Nuclei and Nuclear Energy — Part 2: Fission, fusion and nuclear energy: visual lesson, ISC Class 12 Physics

Lesson

Fission, Fusion and Nuclear Energy

Part 2 — one curve explains why both splitting heavy nuclei and joining light ones can release energy.

In this lesson you will

Use the binding energy per nucleon curve to explain why both fission and fusion release energy and to estimate the energy from a heavy nucleus splitting in two
Write balanced fission equations, describe the chain reaction qualitatively, and state the functions of the parts of a reactor
Explain why fusion needs very high temperatures, calculate the energy of a fusion reaction from masses, and describe the energy source of the Sun and stars

Concept

Binding energy per nucleon, B/A, measures how tightly each nucleon is held. Plotted against A for real nuclides:

It rises steeply for light nuclei (about 1.1 MeV for ²H), with a peak at ⁴He (7.07 MeV).
It reaches a maximum of about 8.8 MeV near A = 56–62 (iron, nickel).
It then falls slowly to about 7.6 MeV for uranium.
Between A about 30 and 170 it is nearly flat, within about 10% of 8.5 MeV.

Concept

The higher B/A, the more tightly bound the nucleus, the more stable. So the middle-order nuclei (A about 50 to 80) are the most stable.

Nearly constant B/A: each nucleon bonds only with its near neighbours (the strong force is short-range and saturates).
The fall beyond A about 60: the electrostatic repulsion between protons acts across the whole nucleus and grows faster than the number of nucleons.
A nucleus moving towards the maximum releases energy.

Table of the three regions of the binding energy per nucleon curve with typical values, the process that raises the binding energy per nucleon and whether energy is released
Region of the BE/A curve	Typical B/A (MeV)	Process that raises B/A	Energy
Light nuclei (A < 20)	about 1 to 8	Fusion into a heavier nucleus	Released
Middle-order nuclei (A about 50 to 80)	about 8.7 to 8.8, the maximum	None: most tightly bound	No energy released
Heavy nuclei (A > 200)	about 7.6 to 7.9	Fission into two middle-order nuclei	Released