Fission and Fusion

Nuclear Fission

Figure 1 - Diagram showing fission

Nuclear fission is a process where a large nucleus is split into smaller parts releasing energy. In a nuclear power station that energy can be used to heat water that turns turbines and creates electricity.

In a nuclear power station, neutrons collide with uranium nuclei splitting them into smaller nuclei and more neutrons. These neutrons then go on to collide with other uranium nuclei continuing the reaction. This is known as a chain reaction.

In order for a chain reaction to continue then there has to be a high enough chance some of the neutrons released from a

previous reaction hitting another uranium nucleus. If there isn’t enough uranium in the reactor then the reaction would stop.

The amount of uranium needed to just sustain a chain reaction is known as its critical mass. If there is more uranium than the critical mass the reaction quickly increases as more and more neutrons are produced, creating more and more energy. In a power station, the amount of fission taking place in the reactor is controlled by special rods which can be used to absorb extra neutrons.

Figure 1 shows a uranium nuclei being split into two smaller nuclei producing energy and three neutrons. The table below shows how the numbers of neutrons would increase after each stage in the chain if all of these went on to create three new neutrons (by colliding with uranium).

Complete the table below. How many stages are needed to create >500 neutrons? How many for >1000?

Stage 1st 2nd 3rd 4th 5th 6th 7thNumber of neutrons 3 9 81

This is an example of exponential growth. In practice this would be impossible to achieve.

Early nuclear weapons, known as atomic bombs, used nuclear fission to create an uncontrolled chain reaction. The bombs were far more powerful than conventional bombs that relied on explosives. Two of these were used to bomb the Japanese towns of Hiroshima and Nagasaki in August 1945 at the end of World War II. This killed thousands of people, mostly civilians.

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Helium

NeutronDeuterium

Tritium

Nuclear Fusion

Nuclear Fusion is the joining or fusing together of two smaller nuclei to form a bigger one, releasing energy in the process. This process occurs naturally in the centre of stars and is the source of the suns energy.

One possible fusion reaction is between two isotopes of hydrogen, Tritium and Deuterium.

Hydrogen has three naturally occurring isotopes:

Hydrogen – with one proton in the nucleus. Deuterium - with one proton and one neutron. Tritium - with one proton and two neutrons.

Figure 2 - Fusion of tritium and deuterium to form helium

Conditions needed for nuclear fusion

Tritium and Deuterium nuclei are both positively charged so in normal conditions they would repel each other. To overcome this repulsion, they need to be heated up to an incredibly high temperatures. This is how fusion occurs in the sun. Here on Earth, this makes fusion very difficult to achieve.

Hydrogen bombs use a fission reaction to create conditions where nuclear fusion can occur. This creates a more powerful bomb than atomic bombs that use fission alone.

Figure 3 - Tokamak Fusion Test Reactor

If we could create controlled fusion reactions we could potentially use it as a power source to create electricity.

Currently fusion reactions have been created in laboratories in machines called tokamaks which use magnetic fields to contain the deuterium and tritium nuclei while heating them up to extremely high temperatures. By doing this they have been able to create fusion in the lab, but so far it requires more energy to create the fusion reaction than is released.

Even though this means it can’t yet be used to generate electricity, experiments like this may allow scientists to discover how to make fusion power a possibility in the future.

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