Nuclear fission and fusion
Introduced| English | Meaning |
|---|---|
| fission/ˈfɪʃn/ | The splitting of a heavy atomic nucleus into lighter nuclei, releasing energy and often additional neutrons. |
| chain reaction/tʃeɪn rɪˈækʃn/ | A process in which products of one reaction trigger further reactions of the same kind, as neutrons can do in nuclear fission. |
| fusion/ˈfjuːʒn/ | Two light nuclei join → a heavier nucleus + energy (powers the Sun). |
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:
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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:
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Fusion = joining two light nuclei into a heavier one, releasing energy. It powers the Sun.
Which process powers the Sun?
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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?
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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.
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Both processes lose a tiny amount of mass, which is released as a large amount of energy.
Worked examples
1. Fission: $^{235}_{\ 92}\text{U} + ^{1}_{0}\text{n} \to ^{141}_{\ 56}\text{Ba} + ^{92}_{36}\text{Kr} + 3\,{}^{1}_{0}\text{n}$. Check the top numbers: $235 + 1 = 141 + 92 + 3 = 236$. Bottom numbers: $92 = 56 + 36$.
2. Fusion: $^{2}_{1}\text{H} + ^{3}_{1}\text{H} \to ^{4}_{2}\text{He} + ^{1}_{0}\text{n}$. Top: $2 + 3 = 4 + 1$. Bottom: $1 + 1 = 2 + 0$.
Fission splits a heavy nucleus; fusion joins light nuclei. Both release energy. Fusion needs a very high temperature, which is why it happens in stars.
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
Write from 6 released neutrons
- Uranium-235 absorbs one neutron and splits into nuclei of nucleon numbers 142 and 88.
- Find how many neutrons are released. Fusion powers the Sun.
Uranium-235 absorbs one neutron and produces nuclei with nucleon numbers 142 and 88. How many neutrons are released, and which process powers the Sun?
235 plus 1 minus 142 minus 88 is 6. The equation balances nucleon number.
How many neutrons are released?
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236 minus 230 is 6 neutrons.