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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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question
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
Uranium-235
  • Radioactive – fissions and emits neutrons
  • Fissionable – breaks when hit by neutrons
  • Rare fraction of natural uranium (0.72%)
uranium 238
Uranium-238
  • Radioactive – emits helium nuclei, some fissions
  • Nonfissionable – absorbs fast neutrons without fission
  • Common fraction of natural uranium (99.27%)
natural uranium
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
thermal neutrons
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
moderators 1
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
moderators 2
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
thermal fission reactors
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
controlling reactors 1
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
controlling reactors 2
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
using nuclear reactors
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
nuclear accidents
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
question14
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