Radiation Hazards: Radiation Hazards


Nuclear Forces: Nuclear Forces At this scale, gravity is utterly insignificant Protons are repelled by electromagnetic force Two types of nuclear forces bind particles together Very short range


Nuclear Decay: Nuclear 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 Weapon


Isotopes: Isotopes 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 neutrons


Radioactive Decay: Half-Life: Radioactive Decay: Half-Life


Radiation and Half-Life: Radiation 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


Curie: Curie 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: 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 curies


Half-Life and Hazard: Half-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 migrate


Types of Radiation: Types 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: 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: 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 internally


Beta Radiation: Beta 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 hazard


Gamma Rays: Gamma 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 hazard


Units of Radiation Dose: Units of Radiation Dose Roentgen – Ability to create a specified electric charge per volume of air Rem (Roentgen equivalent man) –Biological effect of one roentgen of X-rays Rad (Radiation absorbed dose) – Energy absorption: 400,000 rads heat H2O 1 deg For general human exposure, these units are roughly equivalent


Background Radiation: Background Radiation Cosmic Rays Solar Wind Decay of Natural Radioactivity Typical Doses Global Average 0.1 rem/year (80% natural) Some areas up to 1 rem/year Ramsar, Iran: up to 26 rem/year


Human Radiation Sources: Human Radiation Sources Nuclear Fallout from Atmospheric Testing (US and Russia, 1963; France, 1974; China, 1980) Chernobyl 1986 Uranium Mining Radon release from construction and earth-moving Conventional power plants


Human Survival Limits: Human Survival Limits 200 rem (whole body): few immediate fatalities 500 rem (whole body): 50% fatalities 1000 rem (whole body): No survivors


Chain Reaction: Chain Reaction


Nuclear Fission: Nuclear 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 fission


Nuclear Weapons: Nuclear Weapons To 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 explode


Yields of Nuclear Weapons: Yields of Nuclear Weapons Kiloton = 1000 tons of explosives = 4.2 x 1012 joules 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 Magnitude 7 earthquake Impact of 100-m asteroid


“Das war keine gute Idee”: “Das war keine gute Idee”


Effects of Nuclear Weapons: 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) Fallout


Nuclear Winter: Nuclear 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 1991


Controlled Nuclear Fission: Controlled Nuclear Fission Barely achieve critical mass Absorb most neutrons Moderator: water, graphite Allow just enough fissions to occur to keep chain reaction running Heat used to run steam turbines Failure of moderator or coolant can result in meltdown


Nuclear Waste: Nuclear Waste Spent Fuel Breeder Reactors On-site storage Geological storage (100,000 + years) Decommissioned Power Plants Neutrons make reactor walls radioactive Low-Level Waste Medical Universities Smoke detectors (Exempt)


Fusion: Fusion Natural: how stars (and the sun) generate energy Artificial and uncontrolled: Thermonuclear Weapon (hydrogen bomb) Fusion Reactor: controlled “Energy source of the future. Always has been, always will be.”


Uncontrolled Fusion: Uncontrolled Fusion We cannot achieve T and P necessary to use ordinary hydrogen Have to use H-2 (deuterium) or H-3 (tritium) Still need T = 1,000,000 K+ Initiated by a nuclear (fission) weapon Fission weapons yield up to 20 kilotons Fusion (hydrogen or thermonuclear) weapons yield up to 20 megatons


Controlled Fusion: Controlled Fusion Temperatures too high for any material Need to contain by magnetic fields, achieve small-scale reactions for short periods Have not achieved break-even Apparatus will be incredibly complex and expensive Reactions give off neutrons: there will still be radioactive waste No spent fuel or fissionable residue


Plutonium: Plutonium At 24,400 years half-life, much less radioactive than radium (1600 y) or radon (3 days) Not highly soluble Chemical toxicity comparable to many other heavy metals Concentrates in bone marrow Allowed occupational exposure 10-3 microcuries (1.6 x 10-8 gm) per quarter Compare Be, Rh (10-9 gm/m3 of air)