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One way is to split atomic nuclei in a process called nuclear fission.

How do we get energy from atoms?

The energy that holds particles together in a nucleus is much greater than the energy that holds electrons to a nucleus.

This is the why the energy released during nuclear reactions (involving nuclei splitting apart or joining together) is much greater than that for chemical reactions (involving electrons).

Atoms contain huge amounts of energy, and there are two ways in which this energy can be released.

nucleus

electrons

Another way is to join nuclei together in a process called nuclear fusion.


In 1905, Albert Einstein made the connection between energy and matter.

Einstein and E = mc2

Einstein made the prediction that a small amount of matter could release a huge amount of energy.

He expressed this in, what is probably the most famous equation in physics, E = mc2.


In 1933, work by Irène and Frédéric Joliot-Curie proved Einstein’s prediction.

Proof of Einstein’s theory

However, the strongest evidence of Einstein’s theory came with discoveries about nuclear fission and fusion reactions, in which huge amounts of energy are released from atoms.

They produced a photograph, which showed the creation of two particles (mass) when a particle of light (carrying energy) was destroyed.

This was the first proof of the conversion of energy into mass.


Nuclear fission occurs when a stable isotope is struck by a neutron. The isotope absorbs the neutron, becomes unstable and then splits apart, releasing large amounts of energy.

What is nuclear fission?

The fission of 1 kilogram of uranium-235 releases more energy than burning 2 million kilograms of coal!

Unlike natural radioactive decay, fission is not a natural event.

Isotopes that undergo fission include uranium-235 and plutonium-239. These isotopes can both be used in nuclear reactors and in nuclear weapons.


There are two major isotopes of uranium – 238 and 235. Uranium-238 is the major isotope, but it does not undergo nuclear fission.

Only 0.7% of naturally-occurring uranium is uranium-235, which does undergo nuclear fission.

The enriched fuel is made into rods which are used in the reactor.

Before it can be used as the fuel in nuclear power stations, uranium needs to be enriched until it has 3% uranium-235.

How is uranium used in nuclear reactors?


What happens in nuclear fission?


What are the products of fission?

When fission of uranium-235 occurs, it splits into two smaller nuclei, known as daughter nuclei.

Many possible daughter nuclei may be formed in a fission process. One example is shown below.

+ +

neutronuranium235

strontium90

xenon144

fission

+

neutrons+ + + uranium236


In this decay equation, the number of protons and the mass numbers on both sides of the equation balance.

Where does the energy come from?

Barium and krypton are often the daughter nuclei formed by the fission of uranium-235. The decay equation for this is:

The mass that has been lost has turned into energy.

235 1 90 143 192 0 36 56 0

U + n Kr Ba+ n3 +

However, the particles after decay have slightly less mass than the particles before decay.


How does nuclear power work?


Some of the daughter nuclei produced during nuclear fission are stable isotopes, but many are unstable and radioactive.

Unstable daughter nuclei will decay into other radioactive isotopes.

What happens to the daughter nuclei?

An example of an unstable daughter nucleus produced by nuclear fission is strontium-90.

Strontium-90

Zirconium-90

Yttrium-90

half-life = 28 years

half-life = 64 hours

(stable)

The decay process continues until a stable isotope is formed.

This is called a decay series.


Why do fuel rods have to be replaced?

Eventually, the uranium-235 in fuel rods is used up and they have to be replaced.

Plutonium-239 is formed when uranium-238 is bombarded by neutrons. This highly toxic material can be used as a nuclear fuel and to make nuclear weapons.

The spent fuel rods contain fission products, many of which are radioactive. Some waste isotopes are short lived, while others will remain radioactive for thousands of years.

Other significant waste isotopes include strontium-90 and iodine-131, which are easily absorbed by the body.

Why does nuclear waste have to carefully controlled?


How is nuclear waste dealt with?

Spent fuel rods are sent to a reprocessing plant to recover any usable uranium and plutonium.

Long-term storage of nuclear waste is a major problem. Why is it so difficult to find suitable sites?

Many of the isotopes in the remaining waste have no practical purpose and are too dangerous to be released to the environment.

Strict regulations are followed when handling and storing nuclear waste.

Some waste can be stored in cement inside reinforced steel drums.


Nuclear fission – true or false?


What happens in a chain reaction?


What is a chain reaction?

Nuclear fission results in a chain reaction because each time a nucleus splits it releases more neutrons, which can go on and cause more fission reactions to occur... and so on.

+

+

+

+

This is why a chain reaction releases a lot of energy so rapidly.

If a chain reaction is uncontrolled, heat builds up very quickly. A chain reaction must be controlled to maintain a steady output of heat.


What are the stages of a chain reaction?


For nuclear fission to start in a reactor, a uranium-235 atom must absorb a low speed neutron. High speed neutrons are not as readily absorbed by uranium nuclei.

How are neutrons controlled?

However, high speed neutrons are released during fission.

control rod

fuel rod

graphite core

water carrying away heatThe reactor’s graphite core

slows down the released neutrons so the chain reaction can keep going.

Control rods made of boron absorb excess neutrons to prevent chain reactions getting out of control.


Chain reactions can generate a lot of heat and can be extremely dangerous if they are not properly controlled.

Why must chain reactions be controlled?

This is what happened in 1983 in the world’s worst nuclear power accident at Chernobyl, Ukraine.

Most of the control rods had been removed from a reactor during a test.

The chain reactions were uncontrolled and generated too much heat.

The reactor overheated and caused a steam explosion, which blew the building apart and released a lot of radiation into the environment.


Nuclear bombs use uncontrolled chain reactions.

How do nuclear weapons work?

A nuclear weapon works by forcing together two masses of uranium-235 to create a critical mass.

For such a chain reaction to occur, there must be a certain amount of uranium atoms. This is called the critical mass.

This results in uncontrolled chain reactions releasing huge amounts of energy.

This 4 tonne uranium bomb was similar to one used during the second world war in Hiroshima, Japan. It has the samepower as 20,000 tonnes of high explosive.


Chain reactions – key words


Pros and cons of using nuclear fission


Nuclear fusion is the process which powers the Sun and other stars.

What is nuclear fusion?

A worldwide research programme is being carried out to find ways in which nuclear fusion could be harnessed on Earth as a clean and plentiful source of energy.

In this process, small nuclei join together to form larger nuclei and energy is released.

In the Sun’s core, at temperatures of 15 million °C, hydrogen nuclei fuse to form helium nuclei and release vast amounts of energy.


What happens in nuclear fusion?


In nuclear fusion, small nuclei fuse together to form larger nuclei and energy is released.

What are the conditions for nuclear fusion?

Nuclear fusion happens all the time in stars at very high pressures and temperatures. These conditions overcome repulsive forces between the nuclei and force them together.

+ +

tritiumdeuterium helium neutronfusion + +

Scientists have found it difficult to create the extreme conditions needed to carry out nuclear fusion on Earth.

Various fusion reactors are being tested around the world.


How can nuclear fusion be used in the future?

Could nuclear fusion help solve energy problems on Earth?


The largest nuclear fusion experimental reactor is JET (Joint European Torus) in Culham, Oxfordshire.

What does a fusion reactor look like?

Fusion on Earth requires temperatures about six times hotter than the Sun’s core.

The reactor is most efficient as a doughnut shape.

This vessel uses a magnetic field to trap super-hot hydrogen, which has changed from a gas into a high-energy plasma.


In a fusion power station, the hydrogen plasma will be ‘squeezed’ to produce helium and high energy neutrons.

How will a fusion power station work?

The energy of the neutrons will then be transferred by a water cooling loop to a heat exchanger to make steam.

heat exchanger

Then, like fossil fuel and fission power stations, the steam will drives turbines to produce electricity.

It is hoped that the first fusion power station will be ready and working in about 30 years time.

turbines

generatorplasma


There are many advantages of using fusion energy:

Why use nuclear fusion?

Abundant fuels – Deuterium can be extracted from water and tritium is made from lithium, which is readily available.

Clean – No greenhouse or other polluting gases are made.

No weapons material produced – The products are not suitable for making nuclear weapons.

Safe – No need to keep chain reactions under control.

Less radioactive waste – The products of nuclear fusion are not radioactive, although the reactor walls will absorb neutrons and become radioactive.

Small amounts of fuel – 10 grams of deuterium and 15 grams of tritium could produce enough energy for the lifetime of an average person in an industrialized country.


Pros and cons of using nuclear fusion


chain reaction – A self-sustaining series of reactions in which the neutrons produced in one fission cause more fission reactions to occur.

control rods – Tubes of material that absorb neutrons and are used to control chain reactions in a fission reactor.

daughter nuclei – The smaller nuclei formed by the fission of a larger nucleus.

fuel rods – Enriched uranium rods that are used to fuel nuclear fission reactors.

nuclear fission – The splitting of a large nucleus, which creates two smaller nuclei and releases a lot of energy.

nuclear fusion – The joining of two smaller nuclei, which makes a larger nucleus and releases a lot of energy.

Glossary


Anagrams


Comparing nuclear fission and fusion


Multiple-choice quiz

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Documents

nuclear fission

fission of uranium

fission neutrons uranium

fission process

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