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
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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
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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 XRays = 100 Rem (Roentgen equivalent man)
• For general human exposure, Gray and
Sievert are roughly equivalent
Background Radiation
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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 earthmoving
• 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 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
• 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)