Chapter 7 - Fig 7.5 Fission and Energy Fission Isotopes –...

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Unformatted text preview: Fig. 7.5 Fission and Energy Fission Isotopes – forms of the same element that Isotopes differ in the number of neutrons in the nucleus and in mass. nucleus #of protons 1 1 2 1 3 1 12 6 #of neutrons Natural abundance H H H C C C 13 6 14 6 Fission and Energy Fission Isotopes – forms of the same element that Isotopes differ in the number of neutrons in the nucleus and in mass. nucleus 1 1 2 1 3 1 12 6 H H H C C C #of protons 1 1 1 6 6 6 #of neutrons Natural abundance 13 6 14 6 Fission and Energy Fission Isotopes – forms of the same element that Isotopes differ in the number of neutrons in the nucleus and in mass. nucleus 1 1 2 1 3 1 12 6 H H H C C C #of protons 1 1 1 6 6 6 #of neutrons 0 1 2 6 7 8 Natural abundance 13 6 14 6 Fission and Energy Fission Isotopes – forms of the same element that Isotopes differ in the number of neutrons in the nucleus and in mass. nucleus 1 1 2 1 3 1 12 6 H H H C C C #of protons 1 1 1 6 6 6 #of neutrons 0 1 2 6 7 8 Natural abundance 98.9% 1.11% 1 x 10-12 % 13 6 14 6 Fission and Energy Fission Isotopes of interest for nuclear energy: Uranium Fission and Energy Fission Nuclear energy comes from fission – the Nuclear fission 235 splitting of a large nucleus (like 235U) into U) smaller ones accompanied by the release of energy. energy. involves striking with fissionable nucleus with a involves 1 high-energy neutron 0 n high-energy not all isotopes can undergo fission reactions not fission the energy comes from a tiny amount of mass the “lost” “lost” Fig. 7.4 Fission and Energy Fission How can mass be lost? Mass and energy are not individually conserved in Mass individually nuclear reactions nuclear If mass is lost it is converted to energy Governed by Einstein’s equation: E = mc2 Fission and Energy Fission Nuclear fission in power plants 235 235 In fission of 235U about .1% of the mass of 235U is In about is converted to energy converted 235 Example: if we react 3kg of 235U how much energy Example: how is given off? is .1% of 3kg = .001 x 3kg = .003kg E = mc2 = (.003kg)(3.00 x 108 m/s)2 = 2.7 x 1014 (kg m2/s2) or J Note: 3g converted to energy, 2997g of waste Note: products produced. products Fission and Energy Fission How much energy is this? 100,000 Tons of TNT 10,000 Tons of coal 2,100,000 cars lifted 6 miles up 26.1 million gallons of water converted to steam Fission and Energy Fission Nuclear reactions versus chemical reactions chemical reactions same number and type elements on both sides, just rearranged rearranged matter is conserved energy is related to bonds broken and formed nuclear reactions different elements on both sides of reaction different matter is conserved, but small amounts of mass may be converted to energy converted species are written with mass numbers, and often with species atomic numbers atomic 235 235 U or 92 U total of mass numbers on both sides is the same total of atomic numbers on both sides is the same Fission and Energy Fission Example: a typical fission reaction 1 0 n+ 235 92 U→ 141 56 Ba + 92 36 1 Kr + 3 0 n mass numbers: 1 + 235 = 141 + 92 + (3x1) = 236 atomic numbers: 0 + 92 = 56+36+(3x0) = 92 Fission and Energy Fission How are nuclear reactions sustained? 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 n+ n+ n+ n+ n+ n+ n+ n+ n+ 235 92 235 92 235 92 235 92 235 92 235 92 235 92 235 92 235 92 U→ U→ U→ U→ U→ U→ U→ U→ U→ 141 56 139 56 144 54 143 54 142 54 140 54 90 37 Ba + Ba + Xe + Xe + Xe + Xe + Te + Ce + 92 36 94 36 89 38 90 38 92 38 94 38 1 Kr + 3 0 n 1 Kr + 3 0 n 1 Sr + 3 0 n 1 Sr + 3 0 n 1 Sr + 2 0 n 1 Sr + 2 0 n 1 Cs + 2 0 n 1 Zr + 2 0 n 1 Se + 3 0 n Rb + 144 55 97 40 89 34 137 52 144 58 Fission and Energy Fission How are nuclear reactions sustained? 1 neutron in produces 2 or 3 neutrons out Chain reaction when neutrons produced from fission cause fission in nearby nuclei so that the reaction becomes selfnearby sustaining no need for an external source of neutrons to continue no the reaction the Sub-critical mass insufficient mass of fissionable material for a chaininsufficient reaction most neutrons produced escape the mass without most causing more fission causing Fission and Energy Fission How are nuclear reactions sustained? Critical mass Minimum amount of fissionable material needed to Minimum sustain a chain reaction sustain At least one neutron from every fission initiates another At fission (on average) fission Super-critical mass Enough mass that each fission produces 2 or 3 more Enough Very few neutrons escape the mass Reaction speeds up and can become explosive Fission and Energy Fission Nuclear Power Plants Nuclear Nuclear Power Plants Nuclear Non-nuclear part Non-nuclear Steam generator – heat from the nuclear reactor Steam turns water to steam Turbine – steam pressure turns the turbine Turbine (mechanical energy) (mechanical Generator – turbine runs the generator making Generator electricity (electrical energy) electricity Condenser – steam is cooled back to liquid water Nuclear Power Plants Nuclear Nuclear part Nuclear Fuel rods - contain uranium (as UO2 pellets) stacked in fuel rods and bundled in fuel assemblies assemblies Nuclear Power Plants Nuclear Nuclear part Nuclear Primary coolant – usually a solution of boric acid Primary in water in carries heat from the reaction to the steam generator acts as moderator - slows neutrons in the core so that acts they produce more fission reactions they Control rods – made of a neutron-absorbing Control material, usually cadmium; can be adjusted to keep fission self-sustaining or to shut down fission completely fission Nuclear Power Plants Nuclear Nuclear part Nuclear Source of neutrons to start fission Source Radioactive plutonium emits an alpha particle 238 94 Pu → 9 4 234 92 U + 4 He 2 12 6 Alpha particle strikes beryllium target producing a neutron neutron 4 2 He + Be → C+ n+ γ 1 0 0 0 Nuclear Power Plants Nuclear Energy from Nuclear Fusion Energy Fusion – the nuclei of light elements combine Fusion to form new nuclei releasing energy to 2 1 1 1 1 H + 3 H → 4 He + 0 n 1 2 H + 2 H → 3 He 1 2 He + 3 He → 4 He + 21 H 2 2 1 3 2 Happens in stars - Sun is 26% He and 73%H Advantages: Availability of light isotopes Fusion products are generally not radioactive Energy from Nuclear Fusion Energy Fusion Fusion Disadvantages: Very high energies are needed to overcome the repulsion between nuclei (activation energy) between Very high temperatures are needed to overcome this Very activation energy (thermonuclear reactions) activation Use atomic (fission) bomb Hydrogen (fusion) bombs are triggered by atomic (fission) Hydrogen bombs bombs Need some way to contain the nuclear fuel at those temperatures in order to use the reaction for energy temperatures Gravitational confinement – gravity can confine the fuel – Gravitational this happens in stars – too large a mass needed to use for energy generation energy Magnetic confinement – use a magnetic field to contain the Magnetic plasma (hot fuel) plasma Can Power Plants Explode? Can Can a nuclear power plant explode like a bomb? bomb? Both use fission 235 Both (may) use 235U Both Power plant requires slow, controlled reaction Bomb requires rapid, uncontrolled reaction Can Power Plants Explode? Can Nuclear fuel Nuclear 3-5% 235U - fissionable 3-5% 235 97-95% 238U – radioactive and capable of nuclear 97-95% 238 radioactive reaction, but not fissionable reaction, enriched uranium –more than the naturally enriched 235 occurring .7% of 235U can’t get a big enough cascade of neutrons to can’t explode explode can get a big enough cascade of neutrons to get hotter and hotter – may be too much for containment unit Note: for 100kg of U reacted 235 ~5kg is 235U ~.005kg is turned into energy 499.995kg of radioactive waste left over Can Power Plants Explode? Can Atomic Weapons Atomic 90% 235U 90% 235 10% 238U 10% 238 highly enriched uranium Can Power Plants Explode? Can Enriching Uranium – how do we separate two isotopes of uranium that have the same chemical properties? chemical UO2 solid → UF6 gas Gaseous diffusion – force UF6 gas through 235 238 membranes – 235UF6 moves faster than 238UF6 238 Centrifugation – spin in a centrifuge, heavier 238UF6 Centrifugation moves to the bottom moves What about Chernobyl? What What about Chernobyl? What Nuclear explosion? No. Nuclear Cooling water to core interrupted for test Not enough control rods left in reactor Reaction speed increased, core heated up Fuel rods got too hot and ruptured Hot fuel exploded on contact with water Water sprayed on burning graphite (used to Water control the reaction) reacted to produce H2 gas control 2 H2O + C(graphite) → 2 H2 + CO2 Main explosion was a hydrogen gas explosion Blew cover off reactor What about Chernobyl? What Effects Effects Radioactive material released into atmosphere as Radioactive reactor burned reactor Amount equivalent to 100 atomic bombs Estimated 250 million people exposed to radiation Estimated that could shorten their lives that What about Chernobyl? What New Safety Precautions New Seabrook – New Hampshire Seabrook New Safety Precautions New Core contained in reactor vessel – 44 ft. high walls Core of 8-inch thick carbon steel of Inner walls of 4.5 ft. thick reinforced concrete Outer wall 15 in. thick Designed to withstand hurricanes, earthquake, Designed bomber plane crash bomber Newer Reactor Designs Newer Pebble Bed Reactors pebbles of nuclear fuel instead of fuel rod bundles 1mm diameter spheres mixed with graphite 1mm powder to make a 5cm ball powder 9g of U in a 120g mass Coated with silicon carbide to keep fission Coated products contained products Newer Reactor Designs Newer Reactor Vessel Pebbles in a large bin Inert (unreactive) gas line He, N2 or CO2 circulates as primary coolant CO (instead of boric acid in water) (instead Fuel pebbles heat gas, heat Fuel transferred to a secondary coolant gas gas turns turbines Newer Reactor Designs Newer Safety graphite lining containment vessel graphite can withstand 2800ºC can normal reactor temp.: 1200ºC normal extreme temp.: 1600ºC extreme vessel designed to lose heat vessel passively (with no mechanical aid) more quickly than it could be generated – reaction can’t get too hot hot inert gases will not react with inert graphite or metallic core elements graphite Fuel to Weapons? Fuel Yes – but difficult – need to highly enrich it Spent Nuclear Fuel Spent 238 U undergoes a nuclear reaction (NOT fission): 238 92 1 U + 0n → 239 94 Pu + 2 -0 e 1 Pu is fissionable 238 Nuclear fuel (~95% 238U) → spent nuclear fuel Nuclear U) 239 (large amount of 239Pu) Could be used for weapons – or more nuclear fuel Breeder reactors – designed to produce a lot of Breeder 239 239Pu 239 Fuel to Weapons? Fuel Reprocessing Spent Nuclear Fuel Banned by President Carter Ban lifted in 1981, but still not done in US 239 Radiation from 239Pu cannot penetrate skin, Radiation Pu material also not easily absorbed by skin material BUT reacts with O2 → PuO2 dust, easily inhaled, very toxic – 10-6g can cause can lung cancer lung Radioactivity Radioactivity Radioactivity – spontaneous emission of radiation by certain elements radiation Radioactivity Radioactivity Radioactivity – spontaneous emission of radiation by certain elements radiation Radioactivity Radioactivity Radioactivity – spontaneous emission of radiation by certain elements radiation Radioactivity Radioactivity Radioactivity – spontaneous emission of radiation by certain elements radiation Radioactivity Radioactivity Radioactive Decay (another kind of nuclear reaction) reaction) Note: NOT fission or fusion 1 0 n+ 235 92 U→ 141 56 Ba + 92 36 1 Kr + 3 0 n 2 1 1 H + 3 H → 4 He + 0 n 1 2 Radioactivity Radioactivity Hazards of Radiation Hazards electromagnetic radiation can cause Rotation (microwaves) Bond bending and stretching (infrared) Ionization – removal of electrons Ionizing radiation – radiation that can ionize Ionizing water (1216 kJ/mol) water Ionized water forms •OH radical – reactive, attacks Ionized surrounding biomolecules surrounding Hazards of Radiation Hazards Hazard depends on type of radiation α radiation – stopped by skin, but very high radiation energy, so very harmful if ingested or inhaled energy, β radiation – penetrates ~1cm into tissue γ radiation – penetrates skin, tissue, bone and radiation organs, but lower energy organs, Large doses – radiation sickness and death Smaller doses – cancer - damage to the growthregulation mechanism of cells Damage Hazards of Radiation Hazards Curies Curies based on number of radioactive decays per second 1 Ci = 3.7 x 1010 decays /second does not indicate type of radiation or take into account the does size of the person size 1 rad = .01J of energy absorbed per kg of tissue size of person, but not type of radiation accounted for # of rems = quality factor (Q) x # of rads accounts for both size of person and type of radiation 1 Sv = 100 rem Rad – radiation absorbed dose Rem – roentgen equivalent for man Sievert Sievert Hazards of Radiation Hazards National Average Dose: .0036 Sv or .36 Rem Hazards of Radiation Hazards Radioactivity Radioactivity How long are things radioactive? How Rate of decay – depends on identity of the Rate identity radioactive isotope, not on the amount present present Half-life – time required for the level of Half-life radioactivity to fall to half its original level radioactivity not linear – not gone after two half lives exponential decay – radioactivity eventually exponential approaches zero approaches How long are things radioactive? How How long are things radioactive? How How long are things radioactive? How Some common radioactive isotopes Some Radon gas – rocks and soil contain small amounts Radon of uranium, which eventually decays into radonof 222 Iodine-131 Small amounts used to treat hyperthyroidism Large amounts from nuclear fallout or accident can Large cause thyroid cancer cause Strontium-90 Acts like calcium – can be incorporated into bones How long are things radioactive? How Some common radioactive isotopes Some Carbon-14 Used to date ancient biological materials Atmosphere contains a constant ratio of C-14 to C-12 of Atmosphere 1 C-14 to 1012 C-12 C-14 Plants and animals incorporate either – have same ratio Dead organisms stop exchanging carbon – C-14 decays Dead with none to replace it. with Half-life = 5715 years Judge age by C-14 to C-12 ratio Shown to be accurate to within 10% of ages provided by Shown historical records historical Disposal of Nuclear Waste Disposal High-Level radioactive Waste (HLW) High-Level High levels of radioactivity Isotopes with long half-lives Needs permanent isolation Can also be chemically damaging (acidic, basic, Can containing heavy metals) containing Includes spent nuclear fuel (SNF) 99% of US HLW generated by nuclear weapons 99% program program Disposal of Nuclear Waste Disposal Storage of spent nuclear fuel (currently) cooled on-site (at power plant) in pools of water cooled with a neutron absorber with Original plan - reprocessing Disposal of Nuclear Waste Disposal Plan for long-term storage By 2010 – more than 100,000 tons of HLW in US By needing disposal needing Disposal of Nuclear Waste Disposal Requirements for geological storage Isolated from ground water for 10s of thousands Isolated of yrs. of 1000 ft. below ground, 1000ft. above water table salt, basalt or granite salt – dry, self-sealing basalt/granite – can chemically absorb waste Disposal of Nuclear Waste Disposal Disposal of Nuclear Waste Disposal Yucca Mountain in Nevada Disposal of Nuclear Waste Disposal Yucca Mountain in Nevada Disposal of Nuclear Waste Disposal Yucca Mountain in Nevada capacity for 70,000 tons of spent fuel, 8,000 tons capacity military waste military interim proposal – store HLW above ground at interim Yucca Mtn. until deep storage is approved/ready Yucca Low Level Waste Low Low-Level radioactive Waste (LLW) – waste Low-Level contaminated with smaller amounts of radioactive wastes – specifically excludes nuclear fuel nuclear materials contaminated in labs or from medical materials procedures procedures waste from nuclear fuel fabrication, mining, etc. makes up about 90% of all radioactive waste Low Level Waste Low Disposal of LLW buried 10 ft. deep in canisters in lined trenches Nuclear Power Worldwide Nuclear 17% of global electricity (~11% or global 17% energy) produced in 440 power plants energy) Nuclear Power Worldwide Nuclear Nuclear Power – Risks and Benefits Nuclear 100,000 workers killed in coal mines since 100,000 1900 1900 Pollutants released from coal power plants U and Th (both radioactive) are present in coal, and released in exhaust released 1982 – coal power plants released 801 tons U, 1982 1971 tons of Th 1971 4.5 million tons CO2/year/plant 3.5 million tons waste/year/plant compared to nuclear plants – 70 ft3 of waste/year/plant waste/year/plant Nuclear Power – Risks and Benefits Nuclear Nuclear Power – Risks and Benefits Nuclear ...
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This note was uploaded on 04/11/2011 for the course CHEM 102 taught by Professor Henshaw during the Winter '11 term at Grand Valley State.

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