Radioactivity. Attnom Academy

S4.11.1 Chapter 1

Unstable nuclei

Carbon 12 stays the same for ever, but carbon 14 does not. What is different about them?

Your guess

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

Your guess

A Carbon 14 has two more neutrons in its nucleus.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Carbon 14 has two more electrons.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Carbon 14 has two more protons.
You picked C. Read on. At the end, the chapter comes back to your guess.
D Carbon 14 is a different element.
You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain why some isotopes are unstable and decay
Explain that decay is random and cannot be predicted for one atom

Most atoms stay as they are for ever. A few do not. They change by themselves, and this book is about those few. They matter because of what they give out, and because we can use it and must respect it.

Remember the atom. At its centre is a tiny nucleus of protons and neutrons. Protons have a positive charge and neutrons have none. The electrons are outside it. The mass number is the number of protons plus neutrons.

Protons push each other apart, because like charges repel. Something stronger must hold the nucleus together. It is called the strong nuclear force. It pulls protons and neutrons together, but only over a very short distance.

Neutrons help. They add to the pull and add no push. A nucleus holds together when it has the right mix of protons and neutrons. We call that nucleus stable: it stays as it is for ever.

Both have 6 protons, so both are carbon. Carbon 14 has two more neutrons, and its nucleus is unstable.

A nucleus is unstable when the mix is wrong. It may have too many neutrons, or too few, for its protons. Or it may be very large, like uranium. An unstable nucleus can change.

We call an unstable nucleus radioactive. When it changes, we say it decays. It gives out radiation, which is energy or fast particles from the nucleus. The nucleus turns into a new, more stable one, sometimes of another element.

One decay. The nucleus changes once. It does not wear out little by little.

Some real nuclei, with the symbol and mass number. He is helium, Ne neon, Pb lead, C carbon, K potassium and U uranium. No rule on neutrons alone tells the two lists apart.

Worked example

A nucleus of carbon 14 has 6 protons and mass number 14. How many neutrons does it have?

Mass number = protons + neutrons.
neutrons = mass number − protons.
neutrons = 14 − 6 = 8.

Answer: 8 neutrons

Try it

Draw a nucleus

You need: paper, a pencil.

Draw 5 small circles and put a plus sign in each. These are protons.
Draw 6 plain circles beside them. These are neutrons.
Count all 11 circles. This number is the mass number.
Write the name of the element: 5 protons make boron. Then write boron 11.
Now draw the nucleus of boron 10 in the same way. How many neutrons does it have?

What you will see: You can draw any mix of circles you like. Nature does not. Only some mixes hold together, and the rest are unstable. Boron 11 and boron 10 are both stable, but a boron nucleus with many more neutrons is not.

If you cannot try it, this is what happens.

Now a puzzle. Think of a large sample of one unstable kind of nucleus. All the nuclei are identical. Yet they do not all decay together. One decays now, and its neighbour waits for years. Why?

The answer is that decay is random. Nothing makes a nucleus decay at a certain time. Each nucleus has a certain chance of decaying in each second. This chance is the same for every nucleus of that kind. We cannot say when it will happen to one of them.

Random does not mean that nothing can be said. A coin is random when tossed once. Yet 1 000 tosses give about 500 heads. In the same way, we cannot predict one nucleus, but a huge number of them behave in a steady, predictable way. The next chapters use this.

The same source, counted five times for 10 s. Bars 1 to 5 are the five counts. The counts differ because each decay is random.

Worked example

A counter gave these counts in five periods of 10 s from one source: 31, 27, 35, 29, 33. What was the mean count in 10 s?

mean = total ÷ number of readings.
total = 31 + 27 + 35 + 29 + 33 = 155.
mean = 155 ÷ 5 = 31 counts.

Answer: 31 counts

One more fact. Heating a sample does not change how likely its nuclei are to decay. Nor does cooling it, squeezing it, or joining its atoms into a compound. The cause is inside the nucleus, and ordinary changes outside it do not reach there. A very small effect has been measured for a few rare kinds of decay. It does not matter here.

A wrong idea many people have

A substance is radioactive because it has been heated or charged with energy. It will calm down if it is cooled.

Why it feels right: A hot stove glows and gives out heat, and it stops when it cools. It seems radiation works the same way.

The test that breaks it: A radioactive sample kept in a very cold place still decays at the same rate. So does the same sample heated. Neither changes the nucleus.

What is true: Radiation comes from the nucleus. It happens because the nucleus is unstable. Heat, cold and chemistry do not switch it on or off.

A wrong idea many people have

Each unstable nucleus has its own timer, so it decays at a fixed time that we could work out.

Why it feels right: A big sample decays in a steady way, so each nucleus seems to follow a plan.

The test that breaks it: Scientists count the decays from one source several times, for the same length of time. The counts differ each time, as in the bar chart. A plan would give the same count.

What is true: Each decay is random. We can give the chance, and for a huge number of nuclei the total is steady. We cannot say which nucleus goes next.

Key words

nucleus: the tiny centre of an atom, made of protons and neutrons.
stable: a nucleus that stays as it is for ever.
unstable: a nucleus that may change by itself.
radioactive: having unstable nuclei that decay and give out radiation.
decay: the change of an unstable nucleus into a more stable one.
radiation: energy or fast particles given out by an unstable nucleus.
random: not possible to predict for one nucleus, only as a chance.

Back to your guess

The answer is A: Carbon 14 has two more neutrons in its nucleus.

Both are carbon, so both have 6 protons and 6 electrons. Carbon 14 has 8 neutrons and carbon 12 has 6. That mix makes the nucleus of carbon 14 unstable.

Keep it

Copy this into your exercise book.

A nucleus is stable or unstable. An unstable one decays and gives out radiation. The wrong mix of neutrons and protons, or a very large nucleus, can cause it. Decay is random. Heat does not change it.

Draw: Carbon 12 and carbon 14 side by side as circles: 6 protons and 6 neutrons, then 6 protons and 8 neutrons.

S4.11.2 Chapter 2

Alpha, beta and gamma

Radiation comes in three kinds. What are they, and how can we tell them apart?

Your guess

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

Your guess

A They are all the same, but with different amounts of energy.
You picked A. Read on. At the end, the chapter comes back to your guess.
B They differ in what they are made of, and in what stops them.
You picked B. Read on. At the end, the chapter comes back to your guess.
C They are three colours of light.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Compare alpha, beta and gamma by charge, range and what stops them
Write nuclear equations for alpha and beta decay

An unstable nucleus can decay in more than one way. It gives out one of three kinds of radiation: alpha, beta or gamma. The names are the first three letters of the Greek alphabet, α, β and γ.

An alpha particle is 2 protons and 2 neutrons stuck together. That is the same as the nucleus of a helium atom. It has a charge of +2 and is heavy for a particle.

An alpha particle has the same make-up as a helium nucleus.

A beta particle is a fast electron. It comes from the nucleus, not from the shells. Inside the nucleus a neutron changes into a proton and an electron. The proton stays, and the electron flies out. It has a charge of −1.

A tiny neutral particle called an antineutrino also leaves in beta decay. It does not change the numbers we balance, so this book leaves it out. This is called beta minus decay. Some nuclei with too few neutrons decay the other way: a proton changes into a neutron, and a positron leaves. A positron is like an electron with a + charge. This is beta plus decay. In this book, beta means beta minus.

A gamma ray is not a particle. It is an electromagnetic wave, like light, but with far more energy. It has no mass and no charge. After an alpha or beta decay the new nucleus often holds spare energy. It gets rid of it as a gamma ray.

Each kind knocks electrons off the atoms it passes. This is called ionisation, and the atoms that are left become ions. The more ionisation, the more the radiation can harm living cells. It also uses up the energy of the radiation, so it stops.

Alpha ionises the most and gamma the least.

Now see how far each one goes. Alpha loses its energy fast, because it ionises so much. Gamma loses it slowly. So alpha is the most ionising and the least penetrating, and gamma is the least ionising and the most penetrating.

Typical stopping. The exact thickness depends on the energy of the radiation.

In air, alpha goes only a few centimetres. Beta goes from tens of centimetres to a few metres, depending on its energy. Gamma goes very far, and gets weaker with distance. No thickness of lead stops all the gamma rays. Each layer takes away some, and thick lead takes most.

Alpha and beta carry charge, so a magnet or an electric field bends their paths. They bend in opposite ways, because their charges are opposite. Gamma has no charge and goes straight on.

Now the equations. A nuclear equation shows a decay with the mass number on top and the atomic number below each symbol. Both of these must add up to the same total on each side of the arrow. Nothing is lost.

238 = 234 + 4 and 92 = 90 + 2. Both totals match.

Alpha decay: the mass number goes down by 4 and the atomic number goes down by 2. The new nucleus is a different element, two places back in the periodic table. The alpha particle takes 2 protons and 2 neutrons away.

Worked example

A nucleus of radium 226 has atomic number 88. It decays by giving out an alpha particle. Find the mass number and atomic number of the new nucleus.

Alpha decay lowers the mass number by 4 and the atomic number by 2.
New mass number = 226 − 4 = 222.
New atomic number = 88 − 2 = 86.
The element with atomic number 86 is radon, so the new nucleus is radon 222.

Answer: mass number 222, atomic number 86

The worked example, written as an equation.

Beta decay of carbon 14. 14 = 14 + 0 and 6 = 7 + (−1).

Beta decay: the mass number stays the same and the atomic number goes up by 1. An electron has almost no mass, so its mass number is 0. Its charge is −1, so a nucleus that loses it and gains a proton has one more unit of positive charge.

Worked example

A nucleus of strontium 90 has atomic number 38. It decays by giving out a beta particle. Find the mass number and atomic number of the new nucleus.

Beta decay leaves the mass number the same and raises the atomic number by 1.
New mass number = 90.
New atomic number = 38 + 1 = 39.
The element with atomic number 39 is yttrium, so the new nucleus is yttrium 90.

Answer: mass number 90, atomic number 39

Gamma decay changes neither number. The nucleus is the same nucleus, with less energy. Cobalt 60 shows all of this. It gives out a beta particle and becomes nickel 60. The nickel nucleus then gives out gamma rays and settles down.

Try it

Check an equation

You need: paper, a pencil.

Write this decay. A thorium nucleus has mass number 230 and atomic number 90. It gives out an alpha particle.
The new nucleus is radium, with mass number 226 and atomic number 88. Write that too.
Add the mass numbers on the right: the radium and the alpha particle.
Add the atomic numbers on the right in the same way.
Compare each total with the number on the left.
Change the radium mass number to 227 and add again. What goes wrong?

What you will see: With the right numbers, 226 + 4 = 230 and 88 + 2 = 90, so both sides match. Change one number and the totals differ. A real decay never does that, so an equation that does not balance is wrong.

If you cannot try it, this is what happens.

A wrong idea many people have

A beta particle is one of the atom's electrons, knocked out of its shell.

Why it feels right: A beta particle is an electron, and we know atoms have electrons in shells.

The test that breaks it: A decay changes the nucleus. The nucleus of carbon 14 becomes nitrogen. That needs a neutron to change into a proton, and the electron is made in that change.

What is true: A beta particle comes from the nucleus. A neutron changes into a proton and an electron, and the electron leaves. The shell electrons stay where they were.

A wrong idea many people have

Gamma rays change the mass number, because they carry energy away.

Why it feels right: Alpha and beta change the nucleus, so gamma seems to change it too.

The test that breaks it: Look at a gamma ray as a wave. It has no mass and no charge. Take it away and the number of protons and neutrons is still the same.

What is true: Gamma changes neither the mass number nor the atomic number. It only carries away spare energy, so the nucleus stays the same element.

Key words

alpha particle: 2 protons and 2 neutrons, the same as a helium nucleus. It stops in paper.
beta particle: a fast electron from the nucleus. A few millimetres of aluminium stop it.
gamma ray: an electromagnetic wave from the nucleus. Thick lead reduces it a lot.
ionisation: knocking electrons off atoms to make ions.
nuclear equation: a decay written with mass numbers and atomic numbers that add up on both sides.

Back to your guess

The answer is B: They differ in what they are made of, and in what stops them.

Alpha is a helium nucleus. Beta is a fast electron. Gamma is a wave, like light but far more energetic. Paper, aluminium and lead stop them in different ways.

Keep it

Copy this into your exercise book.

Alpha: 2 protons, 2 neutrons, stopped by paper, most ionising. Beta: fast electron, stopped by aluminium. Gamma: wave, only reduced by thick lead. Alpha: mass −4, atomic −2. Beta: mass same, atomic +1. Gamma: no change.

Draw: A source with three rays: alpha stopped at paper, beta stopped at aluminium, gamma reaching lead.

S4.11.3 Chapter 3

Half-life

A sample gives out 800 counts each minute. After 20 minutes it gives out 400. How long until it is down to 0?

Your guess

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

Your guess

A Another 20 minutes, because it falls by 400 every 20 minutes.
You picked A. Read on. At the end, the chapter comes back to your guess.
B It is never exactly 0. It halves again every 20 minutes.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Another 40 minutes, because it falls to 0 twice as slowly.
You picked C. Read on. At the end, the chapter comes back to your guess.
D It stays at 400, because decay stops when half has gone.
You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

Find the half-life from a decay graph
Model random decay by tossing coins and removing the heads

Chapter 1 said that one nucleus decays at a random time. Yet a big sample behaves in a steady way. Its radiation falls away in a pattern that is the same every time. The pattern is described by one number, the half-life.

The half-life of a nucleus is the time it takes for half of the undecayed nuclei in a sample to decay. Each kind of nucleus has its own half-life. It does not change with heat, pressure or chemistry.

Take 8 nuclei with a half-life of 1 hour. After 1 hour, 4 are left undecayed. After 2 hours, half of those 4 are left, which is 2. After 3 hours, 1 is left. Each half-life halves what is left, and does not take away a fixed amount.

See it three ways

With things

Think of 8 nuclei with a half-life of 1 hour. A coin toss for each one in each hour gives the same kind of result.

As a picture

In words

Each half-life, half of the undecayed nuclei decay. Filled circles have not decayed and open circles have. With so few nuclei, a real sample would wander a little from these numbers.

In symbols

8 → 4 → 2 → 1

The decayed nuclei do not vanish. They have become nuclei of another kind, usually of another element. The sample does not lose half its mass at each half-life. It loses half of its undecayed nuclei, and so half of its radiation, if the new nuclei are stable.

We cannot count nuclei, but a detector counts decays. The number it counts each second or minute is the count rate. When there are half as many undecayed nuclei, half as many decays happen each second. So the count rate halves in the same time.

So we can also say: the half-life is the time for the count rate to fall to half. A graph of count rate against time is called a decay curve. It falls steeply at first, and then more and more slowly.

Each dashed pair of lines shows one more half-life. The count rate halves every 20 min.

Worked example

The graph above is the decay curve of a sample. Find its half-life.

The count rate at the start is 800 counts/min. Half of that is 400 counts/min.
Find 400 on the up axis. Go across to the curve, then straight down to the time axis.
The time axis reads 20 min.
The half-life is 20 min. Check: from 400 the curve falls to 200 in another 20 min.

Answer: 20 min

Read the half-life from any part of the curve and you get the same answer. Start at 400 and find where it falls to 200. Start at 200 and find where it falls to 100. Each takes the same time. Only a curve of this kind does that.

Worked example

A sample of iodine 131 has a half-life of 8 days. Its count rate is 640 counts/min. What will the count rate be after 24 days?

Number of half-lives = time ÷ half-life.
Number of half-lives = 24 ÷ 8 = 3.
Halve the count rate 3 times: 640 → 320 → 160 → 80.
The count rate will be 80 counts/min.

Answer: 80 counts/min

After 1 half-life, one half is left. After 2, one quarter. After 3, one eighth. After 4, one sixteenth. The fraction left is one half multiplied by itself once for each half-life. It gets smaller and smaller, but it is never exactly 0.

A wrong idea many people have

After 2 half-lives, all of the radioactive nuclei have gone.

Why it feels right: One half-life takes away half, so it seems that a second half-life takes away the other half.

The test that breaks it: Start with 8 nuclei. After 1 half-life, 4 are left. The second half-life takes half of those 4, which is 2. It does not take all 4.

What is true: Each half-life takes away half of what is left. After 2 half-lives, one quarter is left. After 3, one eighth. A sample never reaches 0 in a set number of half-lives.

Half-lives are very different. Some nuclei last a tiny fraction of a second, and some last billions of years. A short half-life means a lot of decay now, but the sample soon runs down. A long half-life means weak decay that goes on for a very long time.

Some half-lives, which are rounded. Po is polonium, I iodine, Co cobalt, C carbon and U uranium.

A real detector also counts background radiation, which is always there. Take the background reading away before you look at the sample. Only the corrected count rate halves.

Worked example

A detector gives 260 counts/min at the start, 195 after 5 min, 140 after 10 min and 107 after 15 min. The background is 20 counts/min. Find the half-life.

Take away the background: corrected start = 260 − 20 = 240 counts/min.
Half of the corrected start = 240 ÷ 2 = 120 counts/min.
At 10 min the reading is 140. The corrected rate = 140 − 20 = 120 counts/min.
The corrected rate has halved at 10 min, so the half-life is 10 min.

Answer: 10 min

Be careful about one idea. Half-life describes a large number of nuclei. For one nucleus, it gives the chance. In one half-life, it has an even chance of decaying. With few nuclei the halving is rough. You can see this with coins.

Try it

Model decay with coins

You need: 32 coins, or 32 paper discs marked with a cross on one side, a cup, a table, paper and a pencil.

Be safe: Keep small coins away from young children, who may put them in their mouths. Pick up every coin when you finish.

Put all 32 coins in the cup. Write 0 and 32 on your paper.
Shake the cup and tip the coins onto the table.
Take away every coin that shows heads. These have "decayed". Count the coins that are left.
Write the toss number and the number left. Put the left coins back in the cup.
Repeat until you have done 6 tosses, or no coins are left.
Draw a graph of coins left against toss number.

What you will see: About half of the coins go each time, so the numbers roughly halve, for example 32, 16, 8, 4, 2, 1. Yours will not match exactly, because each coin is random. Do it again and you will get different numbers. The pattern is the same. In this model, one toss is one half-life.

If you cannot try it, this is what happens.

A wrong idea many people have

A radioactive sample with a half-life of 1 hour will be safe after 2 hours.

Why it feels right: It feels like 2 half-lives should finish the job, because half and half make a whole.

The test that breaks it: Work out what is left: half of the sample after 1 hour, and a quarter after 2 hours. A quarter of the original radiation is still there.

What is true: After 2 half-lives, a quarter of the radiation remains. How long it takes to be safe depends on how strong the sample was and on what level is safe.

Key words

half-life: the time for half of the undecayed nuclei to decay, or for the count rate to fall to half.
count rate: the number of decays a detector counts each second or minute.
decay curve: a graph of count rate against time. It halves again and again.
background: radiation that is always around us. Take it away from a reading.

Back to your guess

The answer is B: It is never exactly 0. It halves again every 20 minutes.

The count rate halves every 20 minutes: 800, 400, 200, 100, 50 and so on. Each step takes away half of what is left, so it gets very small but never reaches exactly 0.

Keep it

Copy this into your exercise book.

Half-life: the time for half of the undecayed nuclei, or the count rate, to halve. After 1, 2, 3 half-lives: one half, one quarter, one eighth. Never exactly 0. Take away background first. One nucleus is random.

Draw: A decay curve from 800 counts/min, with dashed lines to 400, 200 and 100 and the times 20, 40 and 60 min.

S4.11.4 Chapter 4

Background radiation

A radiation counter reads some counts every minute, often 15 to 30, with no source anywhere near it. Where do they come from?

Your guess

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

Your guess

A The counter is faulty.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Natural sources all around us, such as rocks, air, space and food.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Nuclear power stations, which spread radiation everywhere.
You picked C. Read on. At the end, the chapter comes back to your guess.
D The counter makes the radiation itself.
You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

Name natural sources of background radiation and ones made by people
Explain why background radiation differs from place to place

Radiation is not rare. You are being hit by it as you read this. It comes from the ground, the air, space and even your own body. We call this always present radiation background radiation. Life on Earth has always lived with it.

To describe how much, we need a unit. The sievert (Sv) measures how much harm radiation may do to the body. It takes account of the kind of radiation. The total you receive is your dose.

A sievert is a very large dose, so we use small units. A millisievert (mSv) is one thousandth of a sievert. A microsievert (µSv) is one thousandth of a millisievert. So 1 mSv = 1 000 µSv.

Do not mix up two ideas. A detector gives a count rate, in counts each minute. A dose, in sieverts, is about the effect on a body. They are linked, but they are not the same number.

Most background radiation is natural. These are the sources, with their share of the average natural dose of about 2.4 mSv a year, over the whole world. The figures are rounded world averages from the United Nations committee that studies the effects of radiation.

About half of the natural dose is radon. Each sector is its angle out of 360°. Rounded world averages.

Radon is a radioactive gas. Uranium is in tiny amounts in many rocks and soils. As it decays, it makes radium and then radon. The gas seeps up out of the ground. Radon 222 gives out alpha particles and has a half-life of about 3.8 days.

Out of doors, radon spreads out in the air. In a closed room it can build up.

Radon matters because it is a gas that we breathe in. Alpha particles cannot get through skin, but inside the lungs they strike living cells. In most homes the level is low. Over many years a high level raises the chance of lung cancer, and good ventilation lowers it.

Rocks and soil also give gamma rays, from uranium, thorium and potassium 40. Buildings made of stone, brick or concrete give some too. Food and drink hold potassium 40 and carbon 14, and these are in our bodies, with thousands of decays each second.

Cosmic rays come from space, from the Sun and from far beyond. They are mostly fast charged particles. Air above us takes much of their energy. So the dose rises with height, on a mountain top or in an aeroplane, where there is less air above.

Some background radiation is made by people. By far the largest part of that is medical: X-rays and scans. X-rays are not made by decay, but they ionise like the radiation in this book. The nuclear industry and fallout from past weapon tests add very little on average.

About 2.4 mSv a year is natural and about 0.6 mSv is medical, as world averages. Medical doses vary a lot between people.

Worked example

The chart shows an average natural dose of 2.4 mSv a year and a medical dose of 0.6 mSv a year. Find the total dose and the percentage that is medical.

total = 2.4 + 0.6 = 3.0 mSv.
fraction that is medical = 0.6 ÷ 3.0 = 0.2.
As a percentage, 0.2 × 100 = 20%.

Answer: 3 mSv, 20%

Background radiation is not the same everywhere. A house on granite, which holds more uranium, may have more radon than a house on clay. A few places on Earth have rocks or sands with so much thorium or uranium. There the background is many times the average.

Height changes it too, because of cosmic rays. Buildings change it: a sealed, draught-free room holds more radon than an airy one. Food changes it a little. So a person's dose depends on where they live, how high, and what the house is built of.

Typical values, rounded. A long flight here means one across an ocean. They vary with the place, the machine and the flight.

This gives a sense of size. A chest X-ray gives about three days of natural background. Doctors only use scans when they help, and the benefit of finding an illness is far greater than the small dose. Large doses are another matter. They are the subject of the next chapter.

Try it

Add up a year of doses

You need: paper, a pencil.

Write: background for a year is about 2 400 µSv.
Add 2 chest X-rays. Each is 20 µSv. Write the sum.
Add 1 flight across an ocean, which is 40 µSv.
Add the three lines to get the total for the year.
Work out what share of the total the extra doses are.

What you will see: The extra doses are small next to the natural background, a few percent. But they are real, and they add to it. Doses add up, whatever the source.

If you cannot try it, this is what happens.

A wrong idea many people have

Radiation is only made by people, in nuclear plants, bombs and hospitals.

Why it feels right: We hear about radiation when something goes wrong in an industry, or when we are given a scan.

The test that breaks it: Take a radiation counter far from any plant or hospital. It still counts. The counts come from rocks, air and space. Your own body has potassium 40 in it.

What is true: Radiation was here long before people. Most of the average dose is natural. Human sources add a share, and medical scans are the largest of them.

Key words

background radiation: the radiation that is always present, mostly from natural sources.
dose: the radiation a body receives, measured by the harm it may do.
sievert: the unit of dose. Small doses are in mSv or µSv.
radon: a radioactive gas that comes from uranium in rock.
cosmic rays: fast charged particles from space, stronger at height.

Back to your guess

The answer is B: Natural sources all around us, such as rocks, air, space and food.

Radiation is always around us. Most of it is natural. It comes from rocks and soil, from a gas called radon, from space, and from our food and bodies.

Keep it

Copy this into your exercise book.

Background radiation is always there. Natural: radon (about half), rocks, cosmic rays, food. Made by people: mostly medical. It varies with rock, height and buildings. Dose is in sieverts: 1 mSv = 1 000 µSv.

Draw: A pie chart of natural background with radon about half, then ground, space and food.

S4.11.5 Chapter 5

Radiation and safety

A worker stands near a strong gamma source for an hour and then walks away. Is the worker now radioactive?

Your guess

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

Your guess

A Yes, the radiation has made them radioactive.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Yes, but only until they wash.
You picked B. Read on. At the end, the chapter comes back to your guess.
C No. They were irradiated, and they stay as they were.
You picked C. Read on. At the end, the chapter comes back to your guess.
D No, because gamma rays cannot reach the body.
You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

Compare contamination and irradiation
Explain how distance, shielding and time reduce a dose

The radiation in this book is called ionising radiation, because it ionises atoms. In living cells that can damage the molecules, including DNA. A cell with a little damage usually repairs itself. Too much damage, and the cell may die or divide wrongly.

A very large dose in a short time kills many cells and causes radiation sickness. Smaller doses cause no sickness. A moderate dose adds a small extra chance of cancer, years later. For very small doses, like those from background, the extra chance is too small to measure. Safety rules assume that it is there and that it rises with the dose, so they keep every dose low.

So the aim is clear: keep every dose as low as is reasonable. Some risk is accepted when the gain is greater, as with a medical scan. This chapter shows how people with radioactive materials do it, and it begins with two ways of being exposed.

Irradiation is exposure to radiation from a source outside the body. The source may be a sealed container. While you are near it, your body receives a dose. When you move away, or the source is taken away, it stops. You do not become radioactive. The alpha, beta and gamma rays from a source cannot make your body radioactive. Neutrons can make things radioactive, and they are found inside a working reactor, not near ordinary sources.

Irradiation: the source stays outside. Take the source away and the exposure ends.

Contamination is having radioactive material on or in the body, or on an object. It may be dust or liquid. The source is now on you, and it goes where you go. It keeps on irradiating you until it is washed off, passes out of the body, or decays.

Contamination: the radioactive dust is on the person, so the source goes with them.

Contamination also spreads. Dust can pass from hands to door handles, to food, to other people. That is why workers wear gloves and protective clothing, wash, and leave their outer clothes at the door. Irradiation does not spread in this way.

The kind of radiation matters, and it matters in different ways. Outside the body, alpha is stopped by dead skin and a few centimetres of air, so it does little harm. Gamma and fast beta go in, so a source of those outside the body can be a risk.

Inside the body, it is the other way. A source of alpha that is swallowed or breathed in is next to living cells. The alpha particles ionise strongly and all their energy goes into a very small space. So alpha is the most harmful kind when it is inside.

A wrong idea many people have

Alpha is the most dangerous kind of radiation, in every case.

Why it feels right: Alpha is the most ionising, and ionising is what harms cells.

The test that breaks it: Think about where the particles go. Outside the body, a few centimetres of air and dead skin stop them. Take the same source into the lungs as dust, and nothing stops them.

What is true: Alpha is the most harmful inside the body and the least harmful outside it. Gamma is the opposite: it is a greater risk from outside. Where the source is matters as much as what it is.

Now to lowering a dose. There are three ways, used together. Keep your distance. Put a shield in the way. Spend as little time as you can near the source.

Distance. The rays from a small source spread out in all directions. The further away you are, the thinner they are. The picture counts 16 rays that fall on squares of the same size. At twice the distance they cover four times the area.

Double the distance and each square gets a quarter of the rays.

So for gamma rays from a small source, doubling the distance cuts the dose rate to one quarter. Trebling it cuts it to one ninth. This holds when the air between takes in very little, which is true for gamma. It does not hold for alpha, which stops in a few centimetres of air. Nor does it hold for beta, which the air also takes in.

Shielding. A shield takes in radiation before it reaches you. Paper stops alpha. A few millimetres of aluminium, or about a centimetre of plastic, stop beta. Thick lead or concrete reduce gamma. Each layer takes away a share of the gamma, and none takes it all.

Thick lead takes most of the gamma rays, but some pass through.

Time. While the rate stays the same, the dose is the dose rate multiplied by the time. dose = dose rate × time. Halve the time and you halve the dose.

At a steady 20 µSv an hour, the dose grows in a straight line.

Worked example

A worker stands where the dose rate is 20 µSv an hour. The worker stays for 3 hours. What dose does the worker receive?

dose = dose rate × time.
dose = 20 × 3.
dose = 60 µSv.

Answer: 60 µSv

Try it

Plan a job to lower the dose

You need: paper, a pencil.

Imagine a job of 30 min near a small gamma source. The dose rate at 1 m is 96 µSv an hour.
Work out the dose for 30 min at 1 m. Remember that 30 min is half an hour.
Now stand at 2 m. The dose rate falls to a quarter. Work out the new dose rate and the dose for 30 min.
Now do the job in 15 min at 2 m. Work out the dose.
Write the three doses in a table.

What you will see: Each change lowers the dose. Distance cut it to a quarter, and the shorter time halved it again. A lead shield would lower it further. Workers use all three together.

If you cannot try it, this is what happens.

People who work with radiation also wear a badge that records their dose. There are limits on the dose. Many countries follow the advice of the International Commission on Radiological Protection. A member of the public should get no more than 1 mSv a year from all the ways people use radiation. Natural background and medical care are not counted in this. A radiation worker may get up to 20 mSv a year, as an average over five years. It must never be more than 50 mSv in one year. Workers are allowed more. They are adults who chose the work. They are trained to keep their dose low, and their dose is measured all the time.

Yearly limits advised by the International Commission on Radiological Protection. For workers, 20 mSv is an average over five years. Natural background and medical care are left out.

A wrong idea many people have

A person who has been irradiated is radioactive and a danger to others.

Why it feels right: In films, people who have been hit by radiation glow, and everyone avoids them.

The test that breaks it: Take a person who has had a medical X-ray. A counter held near them reads the same as before. The X-rays passed through and left nothing behind.

What is true: Irradiation does not make a person radioactive. Contamination is different: if radioactive dust is on the person, they can pass it on until it is removed.

Key words

irradiation: exposure to radiation from a source outside the body.
contamination: having radioactive material on or inside the body or an object.
shielding: material that takes in radiation before it reaches you.
dose rate: the dose received each hour or each second.

Back to your guess

The answer is C: No. They were irradiated, and they stay as they were.

Gamma rays do pass into the body and can do harm, but they do not make it radioactive. The worker received a dose and is not a source. It would be different if radioactive dust were on the worker.

Keep it

Copy this into your exercise book.

Irradiation: a source outside, it stops when you leave. Contamination: radioactive material on or in you, it goes with you. Alpha is worst inside, gamma worst outside. Lower a dose by distance, shielding and time. dose = rate × time.

Draw: A person with a source outside and rays arriving, beside a person with dots on their body.

S4.11.6 Chapter 6

Uses of radiation

A doctor gives a patient a small amount of radioactive material to scan the body. Which kind of radiation should it give out?

Your guess

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

Your guess

A Alpha, because it ionises the most.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Beta, because the body stops it.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Alpha or beta, because they are the strongest.
You picked C. Read on. At the end, the chapter comes back to your guess.
D Gamma, because it passes out of the body to a detector.
You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain medical uses, such as tracers and cancer treatment
Explain how carbon dating shows how old remains are

Radiation that can harm can also help, when the dose and place are controlled. People choose the kind of radiation and the half-life to fit the job. This chapter looks at jobs in medicine and industry, and then at dating old remains.

A tracer is a small amount of a radioactive isotope used to follow where something goes. In medicine, a patient swallows or is injected with a tracer. It travels round the body. A gamma camera outside the body picks up the rays and makes a picture.

A tracer shows how an organ works, because the picture shows where the tracer goes and how fast.

The tracer must give out gamma, so the rays leave the body. It must also have a short half-life, of a few hours. A long half-life would leave the patient with a dose for days or years, for no gain. One tracer is technetium 99m, with a half-life of about 6 hours.

Worked example

A tracer with a half-life of 6 hours gives 640 counts/min when it is given. What does it give after 24 hours?

Half-lives = time ÷ half-life.
Half-lives = 24 ÷ 6 = 4.
Halve the count rate 4 times: 640 → 320 → 160 → 80 → 40.
The count rate after 24 hours is 40 counts/min.

Answer: 40 counts/min

Another use is to treat cancer. This is radiotherapy. High doses of radiation kill cells, and cancer cells are killed more easily than most healthy ones. A machine sends a beam of gamma rays or X-rays into the body. The dose is given in many small amounts, so healthy cells can recover.

A beam passes through healthy tissue on its way in and on its way out. One beam gives the tissue in its path about the same dose as the tumour, or more. So the beam is sent from different directions.

One beam. All the tissue in its path gets a dose.

Three beams cross at the tumour. Each path of healthy tissue gets one beam, and the tumour gets all three.

With three beams crossing at the tumour, it gets the dose from all three. Any path of healthy tissue gets only one. Another way is to place a small radioactive source in or beside the tumour. Its radiation travels only a short way.

Gamma rays also kill bacteria. Syringes, dressings and some food are sealed in packets and passed through a beam of gamma rays. This sterilises them. The things are not made radioactive. Its source has a long half-life, such as cobalt 60 at about 5.3 years, so it works for years.

Industry also uses radiation. A paper mill checks its paper with a source of beta particles on one side and a detector on the other. A thicker sheet takes in more of the beta, so the count rate falls. A reading that is too low means the paper is too thick, and the rollers are adjusted.

A thin sheet. Most of the beta particles get through.

A thick sheet takes in more of the beta. The reading falls, so the machine knows the sheet is thick.

Beta is chosen for paper because it goes through thin sheets but not through thick ones. Alpha would be stopped by even thin paper, so the detector would always read about zero. Gamma goes through almost unchanged, so the reading would hardly change with thickness.

A smoke alarm holds a tiny source of americium 241, which gives out alpha. The alpha particles ionise the air between two plates, and a small current flows. Smoke takes in the alpha particles, so the current falls, and the alarm sounds. A sealed source is safe in a home. It must not be taken apart.

The alarm works because alpha is stopped so easily.

A wrong idea many people have

A tracer should have a long half-life, so that it can be followed for longer.

Why it feels right: A source that lasts seems more useful, because it would still be there for a late picture.

The test that breaks it: Think about what a long half-life means. Most of the tracer is still in the body, and still irradiating it, days or years later. The picture was done in hours.

What is true: A medical tracer has a short half-life, of hours, and gives out gamma. The patient gets the picture and a small dose, and the tracer is soon gone. A source for sterilising has a long half-life instead, to work for years.

Now for dating old remains. Carbon dioxide in the air has a tiny, nearly steady share of carbon 14, about one atom in a million million. Cosmic rays make it in the upper air. Plants take it in as they grow. Animals eat the plants, so all living things hold the same share.

When a living thing dies, it takes in no more carbon. The carbon 14 that it holds decays, with a half-life of about 5 700 years. So the share falls. Measure how much carbon 14 is left in a bone or in wood and you can say how long ago it died. This is carbon dating.

Carbon 14 in a dead sample. Living wood gives about 14 decays each minute for each gram of carbon. The graph uses 16 so that the numbers stay whole.

Worked example

Living wood gives 16 counts/min for each gram of carbon. A piece of old wood gives 4. The half-life of carbon 14 is 5 700 years. How old is the wood?

Halve the living rate until you reach the old rate: 16 → 8 → 4.
That is 2 half-lives.
age = 2 × 5 700 years.
age = 11 400 years.

Answer: 11 400 years

Carbon dating has limits. It works only on things that were once alive, such as wood, bone, cloth and charcoal. It cannot date rock or metal. After about 50 000 years, too little carbon 14 is left to measure. Scientists also correct dates using tree rings, because the share in the air has changed a little over time.

Try it

Share out the dose on paper

You need: paper, a pencil.

Draw a large circle for the body. Draw a small circle at its centre for the tumour.
Draw one beam as a straight line right across the body, through the tumour. Shade its path.
Say that each beam gives 1 unit of dose along its path. How many units does the tumour get? How many does a point on the path get?
Now draw 3 beams from different sides. Each must run straight through the tumour.
Count the units again for the tumour, and for a point on the path of one beam.

What you will see: With one beam, the tumour gets 1 unit and so does the healthy tissue in the path. With three beams that cross at the tumour, the tumour gets 3 units and a point on one path gets 1. The tumour now gets three times as much as that tissue.

If you cannot try it, this is what happens.

A wrong idea many people have

Carbon dating can give the age of a rock or a metal tool.

Why it feels right: It is the best known way to date the past, so it seems to work on anything old.

The test that breaks it: Look at what carbon dating measures. It measures carbon 14 taken in while the thing was alive. A rock was never alive and never took any in.

What is true: Carbon dating works only on remains of living things, such as wood, bone and cloth, up to about 50 000 years old. Other methods date rocks.

Key words

tracer: a small amount of a radioactive isotope used to follow where something goes.
radiotherapy: treating cancer with beams of radiation that kill cells.
sterilise: to kill bacteria, for example with gamma rays.
carbon dating: finding the age of remains from the carbon 14 left in them.

Back to your guess

The answer is D: Gamma, because it passes out of the body to a detector.

To make a picture, the radiation must get out of the body and reach a camera. Only gamma passes through the body in large amounts. Alpha and beta would be stopped inside, and give a dose with no picture.

Keep it

Copy this into your exercise book.

Tracers: gamma, short half-life. Radiotherapy: beams cross at the tumour. Beta gauges thickness, alpha works in smoke alarms. Carbon dating: carbon 14 halves every 5 700 years. Only for once-living things, to about 50 000 years.

Draw: A circle for the body with three beams crossing at a small circle for the tumour.

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

A From a chemical reaction between uranium and the air.
You picked A. Read on. At the end, the chapter comes back to your guess.
B From the electrons leaving the uranium atoms.
You picked B. Read on. At the end, the chapter comes back to your guess.
C From nuclei splitting into smaller nuclei, which releases energy.
You picked C. Read on. At the end, the chapter comes back to your guess.
D From the uranium being very hot to start with.
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.

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?

Mass number before the split = 235 + 1 = 236.
The two nuclei have mass number 90 + 143 = 233.
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 One neutron splits one nucleus. Each fission sends out two neutrons, which split two more nuclei.
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?

Each step doubles the number. Step 1 has 1 fission.
Step 2 has 2, step 3 has 4 and step 4 has 8.
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.

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.

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.

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.

Mass numbers on the left: 2 + 3 = 5.
Mass numbers on the right: 4 + 1 = 5.
Atomic numbers on the left: 1 + 1 = 2.
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.

The two ideas side by side.

Try it

Count a chain reaction

You need: paper, a pencil.

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

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.

S4.11.8 Chapter 8

Weighing up nuclear power

People disagree about nuclear power. What is true about it, and what do we have to weigh up?

Your guess

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

Your guess

A It gives out carbon dioxide while making electricity, like coal.
You picked A. Read on. At the end, the chapter comes back to your guess.
B It cannot make steady electricity.
You picked B. Read on. At the end, the chapter comes back to your guess.
C It makes no waste.
You picked C. Read on. At the end, the chapter comes back to your guess.
D It gives almost no carbon dioxide, but it leaves radioactive waste that must be kept safe for a very long time.
You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

Compare nuclear power with fossil fuels and renewables
Explain how nuclear waste is stored safely

This book has described the science. This chapter asks you to use it to judge. There is no perfect way to make electricity. Every source has gains and costs, and sensible people weigh them differently. Here are the facts that go on the scales.

Start with the fuel. Burning coal, oil or gas gives out carbon dioxide, which adds to climate change, and also soot and other pollution. These fuels will run out, because they took millions of years to form. Their power stations are quicker to build than nuclear ones.

Renewable sources, such as wind, sun and water, need no fuel and give out no carbon dioxide while working. But wind and sunshine change with the weather and the time of day. So the supply is not steady, and backup or storage is needed. Hydroelectric power needs a suitable river and valley.

Nuclear power sits between these. A power station gives out almost no carbon dioxide when it runs. The fuel takes little room. One uranium fuel pellet, the size of a fingertip, holds as much energy as about a tonne of coal.

A reactor also runs almost all the time, in any weather, day and night. A typical one is working for more than 8 hours in every 10. That makes it a steady supply, which suits a country whose use never stops.

To make a fair comparison we must count carbon dioxide over the whole life of a source, not only while it runs. Building, mining, making and taking apart all add some. The table gives rounded middle values from reviews of many studies by the Intergovernmental Panel on Climate Change.

Grams of carbon dioxide equivalent for each kWh of electricity, over the whole life. Rounded middle values. Real values vary.

Worked example

A home uses 3 000 kWh of electricity in a year. How much carbon dioxide, in kg, is given out if all of it came from coal, at 820 g for each kWh?

carbon dioxide = grams for each kWh × kWh.
carbon dioxide = 820 × 3 000 = 2 460 000 g.
1 000 g = 1 kg, so 2 460 000 g ÷ 1 000 = 2 460 kg.

Answer: 2 460 kg

The same home on nuclear power, at 12 g for each kWh, would cause 36 kg, which is far less. On wind it would be the same. The table shows that nuclear power and wind have among the lowest values, with solar next, and coal and gas far higher.

Cost is a second item. A nuclear power station is expensive and slow to build, often taking many years. Once built, its fuel is cheap, and it can run for decades. At the end, taking it apart safely, called decommissioning, takes years and money too.

Safety is the third. Serious accidents at nuclear power stations are very rare. The ones at Chernobyl in 1986 and Fukushima in 2011 spread radioactive material, forced people to leave their homes and contaminated land. Yet nuclear power has caused far fewer deaths than coal, oil and gas for each unit of energy. This counts deaths from air pollution. Estimates differ.

The fourth is waste. Burning fuel leaves ash and gives out gases. A reactor gives out no carbon dioxide, but it leaves nuclear waste, which is radioactive. The amount is small, but it must be kept safe for as long as it stays dangerous.

Waste comes in three groups. Low level waste is gloves, overalls and tools that are lightly contaminated. It is only weakly radioactive, so it can be buried in shallow sites. These sites are watched for a few hundred years. Intermediate waste, such as reactor parts and filters, is sealed in concrete or steel.

The three groups of waste. Most of the volume is low level.

High level waste is the used fuel. It holds the fission products and some heavy nuclei made in the reactor. It is very radioactive and hot, but it is a small volume. Most of the waste is low level. Only a small part is high level.

It is stored in steps. First the used fuel is cooled for some years in deep pools of water. The water takes the heat away and shields the radiation. Then it is sealed in thick steel and concrete casks, which are kept above ground, guarded and watched.

The usual steps for high level waste. Deep stores are being built in a few countries.

Why a deep store? The decay of the waste depends on half-lives. Strontium 90 and caesium 137 have half-lives of about 30 years. Some other nuclei last far longer. Plutonium 239 has a half-life of about 24 000 years, and a few have half-lives of millions of years. So the waste must be kept apart for many thousands of years. It goes in a place that stays dry and still, deep in stable rock.

A sample of waste with a half-life of 30 years falls to an eighth in 90 years.

Worked example

A sample of waste has a half-life of 30 years. Its count rate is 800 counts/min. How long until it falls to 100 counts/min?

Halve the count rate until you reach 100: 800 → 400 → 200 → 100.
That is 3 half-lives.
time = 3 × 30 years = 90 years.

Answer: 90 years

There is a link to the half-life. A short half-life means strong radiation now, but it soon fades. A long half-life means weaker radiation, but for a very long time. The waste needs shielding for the first, and isolation for the second.

A wrong idea many people have

All nuclear waste stays dangerous for tens of thousands of years.

Why it feels right: We hear that waste lasts for ages, and the longest half-lives get the most attention.

The test that breaks it: Look at the three groups. Most waste, by volume, is low level and only weakly radioactive. Only the high level waste is strongly radioactive, with very long half-lives.

What is true: Most nuclear waste is low level and only weakly radioactive. A small part is high level, and some of that needs isolation for a very long time. This is why it goes to a deep store.

Try it

Count the carbon dioxide from a lamp

You need: paper, a pencil.

A 100 W lamp is on for 10 hours each day for 100 days. Change the power to kW: 100 W is 0.1 kW.
Work out the hours: 10 × 100 = 1 000 hours.
Energy = power × time. Work out the energy in kWh.
Carbon dioxide in grams = grams for each kWh × kWh. Do this for coal at 820, gas at 490 and wind at 12.
Change each answer to kg by dividing by 1 000. Write the three answers in a table.

What you will see: The energy is 0.1 × 1 000 = 100 kWh for every source. The carbon dioxide is very different: 82 kg for coal, 49 kg for gas and 1.2 kg for wind. The same lamp, the same light, a very different effect.

If you cannot try it, this is what happens.

So how do we decide? A country with plenty of wind, sun or rivers may choose one way. A country that needs a steady supply, with little space or sun, may choose another. Many countries use a mix. The science tells us the gains and costs. The choice is for people to make, with the facts in front of them.

CO₂: carbon dioxide given out while it works. Steady: the output does not depend on the weather.

Key words

nuclear waste: radioactive material left over from a nuclear reactor or from its use.
high level waste: the used fuel, which is very radioactive and needs long isolation.
decommissioning: taking a power station apart safely at the end of its life.

Back to your guess

The answer is D: It gives almost no carbon dioxide, but it leaves radioactive waste that must be kept safe for a very long time.

Nuclear power gives very little carbon dioxide, and a small amount of fuel gives a great deal of energy. But the waste stays radioactive for a long time, and a reactor is costly to build. Each source has gains and costs.

Keep it

Copy this into your exercise book.

Nuclear: almost no CO₂ in use, little fuel, steady, but costly, slow to build, and it leaves waste. Waste: low level, intermediate, high level. High level is cooled, sealed, then kept deep in rock. Every source has gains and costs.

Draw: Three boxes joined by arrows: used fuel in water, then in a steel cask, then in a deep store in rock.

Draw a nucleus

Check an equation

Model decay with coins

Add up a year of doses

Plan a job to lower the dose

Share out the dose on paper

Draw a chain reaction that grows

Count a chain reaction

Count the carbon dioxide from a lamp

Key words, A to Z