Posted in Physics Revision

Nuclear Fission

P2 6.2

I think from our last tutoring session we established this is an area in which a few of you need some extra help, so here it is.

Definition: a reaction in which a large, unstable atomic nucleus splits into smaller nuclei, releasing energy (kinetic energy and heat).

How it works: All atoms are made of protons, neutrons and electrons → the number of each of these is what determines the element an atom is (so, an atom with 6 protons, 6 neutrons and 6 electrons is Carbon).

When nuclear fission occurs, the nucleus of an atom is split → this releases a lot of energy, and is how energy is produced at a nuclear power station, most commonly with uranium-235.

This is caused by two to three neutrons being fired at the nucleus of uranium-235. The nucleus absorbs these neutrons and becomes unstable, so it splits.

As it splits, it releases a number of neutrons, which then are absorbed by another nucleus, which also splits and releases neutrons.

As each reaction causes another one, we call it a chain reaction. It is said to be self-sustaining if one neutron from a fission reaction continues to cause the fission of another nucleus, and so on.

An out-of-control chain reaction is an atomic bomb. That is why they are so completely detrimental (as seen in WW2 in Japan).

As the nucleus has split, it has formed two new elements: Barium and Kryptonium.


Image result for uranium 235 fission diagram


Unfortunately, naturally-occurring uranium contains less than 1% uranium-235, the rest being mainly uranium-238 – this is an issue as Ur-238 is not a nuclear fuel as it doesn’t easily undergo nuclear fission. Engineers have to enrich the uranium to about 3-5% uranium-235.

Plutonium-239 can also be used as a fuel → formed in nuclear reactors, it doesn’t occur naturally, and could be a good way of getting rid of radioactive waste. However, it’s considered dangerous → it was used in the atomic bomb that killed 50 000 people in Nagasaki.

Nuclear Reactors

Image result for nuclear reactor labelled diagram

We can harness the energy released by nuclear fission and use it in the same way we use energy from coal, oil and gas powerstations.

Remember, a fuel is something from which you get energy, not something you burn.

The fuel used here is either uranium-235 or plutonium-239. This is provided in rods.

The rods are surrounded by a moderator → this is either graphite or D2O (heavy water). The moderator slows down neutrons, and so slow down the speed of the chain reaction. When they slow, they are called thermal neutrons.

It is essential that the chain reaction is controlled → remember how dangerous an out-of-control nuclear fission reaction is.

The coolant removes heat from the reactor to be used to turn water into steam to turn turbines (just like a regular powerstation). The coolant could be water or carbon dioxide.

So the main components are the fuel, coolant, and moderator.



  • no greenhouse gasses → no environmental impact (which we all know is extremely important due to the conclusive and undebatable evidence of the very real impacts of global warming).
  • it has the highest energy density of any other fuel.
  • some of the waste product can be used in nuclear medicine (e.g. radiotherapy)


  • it is difficult to dispose of the nuclear waste → we just bury it → can be dangerous as it is highly radioactive.
  • the powerstations are very expensive to build.
  • the building has to be decomissioned (taken down) after 35-40 years, which is another expensive process.
  • there is the danger of nuclear accidents (e.g. Chernobyl 1986).




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