Nuclear fission and fusion

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Apuntes de la lección

Nuclear Fission

Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei.
Isotopes of uranium and plutonium undergo fission and are used as fuels in nuclear power stations.
During fission, a neutron collides with an unstable nucleus, which splits into two smaller daughter nuclei and releases two or three neutrons.
Gamma rays are also emitted during fission.
The products of fission move away very quickly, so energy is transferred from the nuclear potential energy store to the kinetic energy store of the products.

Nuclear fission

Spontaneous and Induced Fission

Spontaneous fission is when a nucleus undergoes fission without additional energy being put in; this is rare.
Usually, for fission to occur, the unstable nucleus must first absorb a neutron — this is induced fission.
Uranium-235 has a very long half-life of 700 million years, so it would release energy very slowly on its own — unsuitable for a power station.
In induced fission, a neutron is absorbed by uranium-235 to make uranium-236, which is very unstable and splits almost immediately.

Chain Reactions

Only one extra neutron is needed to induce a uranium-235 nucleus to split by fission.
Each fission produces two or three neutrons which move away at high speed.
Each of these new neutrons can start another fission reaction, which again creates further excess neutrons — this is a chain reaction.
In a controlled chain reaction, the number of free neutrons is kept constant so the reactor produces energy at the correct rate.
In an uncontrolled chain reaction, the number of neutrons increases quickly, so the number of reactions does too — this releases a huge amount of energy in a short time, as in a nuclear weapon.

Nuclear Fission Diagrams

Fission diagrams show a neutron fired into a target nucleus, causing it to split.
To show a chain reaction, draw the released neutrons hitting other uranium-235 nuclei.
The daughter nuclei do not need to be shown in a chain reaction diagram — only the neutrons and uranium-235 nuclei.
Each fission reaction requires one neutron but releases two or more, so the number of neutrons increases with each reaction.
In these diagrams, time generally moves to the right; clarity of information is more important than artistic quality.

Nuclear fission of uranium-235

Nuclear Reactors

In a nuclear reactor, a chain reaction is maintained and controlled to produce a supply of energy to generate electricity.
Two factors must be controlled: the number of free neutrons and the energy of the free neutrons.
Control rods absorb neutrons without becoming dangerously unstable themselves.
The number of neutrons absorbed is controlled by varying the depth of the control rods in the fuel rods: lowering them decreases the rate of fission, raising them increases it.
In an emergency, control rods can be lowered all the way so no reaction can take place.
The moderator slows down neutrons so they can react efficiently with the uranium fuel.

Energy & Waste

Nuclear fission produces a large quantity of heat which is carried away from the reactor by a coolant (usually pressurised water).
The coolant heats a separate water source, turning it into steam; a separate source reduces the risk of contamination.
The steam drives turbines which turn generators, producing electricity.
The reactor provides thermal energy from fission chain reactions; the boiler uses thermal energy to boil water to create steam; the turbine transfers thermal energy into kinetic energy; the generator transfers kinetic energy into electrical energy; the condenser cools the steam back into water.
The biggest problem with nuclear power is the waste — the unusable fission products from uranium-235 or spent fuel rods.
The daughter nuclei are highly radioactive — more radioactive than the original fuel — and remain radioactive for thousands of years.
Spent fuel rods are also extremely hot and must be handled and stored carefully.
Nuclear waste is stored underground until it is no longer harmful.

Advantages & Disadvantages of Nuclear Power

Nuclear power can scare people if they do not understand it.
It is dangerous if not handled properly, and it is invisible, which can be difficult for some people to comprehend.
With increased education on nuclear energy, society can use this knowledge to inform their own decisions and opinions.

Nuclear Fusion

Nuclear fusion is when two light nuclei join to form a heavier nucleus.
This process requires extremely high temperatures to maintain, which is why it has proven very hard to reproduce on Earth.
Nuclear fusion takes place in the centres of stars, including the Sun.
In most stars, hydrogen nuclei fuse into helium nuclei and release a large amount of energy.
The energy produced during fusion comes from a very small amount of the particle's mass being converted into energy.
Albert Einstein described the mass-energy equivalence with the equation E = mc², where E is energy in joules, m is mass converted into energy in kilograms, and c is the speed of light in metres per second.
The energy from 1 kg of hydrogen that undergoes fusion is equivalent to the energy from burning about 10 million kilograms of coal.

Nuclear fusion of hydrogen

Fusion vs Fission

Fission is the process in which large nuclei are split into two smaller nuclei, releasing energy; this is used in nuclear power stations, where it is well controlled.
Fusion is the opposite: taking smaller nuclei and bringing them together to form a bigger nucleus.
Fission provides less energy per kg of fuel than fusion.
The products of fission are radioactive and hence very dangerous.
Fusion provides more energy per kg of fuel than fission.
The products of fusion are not radioactive and are therefore much safer than the products of fission reactions.

The Conditions for Fusion

Since protons have a positive charge, they repel each other.
To overcome this repulsion, the protons must have very high kinetic energy, travelling towards each other at very high speeds.
The fusion of two protons is only possible through high temperature and pressure.
The gas has to be heated to millions of degrees Celsius — a temperature usually only reached at the centre of a star.
In regular conditions on Earth, the possibility of collisions between nuclei which result in fusion is very low.
High densities (and hence pressures) are needed to increase the number of collisions and hence fusions.
The conditions for fusion are: very high temperature of fuel; very high kinetic energy/speed of nuclei to overcome repulsion; very high density/pressure to increase the possibility of suitable collisions.
The main reasons fusion is not currently used as a source of power on Earth are the difficulties in achieving and maintaining high temperatures and high pressures.
Creating the temperatures needed for fusion requires a great deal of energy, so physicists are still a long way from producing more energy from fusion than the energy needed to start it.

Hydrogen fusion in a star

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Preguntas de práctica

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1.Nuclear fission is defined as:
Easy
- Athe splitting of a large, unstable nucleus into two smaller nuclei
- Bthe joining of two light nuclei to form a heavier nucleus
- Cthe emission of an electron from the nucleus
- Dthe cooling of a star's core
2.The element which undergoes fission in a nuclear reactor is:
Easy
- Ahydrogen-1
- Bhelium-4
- Curanium-235
- Dradium-226
3.The particle released during fission which can initiate a chain reaction is:
Easy
- Aa nucleon
- Ba neutron
- Can α particle
- Da β particle
4.Nuclear fusion is defined as:
Easy
- Athe creation of a larger nucleus from smaller nuclei
- Bthe emission of an electron from the nucleus
- Cthe cooling of the Sun
- Da natural process which occurs in the core of the Earth
5.An element that might undergo fusion is:
Easy
- Apolonium-210
- Bamericium-241
- Curanium-235
- Dhydrogen-1
6.Which of the following are the correct conditions for nuclear fusion to occur?
Easy
- Alow temperature, low pressure
- Blow temperature, high pressure
- Chigh temperature, low pressure
- Dhigh temperature, high pressure
7.How many daughter nuclei are produced from the fission of one nucleus of uranium-235?
Easy
- Atwo
- Bthree
- Cfour
- Dsix
8.In a nuclear reactor, the moderator is used to:
Medium
- Aabsorb neutrons so the chain reaction slows down
- Bslow down neutrons so they can react efficiently with the fuel
- Cspeed up neutrons so they can escape the core
- Dcool the steam back into water after the turbine

Más preguntas

9.In a nuclear reactor, the control rods are used to:
Medium
- Aslow down neutrons by collision
- Babsorb neutrons and so control the rate of fission
- Cincrease the energy of the free neutrons
- Dturn the turbine to generate electricity
10.Which statement about the energy released per kilogram of fuel is correct?
Medium
- AFission provides more energy per kg of fuel than fusion
- BFusion provides more energy per kg of fuel than fission
- CFission and fusion release the same energy per kg of fuel
- DNeither fission nor fusion releases energy per kg of fuel
11.Which statement about the products of fission and fusion is correct?
Medium
- AThe products of fission are radioactive, but the products of fusion are not radioactive
- BThe products of fusion are radioactive, but the products of fission are not radioactive
- CBoth fission and fusion produce radioactive products
- DNeither fission nor fusion produces radioactive products
12.During induced fission, a neutron is absorbed by a uranium-235 nucleus to make uranium-236. What happens next?
Medium
- AThe uranium-236 nucleus is very stable and does not change
- BThe uranium-236 nucleus is very unstable and splits by fission almost immediately
- CThe uranium-236 nucleus emits a proton and becomes uranium-235 again
- DThe uranium-236 nucleus fuses with another nucleus to form a heavier element
13.Which of these is a valid reason why hydrogen fusion is not currently possible on Earth?
Medium
- AHydrogen fusion produces dangerous radioactive waste
- BHydrogen nuclei require very high temperature to fuse together
- CHydrogen is a rare element that would be difficult to get large amounts of
- DHydrogen fusion does not produce enough energy to be commercially viable
14.Nuclear fusion does not happen at low temperatures because of electrostatic repulsion between:
Medium
- Aβ particles
- Belectrons
- Cneutrons
- Dprotons
15.Which of the following statements about fission and fusion are correct? (select all that apply)
Medium
- AFission is the splitting of a large nucleus into daughter nuclei
- BElectrons are released alongside the daughter nuclei in fission
- CFusion is the joining of two smaller nuclei into a heavier nucleus
- DFission releases energy, but fusion does not
- EFusion provides more energy per kg of fuel than fission
16.Which of the following are produced when a uranium-235 nucleus undergoes fission? (select all that apply)
Medium
- Atwo smaller daughter nuclei
- Btwo or three neutrons
- Cgamma rays
- Delectrons
- Eα particles
17.Nuclear fusion is currently used in power stations to generate electricity.
Easy
True or false?
18.It is rare for nuclei to undergo fission without additional energy being put into the nucleus.
Easy
True or false?
19.Control rods slow down neutrons, while the moderator absorbs neutrons.
Easy
True or false?
20.The products of fission move away very quickly, and energy is transferred from the nuclear store to the kinetic store of the products.
Easy
True or false?
21.An uncontrolled chain reaction can be used in a nuclear weapon to release a huge amount of energy in a short period of time.
Easy
True or false?
22.The energy released during nuclear fusion comes from a small amount of the particles' mass being converted into energy.
Easy
True or false?
23.In a nuclear reactor, raising the control rods further out of the fuel rods increases the rate of fission.
Medium
True or false?
24.Match each part of a nuclear reactor to its function.
Medium
- Control rod
- Moderator
- Coolant
- Fuel rod
- Absorbs neutrons to control the rate of fission
- Slows down neutrons so they react efficiently with the fuel
- Carries heat away from the reactor core
- Contains the uranium or plutonium that undergoes fission
25.Match each key term to its correct definition.
Medium
- Nuclear fission
- Nuclear fusion
- Chain reaction
- Spontaneous fission
- The splitting of a large, unstable nucleus into two smaller nuclei
- The joining of two light nuclei to form a heavier nucleus
- A process in which neutrons released by fission go on to cause further fissions
- Fission that occurs without additional energy being put into the nucleus
26.Place the steps of an induced fission chain reaction in the correct order, starting with the first event.
Medium
- A neutron is absorbed by a uranium-235 nucleus to make uranium-236
- The uranium-236 nucleus splits by fission into two daughter nuclei
- Two or three neutrons are released and move away at high speed
- Each released neutron is absorbed by another uranium-235 nucleus
27.Match each term to the correct description.
Medium
- Spontaneous fission
- Induced fission
- Chain reaction
- Moderator
- Fission that occurs without additional energy being put into the nucleus
- Fission that begins when the nucleus absorbs a neutron
- Fission neutrons going on to cause further fissions
- A material that surrounds the fuel rods and slows neutrons down
28.Match each part of a nuclear power station to what it does.
Medium
- Reactor
- Boiler
- Turbine
- Generator
- Provides thermal energy from fission chain reactions
- Uses thermal energy to boil water and create steam
- Uses steam to transfer thermal energy into kinetic energy
- Transfers kinetic energy into electrical energy
29.Match each quantity to the correct value for the reaction in which plutonium-239 splits into palladium-112 and cadmium-124.
Medium
- Mass number of the missing section
- Atomic number of the missing section
- Number of neutrons in the missing section
- Mass number of each neutron
- 3
- 0
- 1
30.Place the events in the correct order for the induced fission of uranium-235 in a reactor.
Medium
- A neutron is absorbed by a uranium-235 nucleus
- A uranium-236 nucleus is formed
- The nucleus splits into two daughter nuclei
- Two or three neutrons are released
- The released neutrons go on to cause further fissions
31.Place the stages of generating electricity in a nuclear power station in the correct order.
Medium
- Fission chain reactions release heat in the reactor
- A coolant carries the heat away
- Water is turned into steam
- Steam drives turbines which turn generators
- Steam is cooled back into water in the condenser
32.Which of the following statements about the moderator in a nuclear reactor are correct? (select all that apply)
Medium
- AIt surrounds the fuel rods and control rods in the reactor core
- BIt slows down fast-moving neutrons
- CIt absorbs neutrons to shut down the reactor
- DIt is made of a material that absorbs neutrons without becoming dangerously unstable
- EIt causes neutrons to lose momentum through collisions
33.Which of the following are correct about nuclear fusion? (select all that apply)
Medium
- AIt is the joining of two light nuclei to form a heavier nucleus
- BIt takes place in the centres of stars
- CIt requires extremely high temperatures
- DIts products are highly radioactive
- EIt is currently used in power stations to generate electricity
34.Which of the following statements correctly compare fission and fusion? (select all that apply)
Hard
- AFission provides less energy per kg of fuel than fusion
- BFusion provides more energy per kg of fuel than fission
- CThe products of fission are radioactive
- DThe products of fusion are radioactive
- EFusion is the splitting of a large nucleus into smaller nuclei
35.Which of the following are needed for fusion of hydrogen nuclei to occur? (select all that apply)
Hard
- AVery high temperature of the fuel
- BVery high kinetic energy of the nuclei
- CVery high density or pressure
- DA supply of neutrons to be absorbed
- ELow pressure so the nuclei can move freely
36.Which of the following statements about nuclear waste are correct? (select all that apply)
Medium
- AThe waste is made up of unusable fission products
- BThe daughter nuclei produced are highly radioactive
- CThe waste is stored underground while it is still radioactive
- DThe spent fuel rods are cool and easy to handle
- EThe waste remains radioactive for only a few days
37.Which of the following statements about a chain reaction are correct? (select all that apply)
Medium
- AOnly one extra neutron is needed to induce a uranium-235 nucleus to split
- BEach fission releases two or three neutrons
- CThe released neutrons can start further fission reactions
- DIn a nuclear weapon the chain reaction is carefully controlled
- EIn a reactor, the number of free neutrons must be kept constant
38.Which of the following are difficulties that must be overcome to produce a working fusion reactor on Earth? (select all that apply)
Hard
- AAchieving the very high temperatures needed
- BMaintaining the very high pressures needed
- CContaining the hot fuel once the conditions are reached
- DFinding enough hydrogen, which is a rare element
- EStopping the fusion products from being highly radioactive
39.Only one extra neutron is required to induce a uranium-235 nucleus to split by fission.
Easy
True or false?
40.The products of nuclear fusion are highly radioactive.
Easy
True or false?
41.Complete the sentences about the parts of a nuclear reactor using the words below.
Easy
The moderator in a nuclear reactor is often made from ____. The moderator is used to ____ neutrons. The control rods in a nuclear reactor are often made from ____. The control rods are used to ____ neutrons.
42.Complete the passage about the properties of fission using the words below.
Easy
The ____ nuclei produced in nuclear fission are highly radioactive. They emit ____ energy neutrons when they decay. Concrete ____ can be used to absorb the neutrons.
43.Complete the passage about chain reactions using the words below. Each word can be used once, more than once or not at all.
Medium
One nucleus of ____ undergoes fission to form ____ daughter nuclei. ____ are released alongside the daughter nuclei. The ____ released by fission are absorbed by other ____ nuclei. This causes the ____ of these ____ nuclei.
44.Complete the sentences about fission and fusion using the words below.
Medium
Nuclear ____ is the splitting of a large, unstable nucleus into two smaller nuclei. Nuclear ____ is the joining of two light nuclei to form a heavier nucleus. In fusion, a small amount of mass is converted into ____.
45.State the name given to the smaller nuclei produced when a large nucleus splits during nuclear fission.
Easy
46.Name the element commonly used as a fuel in nuclear reactors that undergoes fission.
Easy
47.Name the part of a nuclear reactor that surrounds the fuel rods and slows down the fast-moving neutrons produced by fission.
Medium
48.Name the two isotopes of hydrogen that fuse together to form helium in a fusion reactor.
Medium
49.In a nuclear reactor there are 2.0 × 10¹⁷ fission reactions each second. Each fission reaction releases 4.0 × 10⁻¹¹ J of energy. Calculate the energy released in 1 second. Give your answer in joules.
Medium
50.The energy released per kilogram of fuel in a fusion reaction is 845 000 GJ. The energy released per kilogram of fuel in burning oil is 0.0394 GJ. Calculate the ratio of the energy released in fusion compared with the energy released in burning oil. Give your answer to 3 significant figures.
Medium
51.In a fusion reaction the energy released is 4.5 × 10⁻¹² J. Using E = mc², where c = 3.0 × 10⁸ m/s, calculate the decrease in mass. Give your answer in kilograms in standard form.
Hard
52.State the function of the control rods.
Easy
53.Explain how neutrons enable a nuclear chain reaction to take place.
Hard
54.Explain why a very high temperature is needed for two hydrogen nuclei to fuse together.
Hard
55.Describe what happens during a nuclear fusion reaction.
Hard
56.Explain how the graphite core and the movable control rods are used to maintain and control the chain reaction in a nuclear reactor.
Hard
57.State the function of the coolant.
Easy
58.In a nuclear reactor there are 2.0 × 10¹⁷ fission reactions each second. Each fission reaction releases 4.0 × 10⁻¹¹ J of energy. Calculate the energy released in 1 second. Give your answer in joules.
Medium
- A7200000 J
- B9600000 J
- C8800000 J
- D8000000 J
59.The energy released per kilogram of fuel in a fusion reaction is 845 000 GJ. The energy released per kilogram of fuel in burning oil is 0.0394 GJ. Calculate the ratio of the energy released in fusion compared with the energy released in burning oil. Give your answer to 3 significant figures.
Medium
- A25680000
- B19260000
- C23540000
- D21400000
60.In a fusion reaction the energy released is 4.5 × 10⁻¹² J. Using E = mc², where c = 3.0 × 10⁸ m/s, calculate the decrease in mass. Give your answer in kilograms in standard form.
Hard
- A-1 kg
- B1 kg
- C5e-29 kg
- D2 kg

Tarjetas

Define nuclear fission.

Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei.

Which two elements are commonly used as nuclear fuels in power stations?

Isotopes of uranium and plutonium are used as fuels in nuclear power stations.

What happens during induced fission of uranium-235?

A neutron is absorbed by the uranium-235 nucleus to make uranium-236, which is very unstable and splits almost immediately.

What particles are released when a nucleus undergoes fission?

Two or three neutrons and gamma rays are emitted, along with two smaller daughter nuclei.

Why is uranium-235 unsuitable for releasing energy slowly in a power station?

It has a very long half-life of 700 million years, so it has low activity and would release energy very slowly.

What is a nuclear chain reaction?

A chain reaction is when neutrons released by each fission reaction go on to start further fission reactions, which release more neutrons and cause more fissions.

How many neutrons are needed to start the fission of one uranium-235 nucleus?

Only one extra neutron is required to induce a uranium-235 nucleus to split by fission.

What is the purpose of control rods in a nuclear reactor?

Control rods absorb neutrons to control the rate of the chain reaction, without becoming dangerously unstable themselves.

How do control rods control the rate of fission?

Lowering the rods further absorbs more neutrons and decreases the rate of fission; raising them absorbs fewer neutrons and increases the rate.

What is the purpose of a moderator in a nuclear reactor?

A moderator slows down fast-moving neutrons so they can react efficiently with the uranium fuel.

How does a moderator slow down neutrons?

Neutrons collide with the molecules of the moderator, lose momentum and slow down to become thermal neutrons.

What is an uncontrolled chain reaction used for?

An uncontrolled chain reaction is used in a nuclear weapon to release a huge amount of energy in a short time as an explosion.

Define nuclear fusion.

Nuclear fusion is when two light nuclei join to form a heavier nucleus.

Where does nuclear fusion naturally occur?

Nuclear fusion takes place in the centres of stars, including the Sun.

What happens during fusion in stars?

Hydrogen nuclei fuse into helium nuclei and release a large amount of energy.

Why is a very high temperature needed for nuclear fusion?

Protons have a positive charge and repel each other, so the nuclei need very high kinetic energy to overcome this repulsion and fuse.

What are the conditions required for nuclear fusion?

Very high temperature and very high pressure (or density) are needed for fusion to occur.

Why is high pressure needed for nuclear fusion?

High pressure increases the number of collisions between nuclei, making fusion more likely.

What equation relates mass and energy in nuclear fusion?

E = mc², where E is energy in joules, m is mass converted into energy in kilograms, and c is the speed of light in m/s.

How much energy is released from 1 kg of hydrogen undergoing fusion compared to burning coal?

The energy from 1 kg of hydrogen that undergoes fusion is equivalent to burning about 10 million kilograms of coal.

State two differences between nuclear fusion and nuclear fission.

Fusion joins small nuclei and releases more energy per kg of fuel, while fission splits a large nucleus and releases less energy per kg of fuel.

Why is nuclear fusion not currently used to generate electricity on Earth?

It is very difficult to achieve and maintain the extremely high temperatures and pressures needed, and containing them requires a great deal of energy.

What is the main problem with nuclear waste from fission?

The daughter nuclei produced are highly radioactive and remain dangerous for thousands of years, so they must be stored underground.

How is electricity generated in a nuclear power station?

Nuclear fission produces heat, which is carried by a coolant to boil water into steam; the steam drives turbines that turn generators to produce electricity.

What is the difference between a control rod and a moderator?

A control rod absorbs neutrons to control the rate of fission, while a moderator slows down neutrons so they can react efficiently with the fuel.

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