Fission
- There is a lot of energy stored within the nucleus of an atom
- This energy can be released in a nuclear reaction such as fission or fusion
- Nuclear fission is defined as:
The splitting of a large, unstable nucleus into two smaller nuclei
- It is rare for a nucleus to undergo fission without additional energy being put into it
- When nuclear fission occurs in this way it is known as spontaneous fission
- Usually, for fission to occur, the reaction must be induced in fissile nuclei, such as uranium or plutonium
- Fissile nuclei are nuclei which can undergo fission
- During induced fission, a slow-moving neutron is absorbed by a fissile nucleus
- This is very unstable and splits by nuclear fission almost immediately
- The nucleus splits into two smaller nuclei (called daughter nuclei) as well as two or three neutrons
- A large amount of energy in the form of gamma rays is also emitted
Nuclear Fission of Uranium-235
Large nuclei can decay by fission to produce smaller nuclei and neutrons with a lot of kinetic energy
- The products of fission move away very quickly
- Energy transferred is from nuclear potential energy to kinetic energy
Chain Reactions
- Only one extra neutron is required to induce a uranium-235 nucleus to split it by fission
- During the fission, it produces two or three neutrons which move away at high speed
- Each of these new neutrons can start another fission reaction, which again creates further excess neutrons
- This process is called a chain reaction
Visualisation of a Chain Reaction
The neutrons released by each fission reaction can go on to create further fissions, like a chain that is linked several times – from each chain comes two more
Fission Products
- The radioactive isotope uranium-235 is the most commonly used fuel in nuclear reactors
- It has a very long half-life of 700 million years
- Several possible daughter nuclei can be produced from the fission of uranium-235
- These fission products also tend to have a range of half lives, from short to very long, which means:
- They will have dangerously high levels of radioactivity for many years
- Therefore, they must be stored and disposed of very carefully
Worked example
The diagram shows the nuclear fission process for an atom of uranium-235.
Complete the diagram to show how the fission process starts a chain reaction.
Answer:
Step 1: Draw the neutrons to show that they hit other U-235 nuclei
- It is the neutrons hitting the uranium-235 nuclei which causes the fission reactions
- The daughter nuclei do not need to be shown, only the neutrons and uranium-235 nuclei
Step 2: Draw the splitting of the U-235 nuclei to show they produce two or more neutrons
- The number of neutrons increases with each fission reaction
- Each reaction requires one neutron but releases two
- More reactions happen as the number of neutrons increases
Examiner Tip
You need to remember that uranium and plutonium are possible elements for fission, but you do not need to know the specific daughter nuclei that are formed.
