Radiation Hazards

1. Radiation Hazards

2. 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

3. 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

4. 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

5. Radioactive Decay: Half-Life

6. 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 • Generally more energetic

7. 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)

8. 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

9. 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

10. Types of Radiation • Alpha (helium nucleus) • Beta (electrons) • Neutron (nuclear fission only) • X-rays (energetic electromagnetic radiation) • Gamma (more energetic than X-rays)

11. 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)

12. 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

13. 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

14. 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

15. Units 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 equivalent

16. Background 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)/year

17. Human 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 plants

18. Human 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 survivors

21. Chain Reaction

22. 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

23. 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

24. Yields 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 asteroid

25. Largest 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

26. “Das war keine gute Idee”

27. 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

28. 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

29. 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

30. 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)

31. 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.”

32. 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

33. 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

34. 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)