# 4.4.1 Nuclear Fission

### Nuclear 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

• Isotopes of uranium and plutonium both undergo fission and are used as fuels in nuclear power stations
• During fission, when a neutron collides with an unstable nucleus, the nucleus splits into two smaller nuclei (called daughter nuclei) as well as two or three neutrons
• Gamma rays are also emitted

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

#### Exam 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.

Use your knowledge of balancing nuclear equations to work these out.

### Spontaneous Fission

• It is rare for nuclei to undergo fission without additional energy being put into the nucleus
• When nuclear fission occurs in this way it is called spontaneous fission

#### Induced Fission

• Usually, for fission to occur the unstable nucleus must first absorb a neutron
• Take, for example, uranium-235, which is commonly used as a fuel in nuclear reactors
• It has a very long half-life of 700 million years
• This means that it would have a low activity and energy would be released very slowly
• This is unsuitable for producing energy in a nuclear power station
• During induced fission, a neutron is absorbed by the uranium-235 nucleus to make uranium-236
• This is very unstable and splits by nuclear fission almost immediately

#### Worked Example

During a particular spontaneous fission reaction, plutonium-239 splits as shown in the equation below:

Which answer shows the section missing from this equation?

Step 1: Identify the different mass and atomic numbers

• Pu (Plutonium) has mass number 239 and atomic number 94
• Pd (Palladium) has mass number 112 and atomic number 46
• Cd (Cadmium) has mass number 124 and atomic number 48

Step 2: Calculate the mass and atomic number of the missing section

• Mass number is equal to the difference between the mass numbers of the reactants and the products

239 – (112 + 124) = 3

• Atomic number is equal to the difference between the atomic numbers of the reactants and the products

94 – (46 + 48) = 0

• The answer is therefore not B or C

Step 3: Determine the correct notation

• Neutrons have a mass number of 1
• The answer is therefore not A
• Therefore, this must be three neutrons, which corresponds to D

### Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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