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Flashcards in The thermal nuclear reactor Deck (12):
1

What is the reactor core?

The fuel rods and the control rods together with the moderator

2

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.

3

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.

4

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.

5

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.

6

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.

7

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.

8

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.

9

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.

10

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.

11

Low-level waste consists of: 

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

12

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.