Unformatted text preview: UCSD Physics 12 Realities of Nuclear Energy
Resources Waste and Disasters The Promise of Fusion? UCSD Physics 12 Summary of fission U will undergo spontaneous fission if a neutron happens by, resulting in:
235 two sizable nuclear fragments flying out a few extra neutrons gamma rays from excited states of daughter nuclei energetic electrons from beta-decay of daughters The net result: lots of banging around generates heat locally (kinetic energy of tiny particles) for every gram of 235U, get 65 billion Joules, or about 16 million Calories compare to gasoline at roughly 10 Calories per gram a tank of gas could be replaced by a 1-mm pellet of 235U!! Spring 2010 Q 2 UCSD Physics 12 Enrichment Natural uranium is 99.27% 238U, and only 0.72% 235 U 238 U is not fissile, and absorbs wandering neutrons In order for nuclear reaction to self-sustain, must enrich fraction of 235U to 35% interestingly, it was so 3 billion years ago now probability of wandering neutron hitting 235U is sufficiently high to keep reaction crawling forward Enrichment is hard to do: a huge technical roadblock to nuclear ambitions
Spring 2010 3 UCSD Physics 12 Nuclear Fission Reactors Nuclear fission is used simply as a heat source to run a heat engine By controlling the chain reaction, can maintain hot source for periods greater than a year Heat is used to boil water Steam turns a turbine, which turns a generator Efficiency limited by familiar Carnot efficiency: = (Th - Tc)/Th (about 3040%, typically) Spring 2010 4 UCSD Physics 12 Nuclear Plant Layout Spring 2010 5 UCSD Physics 12 The core of the reactor not shown are the control rods that absorb neutrons and thereby keep the process from running away Spring 2010 6 UCSD Physics 12 Fuel Packaging Want to be able to surround uranium with fluid to carry away heat lots of surface area is good Also need to slow down neutrons water is good for this Spring 2010 So uranium is packaged in long rods, bundled into assemblies Rods contain uranium enriched to ~3% 235U Need roughly 100 tons per year for a 1 GW plant Uranium stays in three years, 1/3 cycled yearly 3 Q 7 UCSD Physics 12 Control rod action Simple concept: need exactly one excess neutron per fission event to find another 235 U Inserting a neutron absorber into the core removes neutrons from the pool Pulling out rod makes more neutrons available Emergency procedure is to drop all control rods at once Spring 2010 8 UCSD Physics 12 Our local nuclear plant: San Onofre 10 miles south of San Clemente Easily visible from I-5 2 reactors brought online in 1983, 1984 older decommissioned reactor retired in 1992 after 25 years of service 1.1 GW each PWR type No cooling towers: it's got the ocean for that Spring 2010 9 UCSD Physics 12 The relative cost of nuclear power safety regulations tend to drive cost Spring 2010 10 UCSD Physics 12 Aside on nuclear bombs Since neutrons initiate fission, and each fission creates more neutrons, there is potential for a chain reaction Have to have enough fissile material around to intercept liberated neutrons Critical mass for 235U is about 15 kg, for 239Pu it's about 5 kg need highly enriched (about 90% 235U for uranium bomb) Bomb is dirt-simple: separate two sub-critical masses and just put them next to each other when you want them to explode! difficulty is in enriching natural uranium to mostly 235U Spring 2010 Q 11 UCSD Physics 12 The finite uranium resource Uranium cost is about $23/kg about 1% of cost of nuclear power As we go for more, it's more expensive to get depleted the easy spots 3 million tons available at cost < $230/kg Need 200 tons per GW-yr Now have 100 GW of nuclear power generation in about 100 plants; 1 GW each 3 million tons will last 150 years at present rate only 30 years if nuclear replaced all electricity prod. Spring 2010 12 UCSD Physics 12 Breeder Reactors The finite resource problem goes away under a breeder reactor program Neutrons can attach to the non-fissile 238U to become 239U beta-decays into 239Np with half-life of 24 minutes 239Np beta-decays into 239Pu with half-life of 2.4 days now have another fission-able nuclide about 1/3 of energy in normal reactors ends up coming from 239Pu Reactors can be designed to "breed" 239Pu in a better-than-break-even way
Spring 2010 13 UCSD Physics 12 Breeders, continued Could use breeders to convert all available 238U into 239Pu all the while getting electrical power out Now 30 year resource is 140 times as much (not restricted to 0.7% of natural uranium), or 4200 yr Technological hurdle: need liquid sodium or other molten metal to be the coolant but four are running in the world Enough 239Pu falling into the wrong hands spells: BOOM!! Spring 2010 14 UCSD Physics 12 Reactor Risk Once a vigorous program in the U.S. still so in France: 80% of their electricity is nuclear No new orders for reactors in U.S. since late 70's not coincidentally on the heels of Three-Mile Island Failure modes: criticality accident: runaway chain reactionmeltdown loss of cooling: not runaway, but overheats meltdown reactors are incapable of nuclear explosion steam or chemical explosions are not ruled out meltdown Spring 2010 15 UCSD Physics 12 Risk Assessment Extensive studies by agencies like the NRC 1975 report concluded that: loss-of-cooling probability was 1/2000 per reactor year significant release of radioactivity 1/1,000,000 per RY chance of killing 100 people in an accident about the same as killing 100 people by a falling meteor 1990 NRC report accounts for external disasters (fire, earthquake, etc.) large release probability 1/250,000 per RY 109 reactors, each 30 year lifetime 1% chance Spring 2010 16 UCSD Physics 12 Close to home: Three Mile Island Spring 2010 17 UCSD Physics 12 The Three-Mile Island Accident, 1979 The worst nuclear reactor accident in U.S. history Loss-of-cooling accident in six-month-old plant Combination of human and mechanical errors Severe damage to core but containment vessel held No major release of radioactive material to environment Less than 1 mrem to nearby population less than 100 mrem to on-site personnel compare to 300 mrem yearly dose Instilled fear in American public, fueled by movies like The China Syndrome Spring 2010 18 UCSD Physics 12 The Chernobyl Disaster Blatant disregard for safety plus inherently unstable design spelled disaster Chernobyl was a boiling-water, graphitemoderated design unlike any in the U.S. used for 239Pu weapons production frequent exchange of rods to harvest Pu meant lack of containment vessel like the ones in U.S. positive-feedback built in: gets too hot, it runs hotter: runaway possible once runaway initiated, control rods not effective Spring 2010 19 UCSD Physics 12 Chernobyl, continued On April 25, 1986, operators decided to do an "experiment" as the reactor was powering down for routine maintenance disabled emergency cooling system blatant violation of safety rules withdrew control rods completely powered off cooling pumps reactor went out of control, caused steam explosion that ripped open the reactor many fires, exposed core, major radioactive release Spring 2010 20 UCSD Physics 12 Chernobyl after-effects Total of 100 million people exposed (135,000 lived within 30 km) to radioactivity much above natural levels Expect from 25,000 to 50,000 cancer deaths as a result compared to 20 million total worldwide from other causes 20,000,000 becomes 20,050,000 (hard to notice... ...unless you're one of those 50,000 31 died from acute radiation exposure at site 200 got acute radiation sickness Spring 2010 21 UCSD Physics 12 Nuclear Proliferation The presence of nuclear reactors means there will be plutonium in the world and enriched uranium If the world goes to large-scale nuclear power production (especially breeder programs), it will be easy to divert Pu into nefarious purposes But other techniques for enriching uranium may become easy/economical and therefore the terrorist's top choice Should the U.S. abandon nuclear energy for this reason? perhaps a bigger concern is all the weapons-grade Pu already stockpiled in the U.S. and former U.S.S.R.!! Spring 2010 22 UCSD Physics 12 Nuclear Waste Big Problem Originally unappreciated Each reactor has storage pool, meant as temporary holding place originally thought to be 150 days 35 years and counting Huge variety of radioactive products, with a whole range of half-lives 1GW plant waste is 70 MCi after one year; 14 MCi after 10 years; 1.4 MCi after 100 years; 0.002 MCi after 100,000 years 1 Ci (Curie) is 37 billion radioactive decays per second Spring 2010 23 UCSD Storage Solutions Physics 12 There are none...yet EPA demands less than 1000 premature cancer deaths over 10,000 years!! incredibly hard to design/account Proposed site at Yucca Mountain, NV Very bad choice, geologically: cracks and unstable Worldwide, nobody has worked out a storage solution Spring 2010 24 UCSD Physics 12 Burial Issues Radioactive emissions themselves are not radioactive just light, electrons/positrons and helium nuclei but they are ionizing: they rip apart atoms/molecules they encounter Absorb emissions in concrete/earth and no effect on biology so burial is good solution Problem is the patience of time half lives can be long geography, water table changes nature always outlasts human structures imagine building something to last 10,000 years!!
Q Spring 2010 25 UCSD Physics 12 Fusion: The big nuclear hope Rather than rip nuclei apart, how about putting them together?
alpha (4He) tritium Iron is most tightly bound nucleus Can take loosely bound light nuclei and build them into more tightly bound nuclei, releasing energy Huge gain in energy going from protons (1H) to helium (4He). It's how our sun gets its energy Much higher energy content than fission dueterium proton Spring 2010 26 UCSD Physics 12 Thermonuclear fusion in the sun Sun is 16 million degrees Celsius in center Enough energy to ram protons together (despite mutual repulsion) and make deuterium, then helium Reaction per mole ~20 million times more energetic than chemical reactions, in general
4 protons: mass = 4.029 He nucleus: mass = 4.0015
4 2 neutrinos, photons (light) Spring 2010 27 UCSD Physics 12 E=mc2 balance sheets Helium nucleus is lighter than the four protons! Mass difference is 4.029 4.0015 = 0.0276 a.m.u. 0.7% of mass disappears, transforming to energy 1 a.m.u. (atomic mass unit) is 1.6605 10-27 kg difference of 4.58 10-29 kg multiply by c2 to get 4.12 10-12 J 1 mole (6.022 1023 particles) of protons 2.5 1012 J typical chemical reactions are 100200 kJ/mole nuclear fusion is ~20 million times more potent stuff! works out to 150 million Calories per gram compare to 16 million Cal/g uranium, 10 Cal/g gasoline Spring 2010 28 UCSD Physics 12 Artificial fusion 16 million degrees in sun's center is just enough to keep the process going but sun is huge, so it seems prodigious In laboratory, need higher temperatures still to get worthwhile rate of fusion events like 100 million degrees Bottleneck in process is the reaction:
1 H + 1H 2H + e+ + (or proton-proton deuteron) Better off starting with deuterium plus tritium 2H and 3H, sometimes called 2D and 3T but give up some energy: starting higher on binding energy graph Then:
2 H + 3H 4He + n + 17.6 MeV (leads to 81 MCal/g) Spring 2010 29 UCSD Physics 12 Deuterium everywhere Natural hydrogen is 0.0115% deuterium Lots of hydrogen in sea water (H2O) Total U.S. energy budget (100 QBtu = 1020 J per year) covered by sea water contained in cubic volume 170 meters on a side corresponds to 0.15 cubic meters per second about 1,000 showers at two gallons per minute each about one-millionth of rainfall amount on U.S. 4 gallons per person per year!!! Spring 2010 30 UCSD Physics 12 Tritium nowhere Tritium is unstable, with half-life of 12.32 years thus none naturally available Can make it by bombarding 6Li with neutrons extra n in D-T reaction can be used for this, if reaction core is surrounded by "lithium blanket" Lithium on land in U.S. would limit D-T to a hundred years or so maybe a few thousand if we get lithium from ocean D-D reaction requires higher temperature, but could be sustained for many millennia
Spring 2010 31 UCSD Physics 12 Nasty by-products? Practically none: not like radioactive fission products Building stable nuclei (like 4He) maybe our voices would be higher... Tritium is only radioactive substance energy is low, half-life short: not much worry here Extra neutrons can tag onto local metal nuclei (in surrounding structure) and become radioactive but this is a small effect, especially compared to fission Spring 2010 32 UCSD Physics 12 Why don't we embrace fusion, then? Believe me, we would if we could It's a huge technological challenge, always 50 years from fruition must confine plasma at 50 million degrees!!! 100 million degrees for D-D reaction all the while providing fuel flow, heat extraction, tritium supply, etc. hurdles in plasma dynamics: turbulence, etc. Still pursued, but with decreased enthusiasm, increased skepticism but man, the payoff is huge: clean, unlimited energy Spring 2010
Q 33 UCSD Physics 12 Fusion Successes? Fusion has been accomplished in labs, in big plasma machines called Tokamaks got ~6 MW out of Princeton Tokamak in 1993 but put ~12 MW in to sustain reaction Hydrogen bomb also employs fusion fission bomb (e.g., 239Pu) used to generate extreme temperatures and pressures necessary for fusion LiD (lithium-deuteride) placed in bomb fission neutrons convert lithium to tritium tritium fuses with deuterium Spring 2010 34 UCSD Physics 12 References and Assignments Extra Credit on WebCT: adds 2% to final grade enough to cross grade boundary! More on Three Mile Island: www.nrc.gov/reading-rm/doc-collections/fact-sheets/ More on Chernobyl: www.chernobyl.info/en/Facts/ also NRC link as above Homework #7: due 5/28 Power Plant Tour: Monday 5/24 23 PM (optional) WEAR LONG PANTS and CLOSED SHOES (no toes) preceded by preview in classroom Spring 2010 35 ...
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