. The sideways shifts indicate beta decays. The beta decay only involves changing the atomic number, not the mass number. 28
Biological Effects of Radiation The decay process is harmful to living creatures because of the α, β and γ ray's ability to ionize living tissue. When molecules in the body are ionized (charged), it throws off delicate body chemistry. Large doses of radiation are lethal, small doses which your body encounters naturally can be counteracted by the body with little to no harm. 29
Extent of Harm The extent of harm done depends on the type of radiation: The amount of time of exposure also plays a role: most radiation sickness is acute, caused by short intense exposure to radiation. Chronic exposure, small amounts over long periods of time, is also possible. 30
Measuring Exposure Radiation exposure is measured in at least two ways: Gray (Gy): the amount of ionizing energy needed to deliver 1 J of energy to 1 kg of tissue. Sievert: (Sv): the amount in Grays, multiplied by the relative biological effectiveness (RBE) factor, different for each type of radiation. 1 Gy of α radiation = 20 Sv 1 Gy of β radiation = 1 Sv 1 Gy of γ radiation = 1 Sv *A does of 6 Sv over a short time is fatal. 31
Examples 1. Write the decay equation for the decay of the neutron, producing beta emission. 2. Write the decay equation for positron decay of carbon-11. 3. Write the decay equation for the alpha decay of radium-226. 32
Controlling nuclear chain reactions 33
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When/where is control used? Nuclear power plants work by controlling the rate of the nuclear reactions, and that control is maintained through several safety measures. The materials in a nuclear reactor core and the uranium enrichment level make a nuclear explosion impossible, even if all safety measures failed. On the other hand, nuclear weapons are engineered to produce a reaction that is so fast and intense it cannot be controlled after it has started. When properly designed, this uncontrolled reaction can lead to an explosive energy release. 35
How the chain reaction is controlled In a nuclear fission reaction in a nuclear power plant, the radioactive element Uranium is used in a chain reaction. The fission of splits off two neutrons, which in turn strike two atoms. Two neutrons are split from each of the two atoms. Each of these neutrons then go on to strike another atom. Each of those atoms are split releasing two neutrons, which go on and hit more Uranium atoms. The chain reaction continues on and on, getting bigger and bigger with each split. The things that slow down a chain reaction are the control rods. A control rod is made up of cadmium or boron, which absorb neutrons. If you insert the control rod between the uranium atoms, the amount of neutrons available to cause more splits is reduced. THIS CAN BE DEMONSTRATED WHEN YOU PLACE A RULER IN BETWEEN FALLING DOMINOES 36
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Unstable Nucleus 38 Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 620
NUCLEAR POWER PLANT 39
NUCLEAR FUEL Nuclear fuel is any material that can be consumed to derive nuclear energy. The most common type of nuclear fuel is fissile elements that can be made to undergo nuclear fission chain reactions in a nuclear reactor The most common nuclear fuels are 235U and 239Pu. Not all nuclear fuels are used in fission chain reactions 40
NUCLEAR FISSION When a neutron strikes an atom of uranium, the uranium splits into two lighter atoms and releases heat simultaneously. Fission of heavy elements is an exothermic reaction which can release large amounts of energy both as electromagnetic radiation and as kinetic energy of the fragments 41
NUCLEAR CHAIN REACTIONS A chain reaction refers to a process in which neutrons released in fission produce an additional fission in at least one further nucleus. This nucleus in turn produces neutrons, and the process repeats. If the process is controlled it is used for nuclear power or if uncontrolled it is used for nuclear weapons 42
NUCLEAR REACTOR A nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate, as opposed to a nuclear bomb, in which the chain reaction occurs in a fraction of a second and is uncontrolled causing an explotion. 43
CONTROL RODS Control rods made of a material that absorbs neutrtons are inserted into the bundle using a mechanism that can rise or lower the control rods.. The control rods essentially contain neutron absorbers like, boron, cadmium or indium. 44
STEAM GENERATORS Steam generators are heat exchangers used to convert water into steam from heat produced in a nuclear reactor core. Either ordinary water or heavy water is used as the coolant. 45
STEAM TURBINE A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into useful mechanical Various high-performance alloys and superalloys have been used for steam generator tubing. 46
COOLANT PUMP The coolant pump pressurizes the coolant to pressures of the orderof 155bar. The pressue of the coolant loop is maintained almost constant with the help of the pump and a pressurizer unit. 47
FEED PUMP Steam coming out of the turbine, flows through the condenser for condensation and recirculated for the next cycle of operation. The feed pump circulates the condensed water in the working fluid loop. 48
CONDENSER Condenser is a device or unit which is used to condense vapor into liquid. The objective of the condenser are to reduce the turbine exhaust pressure to increase the efficiency and to recover high qyuality feed water in the form of condensate & feed back it to the steam generator without any further treatment. 49
COOLING TOWER Cooling towers are heat removal devices used to transfer process waste heat to the atmosphere. Water circulating through the condenser is taken to the cooling tower for cooling and reuse 50
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U235 + n fission + 2 or 3 n + 200 MeV If each neutron releases two more neutrons, then the number of fissions doubles each generation. In that case, in 10 generations there are 1,024 fissions and in 80 generations about 6 x 10 23 (a mole) fissions. 52
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مشهدPressurized Water دانشگاه فردوسی Reactor 54
Nuclear Power Plant 55
Reactor Core Hot coolant Control rods of neutron-absorbing substance Uranium in fuel cylinders Incoming coolant 56
Nuclear Power Plant Production of heat Production of electricity 57
Shaft Surface deposits Nuclear Waste Disposal Aquifier Interbed rock layer River Host rock formation Interbed rock layer Aquifier Bedrock Waste package Repository Waste form 58
Half-Life 20 g Start 10 g after 1 half-life 5 g after 2 half-lives 2.5 g after 3 half-lives 59
b emissions Half-Life 131 53 I 89.9% 7.3% 131 54 Xe* g emissions 131 54 Xe 0.500 mg 131 1.00 mg 0.750 mg 0.875 mg 131 53 I 54 Xe 0.500 mg 131 53 I 0.250 mg 0.125 mg 0.00 days 8.02 days 16.04 days 24.06 days 131 I 53 131 54 Xe 0 + b - g -1 + 60
Half-life of Radiation Radioisotope remaining (%) 100 50 25 12.5 t 1/2 Initial amount of radioisotope t 1/ After 1 half-life 2 t 1/2 After 2 half-lives After 3 half-lives 0 1 2 3 4 Number مشهد فردوسی دانشگاهof half-lives 61
Half-Life Plot 20 15 Half-life of iodine-131 is 8 days Amount of Iodine- 131 (g) 10 5 1 half-life 2 half-lives 3 half-lives 4 half-lives etc 0 0 8 16 24 32 40 48 56 Time (days) 62
Half-Life of Isotopes Half-Life and Radiation of Some Naturally Occurring Radioisotopes Isotope Half-Live Radiation emitted Carbon-14 5.73 x 10 3 years b Potassium-40 1.25 x 10 9 years b, g Radon-222 3.8 days a Radium-226 1.6 x 10 3 years a, g Thorium-230 7.54 x 10 4 years a, g Thorium-234 24.1 days b, g Uranium-235 7.0 x 10 8 years a, g Uranium-238 4.46 x 10 9 years a 63
Half-life (t ½ ) Time required for half the atoms of a radioactive nuclide to decay. Shorter half-life = less stable. Potassium Argon Calcium Ratio of Remaining Potassium-40 Atoms to Original Potassium-40 Atoms 1/1 1/2 1/4 1/8 1/16 0 0 1 half-life 1.3 Newly formed rock 2 half-lives 3 half-lives 2.6 3.9 Time (billions of years) 4 half-lives 5.2 64
1. A small piece of fossil is burned in a special furnace. Stable C-12 isotope 2. The burning creates carbon dioxide gas comprised of carbon-12 isotopes and carbon-14 isotopes. Nitrogen Decaying C-14 isotope 3. As the carbon- 14 decays into nitrogen-14, it emits an electron. Electron 4. A radiation counter records the number of electrons emitted. Note: Not to scale. 65 SOURCE: Collaboration for NDT Education MATT PERRY / Union-Tribune
The half-life of carbon-14 is 5730 years. If a sample originally contained 3.36 g of C-14, how much is present after 22,920 years? t 1/2 = 5730 years 22,920 years n = 5,730 years Data Table: Half-life Decay Amount Time # Half-Life 3.36 g 0 y 0 1.68 g 5,730 y 1 0.84 g 11,460 y 2 0.42 g 17,190 y 3 0.21 g 22,920 y 4 n = 4 half-lives (# of half-lives)(half-life) = age of sample (4 half-lives)(5730 years) = age of sample 22,920 years 66
Neutrons (N) 140 130 120 110 100 90 80 70 60 50 Nuclear Stability Decay will occur in such a way as to return a nucleus to the band (line) of stability. 40 30 20 10 0 10 20 30 40 50 60 70 80 90 Protons (Z) 67
Units Used in Measurement of Radioactivity Units Curie (C) Becquerel (Bq) Roentgens (R) Rad (rad) Rem (rem) Measurements radioactive decay radioactive decay exposure to ionizing radiation energy absorption caused by ionizing radiation biological effect of the absorbed dose in humans 68
ADVANTAGES Nuclear power generation does emit relatively low amounts of carbon dioxide (CO 2 ). The emissions of green house gases and therefore the contribution of nuclear power plants to global warming is therefore relatively little. This technology is readily available, it does not have to be developed first. It is possible to generate a high amount of electrical energy in one single plant 69
DISADVANTAGES The problem of radioactive waste is still an unsolved one. High risks: It is technically impossible to build a plant with 100% security. The energy source for nuclear energy is Uranium. Uranium is a scarce resource, its supply is estimated to last only for the next 30 to 60 years depending on the actual demand. 70
DISADVANTAGES Nuclear power plants as well as nuclear waste could be preferred targets for terrorist attacks.. During the operation of nuclear power plants, radioactive waste is produced, which in turn can be used for the production of nuclear weapons. 71
Fusion: 72
What is Nuclear Fusion? Nuclear Fusion is the energy-producing process taking place in the core of the Sun and stars The core temperature of the Sun is about 15 million C. At these temperatures hydrogen nuclei fuse to give Helium and Energy. The energy sustains life on Earth via sunlight 73
Energy Released by Nuclear Reactions Light nuclei (hydrogen, helium) release energy when they fuse (Nuclear Fusion) The product nuclei weigh less than the parent nuclei Heavy nuclei (Uranium) release energy when they split (Nuclear Fission) The product nuclei weigh less than the original nucleus 74
Energy Released by Nuclear Fusion and Fission Fusion reactions release much higher energies than Fission reactions 75
Fusion Reactions Deuterium from water (0.02% of all hydrogen is heavy hydrogen or deuterium) Tritium from lithium (a light metal common in the Earth s crust) Deuterium + Lithium Helium + Energy This fusion cycle (which has the fastest reaction rate) is of interest for Energy Production 76
Fissionable U-235 77
The World, particularly developing countries, needs a New Energy Source Growth in world population and growth in energy demand from increased industrialisation/affluence will lead to an Energy Gap which will be increasingly difficult to fill with fossil fuels Without improvements in efficiency we will need 80% more energy by 2020 Even with efficiency improvements at the limit of technology we would still need 40% more energy 78
Incentives for Developing Fusion Fusion powers the Sun and the stars It is now within reach for use on Earth In the fusion process lighter elements are fused together, making heavier elements and producing prodigious amounts of energy Fusion offers very attractive features: Sustainable energy source (for DT cycle; provided that Breeding Blankets are successfully developed) No emission of Greenhouse or other polluting gases No risk of a severe accident No long-lived radioactive waste Fusion energy can be used to produce electricity and 79 hydrogen, and for desalination
Fusion Energy Disadvantages Fusion reaction is difficult to start! High temperatures (Millions of degrees) in a pure High Vacuum environment are required Technically complex and high capital cost reactors are necessary More Research and Development is needed to bring concept to fruition The physics is well advanced but requires sustained development on a long time scale (20 to 40 years) 80
Fusion Power Station Schematic 81