Nuclear fission and fusion
| English | Chinese | Pinyin |
|---|---|---|
| fission | 裂变 | liè biàn |
| chain reaction | 链式反应 | liàn shì fǎn yìng |
| fusion | 聚变 | jù biàn |
The power in the nucleus
- The nucleus holds a huge amount of energy.
- We release it in two opposite ways: splitting heavy nuclei, or joining light ones.
- These power nuclear stations on Earth and every star in the sky.
Nuclear fission 裂变
- In fission, a heavy nucleus absorbs a neutron and splits into two smaller nuclei, releasing 2 or 3 neutrons and a lot of energy.
- Those neutrons can split more nuclei — a chain reaction 链式反应 — which is what a nuclear power station controls.
The fission chain reaction
A neutron splits a uranium nucleus, which releases energy AND more neutrons — those split more nuclei, and so on.
Nuclear fission is when:
Fission = splitting a heavy nucleus (after it absorbs a neutron) into two smaller nuclei, plus 2–3 neutrons and energy.
Nuclear fusion 聚变
- In fusion, two light nuclei join to make a heavier one, also releasing energy.
- The Sun makes its energy this way, fusing hydrogen into helium. The reaction below — two heavy forms of hydrogen (deuterium and tritium) — is the one used in fusion reactors on Earth:
- Fusion needs enormous temperatures and pressures, which is why it is so hard to do on Earth.
In fusion two light nuclei join into a heavier one, releasing a neutron and energy — the deuterium–tritium reaction used in fusion reactors
Nuclear fusion is when:
Fusion = joining two light nuclei into a heavier one, releasing energy. It powers the Sun.
Which process powers the Sun?
The Sun fuses hydrogen nuclei into helium, releasing huge amounts of energy.
Mass into energy, and balancing
- In both fission and fusion, a tiny amount of mass is lost and turned into energy.
- In any nuclear equation, the top numbers (nucleon numbers) balance, and so do the bottom numbers (proton numbers).
- That lets you find a missing number — for example, how many neutrons are released.
Nuclear fission of a U-235 nucleus
In $^{235}_{\ 92}\text{U} + {}^{1}_{0}\text{n} \rightarrow {}^{141}_{\ 56}\text{Ba} + {}^{92}_{36}\text{Kr} + x\,{}^{1}_{0}\text{n}$, how many neutrons $x$ are released?
Nucleon numbers must balance: $235 + 1 = 141 + 92 + x$, so $236 = 233 + x$ and $x = 3$.
In fission and fusion, a small amount of mass is turned into energy.
Both processes lose a tiny amount of mass, which is released as a large amount of energy.
You've got it
- fission: a heavy nucleus splits → two smaller nuclei + neutrons + energy (chain reaction)
- fusion: two light nuclei join → a heavier nucleus + energy (powers the Sun)
- both convert a little mass into energy
- nuclear equations balance top (nucleon) and bottom (proton) numbers