(c) Fission and fusion
- Syllabus
- 2024
- Topic
- —
- Level
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Learning objectives
7.17Nuclear reactions as energy sourcesKnow that nuclear reactions, including fission, fusion and radioactive decay, can be a source of energy7.18Nuclear fissionUnderstand how a nucleus of U-235 can be split (the process of fission) by collision with a neutron and that this process releases energy as kinetic energy of the fission products7.19Fission productsKnow that the fission of U-235 produces two radioactive daughter nuclei and a small number of neutrons7.20Chain reactionsDescribe how a chain reaction can be set up if the neutrons produced by one fission strike other U-235 nuclei7.21Control rods and moderatorDescribe the role played by the control rods and moderator in the fission process7.22Reactor shieldingUnderstand the role of shielding around a nuclear reactor7.23Fusion and fissionExplain the difference between nuclear fusion and nuclear fission7.24Nuclear fusionDescribe nuclear fusion as the creation of larger nuclei resulting in a loss of mass from smaller nuclei, accompanied by a release of energy7.25Fusion in starsKnow that fusion is the energy source for stars7.26Fusion conditionsExplain why nuclear fusion does not happen at low temperatures and pressures, due to electrostatic repulsion of protons
Recognise nuclear reactions as energy sources
Fission, fusion and radioactive decay are nuclear reactions that can release energy from a nuclear energy store.
| Nuclear reaction | Change in the nucleus | Where released energy appears |
|---|---|---|
| fission | a large nucleus splits | mainly kinetic energy of the fission products |
| fusion | smaller nuclei join to form a larger nucleus | kinetic energy and radiation from the products |
| radioactive decay | an unstable nucleus emits radiation | kinetic energy of emitted particles and/or electromagnetic radiation |
The nuclear energy store decreases as energy is transferred to the reaction products and surroundings. A power station can ultimately transfer this energy into thermal energy and then electrical energy, while a star's fusion energy reaches space as radiation.
A nuclear reaction changes a nucleus; it is not combustion or another chemical reaction involving electron arrangements. The three processes differ in mechanism even though each can release energy.
Trace one uranium-235 fission event
Nuclear fission is the splitting of a large nucleus. In uranium-235 fission, the process begins when a U-235 nucleus absorbs a neutron.
U-235 absorbs a neutron → an unstable U-236 nucleus forms → the unstable nucleus splits into two smaller daughter nuclei → neutrons and energy are released. The energy is carried mainly as kinetic energy of the fast-moving fission products.
The daughter nuclei repel and move apart at high speed. Their kinetic energy is transferred by collisions to surrounding material, increasing its thermal energy; a reactor uses that heating to help generate electricity.
The incoming neutron is absorbed; U-235 does not lose a proton to become U-236. The original nucleus splits—its daughter nuclei do not cause the first split—and 'fission' refers to a nucleus, not a cell or an atom dividing chemically.
Identify the products of uranium-235 fission
A U-235 fission event produces two smaller daughter nuclei, a small number of neutrons and released energy.
| Product | Important feature | Consequence |
|---|---|---|
| two daughter nuclei | smaller than the original uranium nucleus and usually radioactive | they move apart with high kinetic energy and later undergo radioactive decay |
| usually two or three neutrons | uncharged nuclear particles | they may reach other U-235 nuclei and start further fissions |
| energy | carried mainly by moving fission products | collisions heat the reactor material |
The total nucleon number and total proton number are conserved across a complete fission equation. The exact daughter isotopes can vary, but the defining product pattern remains two daughter nuclei plus a few neutrons.
The products are daughter nuclei, not daughter cells. They are not pieces that remain uranium: their proton numbers identify different elements, and their radioactivity creates a waste and shielding problem.
Explain how fission becomes a chain reaction
A fission chain reaction occurs when neutrons released by one U-235 fission cause further U-235 nuclei to split.
- A U-235 nucleus absorbs a neutron and undergoes fission. 2. The fission releases two or three neutrons. 3. Some of these neutrons are absorbed by other U-235 nuclei. 4. Those nuclei split and release still more neutrons. 5. The repeating process forms a chain reaction.
If, on average, one neutron from each fission causes one further fission, the reaction continues at a steady rate. If more than one succeeds, the rate grows; if fewer than one succeeds, the chain reaction dies away.
The chain is carried by released neutrons, not by daughter nuclei splitting again. Not every neutron produces another fission: some escape or are absorbed without splitting U-235, which is why neutron control changes the reaction rate.
Control a reactor with a moderator and control rods
A moderator and control rods both act on neutrons, but they have opposite functional effects: the moderator slows neutrons, while control rods absorb excess neutrons.
| Reactor part | Typical material | Action on neutrons | Effect on chain reaction |
|---|---|---|---|
| moderator | graphite or water | reduces neutron speed and kinetic energy | makes neutrons more likely to cause U-235 fission |
| control rods | boron or another strong neutron absorber | removes neutrons from the chain | controls or reduces the fission rate |
Lowering control rods farther into the core absorbs more neutrons and reduces reactor power. Raising them absorbs fewer neutrons and allows the rate to increase. Their position is adjusted so the reactor produces the required steady output.
The moderator does not absorb the excess neutrons as its main role, and control rods do not slow neutrons for fission. Neither part supplies the fuel: U-235 or plutonium in fuel rods provides the fissile nuclei.
Explain why a nuclear reactor needs shielding
Thick shielding around a nuclear reactor absorbs ionising radiation from the core so much less reaches workers and the environment.
Fission products and radioactive daughter nuclei can emit penetrating radiation, including gamma rays and high-energy neutrons. Without shielding, this radiation could escape, ionise body tissue and increase the risk of cell damage, mutation and cancer.
| Shielding material | Useful role |
|---|---|
| thick concrete | surrounds the reactor and absorbs radiation over a large thickness |
| lead or thick steel | dense barrier that reduces penetrating radiation |
| water | provides distance and helps absorb radiation, including neutrons |
Shielding does not control the chain reaction—that is the control rods' role—and it does not make the radiation harmless. It reduces the intensity outside the barrier by absorbing radiation, so sufficient material and complete coverage matter.
Compare nuclear fission and fusion
Fission and fusion both release nuclear energy, but fission splits a heavy nucleus whereas fusion joins light nuclei.
| Feature | Fission | Fusion |
|---|---|---|
| nuclear change | one large nucleus splits | two small nuclei join to form a larger nucleus |
| typical nuclei | U-235 or plutonium fuel | light nuclei such as hydrogen isotopes |
| trigger or condition | neutron absorption can start U-235 fission | very high temperature and pressure are needed |
| products | two radioactive daughter nuclei and a few neutrons | a larger nucleus with energy released |
| natural or engineered context | controlled chain reaction in a fission reactor | energy source in stars; difficult to maintain in a reactor |
Fusion is not the joining of whole atoms, and fission is not ordinary radioactive decay. Radioactive decay is a spontaneous emission from an unstable nucleus; fission is splitting, and fusion is joining.
Explain how nuclear fusion releases energy
Nuclear fusion joins two smaller nuclei to create a larger nucleus and releases energy.
The total mass of the fusion products is slightly less than the total mass of the original nuclei. This loss of mass corresponds to energy released by the reaction; the energy appears as kinetic energy of products and radiation.
A common fusion pattern joins light hydrogen nuclei into a helium nucleus. The product is larger than either starting nucleus, but its mass is less than the combined starting mass, so energy is released.
The missing mass has not disappeared without consequence: it is associated with the released energy. Fusion does not mean that any two nuclei will join on contact; positively charged nuclei must first get extremely close, which requires the conditions taught later.
Connect fusion to the energy of stars
Nuclear fusion in a star's core is the source of the star's energy.
In a main-sequence star such as the Sun, hydrogen nuclei ultimately combine to form helium nuclei. Each set of fusion reactions releases energy because the products have slightly less mass than the starting nuclei.
Energy transferred from the core moves through the star and is eventually emitted into space as electromagnetic radiation and carried by particles. This sustained release makes a star hot and luminous.
Fusion occurs in the core, where temperature and pressure are high enough; it is not ordinary burning and does not use a chemical energy store. A protostar begins its main-sequence stage when sustained hydrogen fusion starts in its core.
Explain why fusion needs high temperature and pressure
Fusion needs extremely high temperature and pressure because positively charged nuclei repel one another electrostatically.
| Condition | Particle-level effect | Why it helps fusion |
|---|---|---|
| high temperature | nuclei have greater kinetic energy and move faster | collisions can bring nuclei close despite electrostatic repulsion |
| high pressure | nuclei are packed closer together | collisions happen more frequently, increasing the chance of fusion |
At low temperature, nuclei move too slowly to overcome the repulsive effect. At low pressure, they are too far apart and collide too rarely. With both conditions high, some collisions bring nuclei close enough for fusion to occur.
High temperature alone does not guarantee fusion if nuclei rarely collide, and high pressure alone does not provide enough collision energy. The repulsion is between positive nuclei, not neutral atoms as a whole, which is why both conditions are difficult to create and maintain.