Radiation Hazards

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• 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• 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

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

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

• 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• 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

• 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 internally

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

• 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• 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

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

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

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

Chain Reaction

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 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 explode

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

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

“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)• Fallout

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

• 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• 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• 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• 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

• 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• 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)