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Introduction to Nuclear Weapons. Physical Science. I. Nuclear Physics. Key Concepts The atom: Nucleus surrounded by electrons (a.k.a. beta particles). 2. The Nucleus: Protons and Neutrons. Electro-magnetism holds electrons in orbit (electrons are negatively charges, protons are positive)

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i nuclear physics
I. Nuclear Physics
  • Key Concepts
    • The atom: Nucleus surrounded by electrons (a.k.a. beta particles)
2 the nucleus protons and neutrons
2. The Nucleus: Protons and Neutrons
  • Electro-magnetism holds electrons in orbit (electrons are negatively charges, protons are positive)
  • “Strong nuclear force” holds protons and neutrons together (137 times as strong as electro-magnetism)
3 elements
3. Elements
  • Definition: Elements are atoms with the same # of protons in nuclei (their atomic number)
  • Change # protons = change element
  • Atomic weight = protons + neutrons + electrons (trivial weight)
  • Change # neutrons but not protons = same element but different atomic weight  isotope (Carbon-12, Carbon-13, Carbon 14, etc.)
4 the novelty of nuclear weapons
4. The novelty of nuclear weapons
  • 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.
  • Nuclear weapons use the strong nuclear force for destruction  inherently more powerful than any possible chemical reaction (by weight)
b fission splitting a nucleus
B. Fission: Splitting a Nucleus
  • Heavy nuclei are unstable – Put too many protons together and they repel each other. Too many (or too few) neutrons can increase this repulsion.
  • Spontaneous fission: Unstable heavy nuclei can randomly fission – break into two smaller nuclei (different elements).
3 induced fission
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)
4 the fission chain reaction
4. The Fission Chain Reaction
  • More energy is required to hold one heavy nucleus together than two moderate-sized nuclei.
  • Therefore, splitting a heavy nucleus releases a great deal of energy (strong nuclear force).
  • 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.
  • 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)
  • Critical mass varies by element, isotope, shape (spheres work best), and density (so compressing sub-critical mass can make it “go critical” and explode)
c fusion combining nuclei
C. Fusion: Combining Nuclei
  • It takes more energy to hold two light nuclei together than a single moderate-sized nucleus.
  • Therefore, forcing two light nuclei together into one nucleus generates energy.
  • In general, fusion produces more energy than fission (which means bigger bombs)
Curve of Binding Energy: Note energy increase in fusion (light elements) compared to fission (heavy elements)
4 the problem of fusion
4. The problem of fusion
  • Fission is easy – just throw some neutrons at inherently-unstable nuclei and they split
  • Fusion is hard – Hydrogen doesn’t just randomly slam into itself with the energy level of the sun’s core. About 100 million degrees required to overcome strong nuclear force.
  • All efforts to create controlled fusion use more energy to force the nuclei together than they extract from fusion
  • 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)
ii weapon design
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.
3 the gun type nuclear weapon
3. The gun-type nuclear weapon
  • Principle = Quickly mash two sub-critical pieces of U-235 together into one piece above critical mass. Detonation ensues.
  • Simplified design:
4 barriers to building a gun type weapon
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…
gas centrifuges
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 
b the danger of fizzle
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)
c safety problems
c. Safety problems
  • Accident-prone: Two subcritical masses kept in close proximity to explosives
  • 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!
  • Terrorist’s dream: Easy to use U-235 to improvise a nuclear device
b the basic implosion type fission weapon
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
2 the basic components
2. The basic components
  • 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
  • Explosives are shaped, layered, and timed to generate a spherical shock wave
  • Neutron initiator supplies neutrons to begin fission at right moment – too soon causes fizzle, but so does too late (material rebounds after compression)
  • Tamper between explosives and Pu-239 helps to reflect neutrons and hold compression for a moment or two to maximize yield
3. Maximizing Efficiency (Proportion of material that fissions before the whole thing blows itself apart into sub-critical pieces)
  • Neutron reflector: Surrounds fissile material below tamper to bounce stray neutrons back into the core
  • Levitating core: Empty space between tamper and core to allow tamper to build up momentum (standard in today’s weapons)
  • External neutron trigger (particle accelerator outside the sphere) – also useful if you want to put something else in the center of the core….
c boosted fission weapons using fusion to increase power
C. Boosted Fission Weapons: Using Fusion to Increase Power
  • Problem: Most fissile material wasted (only 1%-20% fission before it blows itself apart – Hiroshima bomb was 1.4% efficient). More neutrons needed!
  • 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…
  • 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).
  • 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.
schematic of primary part of boosted fission weapon
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)

c boosted fission weapons using fusion to increase power1
C. Boosted Fission Weapons: Using Fusion to Increase Power
  • Problem: Most fissile material wasted (only 1%-20% fission before it blows itself apart – Hiroshima bomb was 1.4% efficient). More neutrons needed!
  • 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…
  • 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).
  • 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.
d staged fusion weapons the thermonuclear or hydrogen bomb
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
2 inside the secondary
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
3 radiation implosion
3. Radiation Implosion
  • Primary ignites  high-energy X-Rays
  • X-Rays fill the radiation channels, turn polystyrene to plasma
  • Tamper is heated  outside ablates (vaporizes – think of an inside-out rocket). Ablation compresses the nuclear fuel.
  • Plasma helps keep the tamper from blocking the radiation channels, increasing duration of compression
4 the fusion explosion
4. The fusion explosion
  • Compressed fuel must still be heated
  • Plutonium “spark plug” in center of fusion fuel is compressed, becomes super-critical and fissions (raises temperature inside case)
  • Result = huge pressures and temperatures produce fusion, which releases far more energy than fission PLUS “fast fission” of spark plug from fusion-produced neutrons
5 the fuel
5. The fuel
  • Early designs (first US test) used deuterium and tritium – but this required cryogenic machinery (D and T are gases at room temperature)
  • Modern designs use solid Lithium Deuteride instead. Enriched fuel (lots of Li-6) much more effective.
  • 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).
e enhanced fusion weapons
E. Enhanced Fusion Weapons
  • 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)
  • 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)
iii detonation parameters
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.
examples tiny to huge
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)
b height air burst vs ground burst
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):

iv effects of nuclear weapons
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
c biological effects
c. Biological effects

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

ii blindness most far reaching prompt effect
ii. Blindness: Most far-reaching prompt effect
  • Flash blindness (temporary) and retinal burns (permanent) from light focused on retina
iii fire storms
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
2 blast damage
2. Blast damage
  • Heat of fireball causes air to expand rapidly, generating a shock wave
  • Shock wave hits and damages buildings, and is followed by…
  • Low-pressure area follows and sucks everything backwards (blast wind)
d biological effects
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.
3 ionizing radiation
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…
4 electromagnetic pulse emp
4. Electromagnetic Pulse (EMP)
  • High-altitude nuclear bursts generate magnetic fields over large areas (induces current in transistors and integrated circuits)  fried electronics
b fallout
B. Fallout
  • Definition: Radioactive particles fall to earth (fission products, contaminated soil and debris sucked up by explosion)
2 dangers of ionizing radiation
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)
b beta radiation
b. Beta radiation
  • Consists of electrons emitted by radioactive atoms
  • Can burn exposed skin – stopped by clothing, skin, and goggles
  • 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)
c gamma radiation
c. Gamma radiation
  • Extremely high energy photons emitted by the detonation and fallout
  • Penetrating power is high. Needed to reduce exposure by half:
d neutron radiation
d. Neutron radiation
  • Produced by blast itself, insignificant in fallout
  • Induces radioactivity (alpha, beta, gamma) in materials it encounters
  • Shielding requires light elements (hydrogen, lithium)
  • Enhanced-Radiation Weapons, aka “Neutron Bombs” -- permit fusion-produced neutrons to escape, killing people even in armored vehicles (explosions still level civilian structures)
e measures of radiation
e. Measures of Radiation
  • Measurements of exposure: 100 rad = 1 gray
  • Relative biological effectiveness (RBE): alpha = up to 20, neutron varies, beta/gamma/X-Rays = 1
  • Measures of effect: rad * RBE = rem, gray * RBE = sievert
  • Since gamma exposure is likely to be source of most radiation poisoning, rad usually = rem and gray usually = sievert
f radiation poisoning acute radiation syndrome
f. Radiation Poisoning (Acute Radiation Syndrome)
  • Triggered by cumulative exposure – hourly dose * hours exposed
g danger of internal absorption
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
3 distribution of fallout
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
b us ussr predictions
b. US-USSR Predictions
  • Immediate Deaths:
4 half life
4. Half-Life
  • Definition: Time for 50% of a radioactive substance to decay
  • Short half-life: These isotopes are very radioactive but don’t last long
  • Long half-life: These are less radioactive but also long-lived
example 100 kt surface blast fort hood main gate
Example: 100 KT Surface Blast, Fort Hood Main Gate
  • 100 KT = larger than ordinary fission bomb, smaller than largest Russian weapons

15 psi: Virtually all dead

5 psi: 50% dead, 45% injured

2 psi: 5% dead, 45% injured)

1 psi: 25% injured

100 kt surface fallout
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

c global climate
C. Global Climate
  • “Nuclear Winter” – Controversial theory that nuclear war would cause serious global cooling
  • Key variable = soot and smoke from fires ignited by nuclear weapons
  • Targeting cities or heavily forested areas increases risk (major assumption)
  • Recent model (2006) suggests 10-year cooling cycle from “small” nuclear war (100 Hiroshima-sized bombs used on population centers)
5 objections
5. Objections
  • Model assumes carbon lofted into stratosphere – but this process is only confirmed for very small particles (diesel soot)
  • Model assumes urban targeting – bases may be more logical targets
  • Standard objections to climate modeling