June 11, 2004

1
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
• Supported by the John D. and Catherine T.
  MacArthur Foundation,
• the Ploughshares Fund, and the Nuclear Threat
  Initiative

2
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 worlds population lives in a
  nuclear weapon state

3
Intelligence Report from MI5 and CIA

• HUMINT Dissident groups inside the Peoples
  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.

4
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?

5
Explosive Yields

• Typical high-yield conventional military bomb
• 1,000 pounds of TNT explosive equivalent, or
  about ½ ton.
• Low-yield nuclear weapon
• lt 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

6
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

7
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

8
Low-yield Detonation in NYC

• Passage from Jessica Sterns Ultimate Terrorists
• Effects of 1 kiloton nuclear explosion at the
  Empire State Building

9
Technical Background

• Nuclear Physics 101
• Strong nuclear force
• Ionizing radiation
• Half-life
• Fission
• Fusion
• Chain reaction
• Geometric growth of nuclear explosion

10
Neutrons, Protons, and Nuclei

• Nucleus

• Neutron
• Proton

11
Chemical Elements
12
Isotopes
13
Ionizing Radiation
Alpha (a) Helium nucleus 2 neutrons and 2
protons Beta (ß) Highly energetic electron or
positron (positively charged electron) Gamma
(?) Highly energetic particles of light
14
Half-life

• Time required for half the radioactive material
  to decay
• Exponential decay
• Less than 1 of original sample
• after 7 half-lives

15
Nuclear Fission

• A neutron can
• Cause fission
• Be absorbed without resulting in fission
• Escape

16
Nuclear Fusion
17
Curve of Binding Energy
Hydrogen
Uranium Plutonium
Iron (Fe)
18
Chain Reaction
19
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
20
Two Paths to Nuclear Weapons Material Enrich
Uranium or Produce Plutonium
21
Mining Milling
Mining Uranium is found in several types of
minerals Pitchblende, Uranite,
Carnotite, Autunite, Uranophane, Tobernite Also
found in Phosphate rock Lignite Monazite
sands Milling Extraction of uranium oxide from
ore in order to concentrate it
22
World Uranium Resources
23
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.

24
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

25
Grades of Uranium

• Depleted uranium (DU) contains lt 0.7 U-235
• Natural uranium contains 0.7 U-235
• Low-enriched uranium (LEU) contains
• gt 0.7 but lt 20 U-235
• Highly enriched uranium (HEU) contains
• gt 20 U-235
• Weapons-grade uranium contains
• gt 90 U-235
• Weapons-usable uranium

26
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

27
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

28
EMIS (continued)

• Disadvantages
• 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.
• Advantage
• 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.

29
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 LEU.
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
operating diffusion plants.
Uranium hexafluoride UF6 Solid at room
temperature.
30
Gaseous Diffusion Whats 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

31
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

32
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.
  

33
Gas Centrifuge Cascade
34
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

35
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. Khans 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 bombs worth of
  material

36
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.

37
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

38
Atomic Vapor Laser Isotope Separation (AVLIS)

• U metal vaporized
• Powerful copper vapor lasers or NdYAG lasers
  excite red-orange dye lasers
• Dye lasers ionize U-235
• U-235 is collected on a negatively charged plate

39
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

40
Would a proliferant state use LIS?

• Conventional wisdom says no, but think again
  Iran
• Advantages
• Easy to conceal
• Energy costs low compared to centrifuge system
• Disadvantages
• Complex technology
• Hard to acquire or make proper lasers
• Can be significant material losses of U

41
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.

42
Two Paths to Nuclear Weapons Material Enrich
Uranium or Produce Plutonium
43
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.

44
Grades of Plutonium

• Desirable for weapons purposes to have Pu-239
  percentage to be as large as possible.
• Weapon-grade contains lt 7 Pu-240.
• Fuel-grade contains from 7 to 18 Pu-240.
• Reactor-grade contains gt 18 Pu-240.
• Super-grade contains lt 3 Pu-240.
• Weapon-usable refers to plutonium that is in
  separated form and therefore relatively easy to
  fashion into weapons.

45
Fuel Fabrication

• Prepare fissile material to fuel nuclear
  reactors.

46
Cartoon Version of Nuclear Power Plant
Turbine Electricity Production
Heat Source Reactor
Steam Generator
Steam Condensation
Feed Water
Heat Sink External Cooling
47
Nuclear Reactors
48
Operating Nuclear Power Plants
49
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.

50
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.

51
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 acid. 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.
52
PUREX Process
53
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.

54
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
• Dont 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.

55
Nuclear Weapons Types

• Simple
• Gun-type (e.g., Hiroshima bomb)
• Implosion-type (e.g., Nagasaki bomb)
• Sophisticated
• Boosted (fission-fusion)
• Thermonuclear

56
Gun-type Bomb
57
Hiroshima Bomb Little Boy
Gun Type Easiest to design and build
(Hiroshima bomb was never tested) About 13
kiloton explosive yield
58
Implosion Bomb
59
Nagasaki Bomb Fat Man
About 22 kilotons explosive yield Second
detonated implosion weapon Required testing to
prove concept More efficient design than Little
Boy
60
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
61
Thermonuclear Bomb
62
Modern Nuclear Weapons

• Thousands of parts

• Multiple Independently-Targeted Re-entry Vehicle
• (MIRV)

63
Strategic Nuclear Weapons Submarine Launched
Ballistic Missiles (SLBMs)
64
Strategic Nuclear Weapons Intercontinental
Ballistic Missiles (ICBMs)
65
Strategic Nuclear Weapons Bombers, Bombs, and
Air-Launched Cruise Missiles (ALCMs)
ALCM
B-52
B-2
Tu-160 Blackjack Bomber
66
Tactical Nuclear Weapons
B61-11
Davy Crocket
Suitcase Nuke?
Russian Theater Missile
Pershing 2