Fuels in Nuclear Reactors (Oxford AQA IGCSE Physics)

Revision Note

Written by: Caroline Carroll, Physics Subject Lead

Reviewed by: Lucy Kirkham, Head of STEM

Fuels in Nuclear Reactors

Within the core of a nuclear reactor, fuel rods are surrounded by water
The fuel rods release energy which is transferred to the water molecules, heating the water
- The hot water is pumped into a heat exchanger
- Tubes carrying a separate supply of water run through the heat exchanger
- The energy is transferred by heating from the hot water in the heat exchanger to the cooler water in the tubes
- This cools the hot water from the control rods enough to be returned to the reactor
- The water in the tubes is heated and turns to steam
- The steam turns a turbine which generates electricity

Model of a nuclear reactor

Nuclear Reactor Components, for IGCSE & GCSE Physics revision notes — The hot water from the control rods is referred to as coolant. It flows over tubes of colder water as it moves through the heat exchanger. This simultaneously cools the coolant allowing it to be returned to the reactor tank, and boils the water in the tubes turning it to steam.

Isotopes of uranium and plutonium both undergo fission and are used as fuels in nuclear power stations
The two common isotopes used in nuclear reactors are:
- Uranium-235
- Plutonium-239
The majority of nuclear reactors use uranium-235
A possible nuclear equation for the fission of uranium-235 is
${}_{92}^{235}U + {}_{0}^{1}n \to {}_{37}^{96}{Rb} + {}_{55}^{137}{Cs} + 3 {}_{0}^{1}n$

Worked Example

The table shows some information about different nuclei involved in a nuclear fission reaction within a nuclear reactor.

Nucleus	Symbol	Number of protons	Number of neutrons
Uranium	U	92	143
Barium	Ba	56	88
Krypton	Kr	36	53

A nucleus of uranium undergoes fission when bombarded with a slow-moving neutron (n) causing it to split into the daughter nuclei barium and krypton plus the release of further neutrons.

Write a balanced nuclear equation for this fission reaction.

$. . . . . . . . . + . . . . . . . . . \to . . . . . . . . . + . . . . . . . . . + . . . . . . . . .$

Answer:

Step 1: Write a word equation using the information in the question

Underline or highlight the keywords in the question which give information about the nuclei on the left-hand side (LHS):

"A nucleus of uranium undergoes fission when bombarded with a slow-moving neutron"

Underline or highlight the keywords in the question which give information about the nuclei on the right-hand side (RHS):

"causing it to split into the daughter nuclei barium and krypton plus the release of further neutrons"

Write a word equation for the reaction:

uranium + neutron → barium + krypton + neutrons

Step 2: Determine the correct symbols for each nucleus or particle

We need to write each nucleus or particle in ${}_{Z}^{A}X$ notation
It can be useful to draw an extra row and column onto the table to do this (green indicates the changes made to the table):

Nucleus	Symbol ${}_{Z}^{A}X$	Number of protons Z	Number of neutrons	Mass number A
Uranium	${}_{92}^{235}U$	92	143	92 + 143 = 235
Barium	${}_{56}^{144}{Ba}$	56	88	56 + 88 = 144
Krypton	${}_{36}^{89}{Kr}$	36	53	36 + 53 = 89
Neutron	${}_{0}^{1}n$	0	1	0 + 1 = 1

Step 3: Write the nuclear equation using the symbols

${}_{92}^{235}U + {}_{0}^{1}n \to {}_{56}^{144}{Ba} + {}_{36}^{89}{Kr} + {}_{0}^{1}n$

Step 4: Check the total number of protons on each side

Total number of protons (given by the atomic number):
- On the LHS: 92 + 0 = 92
- On the RHS: 56 + 36 + 0 = 92
The total number of protons is the same on both sides

Step 5: Check the total number of nucleons on each side and balance the equation

Total number of nucleons (given by the mass number):
- On the LHS: 235 + 1 = 236
- On the RHS: 144 + 89 + 1 = 234

There is a difference of 2, and the question states there is a "release of further neutrons"
This means there must be 2 extra neutrons released i.e. 3 neutrons in total
To represent this, we put a 3 in front of the neutron symbol on the RHS, so, the fully balanced equation is:

${}_{92}^{235}U + {}_{0}^{1}n \to {}_{56}^{144}{Ba} + {}_{36}^{89}{Kr} + 3 {}_{0}^{1}n$

Examiner Tip

You do not need to know the specific details of how a nuclear reactor generates electricity but you are expected to know the specific isotopes (uranium-235 and plutonium-239) that are used as fuels in nuclear reactors.

Last updated: 4 April 2024

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