Physics

6.7 - Nuclear Fission

8 min

How fission works

Chain reactions

Nuclear reactors

Pros and cons

note 1 of 4

What nuclear fission is

Nuclear fission is the splitting of a large, unstable nucleus (such as uranium-235 or plutonium) into two smaller nuclei, releasing energy.
Spontaneous fission - Where a nucleus splits on its own - is rare. Usually the nucleus has to absorb a neutron first.

How fission is triggered

A slow-moving neutron is absorbed by the nucleus, making it even more unstable and causing it to split.
A neutron is absorbed easily because it has no charge, so it isn't repelled by the positive nucleus.
Nuclei like uranium-235 that can absorb slow neutrons this way are described as fissile.

What fission produces

The heavy nucleus splits into two lighter daughter nuclei, roughly the same size as each other. These are usually radioactive.
Two or three neutrons are also released, along with a large amount of energy.
Most of the energy ends up as kinetic energy of the fast-moving fission products; some is carried away as gamma radiation.

$_{92}^{235} U +_{0}^{1} n \to_{56}^{141} Ba +_{36}^{92} Kr + 3_{0}^{1} n + energy$

A neutron strikes a uranium-235 nucleus of blue and green balls, which stretches into an unstable nucleus, then splits into two daughter nuclei releasing three neutrons and two gamma rays.

note 2 of 4

The chain reaction

Each fission releases two or three neutrons. The diagram below follows the case where two are released and both go on to cause another fission.
If any of these neutrons are absorbed by other nuclei, they cause them to split too, releasing more neutrons and creating a self-sustaining chain reaction.
Left uncontrolled, the number of fissions grows rapidly, and so does the rate at which energy is released.

A neutron hits a heavy nucleus, which splits into a daughter nucleus and two neutrons that each cause further splits, in three columns labelled 1 fission, 2 fissions and 4 fissions.

note 3 of 4

Generating electricity from fission

The energy released by fission heats water into steam.
The steam drives a turbine, which turns a generator to produce electricity.

Using a coolant

In many reactors the energy isn't passed straight to the water.
A coolant (such as carbon dioxide) is pumped around the reactor core to carry heat to a heat exchanger, where it boils the water into steam.
This keeps the water supply separate from the reactor core.

A reactor holding control rods and fuel rods inside shielding, with pipes labelled CO2 in and CO2 out carrying heat through a heat exchanger where water from a water circulator turns to steam, which drives a labelled turbine coupled to a generator with electricity leaving it.

note 4 of 4

Benefits and risks of nuclear power

Benefits	Risks and drawbacks
Reliable, generating large amounts of energy from very small amounts of fuel.	Produces radioactive waste with long half-lives, staying dangerous for thousands of years.
Releases no greenhouse gases during operation, so it doesn't contribute directly to global warming.	High overall cost to build plants and safely decommission them.
Low fuel costs compared to fossil fuels.	Risk of major accidents or radiation leaks.

Disposing of radioactive waste

High-level radioactive waste is sealed in steel canisters or vitrified (set into glass) and stored deep underground.
Stored waste poses a long-term risk if containers leak over time or are damaged by geological activity, contaminating groundwater and soil.

key terms

TermDefinition

Nuclear fissionThe splitting of a large, unstable nucleus into two smaller nuclei, releasing energy.

Daughter nucleiThe smaller nuclei formed when a larger nucleus splits during nuclear fission.

Chain reactionA self-sustaining process where neutrons released by one fission trigger further fissions.

end of notes

Key points to remember

What nuclear fission is

Nuclear fission is the splitting of a large, unstable nucleus (such as uranium-235 or plutonium) into two smaller nuclei, releasing energy.

Spontaneous fission - Where a nucleus splits on its own - is rare. Usually the nucleus has to absorb a neutron first.

How fission is triggered

A slow-moving neutron is absorbed by the nucleus, making it even more unstable and causing it to split.

A neutron is absorbed easily because it has no charge, so it isn't repelled by the positive nucleus.

Nuclei like uranium-235 that can absorb slow neutrons this way are described as fissile.

What fission produces

The heavy nucleus splits into two lighter daughter nuclei, roughly the same size as each other. These are usually radioactive.

Two or three neutrons are also released, along with a large amount of energy.

Most of the energy ends up as kinetic energy of the fast-moving fission products; some is carried away as gamma radiation.

_{92}^{235} U +_{0}^{1} n \to_{56}^{141} Ba +_{36}^{92} Kr + 3_{0}^{1} n + energy

The chain reaction

Each fission releases two or three neutrons. The diagram below follows the case where two are released and both go on to cause another fission.

If any of these neutrons are absorbed by other nuclei, they cause them to split too, releasing more neutrons and creating a self-sustaining chain reaction.

Left uncontrolled, the number of fissions grows rapidly, and so does the rate at which energy is released.

Generating electricity from fission

The energy released by fission heats water into steam.

The steam drives a turbine, which turns a generator to produce electricity.

Using a coolant

In many reactors the energy isn't passed straight to the water.

A coolant (such as carbon dioxide) is pumped around the reactor core to carry heat to a heat exchanger, where it boils the water into steam.

This keeps the water supply separate from the reactor core.

Benefits and risks of nuclear power

Benefits	Risks and drawbacks
Reliable, generating large amounts of energy from very small amounts of fuel.	Produces radioactive waste with long half-lives, staying dangerous for thousands of years.
Releases no greenhouse gases during operation, so it doesn't contribute directly to global warming.	High overall cost to build plants and safely decommission them.
Low fuel costs compared to fossil fuels.	Risk of major accidents or radiation leaks.

Disposing of radioactive waste

High-level radioactive waste is sealed in steel canisters or vitrified (set into glass) and stored deep underground.

Stored waste poses a long-term risk if containers leak over time or are damaged by geological activity, contaminating groundwater and soil.