4.8 Nuclear Fission and Fusion Flashcards
When an uranium atom undergoes fission, it goes on to produce bout 2.5 further neutrons per fission, which go on to do what 3 things?
- Escape without causing further fission.
- Be absorbed by a neighbouring nucleus, without causing further fission
- Cause further fission in a neighbouring nucleus
Define a chain reaction
When enough neutrons go on to cause further fission, and the reaction sustains itself
Define a coolant
The material used to transfer energy away from the nuclear reactor to generate electricity
3 Examples of coolants
Water, CO2 or liquid sodium
Define a moderator
Something which sows down neutrons in between fissions
3 examples of a moderator
Graphite, water or heavy water
What is AGR
A graphite moderated fission reactor
Define critical size
The minimum size if a nuclear reactor core so that sufficient numbers of neutrons are retained within the reactor core to sustain the chain reaction
Define control rods
Used to change the fission rate by capturing excessive neutrons before they can cause further fission
What is reactor shielding
The shield used to prevent potentially dangerous radiation esp, v penetrating gamma rays and neutrons from pprenetrtng the reactors walls
Define nuclear fusion
A nuclear reaction in which atomic nuclei of low atomic no. Fuse to form a heavier nucleus with the release of energy
Why must the plasma be heated to such a high temperature for nuclear fusion
Because the protons must be projected at each other at an extremely high speed to overcome the repulsion between the two positive charges and get close enough to fuse
Define plasma
In a plasma, all the electrons have broken free of the atoms and the gas becomes a mixture of electrons and ions
Name the three ways to confine plasma
Gravitational, Inertial and Magnetic
How does gravitational plasma confinement work in stars
The inward gravitational pull of the stars huge mass balances the outward forces created by the photons mpactng on the particles of the gas plasma
How does Inertial plasma confinement work
Involves directing intense ion or laser beams at a solid fuel pellet. The beams provide energy to heat the pellet. The idea is to produce fusion for long enough to extract the energy before the plasma escapes
How does magnetic plasma confinement work
Magnetic fields are used to hold the charged particles in the plasma in what is sometimes called a magnetic bottle without touching the sides of the vessel.
Plasma containment problems (2)
- Plasma must be kept contained at a high enough temp for long enough to obtain an adequate no. Fusion reactions
- Must produce more energy than it is initially supplied
What is the chemical equation for the Deuterium-Tritium reaction
2.1H + 3.1H arrow 4.2He + 1.0n +17.6MeV
Where is deuterium found
Sea water
Where is tritium obtained
Lithium
Give 2 reasons why the D-T reaction is used for fusion
- Produces a greater energy release than other similar reactions\
- Limited waste products (neutron-irradiated products) so no long-term storage needed
(3. Deuterium and tritium are easily accessible)
How are fusion reactors inherently ‘fail-safe’
The fuel must be continuously fed into them. If the feed is stopped, the reaction ceases.
What does ITER stand for?
International Thermonuclear Experimental Reactor
