Unit Two: Nuclear Fission and Fusion

Question 1

What is nuclear fission?

Answer 1

The splitting up of big atomic nuclei

1) Nuclear power stations generate electricity using nuclear reactors
2) In a nuclear reactor, a controlled chain reaction takes place in which atomic nuclei split up and release energy in the form of heat. This heat is then simply used to hear water to make steam, which is used to drive a steam turbine connected to an electricity generator
3) The “fuel” that’s split is usually uranium-235, though sometimes it’s plutonium-239 (or both)

Question 2

How does nuclear fission happen?

Answer 2

1) A slow moving neutron must be absorbed into a uranium or plutonium nucleus. This addition of a neutron makes the nucleus unstable, causing it to split
2) Each time a uranium or plutonium nucleus splits up, it spits out two or three neutrons, one of which might hit another nucleus, causing it to split also, and thus keeping the chain reaction going
3) Large atoms split in two —> it will form two new smaller nuclei which are usually radioactive cos they have the “wrong” number of neutrons

Question 3

Pros and cons of nuclear fission:

Answer 3

Pros:
• Gives out a lot of energy (more than from a chemical reaction)
• Cheap

Cons:
• Products left are highly radioactive —> can’t just be thrown away —> difficult and expensive to dispose of safely
• Carries the risk of radiation leaks from plants or catastrophes like Chernobyl

Question 4

What’s nuclear fusion?

Answer 4

The joining of small atomic nuclei - two light nuclei can join to create a larger nucleus

Question 5

Pros and cons of nuclear fusion:

Answer 5

Pros:
• Releases a lot more energy than fission
• Doesn’t leave behind a lot of radioactive waste

Cons:
• Can only happen at really high temps (about 10000000 degrees celsius)

Question 6

Lifecycle of stars:

Answer 6

1) Initially form from clouds of dust and gas. The force of gravity makes the dust and gas spiral in together to form a protostar.
2) Gravitational energy is converted into heat energy, so temp rises. When temp gets high enough, hydrogen nuclei undergo nuclear fusion to form helium nuclei and give out massive amounts of heat and light. A star is born.
3) Star enters a long stable period (main sequence star), where the heat created by the nuclear fusion provides outward pressure to balance the force of gravity pulling everything inwards. The star maintains its energy output for millions of years due to massive amounts of hydrogen it consumes.
4) Hydrogen begins to run out. Heavier elements such as iron are made by nuclear fusion of helium. The star then swells into a red giant=small star or red super giant=big star (red because the surface cools)
5) Small-medium sized star (e.g. sun) becomes unstable and ejects its outer layer of dust of gas as a planetary nebula
6) Leaves behind a hot, dense solid core-a White dwarf, which cools down to a black dwarf and eventually disappears