What is nuclear fission?

The splitting of a heavy nucleus into two medium-mass nuclei, releasing energy.

What is nuclear fusion?

The joining of two light nuclei to form a heavier nucleus, releasing energy.

Why does fission of uranium-235 release energy?

The fragments have a higher binding energy per nucleon than uranium-235, so total binding energy increases and the difference is released.

Why does fusion of light nuclei release energy?

The product has a higher binding energy per nucleon, so the total binding energy increases and the difference is released.

Why must nuclei have a very high temperature to fuse?

To have enough kinetic energy to overcome their electrostatic repulsion and get close enough for the strong force to act.

Why is a very high density needed for fusion?

So that nuclei are close together and collide often enough for a useful rate of fusion.

What are the products when hydrogen-2 and hydrogen-3 fuse?

Helium-4 and a neutron: {}^{2}{1}H + {}^{3}{1}H \rightarrow {}^{4}{2}He + {}^{1}{0}n.

How do the conditions for fusion arise in the Sun?

Gravity from the Sun's huge mass compresses the core to a very high density and temperature.

Why is fusion hard to achieve on Earth?

There is no gravity to supply the density, so a higher temperature of about 10⁸ K is needed, and the plasma must be contained away from the walls.

How do you calculate the energy released from binding energy per nucleon data?

Total binding energy of products (A × EB/A for each) minus total binding energy of reactants.

Which releases more energy per nucleon, fission or fusion?

Fusion, because the rise in binding energy per nucleon is much greater (about 6 MeV compared with under 1 MeV).

Nuclear fission and fusionEdexcel A-Level Physics: Flashcards

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Describe the shape of the binding energy per nucleon curve.

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Describe the shape of the binding energy per nucleon curve.: It rises steeply for light nuclei, peaks at about 8.8 MeV near nucleon number 56 (iron-56), then falls slowly for heavier nuclei.
What is nuclear fission?: The splitting of a heavy nucleus into two medium-mass nuclei, releasing energy.
What is nuclear fusion?: The joining of two light nuclei to form a heavier nucleus, releasing energy.
Why does fission of uranium-235 release energy?: The fragments have a higher binding energy per nucleon than uranium-235, so total binding energy increases and the difference is released.
Why does fusion of light nuclei release energy?: The product has a higher binding energy per nucleon, so the total binding energy increases and the difference is released.
Why must nuclei have a very high temperature to fuse?: To have enough kinetic energy to overcome their electrostatic repulsion and get close enough for the strong force to act.
Why is a very high density needed for fusion?: So that nuclei are close together and collide often enough for a useful rate of fusion.
What are the products when hydrogen-2 and hydrogen-3 fuse?: Helium-4 and a neutron: ${}^{2}_{1}\text{H} + {}^{3}_{1}\text{H} \rightarrow {}^{4}_{2}\text{He} + {}^{1}_{0}\text{n}$ .
How do the conditions for fusion arise in the Sun?: Gravity from the Sun's huge mass compresses the core to a very high density and temperature.
Why is fusion hard to achieve on Earth?: There is no gravity to supply the density, so a higher temperature of about 10⁸ K is needed, and the plasma must be contained away from the walls.
How do you calculate the energy released from binding energy per nucleon data?: Total binding energy of products (A × E_B/A for each) minus total binding energy of reactants.
Which releases more energy per nucleon, fission or fusion?: Fusion, because the rise in binding energy per nucleon is much greater (about 6 MeV compared with under 1 MeV).

Exam questions on Nuclear fission and fusion

The binding energy per nucleon of nuclei varies with nucleon number. It rises steeply for the lightest nuclei, reaches a maximum of about 8.8 MeV per nucleon at a nucleon number of about 56 (iron-56), and then falls slowly for heavier nuclei, to about 7.6 MeV per nucleon for uranium-235.
Use the information about the binding energy per nucleon to explain why energy is released when two light nuclei fuse.2 marks
In a proposed fusion reactor, a gas of deuterium (hydrogen-2) and tritium (hydrogen-3) nuclei is heated until it forms a plasma at a temperature of about $1 \times 10^{8}$ K. The nuclei are then held together at a very high density so that they can fuse to form helium-4.
Explain why fusion requires both a very high temperature and a very high density.2 marks
In a nuclear power station, one of the fission reactions of uranium-235 is ${}^{1}_{0}\text{n} + {}^{235}_{92}\text{U} \rightarrow {}^{141}_{56}\text{Ba} + {}^{92}_{36}\text{Kr} + 3\,{}^{1}_{0}\text{n}$ . Binding energy per nucleon: uranium-235, 7.59 MeV; barium-141, 8.33 MeV; krypton-92, 8.51 MeV. Take $1\text{ MeV} = 1.60 \times 10^{-13}$ J, the molar mass of uranium-235 as 235 g mol⁻¹ and $N_A = 6.02 \times 10^{23}$ mol⁻¹.
Use the binding energy per nucleon values to calculate the energy released, in MeV, when one uranium-235 nucleus undergoes this fission.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).