4.4 Nuclear fission and fusion

Question 1

Nuclear fission

Answer 1

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.
Fission is The splitting of a large, unstable nucleus into two smaller nuclei.
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.
The products of fission move away very quickly.
Energy transferred is from nuclear potential energy to kinetic energy.

Question 2

Spontaneous fission

Answer 2

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.

Question 3

Chain reaction

Answer 3

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 4

Controlled chain reactions

Answer 4

In a nuclear reactor, a chain reaction is required to keep the reactor running.
When the reactor is producing energy at the correct rate, the number of free neutrons in the reactor needs to be kept constant.
This means some must be removed from the reactor.
To do this, nuclear reactors contain control rods.
These absorb neutrons without becoming dangerously unstable themselves.

Question 5

Uncontrolled chain reactions

Answer 5

Because each new fission reaction releases energy, uncontrolled chain reactions can be dangerous.
The number of neutrons available increases quickly, so the number of reactions does too.
A nuclear weapon uses an uncontrolled chain reaction to release a huge amount of energy in a short period of time as an explosion.

Question 6

Nuclear fusion

Answer 6

Small nuclei can react to release energy in a process called nuclear fusion.
When two light nuclei join to form a heavier nucleus.
This process requires extremely high temperatures to maintain.
This is why nuclear fusion has proven very hard to reproduce on Earth.
Nuclear fusion takes place in the centres of stars.
Hydrogen nuclei fuse into helium nuclei and release a large amount of energy.
The energy produced during nuclear fusion comes from a very small amount of the particle’s mass being converted into energy.