Nuclear Physics 4 Flashcards
What is a chain reaction
When the neutrons produced by fission then go on to induce fission in more nuclei.
Each fission will go on to cause at least 1 more fission
What is critical mass
What happens when you used less/ exactly the critical mass
Minimum mass of fuel required to maintain a steady chain reaction
Using exactly the critical mass causes 1 induced fission for every fission reaction
Using less than the critical mass causes the chain reaction to eventually stop
What is the purpose of a moderator in a nuclear reactor
Slows down momentum of neutrons that have been produced to speed that they become thermal neutrons
Through elastic collisions of neutrons with atoms in moderator substance
Why is water usually used as a moderator substance, on a molecular level
The closer the size of the moderator atom is to the size of the neutron, the more momentum will be transferred to the moderator atom.
Therefore, less moderator atoms will be needed to get the neutrons to thermal neutron speeds
Water has very small atoms, closest to the size of neutrons
What is the purpose of a control rod in a nuclear reactor
To absorb neutrons in the reactor in order to control chain reactions
What material are control rods usually made of
Materials that can absorb neutrons without undergoing fission
eg. U-238
How is the amount of energy produced in a nuclear reactor controlled
By controlling the height of the control rods
So less/more neutrons are absorbed, controlling the rate at which more fission reactions occur
What is the purpose of a coolant in a nuclear reactor
To absorb the heat released in the fission reaction in the core of the reactor
Then the heat is used to make steam which powers electricity generating turbines
What materials are usually used as a coolant
Why
Water / Molten salt / Gas
They have a high specific heat capacity meaning they can transfer large amounts of thermal energy
What is enriched Uranium
The fuel used in nuclear reactors
Mined uranium that has been enriched to increase the % of U-235 in it to around 5% from 1%
What is the purpose of the thick concrete shielding used in reactors
What are the risks involved with it
Blocks radiation from escaping from the reactor and affecting the workers in the power station.
Though it may become radioactive after a while, as some neutrons may escape the reactor and enter nuclei in the shielding, making them unstable
What is an emergency shut down
When the control rods are entirely dropped into the reactor core, to absorb all the neutrons and stop the nuclear reaction
What is low level waste
Nuclear waste eg. gloves/tools , that contains just short lived radioactivity
So doesn’t take long to stop being radioactive, can be disposed close to the surface
What is high level waste
Give an example of one
Nuclear waste that can stay radioactive for thousands of years
Produced as daughter nuclei produced in fission reactions are very unstable and have high activity
How is high level waste meant to be properly disposed
- Removed and handled remotely
- Placed in a cooling pond for around a year to let it cool down from its very high temperature
- Any reusable plutonium / uranium is removed
- Waste is then vitrified (put in glass), and placed in thick steel casks to be stored in geolocally stable locations
