9.4 Nuclear fission and fusion — Practise

Work through these on your own. Keep your working neat, and keep the units with your numbers.

Both the questions and the answers below come from a set that is checked by computer every time the book is built.

Define nuclear fission.
Describe what happens when a uranium-235 nucleus absorbs a neutron and undergoes fission.
Explain how a chain reaction can happen in a sample of uranium-235.
Explain why the chain reaction in a nuclear reactor must be controlled, and what happens if it is not.
Check that the mass numbers and the atomic numbers balance in the fission equation $^{235}_{92}\text{U} + {}^{1}_{0}\text{n} \rightarrow {}^{141}_{56}\text{Ba} + {}^{92}_{36}\text{Kr} + 3{}^{1}_{0}\text{n}$ .
Define nuclear fusion, and state what happens to some of the mass in the reaction.
Compare fission and fusion. Give one thing they have in common and one way they are different.
State where nuclear fission is used to generate electricity, and where nuclear fusion takes place naturally.
In a fission reaction, energy is released. Explain which particles carry away most of this energy.
Each fission releases 3 neutrons, and every neutron causes one more fission. Calculate the number of neutrons released in the third generation after the first fission.
Explain why fusion needs extremely high temperatures, and why this makes it hard to use fusion in a power station.
A country is choosing how to generate electricity. Say whether a fission power station or a fusion power station can be built today, and give a reason.

Answers and concise solutions

1. Definition of nuclear fission

Nuclear fission is the splitting of a large and unstable nucleus, such as uranium or plutonium.
The nucleus splits into two smaller nuclei that are roughly equal in size, and energy is released.

2. What happens in fission

The unstable nucleus absorbs a neutron, which makes it even less stable.
The nucleus splits into two smaller nuclei that are roughly equal in size.
Two or three neutrons are released, along with gamma rays.
Energy is released in the reaction. The fission products carry away most of it as kinetic energy.

3. How a chain reaction happens

Each fission releases two or three neutrons.
Those neutrons can be absorbed by other uranium-235 nuclei, causing them to fission too.
The number of fissions grows with each generation, so the reaction can spread through the sample.

4. Why the chain reaction is controlled

In a reactor the chain reaction is controlled so that on average only one neutron from each fission causes another fission.
This keeps the energy released steady, so the reactor produces useful power without overheating.
In an uncontrolled chain reaction the number of fissions grows very quickly. This releases a huge amount of energy in a very short time, as in a nuclear weapon.

5. Balancing the fission equation

Answer: The total mass number on the left is $235 + 1 = 236$ , and on the right it is $141 + 92 + 3 = 236$ . The total atomic number on the left is 92, and on the right it is $56 + 36 = 92$ . Both balance.

6. Definition of nuclear fusion

Nuclear fusion is the joining of two light nuclei, such as hydrogen nuclei.
They form a single heavier nucleus.
Some of the mass of the nuclei is converted into the energy of radiation.

7. Fission compared with fusion

Both reactions release a great deal of energy from the nucleus.
Fission splits a large nucleus into smaller ones. Fusion joins light nuclei into a heavier one.

8. Where fission and fusion happen

Fission is used in the nuclear reactors of a power station to generate electricity.
Fusion takes place in the core of a star, such as the Sun, where light nuclei join together.

9. Which particles carry the energy

The two smaller nuclei that are produced, and the neutrons, carry away most of the released energy.
They move away at high speed, so they carry the energy in their kinetic store.

10. Neutrons in the third generation

Answer: The first fission releases $3$ neutrons. The second generation releases $3 \times 3 = 9$ , and the third generation releases $3^3 = 27$ neutrons.

11. Why fusion needs high temperatures

Nuclei are positively charged, so they repel each other.
The nuclei must be moving very fast to get close enough to join, and a very high temperature gives them that energy.
It is difficult to reach and hold such a high temperature in a power station, so fusion is not yet used to generate electricity.

12. Choosing fission or fusion for a power station

A fission power station can be built today, because fission reactors are already used to generate electricity.
Fusion has not yet been made to work as a reliable source of power, because the very high temperature is hard to reach and to contain.