The thermal nuclear reactor

Question 1

What is the reactor core?

Answer 1

The fuel rods and the control rods together with the moderator

Question 2

What is a heat exchanger?

Answer 2

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.

Question 3

What are control rods?

Answer 3

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.

Question 4

What are the fuel rods?

Answer 4

• 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.

Question 5

Why is a moderator used?

Answer 5

• 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 E_K of the neutrons reaches the E_K of the moderator atoms ie water molecules or graphite atoms.
• Energy reduced from 2 MeV → 0.03 MeV in about 50 collisions.

Question 6

What is the use the coolant?

Answer 6

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

Question 7

What are the safety features of a reactor?

Answer 7

• 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.

Question 8

Why is the waste radioactive?

Answer 8

• 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.
• E_K is changed to heat.

Question 9

High-level waste is fuel rods which are:

Answer 9

• 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.

Question 10

Intermediate-level waste includes:

Answer 10

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

Question 11

Low-level waste consists of:

Answer 11

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

Question 12

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

Answer 12

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