Nuclear Fission (Edexcel GCSE Physics)
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Nuclear Fission
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 parent nucleus), the nucleus splits into two smaller nuclei (the daughter nuclei) as well as two or three neutrons
Gamma rays are also emitted
Fission of Uranium-235
Uranium-235 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
A uranium-235 nucleus is struck by a neutron, breaking it into two smaller daughter nuclei and 2 or 3 neutrons
These products of the fission reaction move away very quickly
Energy is transferred from the nuclear store to the kinetic store of the products
Eventually this energy can be used to heat water to produce steam to generate electricity within the nuclear power station
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?
Answer: D
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
Examiner Tips and Tricks
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.
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