Eman 204 Lecture 11: Nuclear (PDF)

Lecture 11 Nuclear EMAN 204 1 Lecture 11 This lecture Basic Physics of Nuclear Energy – Origin of Nuclear Energy: Strong binding force (E=mc2) – Binding energy per nucleon Fission reactors Safety and waste Fusion EMAN 204...

Lecture 11 Nuclear EMAN 204 1 Lecture 11 This lecture Basic Physics of Nuclear Energy – Origin of Nuclear Energy: Strong binding force (E=mc2) – Binding energy per nucleon Fission reactors Safety and waste Fusion EMAN 204 2 Lecture 11  Four Fundamental Forces of Nature W=F x d, Apply a force over a distance…, work is done and therefore the forces will have related energy. electromagnetic strong nuclear gas, fossil fuels, fission, sun, wood, coal, geothermal hydrogen (some of it) batteries electricity gravitational weak nuclear hydro, dams , Nuclear batteries tidal EMAN 204 4 Lecture 11  Origin of Nuclear Energy: Basic Idea Nuclei consist of protons and neutrons The protons repel each other due to electromagnetic force Another force is responsible for binding the nucleus together. The binding force is called the “strong nuclear force” When nuclei break or combine to form more stable nuclei (higher binding energy), energy is released. EMAN 204 5 Lecture 11 Physics of Atoms Atoms contains electrons and a nucleus. – Mass electron = 9 x 10-31 kg Nucleus contains nucleons (nucleon number/mass number): protons (proton number) and neutrons. – Mass proton/neutron/nucleon= 1.7 x 10-27 kg (given symbol u) Examples Elements with different numbers of 1 1His the element hydrogen with neutrons are called isotopes. 1 proton and 1 nucleon, 0 Atoms exist due to subtle balance neutrons. between the nuclear (short range 42He is the element helium with 2 between protons and neutrons) and protons and 4 nucleons. Hence 4-2=2 neutrons. electromagnetic (long range 238 92U is uranium with 92 protons between charges) forces. and 146 (i.e. 238-92) neutrons. EMAN 204 6 Lecture 11 Stability of nuclei N is the number of neutrons. Z is the number of protons. For 𝑍 ≤ 20, the most stable nuclei tend to have 𝑁 = 𝑍. For 𝑍 > 20, stable nuclei have 𝑁 > 𝑍. Black line represents the “valley of stability”. Source: Wikipedia EMAN 204 7 Nuclear Binding Energy Rest mass of nucleus always less than the sum of individual masses of protons and neutrons. Example: Alpha particle Difference in mass 𝚫m is called the mass defect. Mass defect is related to nuclear binding energy holding nucleus together via Einstein’s formula: E= 𝚫mc2 For alpha particle, mass defect 𝚫m=0.0304 u and E= 0.5 x10-11 J =30 x 106 eV (1 eV = 1.6 x 10-19 J) EMAN 204 8 Lecture 11 Breaking apart the nucleus: fission Example nuclear reaction 235 141 92U + 10n → 236 92 U → 92 36 Kr + 56 Ba + 3 1 0n Difference between rest mass of products and reactants: 𝚫m=0.215 u. How much energy is released in reaction? (u= 1.7 x 10-27 kg) How much energy per kg of uranium? EMAN 204 9 Lecture 11 Worked example: U235 How much energy is released in U235 reaction? (𝚫m=0.215 u, u= 1.7 x 10-27 kg) How much energy per kg of U235? E = 𝚫m c2 = 0.215*(1.7 x 10-27 kg) * (3 x 108 m/s)2 = 0.215*1.7*9 x 10(-27+16) kg (m/s)2 ≈ 3 x 10-11 J Calorific value = E/m = 0.215 u c2/ 235 u = 0.215*(3 x 108 m/s)2/235 ≈ 1014 J/kg (coal ~ 30 x 106 J/kg) Nuclear energy - orders of magnitude more energy than chemical reactions: 1 tonne of U235 ≈ 3 M tonnes coal 1 tonne U235 ore ≈ 1,000 tonnes coal EMAN 204 10 Lecture 11 Average binding energy per nucleon 1 eV = 1.6 x 10-19 J Yield from fission The higher on the graph the more “tightly” Yield from fusion bound together the atom is, i.e. more “stable”. Splitting elements above iron gives off energy – this is called fission. Combining elements below iron gives off energy – this is called fusion. If energy can be captured – nuclear power! EMAN 204 11 Lecture 11 Fission: Chain reaction Example reaction 235 141 92U + n0 → 236 92 U → 92 36 Kr + 56 Ba + 3n0 fission products Kinetic energy of particles finally captured as heat. EMAN 204 13 Lecture 11 Fission: some technical points and definitions The reaction requires a “thermal” (low energy) neutron to start. High energy neutrons will whiz past without reacting. The reaction produces several fast neutrons which need to be moderated to continue the chain reaction. EMAN 204 14 Lecture 11 Fission: some technical points and definitions Chain reaction can be sustained and controlled by careful control of the amount of neutrons using control rods (which absorbs neutrons). A working reactor is said to be critical. If there are too many thermal neutrons, reactor overheats and meltdown is said to occur. EMAN 204 15 Lecture 11  Basic Reactor Electricity generated by turbine + 5. Coolant generator Control rod 3.rod 7. 6. Steam generator Water in Condenser 3. Control 4. Shielding tertiary 9. 1. Fuel coolant Fuel circuit (open) Water in secondary coolant circuit (closed) 8. Primary coolant More Heat Less heat circuit (closed) EMAN 204 16 Lecture 11  Pressurized water reactor Source: Energy systems and sustainability: Power for a sustainable future EMAN 204 17 Safety and risks Meltdown occurs when part of the reactor core melts due to overheating. 3 meltdowns have occurred in the past: Fukushima (2011), Chernobyl (1986) and Three Mile Island (1979). Chernobyl power station in 2017 with new confinement structure (source: Wikipedia) EMAN 204 27 Safety and risks Immediate causes of the meltdown were: – Loss of coolant (3MI / Fukushima) – Control rods raised too far (Chernobyl) Meltdowns were followed by non-nuclear explosions. – Steam explosion due to rapid overheating (Chernobyl). – High temperature led to formation of H2 gas, which exploded by chemical reaction (Fukushima). EMAN 204 28 History Rapid growth of nuclear power followed by stagnation which appears to be triggered by a series (3) of major (economical & human) nuclear disasters. Source: Wikipedia EMAN 204 29 Lecture 11 Nuclear waste Radioactive waste comprises reaction products and materials that have been exposed to radiation. There are different levels of waste according to their radioactivity: – High level waste which is mainly the spent fuel – Intermediate level waste such as reactor equipment exposed to neutron bombardment – Low level waste - other materials that are only slightly radioactive. The volume of high level waste is a small fraction of the total, but it accounts for almost all of the radioactivity. EMAN 204 31 Nuclear waste High level waste is initially stored in tanks of water to cool down. After about 50 years it could be stored elsewhere. Cooling, radiation shielding and monitoring will still be required for perhaps another 50 years. Long-term storage is required for at least another 1000 years. The first underground long-term storage facility for high level waste is currently being constructed in Finland. EMAN 204 32 Fusion Fusion of two hydrogen nuclei to form a helium nucleus releases a large amount of energy (difference in binding energies is large). Hydrogen has three main isotopes: – Protium ( 11H) – Deuterium ( 21D) – Tritium ( 31T) EMAN 204 33 Fusion DD reaction: 21D + 21D → 32He + n DT reaction: 21D + 31T → 42He + n DT reaction is preferred since lower temperature is required (108 K compared to 3 x 108 K required for DD). EMAN 204 34 Tokamak reactor Source: Sustainable energy without the hot air EMAN 204 36 Summary Nuclear Energy Physics Concepts – When nuclei break or combine to form more stable nuclei, energy is released – binding energy per nucleon graph. – Energy equal to mass defect: E= 𝚫mc2 – Many orders of magnitude more energy per kg than chemical reactions Power generation though intermediate heat generation and can therefore use standard technologies Safety and waste storage are difficult and controversial issues. Will fusion come to the rescue? EMAN 204 37 Lecture 11