9.4 Nuclear fission and fusion — Learn

Prerequisites: Section 9.2, especially the nucleus and the neutron.

By the end of this section you can: describe nuclear fission and the chain reaction it can start; explain how the chain reaction is controlled in a reactor; describe nuclear fusion; and compare fission with fusion.

1. Nuclear fission

Nuclear fission is the splitting of a large and unstable nucleus, such as a nucleus of uranium or plutonium.

Spontaneous fission is rare. Usually, for fission to happen, the unstable nucleus must first absorb a neutron. The nucleus then splits into two smaller nuclei that are roughly equal in size. The fission releases:

two or three neutrons;
gamma rays;
a large amount of energy.

The two smaller nuclei and the neutrons move away at high speed. They carry away the released energy in their kinetic store.

The equation below shows one fission of uranium-235.

^{235}_{92}\text{U} + {}^{1}_{0}\text{n} \rightarrow {}^{141}_{56}\text{Ba} + {}^{92}_{36}\text{Kr} + 3{}^{1}_{0}\text{n}

Worked example 1. Check that the mass numbers and the atomic numbers balance in the fission equation above.

Mass numbers: on the left, $235 + 1 = 236$ . On the right, $141 + 92 + 3 = 236$ .

Atomic numbers: on the left, 92. On the right, $56 + 36 = 92$ .

Both balance, so the equation is correct.

2. A chain reaction

Each fission releases two or three neutrons. Those neutrons can be absorbed by other uranium-235 nuclei, and each of those nuclei can fission in turn. This keeps the reaction going and is called a chain reaction.

If each fission releases three neutrons and every neutron causes one more fission, then the number of fissions grows quickly.

Generation after the first fission	Neutrons released
First generation	3
Second generation	9
Third generation	27

Worked example 2. Each fission releases 3 neutrons. Calculate the number of neutrons released in the third generation after the first fission.

The first fission releases 3 neutrons. Each neutron causes one more fission, so the next generation releases $3 \times 3 = 9$ neutrons. The third generation releases

3^3 = 27 \ \text{neutrons}

3. Try it: the chain reaction

The figure shows one fission and then the neutrons released in each generation. You can change how many neutrons from each fission go on to cause the next fission.

Predict first. Do not move the slider yet. Write down your answers.

If two neutrons from each fission cause the next fission, what happens to the chain reaction?
If one neutron from each fission causes the next fission, what happens to the chain reaction?
If no neutron causes the next fission, what happens to the chain reaction?

Test. Move the slider from 0 to 3. Watch the numbers in the chart. Then set the slider to 1 and compare the bars.

Explain. In a reactor, control rods absorb neutrons so that on average only one neutron from each fission causes the next fission. Explain why it must be one and not two or three. Write your answer in two or three sentences.

4. Controlling the chain reaction

If every neutron caused another fission, the number of fissions would double or triple at every step. The energy released would grow out of control in a fraction of a second. This is what happens in a nuclear weapon.

In a nuclear reactor, the chain reaction is controlled. On average, only one neutron from each fission goes on to cause the next fission. The energy is then released at a steady rate that can be used to make electricity.

Watch out. Fission and fusion are opposite processes, and their names sound alike. Fission splits a large nucleus. Fusion joins two light nuclei. A nuclear power station uses fission, not fusion. Check by asking whether the nucleus at the start is large and split, or small and joined.

5. Nuclear fusion

Nuclear fusion is the joining of two light nuclei to form a heavier nucleus. In the process, some of the mass of the nuclei is converted into the energy of radiation.

Fusion happens naturally in the core of a star, including the Sun. The very high temperature gives the light nuclei enough energy to move fast and join, even though their positive charges make them repel each other. It is difficult to reach and to hold such a high temperature on the Earth, so fusion is not yet used to generate electricity.

Worked example 3. Compare nuclear fission and nuclear fusion. Give one thing they have in common and one important difference.

Both reactions release a large amount of energy from the nucleus. In fission a large nucleus splits into smaller nuclei. In fusion two light nuclei join to form a heavier nucleus.

6. Fission and fusion side by side

	Fission	Fusion
What happens	A large nucleus splits into two smaller nuclei	Two light nuclei join into one heavier nucleus
What starts it	Absorbing a neutron (usually)	Very high temperature and pressure
Energy	Released	Released
Where it is used or found	A nuclear reactor in a power station	The core of a star

7. Progress check

Return to the chapter problem. The hospital's electricity is generated by a nuclear fission reactor. Explain what happens to a uranium-235 nucleus when it absorbs a neutron, and explain how the chain reaction is kept under control.

You should now be able to

define nuclear fission, and describe what is released in the reaction;
explain how a chain reaction starts and keeps going;
explain why the chain reaction in a reactor is controlled, and what happens if it is not;
define nuclear fusion, and state what happens to some of the mass;
compare fission with fusion, and say where each one happens.