15_nuclear1 - UCSD Physics 12 Nuclear Fission What's it all...

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Unformatted text preview: UCSD Physics 12 Nuclear Fission What's it all about? UCSD Physics 12 What's in a Nucleus The nucleus of an atom is made up of protons and neutrons each is about 2000 times the mass of the electron, and thus constitutes the vast majority of the mass of a neutral atom (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 (hydrogen nucleus) will last forever neutron by itself will "decay" 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 Spring 2010 Q 2 UCSD Physics 12 What holds it together? If like charges repel, and the nucleus is full of protons (positive charges), why doesn't it fly apart? repulsion is from electromagnetic force at close scales, another force takes over: the strong nuclear force The strong force operates between quarks: 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 Spring 2010 3 UCSD Physics 12 What's the deal with neutrons decaying?! 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 an anti-neutrino, a chargeless, nearly massless cousin to the electron proton Poof! neutron neutrino 4 electron Spring 2010 UCSD Physics 12 Insight from the decaying neutron Another force, called the weak nuclear force, mediates these "flavor" 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 Spring 2010 Q 5 UCSD Physics 12 Counting particles A nucleus has a definite number of protons (Z), a definite number of neutrons (N), and a definite total number of nucleons: A = Z + N example, the most common isotope of carbon has 6 protons and 6 neutrons (denoted 12C; 98.9% abundance) 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 Spring 2010 6 UCSD Physics 12 Full notation A fully annotated nucleon symbol has the total nucleon number, A, the proton number, Z, and the neutron number, N positioned around the symbol XN redundancy in that A = Z + N A Z Examples: carbon-12: C6 carbon-14: C8 235 uranium-235: 92 U143 238 uranium-238: 92 U146 239 plutonium-239: 94 Pu145 Q 12 6 14 6 Spring 2010 7 UCSD Physics 12 Radioactivity 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 (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 Certain isotopes of nuclei are radioactively unstable they will eventually change flavor by a radioactive particle emission , , emission constitutes a minor change to the nucleus not as dramatic as splitting the entire nucleus in two large parts Spring 2010 8 UCSD The Physicist's Periodic Table Chart of the Nuclides Physics 12 + 3 2 - Z 1 0 Spring 2010 9 UCSD Physics 12 Radioactivity Demonstration 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: 14 C with half life of 5730 years about 4000 - decays per second in this sample corresponds to 25 ng, or 1015 particles 90 Sr 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 Spring 2010 10 UCSD Physics 12 Natural radioactive dose in mrem/year Source cosmic rays terrestrial (rock) food and water air (mostly radon) air travel house medical X-ray nuclear med. treatment within 50 miles of nuclear plant within 50 miles of coal plant total for no travel/medical 316 Sea Level 28 46 40 200 1 per 1,000 miles traveled 7 if made of stone/brick/concrete 40 each (airport X-ray negligible) 14 each 0.009 0.03 387 Denver 55 90 source: Spring 2010 www.epa.gov/radiation/students/calculate.html 11 UCSD Physics 12 Fission of Uranium Barium and Krypton represent just one of many potential outcomes Spring 2010 12 UCSD Physics 12 Fission There are only three known nuclides (arrangements of protons and neutrons) that undergo fission when introduced to a slow (thermal) neutron: U: 235 U: 239 Pu: 233 hardly used (hard to get/make) primary fuel for reactors popular in bombs Others may split if smacked hard enough by a neutron (or other energetic particle) Spring 2010 13 UCSD Physics 12 How much more fissile is 235U than 238U? Bottom line: at thermal energies (arrow), 235U is 1000 times more likely to undergo fission than 238U even when smacked hard Spring 2010 14 UCSD Physics 12 Uranium isotopes and others of interest Isotope 233 234 235 236 237 238 239 232 Abundance (%) 0 0.0055 0.720 0 0 99.2745 no natural Pu 100 2 Q half-life 159 kyr 246 kyr 704 Myr 23 Myr 6.8 days 4.47 Gyr 24 kyr 14 Gyr decays by: 15 U U U U U U Pu Th Spring 2010 UCSD Physics 12 The Uranium Story No isotope of uranium is perfectly stable: U has a half-life of 704 million years 238 U has a half-life of 4.5 billion years (age of earth) 235 No heavy elements were made in the Big Bang (just H, He, Li, and a tiny bit of Be) Stars only make elements as heavy as iron (Fe) through natural thermonuclear fusion Heavier elements made in catastrophic supernovae massive stars that explode after they're spent on fusion 235 U and 238U initially had similar abundance 16 Spring 2010 UCSD Physics 12 Uranium decay The natural abundance of uranium today suggests that it was created about 6 billion years ago assumes 235U and 238U originally equally abundant Now have 39.8% of original 238U and 0.29% of original 235 U works out to 0.72% 235U abundance today Plutonium-239 half-life is too short (24,000 yr) to have any naturally available Thorium-232 is very long-lived, and is a major contributor to geothermal heat though 238U, 235U, and 40K contribute as well Spring 2010 17 UCSD Physics 12 Why uranium? Why mess with "rare-earth" materials? Why not force lighter, more abundant nuclei to split? though only three "slow-neutron" fissile nuclei are known, what about this "smacking" business? Turns out, you would actually loose energy in splitting lighter nuclei Iron is about the most tightly bound of the nuclides and it's the release of binding energy that we harvest so we want to drive toward iron to get the most out Spring 2010 18 UCSD Physics 12 Binding energy per nucleon Iron (Fe) is at the peak On the heavy side of iron, fission delivers energy On the lighter side of iron, fusion delivers energy This is why normal stars stop fusion after iron Huge energy step to be gained in going from hydrogen (H) to helium-4 via fusion Spring 2010 19 UCSD Physics 12 What does uranium break into? (fish `n chips) Uranium doesn't break into two equal pieces usually one with mass around 95 a.m.u. and one with mass around 140 a.m.u. The fragments are very neutronrich, and some drip off immediately these can spur additional fission events... Even after the neutron-drip, the fragments rapidly undergo radioactive transformations until they hit stable configurations Spring 2010 20 UCSD Physics 12 Chart of the nuclides 235 U daughter 1 daughter 2 Spring 2010 stable nuclide radioactive (unstable) nuclide 21 UCSD Physics 12 Messy details summarized 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 trillion 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 22 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 Bomb is dirt-simple: separate two sub-critical masses and just put them next to each other (quickly) when you want them to explode! difficulty is in enriching natural uranium to mostly 235U Spring 2010 23 UCSD Physics 12 Assignments Continue to read chapter 6 HW #7 will be posted shortly Power Plant tours: TBA Spring 2010 24 ...
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