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
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 • 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 • 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 • 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 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 • 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 • Passage from Jessica Stern’s Ultimate Terrorists • Effects of 1 kiloton nuclear explosion at the Empire State Building
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 • Nucleus • Neutron • Proton
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 • A neutron can: • Cause fission • Be absorbed without resulting in fission • Escape
Curve of Binding Energy Hydrogen Uranium Plutonium Iron (Fe)
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
Two Paths to Nuclear Weapons Material: Enrich Uranium or Produce Plutonium
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
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 • 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 • 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 • 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) • 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) 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.
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.
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? • 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 • 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 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? • 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 • 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 • 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) • 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) • 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? • 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
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.
Two Paths to Nuclear Weapons Material: Enrich Uranium or Produce Plutonium
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 • 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 Prepare fissile material to fuel nuclear reactors.
Cartoon Version of Nuclear Power Plant Turbine: Electricity Production Heat Source: Reactor Steam Generator Feed Water Steam Condensation Heat Sink: External Cooling
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” • 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.