7.3.1-7.3.5 - Fission and fusion

Question 1

How can energy be released in a nuclear reaction?

Answer 1

Fusion
Fission
Radioactive decay

Question 2

What is fusion?

Answer 2

Two small nuclei join together to produce a larger nucleus

Question 3

Where does nuclear fussion occour?

Answer 3

Nuclear fusion does not happen on Earth naturally, but it does in Stars
However, fusion reactors can be made artificially
The fusion of deuterium and tritium (isotopes of hydrogen) fuse to form helium with the release of energy
The amount of energy released during nuclear fusion is huge

Question 4

What is fission?

Answer 4

One large nucleus splits into two smaller nuclei
The large nucleus that splits is often referred to as the parent nucleus
The smaller nuclei that are split from this are referred to as the daughter nuclei

Question 5

What is Nuclear fission

Answer 5

The splitting of a large, unstable nucleus into two smaller nuclei

Question 6

What is spontaneous or induced fission?

Answer 6

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
Usually, for fission to occur the unstable nucleus must first absorb a neutron
This makes the nuclei more unstable, so that it decays almost immediately

Question 7

How does Fission occour in Uranium-235?

Answer 7

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

Question 8

What are the products of nuclear fission?

Answer 8

During fission when a neutron collides with an unstable nucleus the nucleus splits into two smaller nuclei (daughter nuclei) as well as two or 3 neutrons, gamma rays are emitted.

These products of the fission reaction move away very quickly
Energy transferred is from nuclear potential energy to kinetic energy
Eventually this energy can be converted, within the nuclear power station, into electrical energy

Question 9

How does Uranium-235 react to fission?

Answer 9

Only one extra neutron is required to induce a uranium-235 nucleus to split 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

Question 10

How do Chain reactions work in nuclear reactor?

Answer 10

In a nuclear reactor, a chain reaction is required to keep the reactor running
When the reactor is producing energy at the correct rate, two factors must be controlled:
The number of free neutrons in the reactor
The energy of the free neutrons
To do this, nuclear reactors contain control rods and moderators

Question 11

What is the purpose of the control rod?

Answer 11

Purpose of a control rod: To absorb neutrons

Control rods are made of a material which absorb neutrons without becoming dangerously unstable themselves
The number of neutrons absorbed is controlled by varying the depth of the control rods in the fuel rods
Lowering the rods further decreases the rate of fission, as more neutrons are absorbed
Raising the rods increases the rate of fission, as fewer neutrons are absorbed
This is adjusted automatically so that exactly one fission neutron produced by each fission event goes on to cause another fission
In the event the nuclear reactor needs to shut down, the control rods can be lowered all the way so no reaction can take place

Question 12

What is the purpose of a moderator?

Answer 12

The purpose of a moderator: To slow down neutrons

The moderator is a material that surrounds the fuel rods and control rods inside the reactor core
The fast-moving neutrons produced by the fission reactions slow down by colliding with the molecules of the moderator, causing them to lose some momentum
The neutrons are slowed down so that they are in thermal equilibrium with the moderator, hence the term ‘thermal neutron’
This ensures neutrons can react efficiently with the uranium fuel

Question 13

What is shielding?

Answer 13

The entire nuclear reactor is surrounded by shielding materials
The purpose of shielding is to absorb hazardous radiation
The daughter nuclei formed during fission, and the neutrons emitted, are radioactive
The reactor is surrounded by a steel and concrete wall that can be nearly 2 metres thick
This absorbs the emissions from the reactions
It ensures that the environment around the reactor is safe

Question 14

What is nuclear fusion?

Answer 14

When two light nuclei join to form a heavier nucleus

Question 15

Why is nuclear fusion hard on earth?

Answer 15

This process requires extremely high temperatures to maintain
This is why nuclear fusion has proven very hard to reproduce on Earth
Stars, including the Sun, use nuclear fusion to produce energy
Therefore, fusion reactions are very important to life on Earth
In most stars, hydrogen atoms are fused together to form helium and produce lots of energy

Question 16

How is energy produced in nuclear fusion?

Answer 16

The energy produced during nuclear fusion comes from a very small amount of the particle’s mass being converted into energy

Question 17

What are the conditions for nuclear fusion?

Answer 17

Since protons have a positive charge, they repel each other
In order to overcome this repulsion, the protons must have very high kinetic energy in order to be travelling towards each other at very high speeds
In order to make the molecules of a gas travel at such speeds, the gas has to be heated to millions of degrees Celsius – a temperature that is usually only reached at the centre of a star
The conditions for fusion are:
Very high temperature of fuel
Very high kinetic energy / speed of nuclei to overcome repulsion
Very high density / pressure to increase the possibility of suitable collisions

Question 18

Why can’t we obtain energy from fusion yet?

Answer 18

The main reasons why fusion is not currently used as a source of power on Earth are the difficulties in achieving (and maintaining)
High temperatures
High pressures
Creating the temperatures needed for fusion requires a great deal of energy
Hence, physicists are still a long way from the point where they will produce more energy from fusion than the energy needed to start it

Question 19

What are stars made up of?

Answer 19

Stars are huge balls of (mostly) hydrogen gas
In the centre of star, hydrogen atoms undergo nuclear fusion to form helium

Question 20

How do stars have the conditions for Nuclear fusion?

Answer 20

A huge amount of energy is released in the reaction
This provides a pressure that prevents the star from collapsing under its gravity
in larger stars the helium that has been produced can then be fused to form heavier elements

Question 21

What conditions are needed for nuclear fusion?

Answer 21

Within stars the atoms are ionised
This means that the electron that is normally on a hydrogen atom would be lost
A hydrogen ion, or a proton, is therefore positively charged
There exists a repulsive force between protons that has to be overcome for hydrogen fusion to happen
In order to overcome this repulsion, the protons must have very high kinetic energy in order to be travelling towards each other at very high speeds
In order to make the molecules of a gas travel at such speeds, the gas has to be heated to millions of degrees Celsius – a temperature that is usually only reached at the centre of a star
The difficulties in achieving and maintaining such high temperatures and pressures are the main reasons why fusion is not currently used as a source of power on Earth
The conditions for fusion therefore are:
Very high temperature of fuel
Very high kinetic energy / speed of nuclei to overcome repulsion
Very high density / pressure to increase the possibility of suitable collisions