nuclear fuels and fission - st edmund's girls'...

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Nuclear Fuels and Fission


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 in their nuclei.

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 predicted that a

small amount of matter could

release a huge amount of

energy in a nuclear reaction.

He expressed this in what is

probably the most famous

equation in physics: E = mc2.

E is energy produced

m is mass lost

c is the speed of light in a vacuum


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 that 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.

Most nuclear reactors use uranium-235.


There are two major isotopes of uranium – 238 and 235.

Uranium-238 is more common, 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 1

92 0 36 56 0U + n Kr Ba+ n3 +

However, the particles after decay have slightly less mass

than the particles before decay.


How does nuclear power work?


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?


How are neutrons controlled?


Controlling a reactor


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 four tonne uranium bomb is similar to one used during

the Second World War in Hiroshima, Japan. It has the

same power as 20,000 tonnes of high explosive.


Chain reactions – key words


Fukushima Daiichi nuclear disaster

A massive earthquake on

11th March 2011 caused the six

reactors at the Fukushima Dai-ichi

nuclear power plant in Japan to

shut down automatically.

Though fission had stopped, the

reactors continued to produce heat.

The tsunami that followed the

earthquake disabled the pumps driving the cooling

systems, causing the reactors to overheat.

Reactors 1, 2 and 3 subsequently experienced ‘meltdown’.

Why was heat still produced after fission had stopped?


Some of the daughter nuclei produced during nuclear fission

are stable isotopes, but many are unstable and radioactive,

e.g. strontium-90.

Unstable daughter nuclei in the

fuel rods decay into other

radioactive isotopes. The decay

process continues until a stable

isotope is formed.

What happens to the daughter nuclei?

Strontium-90

Zirconium-90

Yttrium-90

half-life = 28 years

half-life = 64 hours

(stable)

This process is what caused

the meltdown at the

Fukushima nuclear plant, as

it produces large amounts of

heat and radiation.


Making materials radioactive

Materials that are placed inside a nuclear reactor can

become radioactive themselves.

This is because their atoms absorb some of the neutrons

released during fission, creating new isotopes.

Some of these isotopes are unstable

and decay, giving out radiation.

All parts of a nuclear reactor must be

carefully disposed of as nuclear waste

when the reactor is decommissioned.

Some substances, like medical tracers,

are deliberately put inside nuclear

reactors to make them radioactive.


Nuclear waste

Waste from spent nuclear fuel rods contains highly toxic

substances and may remain radioactive for thousands

of years. It can harm both people and the environment.

Long-term storage of nuclear waste

is a major problem. Why is it so

difficult to find suitable sites?

Strict regulations are followed when

handling and storing nuclear waste.

Plutonium-239 can be recovered

and used as a nuclear fuel or to

make nuclear weapons. Many of the

other isotopes in the remaining

waste have no practical purpose.


Categorizing nuclear waste


Using nuclear fission


Glossary


Anagrams


Multiple-choice quiz

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