GCSE · Chapter 12
Nuclear fission and fusion
By the end of this chapter you will be able to
- Describe fission and write a simple fission equation
- Explain how a chain reaction is controlled in a reactor
- Describe fusion in stars, including the conditions needed
- Compare fission and fusion in a table
- Evaluate nuclear power, using carbon dioxide, waste and safety
- [H] Calculate the energy released from a loss of mass
You should already know
- Nuclear equations and decay (Chapter 11)
- Energy resources and the energy mix (Chapter 4)
Where this leads
- Energy resources and the global energy supply (Chapter 4)
- Stars and the life cycle (Chapter 21)
GCSE Physics. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Describe nuclear fission, in which a large nucleus such as uranium-235 absorbs a neutron and splits, Explain a chain reaction, and the roles of the moderator and control rods in a reactor, Describe nuclear power: the energy transfers, the waste and the safety issues, Describe nuclear fusion in stars, and why fusion is not yet a commercial energy source, Compare fission and fusion, and evaluate nuclear power against other resources, [H] Use E = mc squared to relate the energy released to the loss of mass.
Exercise P Do you remember?
1
FWhat is 3 x 3 x 3? Write the number.
27
Method. Multiply three times.
- Multiply. 3 x 3 = 9, then 9 x 3 = 27.27
- Write the answer. 27
Check. ✓
2
FWhich particles are in the nucleus of an atom? Write two words.
protons, neutrons
Method. Recall the nuclear model.
- Recall. The nucleus contains protons and neutrons.protons, neutrons
- Write the answer. protons, neutrons
Check. ✓
3
FWhat type of radiation is a helium nucleus? Write the name.
alpha
Method. Recall the radiation.
- Recall. An alpha particle is a helium nucleus.alpha
- Write the answer. alpha
Check. ✓
4
PSWhat is the mass number of uranium-235 minus 1? Write the number.
234
Method. Subtract 1.
- Subtract. 235 - 1 = 234.235 - 1 = 234
- Write the answer. 234
Check. ✓
5
FWhat is 2 to the power 3? Write the number.
8
Method. Multiply 2 by itself three times.
- Multiply. 2 x 2 x 2 = 8.8
- Write the answer. 8
Check. ✓
12.1
Nuclear fission and the chain reaction
Video lesson: Nuclear fission and the chain reactionComing soon
Context. In a nuclear power station, a heavy nucleus is split and the energy is used to make steam. The process is controlled very carefully. “How does it not run out of control?” asks Tom. The answer is the chain reaction, and how it is slowed or stopped.
Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei. In a reactor, a uranium-235 nucleus absorbs a slow neutron, becomes unstable, and splits into two smaller nuclei. Two or three neutrons are also released, along with a large amount of energy as heat. For example:
23592U + 10n → 14156Ba + 9236Kr + 3 10n
The neutrons released can cause more fissions, which is a chain reaction. The moderator (for example, water or graphite) slows the neutrons so that they are more likely to be absorbed. The control rods (for example, boron or cadmium) absorb neutrons, and can be moved in to slow the reaction or out to speed it up.
Nuclear fission: a neutron is absorbed, and the nucleus splits into two smaller nuclei with neutrons and energy
A chain reaction: each fission gives more neutrons to cause further fissions
A simple reactor: control rods absorb neutrons, and the steam drives a turbine that turns a generator
Key ideas
- Fission: a large nucleus absorbs a neutron and splits into two smaller nuclei.
- Each fission releases neutrons and a large amount of energy.
- Chain reaction: released neutrons cause more fissions.
- Moderator: slows the neutrons. Control rods: absorb neutrons to control the rate.
Worked example 12.1.1
In the fission equation, the uranium-235 nucleus absorbs one neutron. How many nucleons are there after absorption? Write the number.
Method. Add the mass numbers of the uranium and the neutron.
- Add. 235 + 1 = 236.235 + 1 = 236
- Check. This is uranium-236, which is unstable and splits.unstable
- Write the answer. 236
Check. ✓
Worked example 12.1.2
In the example fission equation, the products are barium-141 and krypton-92, plus 3 neutrons. Check the mass numbers balance. Write the total on the right.
Method. Add the mass numbers on the right of the equation.
- Add. 141 + 92 + 3 = 236.141 + 92 + 3 = 236
- Compare. The left side is 235 + 1 = 236, so they balance.236 = 236
- Write the answer. 236
Check. ✓
Worked example 12.1.3
Name the part of the reactor that slows neutrons. Write the name.
Method. Recall the moderator.
- Look. Think about what the example in this section describes.look
- Recall. The moderator slows the neutrons so they are more likely to cause fission.moderator
- Write the answer. moderator
Check. ✓
Watch out
Watch out: the control rods absorb neutrons; the moderator slows them. They do different jobs.
Exam tip
Exam tip. When asked how a chain reaction is controlled, say what the control rods do and whether they are moved in or out.
Exercise 12A Fission and the chain reaction
1
FWhat is the name of the process in which a large nucleus splits into two smaller nuclei? Write the word.
fission
Method. Recall the process.
- Recall. Splitting a large nucleus is fission.fission
- Write the answer. fission
Check. ✓
2
FName the isotope of uranium used as the fuel in the example equation in this section. Write the name.
uranium-235
Method. Recall the fuel.
- Recall. Uranium-235 is the fuel that undergoes fission.uranium-235
- Write the answer. uranium-235
Check. ✓
3
FWhat is released by a fission that can cause another fission? Write the particle.
neutron
Method. Recall the particle.
- Recall. The released neutrons cause further fissions.neutron
- Write the answer. neutron
Check. ✓
4
FWhat is the name of the substance that slows neutrons in a reactor? Write the word.
moderator
Method. Recall the part.
- Recall. The moderator slows the neutrons.moderator
- Write the answer. moderator
Check. ✓
5
FWhat are control rods made of, for example? Write one element name.
boron
Method. Recall the material.
- Recall. Control rods are made of a material such as boron that absorbs neutrons.boron
- Write the answer. boron
Check. Cadmium is also used. ✓
6
FIn a reactor, the control rods are moved in. What happens to the rate of the reaction: increases or decreases? Write the word.
decreases
Method. More absorption of neutrons slows the reaction.
- Recall. Moving control rods in absorbs more neutrons, so the reaction slows.decreases
- Write the answer. decreases
Check. ✓
7
FIn one fission, uranium-235 gives 3 neutrons. How many neutrons does the first generation give from one fission? Write the number.
3
Method. Read the number of neutrons from one fission.
- Recall. One fission gives 3 neutrons.3
- Write the answer. 3
Check. ✓
8
PSIn a chain reaction, each fission gives 3 neutrons and each neutron causes one more fission. How many fissions are there in the third generation, starting from 1 fission? Write the number.
9
Method. Each generation multiplies by 3.
- Gen 1. Generation 1: 1 fission.1
- Gen 2. Generation 2: 3 fissions.3
- Gen 3. Generation 3: 3 x 3 = 9 fissions.3 x 3 = 9
- Write the answer. 9
Check. ✓
9
PSIn a chain reaction, how many fissions happen in the fourth generation if each generation gives 3 times as many? Start from 1 fission. Write the number.
27
Method. Multiply by 3 each generation.
- Gen 4. 1 x 3 x 3 x 3 = 27 fissions.3 x 3 x 3 = 27
- Write the answer. 27
Check. ✓
10
RExplain why the moderator is needed, in one sentence. Accept any sentence that says slow neutrons are more likely to be absorbed by a uranium-235 nucleus and cause fission.
Accept any sentence that says slow neutrons are more likely to be absorbed by a uranium-235 nucleus and cause fission.
Method. Think about the speed of the neutrons.
- Speed. The neutrons from fission are fast.fast
- Link. Slower neutrons are more likely to be absorbed and cause fission.slow
- Write the answer. Accept any sentence that says slow neutrons are more likely to be absorbed by a uranium-235 nucleus and cause fission.
Check. ✓
11
RExplain how the control rods keep a reactor from overheating, in one sentence. Accept any sentence that says they absorb neutrons, so fewer fissions happen.
Accept any sentence that says they absorb neutrons, so fewer fissions happen.
Method. Think about the neutrons.
- Absorb. The control rods absorb neutrons.absorb
- Result. So fewer neutrons cause fission, and the reaction slows.slows
- Write the answer. Accept any sentence that says they absorb neutrons, so fewer fissions happen.
Check. ✓
12
FIn fission, the energy is released mainly as which form? Write the word.
heat
Method. Think about how the energy is used in the reactor.
- Recall. Fission releases energy mainly as heat, which heats water to steam.heat
- Write the answer. heat
Check. ✓
13
PSA chain reaction is described as “out of control” if the number of fissions grows each generation. Which is this: a stable or a growing reaction? Write stable or growing.
growing
Method. Think about what each generation does.
- Check. If each generation gives more fissions than the last, the reaction grows.growing
- Write the answer. growing
Check. ✓
14
PSA reactor is designed so that each fission leads to exactly one further fission on average. Is the reaction stable or growing? Write stable or growing.
stable
Method. One for one means no growth.
- Check. If each fission causes one more on average, the number stays the same.stable
- Write the answer. stable
Check. ✓
12.2
Nuclear power: energy transfers, waste and safety
Video lesson: Nuclear power: energy transfers, waste and safetyComing soon
Context. The town council is considering a new nuclear power station. Some people say it is clean because it gives out little carbon dioxide. Others worry about the waste. The council must weigh the evidence before deciding.
In a nuclear power station the energy transfers are: nuclear (fission) to thermal in the reactor, thermal to kinetic as steam drives the turbine, kinetic to kinetic in the generator, and then kinetic to electrical. Nuclear power produces very little carbon dioxide while it is running. However, it produces radioactive waste, which can stay dangerous for thousands of years and must be stored safely. Reactors are built with many safety systems, and the risk of an accident is low but its effects can be serious.
| Feature | Nuclear power | Fossil fuels (coal or gas) |
|---|---|---|
| carbon dioxide while running | very little | large amounts (coal) or some (gas) |
| waste | radioactive waste (long-lived) | ash and gases, some pollution |
| reliable all the time | yes | yes |
| build cost | very high | lower |
| risk | low chance of accident, serious effects | pollution and climate change |
Nuclear power compared with fossil fuels
Key ideas
- Energy transfers: fission to heat, heat to steam, steam to turbine, turbine to generator, electricity.
- Nuclear power produces little carbon dioxide while running.
- Radioactive waste is long-lived and must be stored safely.
- Nuclear power is reliable: it runs all the time, unlike wind or solar.
Worked example 12.2.1
In a nuclear power station, the reactor heats water to make steam. Name the energy transfer from the fission to the water. Write the word.
Method. Think about how the energy reaches the water.
- Look. The fission releases energy as heat in the reactor, which warms the water.heat
- Name. Energy moving from the reactor to the water by heating is heating.heating
- Write the answer. heating
Check. ✓
Worked example 12.2.2
Give one advantage and one disadvantage of nuclear power, in one sentence each. Accept any advantage, such as low carbon dioxide, and any disadvantage, such as radioactive waste.
Method. Think about the table.
- Advantage. Advantage: it produces very little carbon dioxide while running.advantage
- Disadvantage. Disadvantage: it produces long-lived radioactive waste.disadvantage
- Write the answer. Accept any advantage, such as low carbon dioxide, and any disadvantage, such as radioactive waste.
Check. ✓
Worked example 12.2.3
Name the energy transfer that makes the turbine turn in a power station, from the steam. Write the word.
Method. Think about the motion of the turbine.
- Look. The steam pushes the turbine blades, which then move.moving
- Decide. A moving turbine has energy in its kinetic store, so the transfer is mechanical.mechanical
- Write the answer. mechanical
Check. ✓
Watch out
Watch out: “no carbon dioxide” is too strong a claim for nuclear power. It produces very little carbon dioxide while running, but there are emissions from building and mining.
Exam tip
Exam tip. When evaluating nuclear power, give a benefit and a drawback, then a conclusion that compares it with another resource.
Exercise 12B Nuclear power and the energy transfers
1
FWhat is the main energy transfer in the reactor? Write the word.
heating
Method. Think about how the energy is released.
- Recall. The fission releases energy that heats the water.heating
- Write the answer. heating
Check. ✓
2
FWhat does the steam drive in a nuclear power station? Write the word.
turbine
Method. Recall the device.
- Recall. Steam drives a turbine.turbine
- Write the answer. turbine
Check. ✓
3
FWhat turns the turbine to make electricity? Write the word.
generator
Method. Recall the device.
- Recall. A turbine turns the generator, which makes electricity.generator
- Write the answer. generator
Check. ✓
4
FDoes a nuclear power station give out a lot of carbon dioxide while running? Write yes or no.
no
Method. Compare with fossil fuels.
- Recall. Nuclear power produces very little carbon dioxide while running.no
- Write the answer. no
Check. ✓
5
FWhat is the name of the waste that stays dangerous for thousands of years? Write the phrase.
radioactive waste
Method. Recall the waste.
- Recall. Radioactive waste is long-lived and must be stored safely.radioactive waste
- Write the answer. radioactive waste
Check. ✓
6
RGive one reason nuclear power is considered reliable, in one sentence. Accept any sentence that says it runs all the time, unlike wind or solar.
Accept any sentence that says it runs all the time, unlike wind or solar.
Method. Compare with renewables.
- Reliable. Nuclear power runs all the time, day and night, whatever the weather.all the time
- Write the answer. Accept any sentence that says it runs all the time, unlike wind or solar.
Check. ✓
7
PSWrite the energy transfers in a nuclear power station in order, as three words separated by commas: from the reactor to the electricity.
heating, kinetic, electrical
Method. Follow the energy from the reactor.
- Heat. The fission heats the water to steam (heating).heating
- Turn. The steam turns the turbine (kinetic).kinetic
- Electricity. The generator makes electricity (electrical).electrical
- Write the answer. heating, kinetic, electrical
Check. ✓
8
RExplain why nuclear waste must be stored carefully, in one sentence. Accept any sentence that says it is radioactive and dangerous for a very long time.
Accept any sentence that says it is radioactive and dangerous for a very long time.
Method. Think about the waste.
- Waste. The waste is radioactive.radioactive
- Time. It stays dangerous for a very long time, so it must be stored safely.long time
- Write the answer. Accept any sentence that says it is radioactive and dangerous for a very long time.
Check. ✓
9
RGive one safety feature of a nuclear reactor, in one sentence. Accept any sentence that names a feature such as control rods, containment, or cooling systems.
Accept any sentence that names a feature such as control rods, containment, or cooling systems.
Method. Think about how the reactor is kept safe.
- Feature. Control rods stop the reaction if it becomes too fast.control rods
- Write the answer. Accept any sentence that names a feature such as control rods, containment, or cooling systems.
Check. ✓
10
PSA town needs 1000 MWh a year. A nuclear station gives 90% of the town’s need. How much energy is still needed? Write the number of MWh.
100
Method. Find 10% of 1000.
- Remaining. 100% - 90% = 10% of 1000 = 100 MWh.0.1 x 1000 = 100
- Write the answer. 100 MWh
Check. ✓
11
REvaluate: “nuclear power is the best solution for a country because it is clean.” Give one reason for and one against. Accept any answer that says it has low carbon dioxide, but radioactive waste and high build cost count against it.
Accept any answer that says it has low carbon dioxide, but radioactive waste and high build cost count against it.
Method. Weigh the benefits and drawbacks.
- For. It has low carbon dioxide emissions while running.for
- Against. It produces waste, and building it costs a lot.against
- Write the answer. Accept any answer that says it has low carbon dioxide, but radioactive waste and high build cost count against it.
Check. ✓
12.3
Nuclear fusion in stars
Video lesson: Nuclear fusion in starsComing soon
Context. The Sun has been shining for billions of years. Scientists ask: what is the Sun made of, and where does its energy come from? The answer is nuclear fusion in the core.
Nuclear fusion is the joining of two light nuclei to make a heavier nucleus, releasing energy. In stars, hydrogen nuclei are pulled together by gravity. The temperature in the core is very high, about 15 million °C in the Sun, which gives the nuclei enough energy to overcome their repulsion. Hydrogen nuclei fuse to form helium, and energy is released.
Fusion has not yet been used as a commercial power source on Earth, because it needs very high temperatures and pressures, and the hot plasma must be contained without touching the walls of the reactor. Experiments are still going on.
Fusion: two light nuclei join to form a heavier nucleus, releasing energy
| Stage | What happens |
|---|---|
| start | hydrogen nuclei are pulled together by gravity |
| fusion | at very high temperature, hydrogen nuclei join to make helium, releasing energy |
| balance | the outward push of the energy balances gravity, so the star is stable |
The stages of fusion in a star
Key ideas
- Fusion: two light nuclei join to make a heavier one, releasing energy.
- Stars: hydrogen nuclei fuse into helium at very high temperature.
- Fusion needs very high temperature and pressure.
- Fusion is not yet a commercial energy source on Earth.
Worked example 12.3.1
In a star, what is the fuel for fusion? Write the name of the element.
Method. Recall the fuel.
- Look. Think about what the example in this section describes.look
- Recall. Stars fuse hydrogen nuclei into helium.hydrogen
- Write the answer. hydrogen
Check. ✓
Worked example 12.3.2
What is the main product of fusion in the Sun? Write the name of the element.
Method. Recall the product.
- Look. Think about what the example in this section describes.look
- Recall. Hydrogen nuclei join to make helium.helium
- Write the answer. helium
Check. ✓
Worked example 12.3.3
Why does fusion need very high temperature? Write one reason in one sentence. Accept any sentence that says the nuclei need enough energy to overcome their repulsion, so they can get close enough to join.
Method. Think about the nuclei.
- Repel. Positive nuclei repel each other.repel
- Overcome. Very high temperature gives them enough speed to get close and join.high speed
- Write the answer. Accept any sentence that says the nuclei need enough energy to overcome their repulsion, so they can get close enough to join.
Check. ✓
Watch out
Watch out: fusion joins nuclei; fission splits them. Do not mix these two words.
Exam tip
Exam tip. When describing fusion in stars, say what the fuel is, what the product is, and that the temperature and pressure must be very high.
Exercise 12C Fusion and stars
1
FWhat is nuclear fusion? Write the word: joining or splitting.
joining
Method. Recall the definition.
- Recall. Fusion joins two light nuclei together.joining
- Write the answer. joining
Check. ✓
2
FWhat is the fuel used in the fusion in stars? Write the name of the element.
hydrogen
Method. Recall the fuel.
- Recall. Stars fuse hydrogen nuclei.hydrogen
- Write the answer. hydrogen
Check. ✓
3
FWhat is the product of fusion in stars? Write the name of the element.
helium
Method. Recall the product.
- Recall. Hydrogen fuses to make helium.helium
- Write the answer. helium
Check. ✓
4
FWhat force pulls the hydrogen nuclei together in a star? Write the word.
gravity
Method. Recall the force.
- Recall. Gravity pulls the gas together in a star.gravity
- Write the answer. gravity
Check. ✓
5
FIs fusion used as a commercial power source on Earth today? Write yes or no.
no
Method. Recall the current state.
- Recall. Fusion is not yet a commercial power source.no
- Write the answer. no
Check. ✓
6
FWhat is the name of the process that produces the Sun’s energy? Write the word.
fusion
Method. Recall the process.
- Recall. The Sun’s energy comes from nuclear fusion.fusion
- Write the answer. fusion
Check. ✓
7
FIn a star, fusion occurs in which part? Write the word: core or surface.
core
Method. Think about where the temperature is highest.
- Recall. The core is the hottest, densest part where fusion happens.core
- Write the answer. core
Check. ✓
8
RWhy is it difficult to build a fusion power station, in one sentence? Accept any sentence that says the temperature and pressure must be very high, and the hot plasma must be contained.
Accept any sentence that says the temperature and pressure must be very high, and the hot plasma must be contained.
Method. Think about the conditions.
- Conditions. Fusion needs very high temperature and pressure.high temperature
- Containment. The hot plasma must be kept away from the walls of the reactor.contained
- Write the answer. Accept any sentence that says the temperature and pressure must be very high, and the hot plasma must be contained.
Check. ✓
9
RExplain in one sentence why the Sun can shine for billions of years. Accept any sentence that says fusion of hydrogen in the core releases energy steadily over a very long time.
Accept any sentence that says fusion of hydrogen in the core releases energy steadily over a very long time.
Method. Think about the fuel and the process.
- Fuel. The Sun has a huge amount of hydrogen in its core.hydrogen
- Process. Fusion releases energy steadily, so the Sun shines for billions of years.steady
- Write the answer. Accept any sentence that says fusion of hydrogen in the core releases energy steadily over a very long time.
Check. ✓
10
PSWhich element is the product of fusion in stars that is heavier than hydrogen? Write the name.
helium
Method. Recall the product.
- Recall. Helium is heavier than hydrogen and is formed by fusion.helium
- Write the answer. helium
Check. ✓
12.4
Comparing fission and fusion
Video lesson: Comparing fission and fusionComing soon
Context. A pupil says “fusion is just the opposite of fission.” The teacher replies, “They both release energy, but in different ways.” The table shows the key differences.
Both processes release energy by changing the nucleus. Fission splits a large nucleus; fusion joins small nuclei. Fission is used in power stations today. Fusion is what powers stars, and it is an area of research for future power stations, because it produces less long-lived waste.
| Feature | Nuclear fission | Nuclear fusion |
|---|---|---|
| what happens | a large nucleus splits into two smaller nuclei | two small nuclei join to make a larger nucleus |
| fuel | uranium-235 or plutonium | hydrogen isotopes (deuterium and tritium) |
| conditions | a neutron is absorbed; a moderator slows neutrons | very high temperature and pressure |
| radioactive waste | yes, long-lived waste | much less (the reaction products are mostly helium) |
| used in power stations today | yes | not yet, as a commercial supply |
| where it happens | nuclear power stations | the centre of stars, including the Sun |
Fission and fusion compared
Key ideas
- Fission splits a large nucleus; fusion joins small nuclei.
- Fission is used in power stations now; fusion is not yet a commercial power source.
- Fusion produces much less long-lived radioactive waste than fission.
- Both release energy from changes in the nucleus.
Worked example 12.4.1
Which process is used in nuclear power stations today: fission or fusion? Write the word.
Method. Recall which process is used.
- Look. Think about what the example in this section describes.look
- Recall. Nuclear power stations use fission.fission
- Write the answer. fission
Check. ✓
Worked example 12.4.2
Which process releases energy by joining small nuclei? Write the word.
Method. Recall the process.
- Look. Think about what the example in this section describes.look
- Recall. Fusion joins small nuclei and releases energy.fusion
- Write the answer. fusion
Check. ✓
Worked example 12.4.3
Which process releases energy by splitting heavy nuclei: fission or fusion? Write the word.
Method. Recall the definition of each process.
- Look. Splitting a large nucleus is one process, and joining small nuclei is the other.split or join
- Decide. Splitting a heavy nucleus is fission.fission
- Write the answer. fission
Check. ✓
Watch out
Watch out: fusion does produce some radiation, but much less long-lived waste than fission. Do not say fusion produces no radiation at all.
Exam tip
Exam tip. In a comparison, use the same headings for each process, so that the examiner can see both are covered.
Exercise 12D Comparing and evaluating
1
FWhich process splits a large nucleus? Write the word.
fission
Method. Recall the definition.
- Recall. Fission splits a large nucleus.fission
- Write the answer. fission
Check. ✓
2
FWhich process joins small nuclei? Write the word.
fusion
Method. Recall the definition.
- Recall. Fusion joins small nuclei.fusion
- Write the answer. fusion
Check. ✓
3
FWhich process is used in the Sun? Write the word.
fusion
Method. Recall the process in stars.
- Recall. The Sun uses fusion of hydrogen.fusion
- Write the answer. fusion
Check. ✓
4
FWhich process needs very high temperature and pressure to work in a star? Write the word.
fusion
Method. Recall the conditions.
- Recall. Fusion needs very high temperature and pressure.fusion
- Write the answer. fusion
Check. ✓
5
FWhich process produces more long-lived radioactive waste? Write the word.
fission
Method. Compare the waste.
- Recall. Fission produces long-lived radioactive waste.fission
- Write the answer. fission
Check. ✓
6
RExplain in one sentence why fusion is not yet a commercial energy source. Accept any sentence that says it is very difficult to reach and contain the conditions needed for fusion.
Accept any sentence that says it is very difficult to reach and contain the conditions needed for fusion.
Method. Think about the difficulty.
- Difficulty. Fusion needs extreme conditions that are hard to reach and control.difficult
- Write the answer. Accept any sentence that says it is very difficult to reach and contain the conditions needed for fusion.
Check. ✓
7
RA pupil says “fission and fusion both give out energy from changes in the nucleus.” Is the pupil right? Write yes or no.
yes
Method. Both change the nucleus and release energy.
- Check. Both fission and fusion release energy from changes in the nucleus.yes
- Write the answer. yes
Check. ✓
8
REvaluate: “fusion is better than fission because it produces no radioactive waste.” Explain why this is too strong. Accept any answer that says fusion produces much less long-lived waste, but it is not zero and it is not yet a commercial source.
Accept any answer that says fusion produces much less long-lived waste, but it is not zero and it is not yet a commercial source.
Method. Check the claim against the facts.
- Waste. Fusion produces much less long-lived waste, but it is not zero.much less
- Commercial. Fusion is not yet a commercial power source, so the claim is too strong.not yet
- Write the answer. Accept any answer that says fusion produces much less long-lived waste, but it is not zero and it is not yet a commercial source.
Check. ✓
9
RGive one difference between fission and fusion in the nuclei involved, in one sentence. Accept any sentence that says fission splits a large nucleus and fusion joins small nuclei.
Accept any sentence that says fission splits a large nucleus and fusion joins small nuclei.
Method. Compare the nuclei.
- Fission. Fission splits a large nucleus.large nucleus
- Fusion. Fusion joins small nuclei.small nuclei
- Write the answer. Accept any sentence that says fission splits a large nucleus and fusion joins small nuclei.
Check. ✓
10
FWhich process needs a neutron to start the reaction in a reactor? Write the word: fission or fusion.
fission
Method. Recall how the reactor works.
- Recall. In a reactor, uranium-235 absorbs a neutron and splits (fission).fission
- Write the answer. fission
Check. ✓
12.5
[H] Mass, energy and E = mc squared
Video lesson: [H] Mass, energy and E = mc squaredComing soon
[H] Higher tier only. In fission and fusion, the total mass of the products is slightly less than the mass of the original particles. The missing mass is converted into energy. The energy released is found from E = m c2, where m is the mass lost in kg and c is the speed of light, 3.0 x 108 m/s.
Key ideas
- [H] E = m c squared, with m in kg, c = 3.0 x 10^8 m/s, and E in joules.
- [H] The mass lost in a nuclear reaction is converted into energy.
- [H] Even a tiny mass loss releases a very large amount of energy.
- [H] Squaring the speed of light makes the energy very large.
Worked example 12.5.1
[H] A mass of 0.001 kg is lost in a reaction. Calculate the energy released. Write the number of joules.
Method. Use E = m c squared with c = 3.0 x 108 m/s.
- Square. c squared = (3.0 x 108)2 = 9.0 x 1016.9.0 x 10^16
- Multiply. E = 0.001 x 9.0 x 1016 = 9.0 x 1013 J.9.0 x 10^13
- Write the answer. 9.0 x 10^13 J
Check. ✓
Worked example 12.5.2
[H] A reaction loses 2 x 10-6 kg of mass. Calculate the energy released, in joules, to 2 significant figures. Write the answer in standard form.
Method. Use E = m c squared.
- Square. c squared = 9.0 x 1016.9.0 x 10^16
- Multiply. E = 2 x 10-6 x 9.0 x 1016 = 1.8 x 1011 J.1.8 x 10^11
- Write the answer. 1.8 x 10^11 J
Check. ✓
Worked example 12.5.3
[H] A mass of 0.0001 kg is lost in a reaction. Calculate the energy released. Write the answer in standard form.
Method. Use E = m c squared.
- Multiply. E = 0.0001 x 9.0 x 1016 = 9.0 x 1012 J.9.0 x 10^12
- Check. This is 1 x 10-4 kg, which is one tenth of a gram.1 x 10^-4 kg
- Write the answer. 9.0 x 10^12 J
Check. ✓
Watch out
Watch out: [H] the mass must be in kg, and the speed of light is squared. Forgetting to square c makes the answer far too small.
Exam tip
Exam tip. [H] Write E = m c squared first, then substitute the powers of ten carefully.
Exercise 12E [H] Mass-energy calculations
1
F[H] What is the speed of light used in E = m c squared? Write the value in m/s, as 3.0 x 10^8.
3.0 x 10^8
Method. Recall the value of c.
- Recall. The speed of light in a vacuum is about 3.0 x 108 m/s.3.0 x 10^8
- Write the answer. 3.0 x 10^8 m/s
Check. ✓
2
F[H] Calculate c squared when c = 3.0 x 108 m/s. Write the answer in standard form.
9.0 x 10^16
Method. Square both parts.
- Square. (3.0)2 = 9.0 and (108)2 = 1016.9.0 x 10^16
- Write the answer. 9.0 x 10^16
Check. ✓
3
PS[H] Calculate the energy released when 1 kg of mass is lost. Write the answer in standard form.
9.0 x 10^16
Method. E = m c squared with m = 1 kg.
- Multiply. E = 1 x 9.0 x 1016 = 9.0 x 1016 J.9.0 x 10^16
- Write the answer. 9.0 x 10^16 J
Check. ✓
4
PS[H] A mass of 0.5 g is lost. Calculate the energy in joules. Write the answer in standard form. (0.5 g = 0.0005 kg.)
4.5 x 10^13
Method. Convert grams to kilograms, then use E = m c squared.
- Convert. 0.5 g = 0.0005 kg = 5 x 10-4 kg.5 x 10^-4
- Multiply. E = 5 x 10-4 x 9.0 x 1016 = 4.5 x 1013 J.4.5 x 10^13
- Write the answer. 4.5 x 10^13 J
Check. ✓
5
R[H] Explain why a tiny loss of mass gives a very large amount of energy, in one sentence. Accept any sentence that says c squared is a very large number, so even a small mass gives a big energy.
Accept any sentence that says c squared is a very large number, so even a small mass gives a big energy.
Method. Look at the equation.
- Factor. c squared is about 9 x 1016, a very large number.9 x 10^16
- Link. So even a small mass gives a large energy.large energy
- Write the answer. Accept any sentence that says c squared is a very large number, so even a small mass gives a big energy.
Check. ✓
6
PS[H] Calculate the energy released when a mass of 0.01 kg is lost. Write the answer in standard form.
9.0 x 10^14
Method. Use E = m c squared.
- Multiply. E = 0.01 x 9.0 x 1016 = 9.0 x 1014 J.9.0 x 10^14
- Write the answer. 9.0 x 10^14 J
Check. ✓
7
F[H] Which quantity in E = m c squared is squared? Write the symbol.
c
Method. Look at the equation.
- Recall. The speed of light, c, is squared in the equation.c
- Write the answer. c
Check. ✓
8
PS[H] A mass of 2 kg is converted completely into energy. Calculate E. Write the answer in standard form.
1.8 x 10^17
Method. Use E = m c squared with m = 2 kg.
- Multiply. E = 2 x 9.0 x 1016 = 1.8 x 1017 J.1.8 x 10^17
- Write the answer. 1.8 x 10^17 J
Check. ✓
9
F[H] With m in kg and c in m/s, what is the unit of energy from E = m c squared? Write the unit.
J
Method. kg x (m/s) squared gives kg m squared per second squared.
- Recall. kg m2/s2 is the joule.J
- Write the answer. J
Check. ✓
10
PS[H] A reaction loses 3 x 10-3 kg of mass. Calculate the energy released. Write the answer in standard form.
2.7 x 10^14
Method. Use E = m c squared.
- Multiply. E = 3 x 10-3 x 9.0 x 1016 = 2.7 x 1014 J.2.7 x 10^14
- Write the answer. 2.7 x 10^14 J
Check. ✓
Chapter summary
| Skill | Method |
|---|---|
| Fission | Large nucleus absorbs a neutron and splits; releases neutrons and energy as heat |
| Control | Moderator slows neutrons; control rods absorb neutrons |
| Nuclear power | Little CO2 while running; radioactive waste; reliable; high build cost |
| Fusion | Light nuclei join at very high temperature; powers stars; not yet commercial |
| [H] Mass and energy | E = m c squared; c = 3.0 x 10^8 m/s |
Can you…?
- I can describe fission and a chain reaction
- I can explain the roles of the moderator and control rods
- I can describe fusion in stars and its conditions
- I can compare fission and fusion
- I can evaluate nuclear power
- [H] I can calculate energy from a mass loss using E = m c squared
Key terms
- fission
- the splitting of a large nucleus into two smaller nuclei, releasing energy
- fusion
- the joining of two light nuclei to make a heavier nucleus, releasing energy
- chain reaction
- a reaction in which the products of one fission cause further fissions
- moderator
- a material that slows neutrons in a reactor
- control rods
- rods that absorb neutrons, to control the rate of a reaction
- [H] mass defect
- the loss of mass in a reaction, which becomes energy
Exercise 12Z Check-up
1
What is the name of the process in which a large nucleus splits? Write the word.
fission
Method. Recall.
- Recall. Splitting a large nucleus is fission.fission
- Write the answer. fission
Check. ✓
2
What does the moderator do in a reactor? Write the word: slows or absorbs.
slows
Method. Recall.
- Recall. The moderator slows neutrons.slows
- Write the answer. slows
Check. ✓
3
What do the control rods do in a reactor? Write the word: slows or absorbs.
absorbs
Method. Recall.
- Recall. The control rods absorb neutrons.absorbs
- Write the answer. absorbs
Check. ✓
4
Which element is formed by fusion in the Sun? Write the name.
helium
Method. Recall.
- Recall. Hydrogen fuses to make helium.helium
- Write the answer. helium
Check. ✓
5
Name the force that holds stars together, which pulls hydrogen into the core. Write the word.
gravity
Method. Recall.
- Recall. Gravity pulls the gas together.gravity
- Write the answer. gravity
Check. ✓
6
Name the type of energy transfer that makes electricity from the turbine. Write the word.
kinetic
Method. The turbine is turning.
- Recall. The turbine has kinetic energy, which the generator turns into electricity.kinetic
- Write the answer. kinetic
Check. ✓
7
A chain reaction has 1 fission, then 3 fissions in the next generation. What is the number in the generation after that, if each gives 3? Write the number.
9
Method. Multiply by 3.
- Multiply. 3 x 3 = 9.9
- Write the answer. 9
Check. ✓
8
Is nuclear power reliable all the time? Write yes or no.
yes
Method. Recall.
- Recall. Nuclear power is reliable and runs all the time.yes
- Write the answer. yes
Check. ✓
9
Which process releases energy by splitting a nucleus, used in power stations today? Write the word.
fission
Method. Recall.
- Recall. Power stations use fission.fission
- Write the answer. fission
Check. ✓
10
Explain in one sentence why nuclear power is not carbon-free overall. Accept any sentence that mentions the carbon dioxide from building or mining, or any sentence that says it is low in carbon dioxide only while running.
Accept any sentence that mentions the carbon dioxide from building or mining, or any sentence that says it is low in carbon dioxide only while running.
Method. Think about the whole life of the plant.
- Whole life. Building and mining the fuel release carbon dioxide.building and mining
- Running. While running it is low, but not zero.low
- Write the answer. Accept any sentence that mentions the carbon dioxide from building or mining, or any sentence that says it is low in carbon dioxide only while running.
Check. ✓
Total 29 marks. Allow about 29 minutes.
Exercise 12X Exam-style questions
1
FName the process in which a large uranium-235 nucleus absorbs a neutron and splits into two smaller nuclei. Write the word.[1 mark]
fission
Method. Recall the process.
- Recall. Fission is the splitting of a large nucleus.fission
- Write the answer. fission
Check. ✓
2
RExplain the role of the control rods in a nuclear reactor, in one sentence. Accept any sentence that says they absorb neutrons, to slow or control the chain reaction.[2 marks]
Accept any sentence that says they absorb neutrons, to slow or control the chain reaction.
Method. Think about the neutrons.
- Absorb. The control rods absorb neutrons.absorb
- Control. This slows or controls the chain reaction.control
- Write the answer. Accept any sentence that says they absorb neutrons, to slow or control the chain reaction.
Check. ✓
3
RDescribe the process of fusion in a star, using the words hydrogen, helium, temperature and energy. Accept any answer that says hydrogen nuclei join at very high temperature to make helium, releasing energy.[3 marks]
Accept any answer that says hydrogen nuclei join at very high temperature to make helium, releasing energy.
Method. Describe the process.
- Fuel. Hydrogen nuclei are the fuel.hydrogen
- Conditions. They join at very high temperature.temperature
- Products. They make helium and release energy.helium, energy
- Write the answer. Accept any answer that says hydrogen nuclei join at very high temperature to make helium, releasing energy.
Check. ✓
4
REvaluate nuclear power as a source of electricity for a country. Give one benefit and one drawback, and a conclusion. Accept any answer that gives low carbon dioxide while running as a benefit, radioactive waste as a drawback, and a conclusion that compares it with other resources.[4 marks]
Accept any answer that gives low carbon dioxide while running as a benefit, radioactive waste as a drawback, and a conclusion that compares it with other resources.
Method. Weigh the points and conclude.
- Benefit. Benefit: it produces very little carbon dioxide while running and is reliable.benefit
- Drawback. Drawback: radioactive waste is long-lived and must be stored safely.drawback
- Conclusion. So a country might use nuclear with renewables to balance carbon and reliability.conclusion
- Write the answer. Accept any answer that gives low carbon dioxide while running as a benefit, radioactive waste as a drawback, and a conclusion that compares it with other resources.
Check. ✓
5
RCompare fission and fusion, in terms of the nuclei involved and the conditions needed. Accept any answer that says fission splits a large nucleus, usually with a neutron to start, and fusion joins light nuclei at very high temperature and pressure.[4 marks]
Accept any answer that says fission splits a large nucleus, usually with a neutron to start, and fusion joins light nuclei at very high temperature and pressure.
Method. Compare each feature.
- Nuclei. Fission splits a large nucleus; fusion joins small nuclei.split and join
- Start. Fission often starts with a neutron absorbed by uranium-235.neutron
- Conditions. Fusion needs very high temperature and pressure.temperature and pressure
- Product. Both release energy from the nucleus.energy
- Write the answer. Accept any answer that says fission splits a large nucleus, usually with a neutron to start, and fusion joins light nuclei at very high temperature and pressure.
Check. ✓
6
PSA chain reaction starts with 1 fission. Each fission gives 3 neutrons, and each neutron causes one more fission in the next generation. How many fissions are there in generation 4? Write the number.[2 marks]
27
Method. Multiply by 3 each generation.
- Multiply. Generation 1: 1; 2: 3; 3: 9; 4: 27.27
- Write the answer. 27
Check. ✓
7
PS[H] A reaction loses 0.002 kg of mass. Calculate the energy released using E = m c squared. Write the answer in standard form.[3 marks]
1.8 x 10^14
Method. Use E = m c squared with m = 0.002 kg.
- Square. c squared = 9.0 x 1016.9.0 x 10^16
- Multiply. E = 0.002 x 9.0 x 1016 = 1.8 x 1014 J.1.8 x 10^14
- Write the answer. 1.8 x 10^14 J
Check. ✓
8
RExplain why a fusion power station is harder to build than a fission power station, in two sentences. Accept any answer that says the conditions of very high temperature and pressure are very difficult to reach and control, and the plasma must be contained.[3 marks]
Accept any answer that says the conditions of very high temperature and pressure are very difficult to reach and control, and the plasma must be contained.
Method. Explain the two difficulties.
- Conditions. Fusion needs very high temperature and pressure, which are hard to reach.hard to reach
- Containment. The hot plasma must be held away from the walls, which is hard to do.contained
- Write the answer. Accept any answer that says the conditions of very high temperature and pressure are very difficult to reach and control, and the plasma must be contained.
Check. ✓
9
RDescribe the energy transfers in a nuclear power station, from the fission to the electricity. Accept any answer that gives heating (fission to water), kinetic (steam to turbine), and electrical (generator to electricity).[4 marks]
Accept any answer that gives heating (fission to water), kinetic (steam to turbine), and electrical (generator to electricity).
Method. Follow the energy through the station.
- Reactor. Fission releases energy that heats the water, which is a heating transfer.heating
- Turbine. Steam drives the turbine, which is a kinetic store.kinetic
- Generator. The generator turns the turbine motion into electricity.generator
- Electricity. The electricity is an electrical transfer.electrical
- Write the answer. Accept any answer that gives heating (fission to water), kinetic (steam to turbine), and electrical (generator to electricity).
Check. ✓
10
R[H] Explain why the mass of the products of a fission reaction is slightly less than the mass of the original particles, and how this links to the energy released. Accept any answer that says mass is converted to energy, according to E = m c squared.[3 marks]
Accept any answer that says mass is converted to energy, according to E = m c squared.
Method. Link the mass loss to the energy.
- Mass. The products have slightly less mass than the original particles.less mass
- Energy. The missing mass is converted to energy.converted
- Equation. The energy is E = m c squared, so a small mass gives a large energy.E = m c squared
- Write the answer. Accept any answer that says mass is converted to energy, according to E = m c squared.
Check. ✓