# 5.1.3 Nuclear Reactions

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### Nuclear Fission

• Usually large unstable nuclei break up gradually by the process of radioactive decay, but a small number (including Uranium-235, a naturally occurring isotope of Uranium) can break up in one big go – a process known as nuclear fission
• In order to undergo nuclear fission, a nucleus usually requires some energy which can be given by hitting the nucleus with a neutron
(Neutrons are used because they are chargeless and so are not repelled by the positive charge of the nucleus) Nuclear fission: A Uranium-235 nucleus is struck by a neutron, breaking it into two smaller daughter nuclei and 2 or 3 neutrons

• When this happens, the original nucleus breaks apart into two smaller ‘daughter’ nuclei, along with two or three neutrons
• These fission products carry away the energy released in the form of kinetic energy

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### Nuclear Fusion

• Nuclear fusion involves taking small nuclei (such as hydrogen) and colliding them together at high speed to form larger nuclei Fusion is the process in which small nuclei, such as hydrogen, are fused together to form larger nuclei

• This process also releases energy

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### Nuclear Equations

• Nuclear reactions, such as fission and fusion, can be represented using nuclear equations (which are similar to chemical equations in Chemistry)
For example: • The above equation represents a fission reaction in which a Uranium nucleus is hit with a neutron and splits into two smaller nuclei – a Strontium nucleus and a Xenon nucleus, releasing two neutrons in the process
• In the above reaction:
The sum of top (nucleon) numbers on the left-hand side equals the sum of top number on the right-hand side:
`235 + 1 = 236 = 90 + 144 + 2 × 1`

The same is true for the lower (proton) numbers:

`92 + 0 = 92 = 38 + 54 + 2 × 0`
• By balancing equations in this way, you can determine, for example, the number of neutrons emitted by a process like this

Example: • In the above example, balancing the numbers on the top shows that 3 neutrons must be released in the reaction (i.e. N = 3) ### Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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