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Making the Bomb: Understanding Nuclear Weapons June 11, 2004 Teaching Nonproliferation Summer Institute University of North Carolina, Asheville Dr. Charles D. Ferguson Scientist-in-Residence Center for Nonproliferation Studies Monterey Institute of International Studies

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Making the Bomb: Understanding Nuclear Weapons

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making the bomb understanding nuclear weapons

Making the Bomb: Understanding Nuclear Weapons

June 11, 2004

Teaching Nonproliferation Summer Institute

University of North Carolina, Asheville

Dr. Charles D. Ferguson


Center for Nonproliferation Studies

Monterey Institute of International Studies

Supported by the John D. and Catherine T. MacArthur Foundation,

the Ploughshares Fund, and the Nuclear Threat Initiative

snapshot of nuclear proliferation today
Snapshot of Nuclear Proliferation Today
  • Some 30,000 nuclear weapons in the world
  • 5 de jure nuclear weapon states: China, France, Russia, the U.S., and the UK
  • 4 de facto nuclear weapon states:

India, Israel, North Korea, and Pakistan

  • About half the world’s population lives in a nuclear weapon state
intelligence report from mi5 and cia
Intelligence Report from MI5 and CIA
  • HUMINT: Dissident groups inside the People’s Republic of Plutostan report that Plutostani engineers are constructing a heavy water plant.
  • SIGINT: Intercepted communications suggest that Plutostani authorities are trying to purchase maraging steel and tributyl phosphate (TBP).
  • Other NTM: Krypton-85 emissions detected from inside Plutostan.
problem and mission
Problem and Mission
  • Is Plutostan embarked on a nuclear weapons program or does it just want to develop civil nuclear technologies?
  • Your Mission: Take a crash course on nuclear weapons technology to begin to determine if Plutostan is making nuclear weapons or is engaged in peaceful pursuits?
explosive yields
Explosive Yields
  • Typical “high-yield” conventional military bomb:

1,000 pounds of TNT explosive equivalent, or about ½ ton.

  • “Low-yield” nuclear weapon:

<= 5 kilotons or 5,000 tons

  • Hiroshima bomb:

≈13 kilotons or 13,000 tons

  • Typical nuclear weapon in U.S. arsenal:

100 to 300 kilotons or

100,000 to 300,000 tons

nuclear weapons vs conventional weapons
Nuclear Weapons vs. Conventional Weapons
  • Nuclear weapons are not just bigger versions of conventional weapons
  • Nuclear force orders of magnitude greater than electromagnetic force
  • Much greater energy release in much shorter time
  • Nuclear weapons are qualitatively different
nuclear weapon effects
Nuclear Weapon Effects
  • Blast ≈ 50% energy -- within seconds after detonation
  • Thermal radiation ≈ 40-45% energy -- within seconds after detonation
  • Neutrons – prompt radiation
  • X-rays and gamma rays (≈50% energy immediately – milliseconds – after detonation)
  • Electromagnetic pulse (EMP)
  • Ionization of the upper atmosphere – depletion of ozone layer
  • Radioactive Fallout  long term effect
low yield detonation in nyc
“Low-yield” Detonation in NYC
  • Passage from Jessica Stern’s Ultimate Terrorists
  • Effects of 1 kiloton nuclear explosion at the Empire State Building
technical background
Technical Background
  • Nuclear Physics 101
    • Strong nuclear force
    • Ionizing radiation
    • Half-life
    • Fission
    • Fusion
    • Chain reaction
    • Geometric growth of nuclear explosion
neutrons protons and nuclei
Neutrons, Protons, and Nuclei
  • Nucleus
  • Neutron
  • Proton
ionizing radiation
Ionizing Radiation

Alpha (α): Helium nucleus: 2 neutrons and 2 protons

Beta (β): Highly energetic electron or positron (positively charged electron)

Gamma (γ): Highly energetic particles of light

half life
  • Time required for half the radioactive material to decay
  • Exponential decay
  • Less than 1% of original sample

after 7 half-lives

nuclear fission
Nuclear Fission
  • A neutron can:
  • Cause fission
  • Be absorbed without resulting in fission
  • Escape
curve of binding energy
Curve of Binding Energy




Iron (Fe)

growth of nuclear chain reaction
Growth of NuclearChain Reaction

Number of Fissions = 2Generation

After 80 generations, 280 fissions or about 1024 have occurred.

This number of fissions is required to produce the explosive energy in a typical nuclear weapon – within a small fraction of a second – within microseconds.

Exponential growth

# Fissions

Linear growth

Time or # Generations

mining milling
Mining & Milling


Uranium is found in several

types of minerals:

Pitchblende, Uranite, Carnotite,

Autunite, Uranophane, Tobernite

Also found in:

Phosphate rock


Monazite sands

Milling: Extraction of uranium oxide from ore in order to concentrate it

why enrich uranium
Why enrich uranium?
  • Most commercial and research reactors and all nuclear weapons that use uranium for fission require enriched uranium.
  • Only 0.72% of natural uranium is U-235 – the fissile isotope. A tiny fraction is U-234.

Over 99% is U-238.

  • Without a very efficient moderator, such as heavy water or very pure graphite, a chain reaction cannot be sustained in natural uranium – U-235 is too sparsely distributed.
why enrichment is difficult
Why enrichment is difficult
  • Chemical properties of U-235 and

U-238 are essentially identical

  • Have to rely on physical separation processes
  • These typically require more energy and resources than chemical reaction methods
grades of uranium
Grades of Uranium
  • Depleted uranium (DU) contains < 0.7% U-235
  • Natural uranium contains 0.7% U-235
  • Low-enriched uranium (LEU) contains

> 0.7% but < 20% U-235

  • Highly enriched uranium (HEU) contains

> 20% U-235

  • Weapons-grade uranium contains

> 90% U-235

  • [Weapons-usable uranium]
uranium enrichment methods
Uranium Enrichment Methods
  • Electromagnetic Isotope Separation (EMIS)
  • Gaseous Diffusion
  • Gas Centrifuge
  • Aerodynamic Process
  • Laser Isotope Separation:
    • Atomic Vapor Laser Isotope Separation (AVLIS)
    • Molecular Laser Isotope Separation (MLIS)
  • Thermal Diffusion
electromagnetic isotope separation emis
Electromagnetic Isotope Separation (EMIS)
  • Uranium tetrachloride (UCl4) is vaporized and ionized.
  • An electric field accelerates the ions to high speeds.
  • Magnetic field exerts force on UCl4+ ions
  • Less massive U-235 travels along inside path and is collected
emis continued
EMIS (continued)


  • Inefficient: Typically less than half the feed is converted to U+ ions and less than half are actually collected.
  • Process is time consuming and requires hundreds to thousands of units and large amounts of energy.
  • UCl4 is very corrosive.
  • Many physicists, chemists, and engineers needed.


  • Could be hidden in a shipyard or factory – could be hard to detect
    • Although all five recognized nuclear-weapon states had tested or used EMIS to some extent, this method was thought to have been abandoned for more efficient methods until it was revealed in 1991 that Iraq had pursued it.
gaseous diffusion
Gaseous Diffusion

Relies on molecular effusion (the flow of gas

through small holes) to separate U-235 from U-238.

The lighter gas travels faster than the heavier gas.

The difference in velocity is small (about 0.4%).

So, it takes many cascade stages to achieve even


U.S. first employed this enrichment

technique during W.W. II. Currently,

only one U.S. plant is operating to

produce LEU for reactor fuel.

China and France also

still have


diffusion plants.

Uranium hexafluoride UF6: Solid at

room temperature.

gaseous diffusion what s needed for a bomb a year 25 kilograms of heu
Gaseous Diffusion: What’s Needed for a Bomb a Year: 25 kilograms of HEU
  • At least one acre of land
  • 3.5 MW of electrical power
  • Minimum of 3,500 stages, including:
    • Pumps, cooling units, control valves, flow meters, monitors, and vacuum pumps
  • 10,000 square meters of diffusion barrier with sub-micron-sized holes
would a proliferant state choose gaseous diffusion
Would a proliferant state choose gaseous diffusion?
  • Hard to conceal in a country that was not very industrialized
  • Many parts are very difficult to obtain
  • Large volume purchases could be hard to keep secret
  • Costs more energy than centrifuge method
gas centrifuge
Gas Centrifuge
  • Uses physical principle of centripetal force to separate U-235 from U-238
  • Very high speed rotor generates centripetal force
  • Heavier 238UF6 concentrates closer to the rotor wall, while lighter 235UF6 concentrates toward rotor axis
  • Separation increases with rotor speed and length.
gas centrifuge main components
Gas Centrifuge Main Components

Rotating components:

Thin-walled cylinders, end caps, baffles, and bellows

Made of high-strength materials: Maraging steel, Aluminum alloys, or Composite materials (e.g., graphite fiber)

Other key components

Magnetic suspension bearings, vacuum

pumps, and motor stators

what centrifuge gear is needed for a bomb a year
What Centrifuge Gear is Needed for a Bomb a Year?
  • Minimum of 350 very high-efficiency units
  • Alternatively, about 5,000 low-efficiency units  Most likely that a developing proliferant state would have the most access to these units, for example, A. Q. Khan’s nuclear black market
  • About 0.5 MW of electrical power to operate low-efficiency system (compared to about 3.5 MW for gaseous diffusion plant) for bomb’s worth of material
aerodynamic processes
Aerodynamic Processes
  • Developed and used by South Africa with German help for producing both LEU for reactor fuel and HEU for weapons.
  • Mixture of gases (UF6 and carrier gas: hydrogen or helium) is compressed and directed along a curved wall at high velocity.
  • Heavier U-238 moves closer to the wall.
  • Knife edge at the end of the nozzle separates the U-235 from the U-238 gas mixture.
  • Proliferant state would probably need help from Germany, South Africa, or Brazil to master this technology.
laser isotope separation
Laser Isotope Separation
  • Uses lasers to separate U-235 from U-238
  • Lasers are tuned to selectively excite one isotope
  • Technology and equipment are highly specialized
atomic vapor laser isotope separation avlis
Atomic Vapor Laser Isotope Separation (AVLIS)
  • U metal vaporized
  • Powerful copper vapor lasers or Nd:YAG lasers excite red-orange dye lasers
  • Dye lasers ionize U-235
  • U-235 is collected on a negatively charged plate
molecular laser isotope separation mlis
Molecular Laser Isotope Separation (MLIS)
  • 16 micron wavelength IR laser excites uranium-235 hexafluoride gas
  • Another laser (either IR or UV) dissociates a fluorine atom to form uranium-235 pentafluoride, which precipitates out as a white powder
would a proliferant state use lis
Would a proliferant state use LIS?
  • Conventional wisdom says no, but think again: Iran


  • Easy to conceal
  • Energy costs low compared to centrifuge system


  • Complex technology
  • Hard to acquire or make proper lasers
  • Can be significant material losses of U
thermal diffusion
Thermal Diffusion
  • Uses difference in heating to separate light particles from heavier ones.
  • Light particles preferentially move toward hotter surface.
  • Not energy efficient compared to other methods.
  • Used for limited time at Oak Ridge during WW II to produce approximately 1% U-235 feed for EMIS. Plant was dismantled when gaseous diffusion plant began operating.
plutonium production
Plutonium Production
  • Because of its relatively short half-life (about 22,000 years for Pu-239), plutonium exists in only trace quantities in nature.
  • Therefore, it must be produced through manmade processes, such as using U-238 as fertile material in a nuclear reactor.
  • Pu-239 is readily fissionable and more so than U-235. Pu-239 also has a much higher rate of spontaneous fission than U-235.
  • The complete detonation of 1 kg of plutonium is equivalent to about 20,000 tons of chemical explosive – about the explosive yield of the bomb dropped on Nagasaki.
grades of plutonium
Grades of Plutonium
  • Desirable for weapons purposes to have Pu-239 percentage to be as large as possible.
  • Weapon-grade contains < 7% Pu-240.
  • Fuel-grade contains from 7 to 18% Pu-240.
  • Reactor-grade contains > 18% Pu-240.
  • “Super-grade” contains < 3% Pu-240.
  • “Weapon-usable” refers to plutonium that is in separated form and therefore relatively easy to fashion into weapons.
fuel fabrication
Fuel Fabrication

Prepare fissile material to fuel nuclear reactors.

cartoon version of nuclear power plant
Cartoon Version of Nuclear Power Plant




Heat Source:








Heat Sink:

External Cooling

assessing the proliferation potential of a reactor
Assessing the Proliferation Potential of a Reactor
  • 1 Megawatt-day (thermal energy, not electricity output) of operation produces roughly 1 gram of plutonium in many reactors using 20% or lower enriched uranium.
  • So, a 100 MWth would produce about 100 grams of Pu per day and could produce roughly enough plutonium for one weapon every 2 months.
reactor fuel burnup
Reactor fuel “burnup”
  • Low burnup (typically 400 MW-days/thermal) is ideal to produce weapon-grade plutonium  Less time for a buildup of Pu-249 and other non-Pu-239 plutonium isotopes.
  • Reactors fueled with natural uranium have much lower burnups than reactors fueled with LEU: 3,000-8,000 MWd/t compared to 30,000-40,000 MWd/t.  Natural uranium reactors are much better suited for weapon-grade plutonium production.
  • Natural uranium fueled reactors can be refueled while operating.
reprocessing spent fuel to extract plutonium
Reprocessing Spent Fuel to Extract Plutonium

PUREX = plutonium-uranium extraction

Three main stages:

1. Spent fuel assemblies are dismantled

and fuel rods are chopped up.

2. Extracted fuel is dissolved in hot nitric


3. (Most complex stage) Pu and U are

separated from other actinides

and fission products, and then

from each other. Technique is

known as “solvent extraction.”

Tributyl phosphate (TBP) is the

typical organic solvent.

iaea significant quantities
IAEA Significant Quantities
  • Approximate amount of fissile material needed to make a nuclear explosive
  • For plutonium, SQ is 8 kg of total plutonium.
  • For U-233, SQ is 8 kg.
  • For HEU, SQ is 25 kg of contained U-235.

Some (e.g., Cochran and Paine of NRDC) have argued that the SQs are much too high. For instance, low-yield weapons would require much less fissile material.

But IAEA has relied on input from nuclear weapons states to determine what is a significant quantity.

what path is best for weapons production
What path is best for weapons production?
  • HEU:
    • Can be used in simplest type of nuclear bomb
    • Enrichment can be a resource intensive process
    • Enrichment can only be justified under LEU fuel program for civilian reactors
  • Plutonium:
    • Cannot be used in simplest bomb
    • Don’t need as much material as in an HEU bomb
    • Need reactor, but research reactor will do – could be relatively easy to justify this type of reactor
    • Reprocessing relies on well-known chemical process, but requires specialized equipment and TBP
  • If in doubt and resources are not constrained, try both paths.
nuclear weapons types
Nuclear Weapons Types
  • Simple:
    • Gun-type (e.g., Hiroshima bomb)
    • Implosion-type (e.g., Nagasaki bomb)
  • Sophisticated
    • Boosted (fission-fusion)
    • Thermonuclear
hiroshima bomb little boy
Hiroshima Bomb – “Little Boy”

Gun Type – Easiest to design and build

(Hiroshima bomb was never tested)

About 13 kiloton explosive yield

nagasaki bomb fat man
Nagasaki Bomb – “Fat Man”

About 22 kilotons explosive yield

Second detonated implosion weapon –

Required testing to prove concept

More efficient design than “Little Boy”

schematic of primary part of implosion bomb
Schematic of Primary Part of Implosion Bomb

Hollow core, where D and T are injected for boosting.

Fissile material (WgU or WgPu)

WgU: 12 kg, 7 cm outside,

1.23 cm thick

WgPu: 4 kg, 5 cm outside,

0.75 cm thick

Beryllium reflector (2 cm)

Tamper (tungsten or

uranium) (3 cm)

High explosive (10 cm)

Aluminum case (1 cm)

Source: Steve Fetter et al., “Detecting Nuclear Weapons,” 1990

modern nuclear weapons
Modern Nuclear Weapons
  • Thousands of parts
  • Multiple Independently-Targeted Re-entry Vehicle
  • (MIRV)
strategic nuclear weapons bombers bombs and air launched cruise missiles alcms
Strategic Nuclear Weapons: Bombers, Bombs, and Air-Launched Cruise Missiles (ALCMs)




Tu-160 Blackjack Bomber

tactical nuclear weapons
Tactical Nuclear Weapons


Davy Crocket

Suitcase Nuke?

Russian Theater Missile

Pershing 2