Nuclei — Part 2: Fission and fusion: visual lesson, CBSE Class 12 Physics

Lesson

Fission and fusion

Part 2 — splitting heavy nuclei and joining light ones: the two ways to unlock nuclear energy.

In this lesson you will

Explain nuclear fission and estimate the energy released using binding energies or masses
Explain nuclear fusion and why it needs very high temperatures
Use the binding-energy curve to explain why both processes release energy

smooth trend (semi-empirical formula)
nuclides (from measured masses)

Fusion of light nuclei and fission of heavy nuclei both move towards the peak, so both release energy.

Diagram

Nuclear fission

A slow neutron absorbed by $\,{}^{235}\text{U}$ makes an unstable $\,{}^{236}\text{U}$ nucleus that splits into two medium-mass fragments and 2–3 neutrons, for example $\,{}^{1}_{0}\text{n} + {}^{235}_{92}\text{U} \to {}^{141}_{56}\text{Ba} + {}^{92}_{36}\text{Kr} + 3\,{}^{1}_{0}\text{n} + Q$

Nucleon number and charge are conserved: $1 + 235 = 141 + 92 + 3$ and $92 = 56 + 36$ .
About 200 MeV is released per fission, mostly as kinetic energy of the fragments.

Worked example

Worked example 4: Q of a fission from masses

Find the energy released in $\text{n} + {}^{235}\text{U} \to {}^{141}\text{Ba} + {}^{92}\text{Kr} + 3\text{n}$ . Atomic masses: U-235 235.043928 u, Ba-141 140.914411 u, Kr-92 91.926156 u; $m_n = 1.008665$ u.

Step 1 — mass before $= 235.043928 + 1.008665 = 236.052593$ u; after $= 140.914411 + 91.926156 + 3 \times 1.008665 = 235.866562$ u.
Step 2 — $\Delta m = 0.1860$ u; $Q = 0.1860 \times 931.5 \approx 173$ MeV.
Note — the fragments later decay, releasing more energy: about 200 MeV in all.

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