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Flashcards in The thermal nuclear reactor Deck (12):

What is the reactor core?

The fuel rods and the control rods together with the moderator


What is a heat exchanger?

A steel vessel containing pipes through which hot coolant in a sealed circuit is pumped, causing water passing through the steel vessel in separate pipes to turn to steam which is used to drive turbines.


What are control rods?

Rods made of a neutron-absorbing substance such as cadmium or boron that are moved in or out of the core of a nuclear reactor to control the rate of fission events in the reactor.

They maintain a rate of 1 neutron per fission.


What are the fuel rods?

  • Rods made of mostly U238 which does not fission and U235.
  • Enriched uranium contains more U235 than naturally occuring uranium.
  •  The rods are bombarded with neutrons which causes fission reactions and the releases energy.


Why is a moderator used?

  • Fission is more likely to occur with slow moving neutrons.
  • Reduce speed of neutrons to ‘thermal’ neutrons by elastic collisions with moderator atoms 
  • Where the EK of the neutrons reaches the EK of the moderator atoms ie water molecules or graphite atoms.
  • Energy reduced from 2 MeV → 0.03 MeV in about 50 collisions.


What is the use the coolant?

Take away heat from reactor to heat exchanger. water is used as it has a high specific heat capacity.


What are the safety features of a reactor?

  • The reactor core is a thick steel vessel designed to withstand the high pressure and temperature of the core.
  • Core is in a building with thick concrete that absorbs neutrons and gamma radiation that escape from the reactor.
  • Reactor has emergyency shut down system that inserts control rods fully into the core to stop fission completely.
  • Sealed fuel rods are inserted and removed remotely from the core.


Why is the waste radioactive?

  • Neutrons are absorbed by the atoms of fission fragments. 
  • This makes nuclei fragments unstable. 
  • They become beta and/or gamma emitters. 
  • Some have short half-lives, so have high activities. 
  • Beta/gamma radiation is absorbed by fuel rods. 
  • EK is changed to heat.


High-level waste is fuel rods which are: 

  • Removed by remote control 
  • Placed in cooling ponds for up to a year 
    • Still releasing heat due to radioactive decay 
  • After a year, they are stripped of casing 
  • Unused uranium and plutonium is removed from active waste, but it still contains the fission products 
  • Remaining waste stored in sealed containers 
  • Vitrification may be used to reduce volume 
  • Transported in impact resistant flasks 
  • Buried in concrete chambers underground.


Intermediate-level waste includes: 

  • Radioactive materials with low activity 
  • Containers of radioactive materials.
  • This waste is encased in concrete and stored in concrete walled buildings.


Low-level waste consists of: 

  • Laboratory equipment 
  • Protective clothing.
  • This is sealed in metal drums and buried in large trenches.


The storage of radioactive waste is an environmental problem caused by: 

  • The volume of waste 
  • The concentration of radioactivity 
  • The type of waste 
  • The possibility of leaks 
  • Security risks.