The Nuclear Fuel Cycle and Nuclear Weapons

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The Nuclear Fuel Cycleand Nuclear Weapons

• Fred Wehling
• Center for Nonproliferation Studies

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Presentation Outline

• Brief introduction to nuclear energy
• Overview of nuclear fuel cycle and fissile
  material production
• Overview of nuclear weapon types and designs

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Nuclear Fission

• The nuclei of some isotopes of heavy elements
  such as uranium and plutonium can split when they
  absorb a neutron. This splitting is known as
  nuclear fission.
• Nuclear fission releases a great amount of
  energy in the form of heat and radiation. It also
  expels neutrons which can be absorbed by other
  nuclei, which may then fission and give off more
  energy and neutrons.
• This process is known as a fission chain
  reaction.

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Fission Fast and Slow

• In nuclear reactors, fission chain reactions take
  place slowly and in a controlled manner. The
  resulting energy can be used to generate
  electricity.
• In a nuclear weapon, uncontrolled fission chain
  reactions occur rapidly. This results in an
  explosive release of energy in several forms
• Heat
• Blast
• Prompt radiation
• Electromagnetic pulse (EMP)

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Nuclear Fusion

• Under conditions of extreme heat and pressure the
  nuclei of two isotopes of hydrogen, deuterium and
  tritium, can merge together to form an atom of
  helium. This merging is known as nuclear fusion,
  which also releases a large amount of energy.
• Nuclear fusion produces the light and other
  energy released by the sun. Because the sun is
  the original source of all the energy used by
  living organisms, nuclear fusion is the source of
  energy for all life on earth.
• Methods for using nuclear fusion slowly and
  safely to produce energy in reactors have not yet
  been developed.

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Fissionable Material

• Isotopes capable of fission are called
  fissionable isotopes.
• The most common fissionable isotope is
  uranium-238 (often referred to by its chemical
  symbol, U238).
• Large quantities of U238 exist in nature, but
  U238 does not fission very easily.
• For this reason, while U238 can be used to fuel
  some types of nuclear reactors, it cannot be used
  to create a nuclear explosion.

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Fissile Material

• Some fissionable isotopes such as uranium-235 or
  plutonium-239 fission much more easily. These
  isotopes are called fissile isotopes.
• Because they fission easily, fissile isotopes are
  very useful for producing energy in nuclear
  reactors
• They can also be used to create a nuclear
  explosion
• However, all fissile isotopes are extremely rare,
  and most do not exist in nature

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Fissile Material HEU

• In order to be used to create a nuclear
  explosion, uranium metal must be enriched to
  contain a much higher proportion of U235 than
  natural uranium
• Natural uranium about .07 U235
• Low-enriched uranium (LEU) .07,
• High-enriched uranium (HEU) 20 U235
• Weapons-grade uranium 90 U235
• Natural uranium, LEU, or HEU can be used in
  nuclear reactors (depending upon reactor design)
• Only HEU can be used in nuclear weapons
• Does NOT have to be weapons-grade

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Fissile Material Plutonium

• Plutonium is produced in nuclear reactors by the
  irradiation of natural uranium
• Plutonium is extracted from spent reactor fuel
  through reprocessing
• Plutonium may be used in both nuclear reactors
  and in nuclear weapons
• Reactor grade plutonium
• Weapons-grade plutonium 90 Pu239
• Reactor-grade plutonium MAY be used in nuclear
  weapons, but weapons-grade plutonium is preferable

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The Nuclear Fuel Cycle

• The nuclear fuel cycle describes the production,
  utilization, and disposition of nuclear materials
• In most stages of the fuel cycle, the processes
  and technology for producing fissile material for
  military and civilian purposes is essentially
  similar
• There are therefore several points in the fuel
  cycle where nuclear material could be diverted
  from peaceful uses to make nuclear weapons

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Once-Through Fuel Cycle
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Reprocessing Fuel Cycle
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Diversion to Nuclear Weapons
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The nuclear fuel cycle can become very complex.

• Nuclear materials have potential dual use, both
  civilian and military.
• The fuel cycle may involve several countries.

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Bomb Basics

• The simplest type of nuclear weapon is a
  gun-type fission weapon.
• A gun-type weapon uses chemical explosives to
  shoot one mass of HEU into another at high
  speed, much as a bullet is shot from a gun
• When the two masses collide, they form a critical
  mass which produces a nuclear explosion

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Gun-type Weapons

• Gun-type weapons are the easiest type of nuclear
  weapon to design and manufacture
• Do not require sophisticated explosive or
  electronic components
• Would not require testing
• However, gun-type weapons cannot use plutonium,
  and they require a relatively large amount of HEU
  (probably at least 50 kg, or 112.5 lbs).
• Gun-type weapons must therefore be fairly large
  and heavy, making them more difficult to bring to
  an intended target.

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Gun-type Weapons

• The Little Boy bomb used against Hiroshima on
  August 6, 1945 was a gun-type fission weapon with
  an explosive yield of about 15 kilotons.
• South Africa built six gun-type weapons starting
  in 1979, but destroyed all of them by 1993.

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Implosion Weapons

• An implosion-type fission weapon is a more
  sophisticated fission weapon.
• An implosion-type weapon uses a complex
  arrangement of explosives to rapidly compress one
  or more pieces of fissile material into a
  critical mass.
• The first nuclear weapon ever exploded, at the
  Trinity test near Alamogordo, New Mexico on
  July 16, 1945, was an implosion-type weapon.
• The Fat Man bomb dropped on Nagasaki on August
  9, 1945, was also an implosion-type weapon, with
  an explosive power of about 21 kt.

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Implosion Weapons

• Implosion-type weapons are much more difficult to
  design and build than gun-type weapons.
• Often require advanced explosive components and
  sophisticated fusing systems
• Would require testing to prove reliability of
  design
• Implosion weapons can use plutonium as well as
  HEU
• Typically require much less fissile material than
  gun-type weapons
• A well-designed implosion weapon would require
  only about 8 kg (18 lbs) of highly-enriched
  uranium or 4 kg (9 lbs) of plutonium.
• Can be smaller and light in weight than gun-type
  weapons

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Boosted Weapons

• A boosted fission weapon uses nuclear fusion to
  increase the explosive power of a fission weapon
• A boosted fission weapon is fission weapon with a
  small amount of deuterium and tritium placed
  inside the weapons core.
• When the fissile material inside the weapon is
  compressed into a critical mass, the deuterium
  and tritium undergo nuclear fusion.
• This produces additional explosive energy, but
  more importantly, releases more neutrons. These
  neutrons cause more of the fissile material in
  the weapon to fission before it is dispersed by
  the explosion.

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Thermonuclear Weapons

• A thermonuclear weapon derives most of its
  explosive force from nuclear fusion
• In a thermonuclear weapon, radiation from a
  fission explosion heats and compresses deuterium
  and tritium, which then undergo fusion
• The fission component of a thermonuclear weapon
  is called the primary. The fusion component is
  called the secondary
• Pure fusion weapons may be possible, but none
  have yet been developed or tested
• Thermonuclear weapons are significantly more
  difficult to design, build, and maintain than
  fission weapons

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Thermonuclear Weapons

• Thermonuclear weapons can be extremely powerful,
  with yields measured in megatons
• The largest nuclear weapon ever produced was the
  Tsar Bomba tested by the Soviet Union on October
  31, 1961.
• The USSR claimed that the designed yield of the
  Tsar Bomba was 100 megatons, but the yield of the
  weapon was reduced to 50 megatons for safety
  reasons.
• Most U.S. experts conclude that the total yield
  of the weapon was about 57 megatons.

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Conclusions

• Nuclear weapons require fissile material (HEU or
  plutonium)
• Producing fissile material in sufficient quantity
  and quality for nuclear weapons presents
  formidable technical challenges
• Without safeguards, fissile material produced for
  peaceful purposes can easily be diverted for use
  in nuclear weapons

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Nuclear Links

• World Nuclear Association
• http//www.world-nuclear.org/education/nfc.htm
• How Nuclear Power Works http//science.howstuffwo
  rks.com/nuclear-power.htm
• How Nuclear Weapons Work http//science.howstuffw
  orks.com/nuclear-bomb.htm
• Atomic Archive http//www.atomicarchive.com
• Federation of American Scientists nuclear weapon
  design basics http//www.fas.org/nuke/intro/nuke/
  design.htm

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• Dr. Fred WehlingCenter for Nonproliferation
  StudiesMonterey Institute of International
  Studies425 Van Buren StreetMonterey, CA 93940
  USAphone 1 831-647-6576fax 1
  831-647-6522email fwehling_at_miis.eduhttp//cns.m
  iis.edu