Chapter 7 of 8

All 8 chapters
  1. 1Unstable nuclei, readunderstood
  2. 2Alpha, beta and gamma, readunderstood
  3. 3Half-life, readunderstood
  4. 4Background radiation, readunderstood
  5. 5Radiation and safety, readunderstood
  6. 6Uses of radiation, readunderstood
  7. 7Fission and fusion, readunderstood
  8. 8Weighing up nuclear power, readunderstood

S4.11.7 Chapter 7

Fission and fusion

A lump of uranium 235 can give out far more energy than a lump of coal of the same mass. Where does the energy come from?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

  • You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Describe how a chain reaction works in a nuclear reactor
  • Explain how fusion powers the Sun

Burning a fuel moves electrons between atoms. The nuclei stay as they are. Nuclear reactions are different. They change the nuclei themselves, and they release millions of times more energy for each kilogram of fuel.

Nuclear fission is the splitting of a large nucleus into two smaller ones. It happens when a nucleus of uranium 235 absorbs a neutron. The nucleus becomes unstable and splits. This is the reaction in nuclear power stations.

Three boxes joined by arrows going down. The first says a neutron hits a uranium 235 nucleus. The second says the nucleus splits into two smaller nuclei. The third says 2 or 3 neutrons and energy are released.a neutron hits a uranium 235nucleusthe nucleus splits into twosmaller nuclei2 or 3 neutrons and energyare released
One fission. The two smaller nuclei are called fission products, and they are radioactive.

An example is uranium 235 plus a neutron, which gives zirconium 94, tellurium 139 and 3 neutrons. The numbers balance. On the left the mass numbers are 235 + 1 = 236. On the right they are 94 + 139 + 3 = 236.

Worked example

A nucleus of uranium 235 absorbs a neutron and splits into strontium 90 and xenon 143, plus some neutrons. How many neutrons are released?

  1. Mass number before the split = 235 + 1 = 236.
  2. The two nuclei have mass number 90 + 143 = 233.
  3. neutrons = 236 − 233 = 3.

Answer: 3 neutrons

Most of the energy is the movement of the two new nuclei, which fly apart fast. They hit the atoms round them, and the uranium gets hot. The pieces together have slightly less mass than the nucleus before. That missing mass is what became energy.

The neutrons released matter just as much. Each can hit another uranium 235 nucleus and make it split. That releases more neutrons, and so on. This is a chain reaction.

Draw it first

Draw a chain reaction that grows

  1. A strip in two steps. Step 1 shows a neutron arrow, labelled neutron, going into one hatched disc that splits and sends out two neutron arrows to two discs. The counts under the steps are 1 and 2. Step 2 adds a third step in which each of those two discs sends out two more, so the counts are 1, 2 and 4. A key says nucleus splits.neutron12nucleus splits

    1 One neutron splits one nucleus. Each fission sends out two neutrons, which split two more nuclei.

  2. A strip in two steps. Step 1 shows a neutron arrow, labelled neutron, going into one hatched disc that splits and sends out two neutron arrows to two discs. The counts under the steps are 1 and 2. Step 2 adds a third step in which each of those two discs sends out two more, so the counts are 1, 2 and 4. A key says nucleus splits.neutron124nucleus splits

    2 Each of those two sends out two neutrons again. Now four nuclei split. The number doubles at every step.

Worked example

In a chain reaction, each fission causes 2 more. Step 1 is one fission. How many fissions happen in step 5?

  1. Each step doubles the number. Step 1 has 1 fission.
  2. Step 2 has 2, step 3 has 4 and step 4 has 8.
  3. Step 5 has 2 × 8 = 16 fissions.

Answer: 16 fissions

Left alone, this grows very fast. Fifty steps would make more than a million million fissions. All the energy is released in a moment. This is an uncontrolled chain reaction, as in a nuclear bomb.

In a reactor, the chain reaction is controlled. Control rods, made of a material such as boron, absorb neutrons. They are lowered into the core until, on average, each fission leads to exactly one more. Then the number of fissions stays steady, and the heat output is steady.

A chain reaction that stays steady. A neutron arrow labelled neutron goes into the first hatched disc. Each disc sends one neutron arrow on to the next disc and one up into a bar labelled control rods. The counts under the four steps are 1, 1, 1 and 1. A key says nucleus splits.control rodsneutron1111nucleus splits
The control rods take one neutron from each fission, so the count stays at one.

A reactor has other parts too. The fuel is uranium that has been enriched so that a few in every hundred atoms are uranium 235. Natural uranium has less than 1 in 100. A moderator, such as water or graphite, slows the neutrons, because slow neutrons are more likely to split uranium 235.

A coolant, often water, carries the heat from the fuel. The heat makes steam. The steam turns a turbine and the turbine turns a generator. A thick concrete shield round the reactor takes in the radiation.

Five boxes joined by arrows going down. The first says fission in the fuel makes heat. The second says the coolant carries the heat away. The third says the heat makes steam. The fourth says the steam turns a turbine. The fifth says a generator makes electricity.fission in the fuel makesheatthe coolant carries the heatawaythe heat makes steamthe steam turns a turbinea generator makes electricity
From the nucleus to the electricity supply. The last steps are the same as in a coal power station.

A wrong idea many people have

A nuclear power station can explode like a nuclear bomb.

Why it feels right: Both use uranium and a chain reaction, and the word nuclear is the same.

The test that breaks it: Look at the fuel. A bomb needs almost pure uranium 235 or plutonium, packed together. Reactor fuel has only a few in every hundred atoms of uranium 235, and control rods hold the chain steady.

What is true: A reactor cannot explode like a bomb. A fault can still overheat the fuel, so reactors have many safety systems, but this is not a bomb.

Nuclear fusion is the opposite process. Two light nuclei join to make a heavier one, and energy is released. Research for power stations studies a fusion of two forms of hydrogen. These are deuterium, which is hydrogen 2, and tritium, which is hydrogen 3.

A nuclear equation for fusion. Hydrogen, symbol H with 2 at the top and 1 at the bottom, plus hydrogen, symbol H with 3 and 1, gives helium, symbol He with 4 and 2, plus a neutron, symbol n with 1 and 0.21H31H42He10n
Deuterium plus tritium gives helium 4 and a neutron, with energy. The numbers balance.

Worked example

Check that the fusion of deuterium (hydrogen 2) and tritium (hydrogen 3) into helium 4 and a neutron balances.

  1. Mass numbers on the left: 2 + 3 = 5.
  2. Mass numbers on the right: 4 + 1 = 5.
  3. Atomic numbers on the left: 1 + 1 = 2.
  4. Atomic numbers on the right: 2 + 0 = 2. Both totals match, so it balances.

Answer: mass numbers 5 and 5, atomic numbers 2 and 2

Nuclei are positive, so they repel one another. To fuse, they must be driven together very fast, which needs a very high temperature, of millions of degrees. It also needs a high pressure so that they meet often. At this heat, matter is a gas of nuclei and electrons, called a plasma.

The Sun does this all the time. Its core is about 15 million °C, and the weight of the Sun squeezes it. Hydrogen nuclei fuse, in steps, into helium. The Sun turns about 4 million tonnes of matter into energy each second. It has enough hydrogen for billions of years more.

On Earth, fusion has been made in research machines, but at the time of writing no fusion power station supplies electricity. The hard part is holding a gas that hot for long enough. Fusion fuel is hydrogen. Fusion makes no fission products, though the walls of the machine do become radioactive, because the neutrons from fusion strike them.

A table with the columns Idea, Fission and Fusion. Under nuclei, fission splits and fusion joins. Under example, fission uses uranium and fusion uses hydrogen. Under where, fission is in reactors and fusion is in the Sun.IdeaFissionFusionnucleisplitjoinexampleuraniumhydrogenwherereactorsthe Sun
The two ideas side by side.

Try it

Count a chain reaction

You need: paper, a pencil.

  1. Draw one circle. This is a nucleus that splits in step 1.
  2. Say that each split causes 3 more. Draw 3 circles for step 2 and join them with arrows.
  3. Draw the circles for step 3. How many splits are there in step 3?
  4. Write the number of splits in steps 1, 2 and 3. Work out step 4 without drawing it.
  5. Now suppose control rods absorb two of the three neutrons. How many splits are there in each step?

What you will see: With 3 more for each split the numbers grow very fast, because each step multiplies the number by 3. With the control rods, each split causes only one more, and the count stays at 1.

If you cannot try it, this is what happens.
A table with the columns Step and Splits. Step 1 has 1. Step 2 has 3. Step 3 has 9. Step 4 has 27.StepSplits112339427

A wrong idea many people have

The Sun is a huge fire, burning fuel in the air.

Why it feels right: The Sun looks like flames and gives light and heat, just as a fire does.

The test that breaks it: Burning needs oxygen, and space has almost none. Even if the Sun were made of the best fuel, burning would use it up in thousands of years. The Sun has shone for billions.

What is true: The Sun shines by nuclear fusion in its core. Hydrogen nuclei join to make helium, and the energy that comes out is far greater than any burning could give.

Key words

nuclear fission
the splitting of a large nucleus into two smaller ones, with neutrons and energy.
chain reaction
fission in which the neutrons released go on to cause more fissions.
control rods
rods that absorb neutrons, to keep the chain reaction steady.
moderator
a material that slows neutrons so that they are more likely to cause fission.
nuclear fusion
two light nuclei joining to make a heavier one, with energy released.

Back to your guess

The answer is C: From nuclei splitting into smaller nuclei, which releases energy.

Energy comes from the nuclei themselves. When a large nucleus splits, the pieces have a little less mass than it had. That missing mass is released as energy.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. What happens in nuclear fission?

    Tap your answer.

    1. Yes, that is right. In fission, a nucleus such as uranium 235 absorbs a neutron and splits. It releases 2 or 3 neutrons and a large amount of energy.

    2. Not this one. That is nuclear fusion, which is the opposite process. Try another.

    3. Not this one. That is ionisation. Fission changes the nucleus, not the electrons. Go to S4.11.2. Try another.

    4. Not this one. That is gamma decay. In fission the nucleus splits into two. Go to S4.11.2. Try another.

    See the answer

    A large nucleus splits into two smaller nuclei, and neutrons and energy are released.

    In fission, a nucleus such as uranium 235 absorbs a neutron and splits. It releases 2 or 3 neutrons and a large amount of energy.

    • If you chose B: That is nuclear fusion, which is the opposite process.
    • If you chose C: That is ionisation. Fission changes the nucleus, not the electrons. Go to S4.11.2.
    • If you chose D: That is gamma decay. In fission the nucleus splits into two. Go to S4.11.2.

  2. What do the control rods in a nuclear reactor do?

    Tap your answer.

    1. Not this one. That is the job of the moderator. Try another.

    2. Not this one. That is the job of the coolant. Try another.

    3. Not this one. They do not hold the fuel. They are lowered between the fuel to take in neutrons. Try another.

    4. Yes, that is right. Control rods take in neutrons, so that each fission causes exactly one more on average. Lowering them slows the reaction and raising them speeds it up.

    See the answer

    They absorb neutrons, to keep the chain reaction steady.

    Control rods take in neutrons, so that each fission causes exactly one more on average. Lowering them slows the reaction and raising them speeds it up.

    • If you chose A: That is the job of the moderator.
    • If you chose B: That is the job of the coolant.
    • If you chose C: They do not hold the fuel. They are lowered between the fuel to take in neutrons.

  3. What does the moderator in a reactor do?

    Tap your answer.

    1. Not this one. That is the job of the control rods. Try another.

    2. Yes, that is right. Slow neutrons are more likely to be taken in by a uranium 235 nucleus and split it. Water and graphite slow neutrons.

    3. Not this one. The heat from the fuel makes the steam. The moderator slows neutrons. Try another.

    4. Not this one. The thick concrete round the reactor does that. The moderator is inside the core. Try another.

    See the answer

    It slows the neutrons, so that they are more likely to cause fission.

    Slow neutrons are more likely to be taken in by a uranium 235 nucleus and split it. Water and graphite slow neutrons.

    • If you chose A: That is the job of the control rods.
    • If you chose C: The heat from the fuel makes the steam. The moderator slows neutrons.
    • If you chose D: The thick concrete round the reactor does that. The moderator is inside the core.

  4. Each fission in a sample causes two more fissions. What happens to the number of fissions at each step?

    Tap your answer.

    1. Not this one. It would stay the same only if each fission caused exactly one more. Try another.

    2. Not this one. It falls only if each fission causes less than one more. Try another.

    3. Yes, that is right. The number doubles every step: 1, 2, 4, 8, 16 and so on. That is an uncontrolled chain reaction.

    4. Not this one. It doubles. Each fission is replaced by two. Try another.

    See the answer

    It grows very fast.

    The number doubles every step: 1, 2, 4, 8, 16 and so on. That is an uncontrolled chain reaction.

    • If you chose A: It would stay the same only if each fission caused exactly one more.
    • If you chose B: It falls only if each fission causes less than one more.
    • If you chose D: It doubles. Each fission is replaced by two.

  5. Why does fusion need a very high temperature?

    Tap your answer.

    1. Not this one. Temperature does not add particles. The nuclei join to become heavier. Try another.

    2. Not this one. Nuclei are positive and stay positive. That is why they repel. Try another.

    3. Not this one. The fuel is already a gas. Melting is not the problem. Try another.

    4. Yes, that is right. All nuclei are positive, so they repel. Only very fast nuclei, at a very high temperature, get close enough to join.

    See the answer

    So the nuclei move fast enough to get close despite their repulsion.

    All nuclei are positive, so they repel. Only very fast nuclei, at a very high temperature, get close enough to join.

    • If you chose A: Temperature does not add particles. The nuclei join to become heavier.
    • If you chose B: Nuclei are positive and stay positive. That is why they repel.
    • If you chose C: The fuel is already a gas. Melting is not the problem.

  6. Uranium 235 absorbs a neutron and splits into strontium 94, xenon 140 and some neutrons. How many neutrons are released?

    Tap your answer.

    1. Yes, that is right. Before: 235 + 1 = 236. The two nuclei: 94 + 140 = 234. neutrons = 236 − 234 = 2.

    2. Not this one. You forgot the neutron that started the split. Before the split the mass number is 235 + 1 = 236. Try another.

    3. Not this one. Check the sum: 94 + 140 = 234, and 236 − 234 = 2. Try another.

    4. Not this one. Nothing is released only if the mass numbers match. 236 does not equal 94 + 140 = 234. Try another.

    See the answer

    2

    Before: 235 + 1 = 236. The two nuclei: 94 + 140 = 234. neutrons = 236 − 234 = 2.

    • If you chose B: You forgot the neutron that started the split. Before the split the mass number is 235 + 1 = 236.
    • If you chose C: Check the sum: 94 + 140 = 234, and 236 − 234 = 2.
    • If you chose D: Nothing is released only if the mass numbers match. 236 does not equal 94 + 140 = 234.

  7. From an earlier chapter

    A bone has an eighth of the carbon 14 of living bone. The half-life of carbon 14 is 5 700 years. How old is the bone?

    Tap your answer.

    1. Not this one. That is one half-life, which leaves half. An eighth is three half-lives. Go to S4.11.3. Try another.

    2. Not this one. That is two half-lives, which leaves a quarter. An eighth is three. Go to S4.11.3. Try another.

    3. Yes, that is right. An eighth is three halvings: 1 → 1/2 → 1/4 → 1/8. So the age is 3 × 5 700 = 17 100 years.

    4. Not this one. That is four half-lives, which leaves a sixteenth. An eighth is three. Go to S4.11.3. Try another.

    See the answer

    17 100 years

    An eighth is three halvings: 1 → 1/2 → 1/4 → 1/8. So the age is 3 × 5 700 = 17 100 years.

    • If you chose A: That is one half-life, which leaves half. An eighth is three half-lives. Go to S4.11.3.
    • If you chose B: That is two half-lives, which leaves a quarter. An eighth is three. Go to S4.11.3.
    • If you chose D: That is four half-lives, which leaves a sixteenth. An eighth is three. Go to S4.11.3.

  8. The picture shows a fusion of two deuterium nuclei, with one particle hidden. What is the mass number of the particle in the box?

    A nuclear equation for fusion with one term hidden. Hydrogen, symbol H with 2 at the top and 1 at the bottom, plus another hydrogen, symbol H with 2 and 1, gives helium, symbol He with 3 and 2, plus a particle hidden behind a ?.21H21H32He?
    See the answer

    1

    The mass numbers on the left add to 2 + 2 = 4. On the right, helium 3 has 3, so the hidden particle has 4 − 3 = 1. It is a neutron.

  9. Uranium 235 absorbs a neutron and splits into barium 144, a krypton nucleus and 3 neutrons. What is the mass number of the krypton nucleus?

    See the answer

    89

    Before: 235 + 1 = 236. Barium and the neutrons: 144 + 3 = 147. krypton = 236 − 147 = 89.

  10. In a chain reaction each fission causes 2 more. Step 1 is one fission. How many fissions happen in step 6?

    See the answer

    32 fissions

    Step 1: 1, step 2: 2, step 3: 4, step 4: 8, step 5: 16. Step 6: 2 × 16 = 32 fissions.

Keep it

Copy this into your exercise book.

Fission: a large nucleus splits, releasing neutrons and energy. Each neutron can cause more fission, a chain reaction. Control rods absorb neutrons to keep it steady. Fusion: light nuclei join. It needs great heat. The Sun is powered by fusion.

Draw: Discs joined by arrows, with 1, then 2, then 4 splitting nuclei, and a bar labelled control rods above a steady chain.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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