Nuclear fusion and fission
Power from the nucleus
- The Sun shines by fusion; a power station runs on fission.
- Both release energy locked inside the nucleus.
- The binding-energy curve tells us which way releases energy.
Toward the iron peak
- Energy is released when nuclei move toward the iron peak (higher binding energy per nucleon).
- Joining light nuclei, or splitting heavy ones, both head that way.
Energy is released when nuclei move:
Climbing the B/A curve toward iron means the products are more tightly bound, so energy is released.
Fusion
- Fusion joins two light nuclei into a heavier one (with higher B/A) — releasing energy.
- It needs very high temperatures so nuclei beat their electrostatic repulsion. Fusion powers stars.
Fusion needs very high temperatures to overcome the electrostatic repulsion between nuclei.
The nuclei must get close enough for the strong force to act, despite repelling — hence the millions of kelvin in stars.
Fission
- Fission splits a heavy nucleus into two lighter ones, plus a few neutrons.
- Those neutrons can split more nuclei — a chain reaction in a large enough mass (the critical mass).
Match each process to what it does.
Both move toward the iron peak, so both can release energy.
The extra neutrons released in fission can trigger a ____ reaction.
Each fission releases neutrons that cause more fissions — a chain reaction in a critical mass of fuel.
A sustained chain reaction needs a critical mass of fuel.
Below the critical mass too many neutrons escape, and the reaction dies out.
Energy released
- Find the mass change $\Delta m$ = mass of reactants − mass of products.
- Energy released $\Delta E = c^{2}\Delta m$, or in atomic units $\Delta E\,(\text{MeV}) = \Delta m\,(\text{u}) \times 931$.
A reaction has a mass change of $0.020\ \text{u}$. How much energy is released, in MeV?
$\Delta E = \Delta m\,(\text{u}) \times 931 = 0.020 \times 931 \approx 18.6\ \text{MeV}$.
You've got it
- energy is released moving toward iron: fusion (light join) or fission (heavy split)
- fusion needs high temperature; fission's neutrons can start a chain reaction
- energy released $\Delta E = c^{2}\Delta m$ (or $\Delta m\,(\text{u}) \times 931\ \text{MeV}$)