Radiation HazardsNuclear ForcesNuclear DecayIsotopesRadioactive Decay: Half-LifeRadiation and Half-LifeCurieSlide 8Half-Life and HazardTypes of RadiationHazards of RadiationAlpha RadiationBeta RadiationGamma RaysUnits of Radiation DoseBackground RadiationHuman Radiation SourcesHuman Survival LimitsPowerPoint PresentationSlide 20Chain ReactionNuclear FissionNuclear WeaponsYields of Nuclear WeaponsLargest Chemical Explosions“Das war keine gute Idee”Effects of Nuclear WeaponsNuclear WinterControlled Nuclear FissionNuclear WasteFusionUncontrolled FusionControlled FusionPlutoniumRadiation HazardsNuclear Forces•At this scale, gravity is utterly insignificant•Protons are repelled by electromagnetic force•Two types of nuclear forces bind particles together–Very short rangeNuclear Decay•Too many protons (>83, Bi): nuclear forces cannot hold nucleus together•Too many neutrons also unstable•Unstable nuclei emit particles and energetic radiation (gamma rays)•Massive nuclei can sometimes split catastrophically (fission)–Natural or Spontaneous–Nuclear Reactor–Nuclear WeaponIsotopes•Atoms of element with different number of neutrons•Protons = Atomic Number•Protons + Neutrons = Atomic Weight•Example: Uranium-238–92 protons by definition–238-92 = 146 neutrons•Carbon-14–6 protons (by definition), 8 neutronsRadioactive Decay: Half-LifeRadiation and Half-Life•Decay Constant: fraction of atoms that decay/time•Half-life = 0.693/Decay Constant•Example: 10% decay per hour: Half Life = 0.693/(0.1/hour) = 6.9 hours•Shorter Half Life = More Radiation Per Unit Time–Generally more energeticCurie•Unit of radioactivity•3.7 x 1010 decays/second•Rn-222 3.8 days .000006 grams•Co-60 5.26 yr .0013 grams•Sr-90 28 yr .007 grams•Ra-222 1600 yr 1 gram•Pu-239 24400 yr 16 grams•U-238 4.5 b.y. 3,000,000 gm (3 tons)Radiation Hazards•Three Mile Island: 50 curies–About ½ gram•Chernobyl (1986) 50,000,000 curies–About 500,000 grams (half a ton)•Russian Deep Waste Injection Program: 3,000,000,000 curiesHalf-Life and Hazard•Very short half-life (days or less)–Extremely high radiation hazard–Decays very quickly–Probably won’t move far during lifetime•Extremely long half-life (geological)–Radiation hazard negligible–Chemical toxicity is worst hazard–Daughter products (radon) can be a problem•Medium half-lives (years to 1,000’s years)–Last long enough to migrateTypes of Radiation•Alpha (helium nucleus)•Beta (electrons)•Neutron (nuclear fission only)•X-rays (energetic electromagnetic radiation)•Gamma (more energetic than X-rays)Hazards of Radiation•Direct damage to organic molecules•Creation of reactive molecules and free radicals•DNA mutations–Birth Defects–Sterility–Cancer•Dangers of Radiation Types–Penetrating Ability–Ability to create electric charges (ionize)Alpha Radiation•Given off by decay of uranium and thorium and daughter products (including radon and radium)•Cannot penetrate skin•+2 electric charge = high ionizing ability•Least dangerous externally, most dangerous internallyBeta Radiation•Given off by light and medium nuclei, including most fission products (fallout and reactor waste)•Can penetrate a few mm into tissue•Electrons, -1 charge = moderately high ionizing ability•Minor external hazard, fairly serious internal hazardGamma Rays•Produced by all nuclear decays•Need not be accompanied by particle emission•Penetrates tissue easily, requires 1 cm lead to reduce by ½•Most serious external hazardUnits of Radiation Dose•Roentgen – Ability to create a specified electric charge per volume of air•Gray (Gy): 1 Joule/kg = 100 Rad (Radiation absorbed dose) •Sievert (Sv)= Biological Effect of 1 Gray of X-Rays = 100 Rem (Roentgen equivalent man) •For general human exposure, Gray and Sievert are roughly equivalentBackground Radiation•Cosmic Rays•Solar Wind•Decay of Natural Radioactivity•Typical Doses–Global Average 1 mSv (0.1 rem)/year (80% natural)–Some areas up to 10 mSv (1 rem)/year–Ramsar, Iran: up to 0.26 Sv (26 rem)/yearHuman Radiation Sources•Nuclear Fallout from Atmospheric Testing (US and Russia, 1963; France, 1974; China, 1980)•Chernobyl 1986, Fukushima 2011•Uranium Mining•Radon release from construction and earth-moving•Conventional power plantsHuman Survival Limits•2 Sievert = 200 rem (whole body): few immediate fatalities•5 Sievert = 500 rem (whole body): 50% fatalities•10 Sievert = 1000 rem (whole body): No survivorsChain ReactionNuclear Fission•Chain reaction requires a critical mass to proceed•10 kg U-235 = 2.5 x 1025 atoms•1,2,4,8 … 2.5 x 1025 = 85 steps•@ 1/1,000,000 sec per step = 1/10,000 sec•After 64 steps, T = 10,000 K (twice as hot as sun)•Have only completed 1/1,000,000 of fissionNuclear WeaponsTo get a nuclear explosion, you have to•Assemble a critical mass in millionths of a second•Retain a high percentage of the neutrons•Hold the material together against temperatures hotter than the Sun•Imposes limits on yield of weapon•Unless something is specifically designed to be a nuclear weapon, it will not explodeYields of Nuclear Weapons•Kiloton = 1000 tons of explosives = 4.2 x 1012 joules (1 trillion calories)–Texas City, Texas, April 16-17, 1947–Collapse of World Trade Center–Impact of 10-m asteroid•Megaton = 1,000,000 tons of explosives = 4.2 x 1015 joules (1000 trillion calories)–Magnitude 7 earthquake–Impact of 100-m asteroidLargest Chemical Explosions•Many Chemical Explosions Have Overlapped Nuclear Weapon Yields–Wartime Events–Ammunition Handling Mishaps–Disposal of Explosives–Simulation of Nuclear Explosions–Excavation–Industrial Accidents“Das war keine gute Idee”Effects of Nuclear Weapons•Direct ionizing radiation•Heat (Fireball)–Rising fireball sucks dust upward, creates “mushroom cloud”–Any large explosion will create a “mushroom cloud”•Blast (Expansion of Fireball)•FalloutNuclear Winter•Publicized by Carl Sagan and others in 1980’s•Global nuclear exchange would raise large amounts of dust and soot into upper atmosphere•Would absorb or reflect sunlight, cooling the surface•Would be above most precipitation processes•Did not happen in Gulf War 1991Controlled Nuclear Fission•Barely achieve critical mass•Absorb most neutrons–Moderator: water, graphite•Allow just enough fissions to occur to keep chain reaction