Nuclear Energy 6/04/08Lecture 22 1UCSDPhysics 10Nuclear EnergyNuclear EnergyFission, Fusion, the SunFission, Fusion, the Sun’’s Energys EnergySpring 2008 2UCSDPhysics 10WhatWhat’’s in a Nucleuss in a Nucleus••The nucleus of an atom is made up of The nucleus of an atom is made up of protonsprotons and andneutronsneutrons– each is about 2000 times the mass of the electron, andthus constitutes the vast majority of the mass of a neutralatom (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 spaceSpring 2008 3UCSDPhysics 10What holds it together?What holds it together?••If like charges repel, and the nucleus is full ofIf like charges repel, and the nucleus is full ofprotonsprotons (positive charges), why doesn (positive charges), why doesn’’t it flyt it flyapart?apart?– repulsion is from electromagnetic force– at close scales, another force takes over: the strongnuclear force••The The strong forcestrong force operates between quarks: the operates between quarks: thebuilding blocks of both protons and neutronsbuilding blocks of both protons and neutrons– it’s a short-range force only: confined to nuclear sizes– this binding overpowers the charge repulsionSpring 2008 4UCSDPhysics 10WhatWhat’’s the deal with neutrons decaying?!s the deal with neutrons decaying?!••A A neutronneutron, which is heavier than a , which is heavier than a protonproton, can, can(and will!) decide to switch to the lower-energy(and will!) decide to switch to the lower-energystate of the state of the protonproton••Charge is conserved, so produces an electron tooCharge is conserved, so produces an electron too– and an anti-neutrino, a chargeless, nearly masslesscousin to the electronneutronprotonneutrinoelectronPoof!Nuclear Energy 6/04/08Lecture 22 2Spring 2008 5UCSDPhysics 10Insight from the decaying neutronInsight from the decaying neutron••Another force, called the weak nuclear force, mediatesAnother force, called the weak nuclear force, mediatesthese these ““flavorflavor”” changes changes– referred to as beta decay••Does this mean the neutron is Does this mean the neutron is mademade from an electron and from an electron andproton?proton?– No. But it will do you little harm to think of it this way••Mass-energy conservation: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.– 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 particlesSpring 2008 6UCSDPhysics 10Counting particlesCounting particles••A nucleus has a definite number of A nucleus has a definite number of protonsprotons ( (ZZ), a), adefinite number of definite number of neutronsneutrons ( (NN), and a definite), and a definitetotal number of total number of nucleonsnucleons: : AA = = ZZ + + NN– example, the most common isotope of carbon has 6protons and 6 neutrons (denoted 12C; 98.9% abundance)• Z = 6; N = 6; A = 12– another stable isotope of carbon has 6 protons and 7neutrons (denoted 13C; 1.1% abundance)• Z = 6; N = 7; A = 13– an unstable isotope of carbon has 6 protons and 8neutrons (denoted 14C; half-life is 5730 years)• decays via beta decay to 14N••IsotopesIsotopes of an element have same of an element have same ZZ, differing , differing NNQSpring 2008 7UCSDPhysics 10Fission of UraniumFission of UraniumBarium and Krypton represent just one of many potential outcomesResulting mass products add up to 99.9% of the mass that went inSpring 2008 8UCSDPhysics 10FissionFission••There are only three known There are only three known nuclidesnuclides (arrangements (arrangementsof protons and neutrons) that undergo fission whenof protons and neutrons) that undergo fission whenintroduced to a slow (thermal) neutron:introduced to a slow (thermal) neutron:–233U: hardly used (hard to get/make)–235U: primary fuel for reactors–239Pu: popular in bombs••Others may split if smacked hard enough by aOthers may split if smacked hard enough by aneutron (or other energetic particle)neutron (or other energetic particle)Nuclear Energy 6/04/08Lecture 22 3Spring 2008 9UCSDPhysics 10How much more How much more fissilefissile is is 235235U than U than 238238U?U?Bottom line: at thermal energies (arrow), 235U is 1000 times more likelyto undergo fission than 238U even when smacked hardSpring 2008 10UCSDPhysics 10Uranium isotopes and others of interestUranium isotopes and others of interest14 14 GyrGyr100100232232ThTh24 24 kyrkyrno natural no natural PuPu239239PuPu4.47 4.47 GyrGyr99.274599.2745238238UU--6.8 days6.8 days00237237UU23 23 MyrMyr00236236UU704 704 MyrMyr0.7200.720235235UU246 246 kyrkyr0.00550.0055234234UU159 159 kyrkyr00233233UUdecays by:decays by:half-lifehalf-lifeAbundance (%)Abundance (%)IsotopeIsotopeSpring 2008 11UCSDPhysics 10The Uranium StoryThe Uranium Story••NoNo isotope of uranium is perfectly stable: isotope of uranium is perfectly stable:–235U has a half-life of 704 million years–238U has a half-life of 4.5 billion years (age of earth)••No heavy elements were made in the Big BangNo heavy elements were made in the Big Bang(just H, He, Li, and a tiny bit of Be)(just H, He, Li, and a tiny bit of Be)••Stars only make elements as heavy as iron (Fe)Stars only make elements as heavy as iron (Fe)through natural thermonuclear fusionthrough natural thermonuclear fusion••Heavier elements made in catastrophic supernovaeHeavier elements made in catastrophic supernovae– massive stars that explode after they’re spent on fusion••235235U and U and 238238U initially had similar abundanceU initially had similar abundanceSpring 2008 12UCSDPhysics 10Uranium decayUranium decay••The natural abundance of uranium today suggestsThe natural abundance of uranium today suggeststhat it was created about 6 billion years agothat 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 original235U–