Nuclear Technology Part II: The Nuclear Fuel Cycle and Nuclear Material Production

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Nuclear Technology Part IIThe Nuclear Fuel
Cycle and Nuclear Material Production

• Fred Wehling
• IP 574
• Spring 2006

With thanks to Adam Bernstein, Charles Ferguson,
Lance Lesher, and Mary Beth Ward
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World Uranium Resources
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Mining Milling
Mining Uranium is found in several types of
Minerals (pitchblende, uranite, carnotite,
autunite, uranophane, tobernite) Also found in,
phosphate rock, ignite, and monazite
sands Milling Extraction of uranium oxide from
ore in order to concentrate it Milling produces
yellowcake, Concentrated uranium oxide (U308)
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Uranium Conversion

• Yellowcake (U3O8) is converted to other chemical
  compounds of U suitable for enrichment or for
  fuel fabrication
• Converted forms
• Uranium hexafluoride (UF6)
• Uranium metal (U)
• Uranium dioxide (UO2)
• Uranium tetrachloride (UCl4)

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Five 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

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Why enrich uranium?

• Except for certain types of nuclear reactors,
  such as heavy-water moderated reactors, 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.

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The Problem with Enrichment

• The chemical properties of 235U and 238U are
  essentially identical
• To increase the percentage of 235U, enrichment
  must exploit minute differences in the physical
  properties of the two isotopes (mainly the fact
  that 235U is slightly lighter in weight)

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Uranium Enrichment Methods

• Electromagnetic Isotope Separation (EMIS)
• Gaseous Diffusion
• Gas Centrifuge
• Aerodynamic Process
• Atomic Vapor Laser Isotope Separation (AVLIS) or
  Molecular Laser Isotope Separation (MLIS)
• Thermal Diffusion

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Electromagnetic Isotope Separation (EMIS)
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EMIS Process

• Uranium tetrachloride (UCl4) is electrically
  heated to produce UCl4 vapor, which is bombarded
  with electrons to ionize it.
• An electric field accelerates the ions to high
  speeds.
• A perpendicular magnetic field causes the
  accelerated ions to bend in a circular path. The
  lighter U-235 ions will follow a path with a
  shorter radius than the U-238 ions.
• The two types of ions then go through apertures
  leading to different collectors.

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EMIS Proliferation Potential

• Inefficient -- Typically less than half the feed
  is converted to U ions and less than half are
  actually collected.
• The process is time consuming and requires
  hundreds to thousands of units and large amounts
  of energy.
• Feed material is corrosive.
• The U.S. used calutrons at the Oak Ridge Y-12
  plant in two stages.
• Although all five recognized nuclear weapons
  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
  that Iraq had pursued it.

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Iraqi EMIS machine
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Gaseous Diffusion
Relies on the 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.
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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

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Gaseous Diffusion Proliferation Potential

• Unlikely to be the preferred technology of a
  proliferator due to
• large energy consumption,
• the specialized equipment, and
• the difficulty in hiding the operation because
  the facility tends to be large.

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Gas Centrifuge

• Uses physical principle of centripetal force to
  separate U-235 from U-238 (slight mass
  difference)
• Very high speed rotor generates centripetal force
• Heavier uranium hexafluoride gas 238UF6
  concentrates closer to the rotor wall, while
  lighter 235UF6 concentrates toward rotor axis
• Separation increases with rotor speed and length.
  

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Gas Centrifuge Unit
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Gas Centrifuge Cascade
Enricher stages increase product assay Stripper
stages decrease waste assay.
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Gas Centrifuge Cascade and Components
Key Components Rotating components Thin-walled
cylinders (3- to 16-in. diameter), end caps,
baffles, and bellows Made of high-strength
materials Maraging steel Aluminum
alloys Composite materials (e.g., graphite
fiber) Other components Magnetic suspension
bearings, bearings/dampers, vacuum pumps, and
motor stators Auxiliary equipment High-frequency
motor power supplies Rotor balancing
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What Centrifuge Gear is Needed for a Bomb a Year?

• Minimum of 350 very high-efficiency units (P-2 or
  better)
• Alternatively, about a few thousand
  low-efficiency units (P-1 type)
• 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 one bombs worth
  of material

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Atomic Vapor Laser Isotope Separation (AVLIS) and
Molecular Laser Isotope Separation (MLIS)

• Uses lasers to separate U-235 from U-238
• Lasers are tuned to selectively excite one
  isotope
• Technology and equipment are highly specialized
• U.S. cancelled its AVLIS program in 1999 for
  economic reasons

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Would Proliferators use AVLIS or MLIS?

• Conventional wisdom says no, but over 20
  countries have experimented with AVLIS and/or
  MLIS
• 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

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Alternative Enrichment Processes

• Aerodynamic
• Becker nozzle developed at the Karlsruhe
  Nuclear Research Center in Germany.
• Used by South Africa for producing both LEU for
  reactor fuel and HEU for weapons.
• Mixture of gases (UF6 and hydrogen or helium) is
  compressed and directed along a curved wall at
  high velocity.
• Heavier U-238 moves closer to the wall.
• 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

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Need SWU to Enrich U

• Enrichment effort (energy expenditure) is
  measured in terms of separative work units
  (SWU)
• About 4 SWU ? 1 kg of LEU (3) from about 6 kg
  of natural uranium
• About 200 SWU ? 1 kg weapons-grade HEU from about
  200 kg of natural U
• About 5,000 SWU ? 1 weapon from about 5,000 kg of
  natural U
• About 100,000 SWU ? fuel for 1,000 MW(e) LWR for
  1 years operation (e.g. Irans Bushehr reactor)
• Important to realize that a majority of the
  SWU/kg work to produce weapons-grade HEU is
  already done in producing LEU from natural U
  starting from LEU would give a proliferator a
  huge head start
• Assumes a nominal waste assay of 0.25 U

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Plutonium Production

• Because of its relatively short half-life (about
  22,000 years for 239Pu), plutonium exists in only
  trace quantities in nature.
• Therefore, it must be produced through manmade
  processes, such as using 238U as fertile material
  in a nuclear reactor.
• U-238 n ? U-239 ? Np-239 ? Pu-239
• Relatively rapid decays of U-239 (23 min.) and
  Neptunium-239 (Np-239) (2.35 days)

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Grades of Plutonium

• Desirable for weapons purposes to have 239Pu
  percentage to be as large as possible.
• Weapon-grade contains lt 6 240Pu and other non-
  239Pu isotopes.
• Fuel-grade contains 6-18 240Pu etc
• Reactor-grade contains gt 18 240Pu
• Super-grade contains lt 97 240Pu
• Weapon-usable refers to plutonium that is in
  separated form and therefore relatively easy to
  fashion into weapons any of the above

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Extracting Plutonium from Spent Nuclear Fuel

• Plutonium content is typically about 1
• Chemical processing (reprocessing) extracts the
  plutonium and remaining uranium from the
  radioactive fission products in spent fuel
• Highly radioactive material ? requires remote
  handling

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PUREX Method

• PUREX (plutonium-uranium extraction) is the most
  widely used method for plutonium extraction (only
  method used commercially)
• First used at Savannah River in U.S. in 1954
• Spread throughout the world, helped in part by
  Atoms for Peace programs
• Three main stages
• Spent fuel assemblies are dismantled and fuel
  rods are chopped up.
• 2. Extracted fuel is dissolved in hot nitric
  acid.
• 3. Solvent extraction Pu and U are separated
  from other actinides and fission products, and
  then from each other.
• Most complex stage
• Tributyl phosphate (TBP) is the typical organic
  solvent.

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Alternative Plutonium Separation Processes

• Bismuth phosphate
• REDOX
• BUTEX
• Ion Exchange
• Pyroprocessing

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Two Paths to Nuclear Weapons Material Enrich
Uranium or Produce Plutonium
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Assessing the Proliferation Potential of a
Reactor

• 1 Megawatt-day (thermal energy, not electricity
  output) of operation produces 1 gram of plutonium
  in any reactor using 20 or lower enriched
  uranium.
• So, a 100 MW(th) reactor produces 100 grams of Pu
  per day and could produce roughly enough
  plutonium for one weapon every 2 to 3 months
  depending on hours per day of operation.
• A 25 MW(th) reactor produces approximately 1
  bombs worth of Pu per year. e.g. the North
  Korean 5MW(e) reactor

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Reactors and Proliferation Potential

• Reactor types that pose the biggest proliferation
  potential
• Heavy water research reactor of 25 MW(th) or
  greater power rating (e.g. Israel and Iran)
• Graphite reactor of 25 MW(th) or greater power
  rating (e.g. North Korea)
• Light water research reactor of 25 MW(th) or
  greater power rating

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Civil Nuclear Energy and Proliferation (Dual-Use
Dilemma)

• Almost all reactors produce plutonium (large
  commercial reactors produce about 1 kg/day)
• Research reactors are in dozens of countries, but
  most are low-power (lt 1 MW)
• Reprocessing of commercial fuel has not spread as
  broadly as originally forecast
• Usually not economical to develop U enrichment
  capability for a modest nuclear power program
  e.g., Iran

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Proliferation and Reactor Choices

• Reactor design info readily available
• Natural U-fueled reactors do not need enrichment
  plants
• Heavy water reactors are smaller and have greater
  neutron flux than graphite reactors.
• But nuclear-grade graphite is usually easier to
  obtain than heavy water. Both can operate on
  natural U.
• Possible to divert fuel from HEU-fueled reactors
  e.g., Iraq considered doing this.
• No proliferator has built and operated a
  clandestine reactor ? hard to hide an operating
  reactor ? need plausible dual-use story such as
  scientific research or isotope production

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Proliferation Choices

• Heavy water, natural U-fueled reactor
• India
• Israel
• Pakistan
• Graphite, natural U-fueled reactor
• North Korea
• Light water research reactor
• Iraq (destroyed by Israel)
• No reactor (HEU path)
• South Africa
• Pakistan (until recently)
• Iraq (1982-1991)
• Iran

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Nuclear Fuel Cycle Links

• World Nuclear Association
• http//www.world-nuclear.org/education/nfc.htm
• How Nuclear Power Works http//science.howstuffwo
  rks.com/nuclear-power.htm
• Wikipedia Nuclear Power http//en.wikipedia.org/
  wiki/Nuclear_power