Nuclear Fission and Fusion Flashcards
Nuclear fission
When one large, unstable nucleus splits into two smaller nuclei
A neutron is fired at the parent nucleus for this to happen
(parent nucleus -> daughter nuclei)
Common elements for fission
uranium, plutonium
Nuclear chain reaction (Uranium-235)
When the neutrons released from one fission reaction are absorbed by other Uranium nuclei, causing them to also undergo fission
Critical mass
The minimum mass of fissile material required to set up a chain reaction (exceeding this, rate of reaction accelerates)
Nuclear fission equation of Uranium-235
Why is it used?
[235|92 U] + [1|0 n] -> daughter nuclei + x[1|0 n] + energy (gamma)
Neutron is fired creating U-236 which is very unstable and splits
Can produce a lot of energy in nuclear reactors
Fuel rods function
Made of Uranium
Provides energy/material for fission reactions
Control rods function
Made of Cadmium/Boron
Able to absorb neutrons (decrease fission) without becoming dangerously unstable
Can be put in/taken out/raised/lowered to adjust rate of fission
Moderator function
Made of graphite (or water), surrounding fuel and control rods
Slows down neutrons so they are more likely to be absorbed
The fast-moving neutrons collide with the moderator, losing momentum
Shielding function
Made of thick concrete/steel - entire nuclear reactor surrounded by shielding
Absorbs hazardous radiation as shielding can be nearly 2m thick
Ensures that environment around reactor is safe for workers
Nuclear fusion
When two small nuclei fuse together to form a heavier nucleus (at very high temperatures and pressures), resulting in a loss of mass converted into energy
Differences between fusion and fission:
Nuclei joining together vs nuclei splitting
Small nuclei (e.g. hydrogen) vs large nuclei e.g. uranium
Occurs in stars vs occurs in nuclear reactors
Produces larger nuclei (usually stable) vs smaller daughter nuclei (unstable) + neutrons
Requires high temp/pressures vs requires thermal neutrons to induce fission
Advantages/disadvantages of fusion
Capable of generating more energy than fission reactions (/kg of fuel)
Nuclear fuel required for fusion (isotopes of hydrogen) is more abundant than fuel required for fission (uranium/plutonium)
No long-lived nuclear waste products
But: Conditions for fusion much harder to achieve and maintain on Earth than fission
Fusion in stars
Stars are huge balls of (mostly) hydrogen gas
In the centre of a star, hydrogen nuclei undergo fusion to form helium nuclei
E.g. [2|1 H] + [3|1 H] -> [4|2 He] + [1|0 n]
The huge amount of energy/heat released from fusion provides a pressure that prevents the star from collapsing under its own gravity
Why must fusion occur at such high temperatures and pressures
Due to electrostatic repulsion between protons - positive charge, repel each other
Protons need very high kinetic energies to overcome this repulsion
