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Nuclear Reactors PowerPoint PPT Presentation

Nuclear Reactors Question: A nuclear reactor is powered by nuclear fuel rods. After being used for a while, those nuclear fuel rods are Heavier than when they were new Lighter than when they were new The same weight as when they were new Uranium-235 Radioactive – fissions and emits neutrons

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Nuclear Reactors

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Nuclear reactors l.jpg

Nuclear Reactors


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Question:

A nuclear reactor is powered by nuclear fuel rods. After being used for a while, those nuclear fuel rods are

  • Heavier than when they were new

  • Lighter than when they were new

  • The same weight as when they were new


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Uranium-235

  • Radioactive – fissions and emits neutrons

  • Fissionable – breaks when hit by neutrons

  • Rare fraction of natural uranium (0.72%)


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Uranium-238

  • Radioactive – emits helium nuclei, some fissions

  • Nonfissionable – absorbs fast neutrons without fission

  • Common fraction of natural uranium (99.27%)


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Natural Uranium

  • Contains mostly 238U, with some 235U

  • Fissioning uranium nuclei emit fast neutrons

  • 238U absorbs fast neutrons

  • Most fission neutrons are absorbed by 238U

  • Chain reactions won’t work in natural uranium


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Thermal Neutrons

  • 238U doesn’t absorb slow (thermal) neutrons!

  • Slowed neutrons bypass 238U

  • A 235U chain reaction can occur in natural uranium if the neutrons are slowed by a moderator

  • Moderator nuclei

    • Small nuclei that don’t absorb neutrons

    • Extract energy and momentum when struck by neutrons

    • Slow neutrons down


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Moderators 1

  • Hydrogen nuclei (protons)

    • Good mass match with neutron

    • Excellent energy and momentum transfer

    • Slight possibility of absorbing neutron

  • Deuterium nuclei (heavy hydrogen isotope)

    • Decent mass match with neutron

    • Good energy and momentum transfer

    • No absorption of neutrons


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Moderators 2

  • Carbon

    • Adequate mass match with neutron

    • Adequate energy and momentum transfer

    • Little absorption of neutrons

  • Choosing a moderator

    • Deuterium is best, but it’s rare and reactive (hydrogen)

    • Hydrogen is next best, but its reactive

    • Carbon is acceptable and a convenient solid


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Thermal Fission Reactors

  • Reactor core contains huge amount of uranium

  • Uranium is natural or slightly enriched

  • Moderator is interspersed throughout core

  • Moderator quickly slows neutrons down

  • Nuclear chain reactions occur only among 235U

  • Critical mass is controlled by size & shape of core, type of fuel, location and quality of moderator, and positions of neutron-absorbing control rods


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Controlling Reactors 1

  • Critical mass

    • Below it, fission rate diminish with each generation

    • Above it, fission rate increases with each generation

    • Generation rate of prompt neutrons is very short

    • Controlling prompt-neutron fission is difficult!

  • Delayed fission

    • Some fissions produce short-lived radioactive nuclei

    • These radioactive nuclei emit neutrons after a while

    • Delayed neutrons contribute to the chain reactions


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Controlling Reactors 2

  • There are two different critical masses

    • Prompt critical: prompt neutrons sustain chain reaction

    • Delayed critical: prompt and delayed neutrons required

  • Reactors operate

    • Below prompt critical mass

    • Above delayed critical mass

  • Control rods govern the fission rate


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Using Nuclear Reactors

  • Fissions release thermal energy

  • Thermal energy is extracted by a coolant

  • Coolant is used to power a heat engine

  • Heat engine produces power


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Nuclear Accidents

  • Windscale Pile 1 (Britain)

    • Carbon moderator burned during annealing

  • Three Mile Island (US)

    • Cooling pump failed and core overheated (while off)

  • Chernobyl Reactor 4 (USSR)

    • Coolant boiled in overmoderated graphite reactor

    • Exceeded prompt critical


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Question:

A nuclear reactor is powered by nuclear fuel rods. After being used for a while, those nuclear fuel rods are

  • Heavier than when they were new

  • Lighter than when they were new

  • The same weight as when they were new


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