Nuclear fusion and fission
| English | Chinese | Pinyin |
|---|---|---|
| fusion | 熔化 | róng huà |
| fission | 裂变 | liè biàn |
| chain reaction | 链式反应 | liàn shì fǎn yìng |
| electrostatic repulsion | 静电排斥 | jìng diàn pái chì |
| critical mass | 临界质量 | lín jiè zhì liàng |
Power from the nucleus
- The Sun shines by fusion 熔化; a power station runs on fission 裂变.
- 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.
In an uncontrolled chain reaction 链式反应 each fission of uranium-235 releases neutrons that cause more fissions
Nuclear fission chain reaction
A neutron splits a heavy nucleus, releasing energy and more neutrons — which split more nuclei.
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 静电排斥. 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 临界质量).
Mass and energy are equivalent and can change into each other
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 $\Delta m\,(\text{u}) \times 931\ \text{MeV}$)