3.8.1.7
Fission splits heavy nuclei into smaller, more stable fragments
Nuclear Energy & Binding Energy — AQA A-Level Physics
Key Definition
Nuclear fission — The splitting of a large, unstable nucleus into two smaller daughter nuclei, accompanied by the release of neutrons and energy.
- Occurs in heavy nuclei (A > 56) where the products have a higher binding energyThe capacity to do work. Measured in joules (J). per nucleonThe binding energyThe capacity to do work. Measured in joules (J).The energyThe capacity to do work. Measured in joules (J). required to completely separate a nucleus into its individual protons and neutrons. Equal to the mass defectThe difference between the total mass of the individual nucleons and the actual mass of the nucleus. This mass is converted to binding energy. multiplied by c². of a nucleus divided by its nucleon number (mass number). Higher values indicate greater nuclear stability..
- The daughter nuclei plus neutrons have less total mass than the original nucleus + absorbed neutron — the difference appears as kinetic energyThe energy an object possesses due to its motion..
- The released neutrons can go on to cause further fission events in a chain reaction.
- Both fission and fusion release energy because the products have higher binding energy per nucleonThe binding energyThe energy required to completely separate a nucleus into its individual protons and neutrons. Equal to the mass defectThe difference between the total mass of the individual nucleons and the actual mass of the nucleus. This mass is converted to binding energy. multiplied by c². of a nucleus divided by its nucleon number (mass number). Higher values indicate greater nuclear stability. than the reactants.
Worked Example
U-235 absorbs a neutron and splits into Tc-112 and In-122 plus 2 neutrons. Binding energy per nucleonThe binding energyThe energy required to completely separate a nucleus into its individual protons and neutrons. Equal to the mass defectThe difference between the total mass of the individual nucleons and the actual mass of the nucleus. This mass is converted to binding energy. multiplied by c². of a nucleus divided by its nucleon number (mass number). Higher values indicate greater nuclear stability.: U-235 = 7.59 MeV, Tc-112 = 8.36 MeV, In-122 = 8.51 MeV. Calculate the energy released.
Show Solution
1
Calculate total binding energy before
$$BE_{\text{before}} = 235 \times 7.59 = 1784 \text{ MeV}$$
(Neutrons have zero binding energy.)
2
Calculate total binding energy after
$$BE_{\text{after}} = (112 \times 8.36) + (122 \times 8.51) = 936 + 1038 = 1975 \text{ MeV}$$
3
Energy released = increase in binding energy
$$\Delta E = 1975 - 1784 = 191 \text{ MeV}$$
Answer
Energy $released = 191 MeV$