(7.17-7.26) Fission and Fusion

Name three types of nuclear reactions.

Fission

Fusion

Radioactive decay

What do nuclear reactions release?

Energy

Why do nuclear reactions produce such great amounts of energy?

Nuclei contain huge amounts of energy. When reactions occur, there is potential for large amounts of energy to be released.

Define fission.

When one large, unstable nucleus splits into two smaller nuclei

What isotopes undergo fission? What are they used for?

Uranium-235 and Plutonium-239

These are often used as fuels in nuclear power stations.

What is induced fission?

Nuclei rarely undergo fission without additional energy (a neutron) being put into the nucleus, making it more unstable and speeding up decay. This process is called induced fission.

Describe the process of induced fission.

A neutron is absorbed by the nucleus, making it very unstable and causing almost immediate splitting.

What are the products of nuclear fission?

Two smaller nuclei

A small number of neutrons

What are the nuclei involved in fission often referred to as?

The large nucleus is often the parent nucleus.

The smaller nuclei are the daughter nuclei.

How is energy released in fission?

The products of the reaction move away very quickly.

Nuclear potential energy becomes kinetic energy.

This kinetic energy can be converted into electrical energy in a nuclear power station.

How does one fission reaction prompt a chain reaction?

The neutrons produced in one reaction can go on to strike other U-235 nuclei, triggering another reaction.

As one reaction releases multiple neutrons, the number of reactions continues to grow.

What factors need to be controlled while a fission reactor is producing energy at the correct rate?

• the number of free neutrons in the reactor

• the energy of the free neutrons

What is the purpose of controls rods in a nuclear fission reactor?

To absorb neutrons to maintain the right number to control the rate of fission.

How do control rods in fission reactors work?

• They are made of a material that absorb neutrons without becoming unstable itself

• Lowering the rods = more neutron absorption = decreased fission rate

• Raising the rods = fewer neutrons absorbed = higher fission rate

• If a reactor needs to shut down, the rods can be lowered all the way to prevent any reactions.

What is the purpose of a moderator in a nuclear fission reactor?

To slow down neutrons to enable efficient reactions

Why do neutrons need to be slow during fission?

They need to be slow to be absorbed, otherwise they will not react with the large nuclei.

How does a moderator in a fission reactor work?

• A material that surrounds the fuel and control rods inside the reactor core.

• The fast moving neutrons collide with the molecules of the moderator, causing them to lose momentum.

Shielding around a nuclear reactor- why is it there and how does it work?

• Shielding materials surround the entire reactor in order to absorb hazardous radiation.

• It consists of a steel and concrete wall that can be nearly 2m thick

• It absorbs emissions from the reactions, ensuring the environment around the reactor is safe.

Define nuclear fusion.

The creation of larger nuclei resulting in a loss of mass from smaller nuclei, accompanied by a release of energy

Where does the energy released in fusion come from?

It comes from a very small amount of the particle’s mass being converted into energy

(E = mc² )

What environment does fusion occur in?

Fusion requires high temperatures so often occurs in stars, making it difficult to replicate on Earth

What does fusion do for stars?

It’s the energy source for stars. In most stars, hydrogen atoms are fused together to form helium and energy.

Why does nuclear fusion require high temperatures?

• Since protons have a positive charge, they repel each other.

• They need to have a very high kinetic energy to overcome this repulsion.

• To make gas molecules travel at such high speeds, it has to be millions of degrees.

State the conditions for fusion.

• Very high temperature of fuel to create

• Very high kinetic energy to overcome repulsion

• Very high density/pressure to increase the possibility of collisions