Introduction to Nuclear Weapons

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Introduction to Nuclear Weapons

• Physical Science

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I. Nuclear Physics

• Key Concepts
• The atom Nucleus surrounded by electrons
  (a.k.a. beta particles)

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2. The Nucleus Protons and Neutrons

1. Electro-magnetism holds electrons in orbit
   (electrons are negatively charges, protons are
   positive)
2. Strong nuclear force holds protons and neutrons
   together (137 times as strong as
   electro-magnetism)

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3. Elements

1. Definition Elements are atoms with the same of
   protons in nuclei (their atomic number)
2. Change protons change element
3. Atomic weight protons neutrons electrons
   (trivial weight)
4. Change neutrons but not protons same element
   but different atomic weight ? isotope (Carbon-12,
   Carbon-13, Carbon 14, etc.)

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4. The novelty of nuclear weapons

1. Chemistry Elements are combined into compounds
   (atoms become molecules), which can release
   electro-magnetic energy as heat, light, etc. ALL
   weapons before 1945 use chemistry explosives,
   napalm, toxins, etc.
2. Nuclear weapons use the strong nuclear force for
   destruction ? inherently more powerful than any
   possible chemical reaction (by weight)

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B. Fission Splitting a Nucleus

1. Heavy nuclei are unstable Put too many protons
   together and they repel each other. Too many (or
   too few) neutrons can increase this repulsion.
2. Spontaneous fission Unstable heavy nuclei can
   randomly fission break into two smaller nuclei
   (different elements).

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3. Induced fission

• Throw a neutron at an unstable nucleus and
• It might escape (pass by without being captured
  by nucleus)
• Be absorbed into the nucleus
• Trigger fission of the nucleus into two nuclei
  (shown)

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4. The Fission Chain Reaction

1. More energy is required to hold one heavy nucleus
   together than two moderate-sized nuclei.
2. Therefore, splitting a heavy nucleus releases a
   great deal of energy (strong nuclear force).
3. If neutrons cause fission, and fission creates
   more neutrons, a chain reaction may ensue. Small
   initial energy (a few neutrons) cascades to
   trillions of split nuclei.
4. Uncontrolled chain reaction fission explosion.
   Requires Critical Mass (enough nuclei close
   together for neutrons to be more likely to hit
   nuclei than fly out of the mass without hitting
   anything)
5. Critical mass varies by element, isotope, shape
   (spheres work best), and density (so compressing
   sub-critical mass can make it go critical and
   explode)

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Example Chain Reaction in U-235
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C. Fusion Combining Nuclei

1. It takes more energy to hold two light nuclei
   together than a single moderate-sized nucleus.
2. Therefore, forcing two light nuclei together into
   one nucleus generates energy.
3. In general, fusion produces more energy than
   fission (which means bigger bombs)

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Curve of Binding Energy Note energy increase in
fusion (light elements) compared to fission
(heavy elements)
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4. The problem of fusion

1. Fission is easy just throw some neutrons at
   inherently-unstable nuclei and they split
2. Fusion is hard Hydrogen doesnt just randomly
   slam into itself with the energy level of the
   suns core. About 100 million degrees required
   to overcome strong nuclear force.
3. All efforts to create controlled fusion use more
   energy to force the nuclei together than they
   extract from fusion
4. BUT we do have one tool to generate huge amounts
   of uncontrolled energy a fission chain
   reaction! (Even this just barely provides enough
   energy limiting fusion weapons to very light
   elements like hydrogen)

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II. Weapon Design

• The most basic fission weapon (aka atomic bomb)
  The U-235 weapon
• U-235 is fissile Only low-energy neutrons are
  needed to split the nucleus. Other types of
  uranium (U-238, the most common type) require
  very high-energy neutrons for fission ( nearly
  impossible to create a chain reaction)
• Critical mass of U-235 50 kg (about 110 pounds)
  in a sphere.

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Advantage of U-235 over U-238
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3. The gun-type nuclear weapon

1. Principle Quickly mash two sub-critical pieces
   of U-235 together into one piece above critical
   mass. Detonation ensues.
2. Simplified design

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4. Barriers to building a gun-type weapon

• Getting the U-235
• 99.3 of Uranium is U-238. Must enrich uranium
  to increase of U-235
• Combine uranium with fluorine to make uranium
  hexafluoride gas (hex). Then put hex in a
  container surrounded by a membrane. Slightly
  more U-235 will diffuse out than U-238. Also
  useful

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Gas Centrifuges

• Since U-235 is lighter than U-238, spinning hex
  rapidly pulls the U-238 to the edge and leaves
  more U-235 in the middle
• US cascade of centrifuges ?

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b. The danger of fizzle

• Difficult to eliminate the last U-238 from the
  U-235 (Hiroshima bomb was 80 U-235 / 20 U-238)
• U-238 spontaneously fissions, generating neutrons
• Danger chance that U-238 will start a partial
  chain reaction just before critical mass is
  reached. Blows U-235 apart before most of it has
  a chance to fission. Result small explosion.
• Solution assemble critical mass so quickly that
  U-238 is unlikely to spontaneously fission at the
  wrong moment (we now know Hiroshima bomb had just
  under a 10 chance of fizzle the U-238 in the
  weapon spontaneously fissioned about 70
  times/second)
• Similar problem makes U-233 gun-type bombs
  difficult to build (contaminated with U-232,
  which fissions too rapidly) and Pu-239 ones
  impossible (contaminated with Pu-240)
• More complex designs reduce but do not
  eliminate chance of fizzle. DPRK test probably
  fizzled (very small blast)

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c. Safety problems

1. Accident-prone Two subcritical masses kept in
   close proximity to explosives
2. Accidental moderation Seawater moderates
   (slows) neutrons, and slower neutrons are more
   likely to cause fission before escaping the core.
   Result drop bomb in seawater potential
   detonation!
3. Terrorists dream Easy to use U-235 to improvise
   a nuclear device

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B. The Basic Implosion-Type Fission Weapon

• Why bother?
• Desire to use Pu-239 (can be made using nuclear
  reactors, so no separation necessary)
• Compressing material takes 1/10 the time of
  slamming it together (helps prevent fizzle)
• Less fissile material is required if it can be
  compressed
• Much safer accidental detonation can be made
  impossible
• Allows flexibility some or all charges can be
  detonated, compressing material to different
  degrees

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Advantage of Pu-239 ?
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2. The basic components

1. Subcritical mass of Plutonium (any isotope),
   U-233 (rarely), U-235, Np-237 (similar to U-235
   but easier to obtain), or Am-241 (theoretically)
   surrounded by explosives ? nearly all designs use
   Pu-239 or U-235
2. Explosives are shaped, layered, and timed to
   generate a spherical shock wave
3. Neutron initiator supplies neutrons to begin
   fission at right moment too soon causes fizzle,
   but so does too late (material rebounds after
   compression)
4. Tamper between explosives and Pu-239 helps to
   reflect neutrons and hold compression for a
   moment or two to maximize yield

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Simplified Implosion Design
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3. Maximizing Efficiency (Proportion of material
that fissions before the whole thing blows itself
apart into sub-critical pieces)

1. Neutron reflector Surrounds fissile material
   below tamper to bounce stray neutrons back into
   the core
2. Levitating core Empty space between tamper and
   core to allow tamper to build up momentum
   (standard in todays weapons)
3. External neutron trigger (particle accelerator
   outside the sphere) also useful if you want to
   put something else in the center of the core.

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C. Boosted Fission Weapons Using Fusion to
Increase Power

1. Problem Most fissile material wasted (only
   1-20 fission before it blows itself apart
   Hiroshima bomb was 1.4 efficient). More
   neutrons needed!
2. Solution fill core with isotopes of H that fuse
   easily Deuterium (D or H-2 -- 1 proton, 1
   neutron) and Tritium (T or H-3 -- 1 proton, 2
   neutrons) can fuse into He-4 (2 protons, 2
   neutrons), creating energy and 1 extra neutron.
   Fusion energy generated is trivial in these
   weapons, but
3. The boost Extra neutrons hit the fissile
   material and cause more of it to fission before
   blowing itself apart. Result much larger
   explosion (about double the explosive power).
4. Advantages Higher yield for equal mass which
   also means weapons can be miniaturized (up to a
   point), dial-a-yield through control of D/T
   injected into center.

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Schematic of Primary Part of Boosted Fission
Weapon
Hollow core, where D (H-2) and T (H-3) are
injected for boosting.
Fissile material (U-235 or Pu-239)
Beryllium reflector (2 cm)
Tamper (tungsten or uranium) (3 cm)
High explosive (10 cm)
Aluminum case (1 cm)
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C. Boosted Fission Weapons Using Fusion to
Increase Power

1. Problem Most fissile material wasted (only
   1-20 fission before it blows itself apart
   Hiroshima bomb was 1.4 efficient). More
   neutrons needed!
2. Solution fill core with isotopes of H that fuse
   easily Deuterium (D or H-2 -- 1 proton, 1
   neutron) and Tritium (T or H-3 -- 1 proton, 2
   neutrons) can fuse into He-4 (2 protons, 2
   neutrons), creating energy and 1 extra neutron.
   Fusion energy generated is trivial in these
   weapons, but
3. The boost Extra neutrons hit the fissile
   material and cause more of it to fission before
   blowing itself apart. Result much larger
   explosion (about double the explosive power).
4. Advantages Higher yield for equal mass which
   also means weapons can be miniaturized (up to a
   point), dial-a-yield through control of D/T
   injected into center.

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D. Staged Fusion Weapons The Thermonuclear or
Hydrogen Bomb

• Parts
• The primary stage A fission device
• The secondary stage designed to fuse when
  bombarded with radiation
• The casing Usually made of U-238

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2. Inside the Secondary

• Radiation channels filled with polystyrene foam
  surround the capsule
• The capsule walls are made of U-238
• Spark plug of plutonium boosts fusion

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3. Radiation Implosion

1. Primary ignites ? high-energy X-Rays
2. X-Rays fill the radiation channels, turn
   polystyrene to plasma
3. Tamper is heated ? outside ablates (vaporizes
   think of an inside-out rocket). Ablation
   compresses the nuclear fuel.
4. Plasma helps keep the tamper from blocking the
   radiation channels, increasing duration of
   compression

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4. The fusion explosion

1. Compressed fuel must still be heated
2. Plutonium spark plug in center of fusion fuel
   is compressed, becomes super-critical and
   fissions (raises temperature inside case)
3. Result huge pressures and temperatures produce
   fusion, which releases far more energy than
   fission PLUS fast fission of spark plug from
   fusion-produced neutrons

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5. The fuel

1. Early designs (first US test) used deuterium and
   tritium but this required cryogenic machinery
   (D and T are gases at room temperature)
2. Modern designs use solid Lithium Deuteride
   instead. Enriched fuel (lots of Li-6) much more
   effective.
3. The fusion process Neutrons from fission turn
   some D into T, which then fuse together,
   generating more neutrons. Some D and T also
   fuses with Lithium (but this generates less
   energy).

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E. Enhanced Fusion Weapons

1. Fission-Fusion-Fission designs Make the bomb
   case out of U-238 or even U-235 and it will
   detonate when neutrons from the fusion capsule
   hit it, greatly enhancing yield (doubling power
   is easy)
2. Multi-stage weapons Use the secondary stage to
   compress a tertiary stage, and so forth. Each
   stage can be 10-100 times larger than previous
   stage ( unlimited explosive potential)

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III. Detonation Parameters

• Yield A measure of explosive power
• Expressed as kt or Mt of TNT
• Measures power not weight 20 kt weapon is
  equivalent to detonating 20,000 TONS of TNT all
  at once. 1 Mt means the equivalent of a million
  tons of TNT detonating at once.

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Examples Tiny to Huge

• Oklahoma City non-nuclear bomb (.002 Kt)
• Davy Crockett nuclear rifle (.01 kt)
• British tactical nuclear weapon (1.5 kt)
• The nuclear cannon (15 kt)
• Hiroshima (15 kt) and Nagasaki (20 kt)
• Max pure fission Orange Herald (720 kt)
• Chinese (3 Mt) and British (1.8 Mt) H-Bombs
• Largest deployed weapon (25 Mt)
• Tsar Bomba, the largest bomb tested (58 Mt)

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Comparative fireballs by yield
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B. Height Air-Burst vs. Ground-Burst

• Zones of destruction (1 Mt weapon)
• Groundburst (energy concentrated at ground zero)
• Airburst (energy distributed over wider area)

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IV. Effects of Nuclear Weapons

• Prompt effects
• Thermal and visible radiation (heat and light)
• Initial pulse 1/10 second (too quick for eyes
  to react). Few killed, but many blinded
• Second pulse most heat damage, lasts up to 20
  seconds for large weapons

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c. Biological effects

• i. Flash burns Most prominent on exposed
  areas (i.e. dark areas of kimono worn by this
  victim)

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Burns 1.5 miles from hypocenter in Nagasaki
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Add 20 for 1st degree burn range, subtract 20
for 3rd degree burn range
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ii. Blindness Most far-reaching prompt effect

• Flash blindness (temporary) and retinal burns
  (permanent) from light focused on retina

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iii. Fire Storms

• Heat ignites flammable materials
• If large enough area burns, it creates its own
  wind system, sucking in oxygen to feed the flames
• Natural example in Peshtigo, WI (1871) A wall
  of flame, a mile high, five miles (8 km) wide,
  traveling 90 to 100 miles (200 km) an hour,
  hotter than a crematorium, turning sand into
  glass.
• Firestorms in Hiroshima (but not Nagasaki),
  Dresden, Tokyo in World War II.
• Result Large numbers of people not burned by
  nuclear detonation will be burned by subsequent
  firestorms sweeping through city

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2. Blast damage

1. Heat of fireball causes air to expand rapidly,
   generating a shock wave
2. Shock wave hits and damages buildings, and is
   followed by
3. Low-pressure area follows and sucks everything
   backwards (blast wind)

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Note the Mach Front
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1 Mt
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d. Biological Effects

• Few likely to die from blast wave itself, but
  flying debris may kill many
• Lung damage occurs at about 70 KPa (double the
  pressure needed to shatter concrete walls)
• Ear damage begins at 22 KPa (as brick walls
  shatter)
• In general, heat will kill anyone close enough to
  experience primary blast damage. Crushed
  buildings will kill many outside this zone.

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3. Ionizing Radiation

• For most weapons, immediate radiation (gamma rays
  and neutrons) will only kill those very close to
  the explosion
• More on biological effects later

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Hiroshima Health Dept Estimates
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4. Electromagnetic Pulse (EMP)

• High-altitude nuclear bursts generate magnetic
  fields over large areas (induces current in
  transistors and integrated circuits) ? fried
  electronics

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B. Fallout

• Definition Radioactive particles fall to earth
  (fission products, contaminated soil and debris
  sucked up by explosion)

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2. Dangers of Ionizing Radiation

• Alpha radiation
• Composed of Helium nuclei (2 protons, 2 neutrons)
• Little danger unless inhaled or ingested
  stopped by a piece of paper (or skin)
• Very destructive if inhaled or ingested (only
  known example Alexander Litvinenko, poisoned
  with alpha-emitter Po-210)

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b. Beta radiation

1. Consists of electrons emitted by radioactive
   atoms
2. Can burn exposed skin stopped by clothing,
   skin, and goggles
3. Effective range is only a few feet, so exposure
   to radioactive dust is most likely source of
   damage (no known fatalities from beta exposure at
   Hiroshima or Nagasaki)

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c. Gamma radiation

1. Extremely high energy photons emitted by the
   detonation and fallout
2. Penetrating power is high. Needed to reduce
   exposure by half

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d. Neutron radiation

1. Produced by blast itself, insignificant in
   fallout
2. Induces radioactivity (alpha, beta, gamma) in
   materials it encounters
3. Shielding requires light elements (hydrogen,
   lithium)
4. Enhanced-Radiation Weapons, aka Neutron Bombs
   -- permit fusion-produced neutrons to escape,
   killing people even in armored vehicles
   (explosions still level civilian structures)

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e. Measures of Radiation

1. Measurements of exposure 100 rad 1 gray
2. Relative biological effectiveness (RBE) alpha
   up to 20, neutron varies, beta/gamma/X-Rays 1
3. Measures of effect rad RBE rem, gray RBE
   sievert
4. Since gamma exposure is likely to be source of
   most radiation poisoning, rad usually rem and
   gray usually sievert

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f. Radiation Poisoning (Acute Radiation Syndrome)

1. Triggered by cumulative exposure hourly dose
   hours exposed

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• ii. LD 50 is 4.5 Grays

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g. Danger of Internal Absorption

• Strontium-90 is chemically similar to Calcium ?
  incorporated into bones
• Iodine 131 is absorbed by the thyroid
• Cesium 137 is chemically similar to potassium and
  absorbed throughout the body

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3. Distribution of Fallout

• Fallout point-source pollutant (exposure
  almost always decreases with distance)
• Key variables speed and direction of wind.
• Closer to source usually more dangerous but
  downwind hot spots are possible

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1 Mt Surface Burst Cumulative and Hourly
Radiation Exposure
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Hot Spots from Castle Bravo Test
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b. US-USSR Predictions

• Immediate Deaths

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Fallout (1977 estimates)
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Fallout (1990 Estimate)
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Fallout (USSR Estimate)
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4. Half-Life

1. Definition Time for 50 of a radioactive
   substance to decay
2. Short half-life These isotopes are very
   radioactive but dont last long
3. Long half-life These are less radioactive but
   also long-lived

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Example 100 KT Surface Blast, Fort Hood Main Gate

• 100 KT larger than ordinary fission bomb,
  smaller than largest Russian weapons

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15 psi Virtually all dead 5 psi 50 dead, 45
injured 2 psi 5 dead, 45 injured) 1 psi 25
injured
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Compare 1 MT Surface Blast
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Compare 20KT Surface Blast
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100 KT Surface Fallout
1 hour Lethal
2 hours Lethal
3 hours Lethal
4 hours Lethal and 50 Lethal
5 hours Lethal and 50 Lethal
Possible Zone of Sickness
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C. Nuclear Winter

1. Theory that nuclear war would cause global
   cooling ? bigger nuclear wars more and longer
   cooling
2. Mechanism Soot and smoke from urban firestorms
   and forest fires rises to stratosphere, carried
   around globe, remains for prolonged time, blocks
   sunlight

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Nuclear Holocaust
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3. Technical Issues (See Pry)

1. Initial TTAPS study (dramatized here) was poor
2. Models assume carbon lofted into stratosphere
   but this process is only confirmed for very small
   particles (diesel soot)
3. Models assume urban/forest targeting bases may
   be more logical targets
4. Standard objections to climate modeling (no
   global climate models are perfect)

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4. Political Issues Why Hard-Liners (such as
Pry) Opposed the Theory

1. Theory undermines conventional deterrence if
   nuclear winter is believed by policymakers, the
   world is safe for conventional war
2. Theory undermines nuclear deterrence Irrational
   to retaliate if doing so makes nuclear winter
   worse for everyone (including ones own people)
3. Theory undermines rationale for nuclear arms
   race more weapons threaten human extinction if
   used (early studies come from left-wing
   scientists and environmentalists)

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5. Scientific Analysis

• Cold War studies Better science generally found
  smaller nuclear winter effects
• (note that most studies were excluded from Prys
  chart on p. 203 which was taken from the
  conservative National Review)

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b. Post-Cold War Studies

• Almost no studies 1990-2005 Why?
• 2006 study 100 Hiroshima-sized bombs on 100
  subtropical cities (obviously talking about India
  and Pakistan)

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b. Post-Cold War Studies

• Almost no studies 1990-2005 Why?
• 2006 study 100 Hiroshima-sized bombs on 100
  subtropical cities (obviously talking about India
  and Pakistan)
• Predicts that some tropospheric soot (which
  usually rains out quickly) would be heated by the
  sun and enter the stratosphere (where no rain
  occurs)
• Predicts reduced cooling but lasts longer (up to
  10 years)

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Chief danger Food Supply

• Summer wont turn to winter but it may turn to
  autumn, with repeated freezes threatening crops

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Chief danger Food Supply

• Summer wont turn to winter but it may turn to
  autumn, with repeated freezes threatening crops
• Besides temperature, ozone depletion, changes in
  precipitation, and reduced sunlight all reduce
  productivity

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2008 Study A SORT War

• Imagines 2012 global nuclear war using arsenals
  which have been reduced by (existing) arms
  control agreements

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• SORT war scenario open to question

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Comparison Regional (5 Tg) vs. SORT (150 Tg or
more)
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D. Popular Perceptions and Propaganda

• 1. Examples of anti-nuclear research and culture
• Nuclear Winter Theory popularized by Carl Sagan
  before academic publication (PARADE Magazine)
• Film treatments dramatize dangers in 1980s
• The Day After
• Threads
• When the Wind Blows

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Soviet Propaganda Examples

• Two worlds - two goals. We are planning new
  life. They are planning death.

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Soviet Propaganda Examples

• A Christmas present for the people

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Soviet Propaganda Examples

• What dangerous madness!

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Soviet Propaganda Examples

• Myth and reality.

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D. Popular Perceptions and Propaganda

• 1. Examples of anti-nuclear research and culture
• Nuclear Winter Theory popularized by Carl Sagan
  before academic publication (PARADE Magazine)
• Film treatments dramatize dangers in 1980s
• The Day After
• Threads
• When the Wind Blows
• Soviet Propaganda
• Responses
• Indictments of the TTAPS study (long after others
  have moved on)
• Pry, Societal Survival (Assigned)
• Most responses focused on elites, not public (no
  counter-films, for example). Why?