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  1. 1. P6 Radioactive materials254
  2. 2. P6: Radioactive materialsWhy study radioactivematerials? People make jokes about radioactivity. If you have hospital treatment with radiation, they may say you will ‘glow in the dark’. People may worry about radioactivity when they don’t need to. Most of us take electricity for granted. But today’s power stations are old. Should nuclear power stations be built as replacements? Should we research nuclear fusion as a long-term energy solution?What you already know The Science• Some materials are radioactive, and naturally The discovery of emit gamma rays. radioactivity changed ideas about matter and• Gamma rays are ionising radiation. atoms. The nuclear model• Ionising radiation can damage living cells. of the atom helped• Nuclear power stations produce radioactive scientists explain many waste. observations – including radioactivity and the• Contamination by a radioactive material is colour of stars. more dangerous than a short period of irradiation. Ideas aboutFind out about Science• radioactive materials and emissions Physicists’ understanding of radioactivity has• radioactive materials being used to treat enabled them to develop cancer many applications – from• ways of reducing risks from radioactive nuclear power stations materials to cancer detection and treatment. Knowledge of• nuclear power stations and fusion research. radioactivity is essential to develop safe ways of working. 255
  3. 3. A Radioactive materials What do these elements have in common? Find out about D radioactive decay D what makes an atom radioactive D types of radiation Radon is a radioactive gas. Radium is a radioactive metal. in the early 1900s, these bulbs were used to make drinking water radioactive. uranium ore – uranium is used as a nuclear fuel in power stations. They are all radioactive. If you test them with a Geiger counter you will hear it click. When radioactivity was first discovered, people did not know that the radiation was ionising and could damage or kill living cells. They thought that it was natural and healthy. Manufacturers this scientist is measuring made all kinds of products using radioactive materials. When the radioactivity of the fruit scientists realised the danger the products were banned. from plants grown with radioactive water. Safety rules for using radioactive materials were introduced. Radioactive elements can be naturally occurring, or they can be man-made. The man-made elements may be produced because they are useful. For example, the radioactive hydrogen in the water molecules is used to water the plants in the photograph. They may be a waste product, like waste from nuclear power stations.256
  4. 4. P6: Radioactive mateRialsChanges inside the atomMany elements have more than one type of atom. For example, some carbon atoms are radioactive. In most ways they are identical to other carbon atoms. All can:• be part of coal, diamond, or graphite• be a part of molecules• take point in chemical reactions, for example, burn to form carbon dioxide. a cut diamond sitting on a lump of coal. each of these isRadioactive decay made of carbon atoms. some of the atoms will be radioactive.The main difference is that most carbon atoms do not change. They are stable. Radiation What it isRadioactive carbon atoms randomly give out energetic alpha small, high speedradiation. Each atom does it only once. And what is left particles particle withafterwards is not carbon, but a different element . The process (␣) ϩ chargeis called radioactive decay. It is not a chemical change; it is a beta smaller, higherchange inside the atom. particles speed particle with (␤) Ϫ chargeWhat makes an atom radioactive? gamma high energy radiation electromagneticAtoms have a tiny core called the nucleus. In some atoms, the (␥) radiationnucleus, is unstable. The atom decays to become more stable. It emits energetic radiation and the nucleus changes. This is why the word ‘nuclear’ appears in nuclear reactor, nuclearmedicine, and nuclear weapon.Three types of radiation are emitted, called alpha, beta, and gamma. Questions Summary box 1 how can you test to see if something is radioactive? D Some atoms are 2 Why is ionising radiation dangerous? radioactive. They decay by emitting 3 What are the three different types of radiation from alpha particles or radioactive materials? gamma radiation. A: Radioac tive mateRials 257
  5. 5. B Atoms and nuclei How do scientists know about the structure Find out about of atoms? DDmodels of the atom The ‘solar system’ model of the atom dates back to 1910, and DDhow alpha particle an experiment thought up by Ernest Rutherford. Scientists scattering reveals were beginning to understand radioactivity, and were the existence of the experimenting with radiation. Rutherford realised that alpha atomic nucleus particles were smaller than atoms, and so they might be useful tools for investigating the structure of atoms. So he designed a suitable experiment, and it was carried out by his assistants, Hans Geiger and Ernest Marsden. Here is how to do it: • Start with a metal foil. Use gold, because it can be rolled out very thin, to a thickness of just a few atoms. • Direct a source of alpha radiation at the foil. Do this in a vacuum, so that the alpha particles are not absorbed by air. • Watch for flashes of light as the alpha particles strike the detecting material on the screen at the end of the microscope. The ‘solar system’ model of • Work all night, counting the flashes at different angles, to the atom has a nucleus at see how much the alpha radiation is deflected. the centre and electrons whizzing around like miniature planets. gold foil screen microscope alpha source Rutherford’s experiment. An alpha particle striking the detecting material gives a tiny flash of light.258
  6. 6. P6: Radioactive materialsResults and interpretation most radiationThis is what Geiger and Marsden observed: alpha radiation only slightly• Most of the alpha particles passed straight through deflected back-scattered the gold foil, deflected by no more than a few degrees. gold foil• A few of the alpha particles were actually reflected back towards the direction from which they had come.And here is what Rutherford said:‘It was as if, on firing a bullet at a sheet of tissue paper,the bullet were to bounce back at you!’In fact, very few alpha particles were back-scattered, butit still needed an explanation. alpha radiation gold nucleusRutherford realised that there must be something withpositive charge repelling the alpha particles becausethey also have positive charge. And it must also have alot of mass, or the alpha particles would just push it outof the way. Only alpha particles passing closeThis ‘something’ is the nucleus of a gold atom. It contains all of to a nucleus are deflected by athe positive charge within the atom, and most of the mass. large amount.Rutherford’s nuclear model is a good example of a scientist usingcreative thinking to develop an explanation of the data.His analysis of his data showed that the nucleus was very tiny,because most alpha particles flew straight past without beingaffected by it. The diameter of the nucleus of an atom isroughly a hundred-thousandth of the diameter of the atom. Questions 1 What charge do the following have: a the atomic nucleus? b alpha radiation? Summary box c electrons? DDThe alpha particle experiment should 2 Put these in order, from least mass to greatest: gold that atoms have a atom, alpha particle, gold nucleus, electron. small massive nucleus with 3 Describe and explain what happened to alpha particles positive charge. that were directed: DDRutherford used a straight towards a gold nucleus creative thinking to b slightly to one side of a gold nucleus develop the nuclear c midway between two nuclei. model explanation. B: Atoms and nuclei 259
  7. 7. C Inside the atom Atoms are small – about a ten millionth of a millimetre across. Find out about Their outer layer is made of electrons. Most of their mass is D protons and concentrated in a tiny core, called a nucleus. neutrons D ␣ and ␤ particles The nucleus The tiny nucleus contains two types of particle: protons and neutrons. Carbon-11 and carbon-12 are different forms of carbon. Carbon-12 has 12 particles in the nucleus: six protons and six neutrons. Carbon-11 has 11 particles in the nucleus: six 6p protons and five neutrons. These different forms are called 6p 6n 5n isotopes of carbon. Carbon-11 will give out its radiation whether it is in diamond, coal, or graphite. You can burn it or vaporise it and it will still carbon-12 carbon-11 be radioactive.6p 6p6n 5non-12 carbon-11 carbon-11 has 11 particles in its compared to the nucleus: 6 protons and 5 neutrons. whole atom, the tiny the nucleus of carbon-12 has 6 nucleus is like a protons and 6 neutrons. pinhead in a stadium. Making gold When radioactive platinum decays it turns into a new element – gold. A good way to make money? No. The price of gold is only half the price of platinum. beta radiation 260
  8. 8. P6: Radioactive materialsRadioactive changesSome nuclei that are unstable can become more stable byemitting an alpha particle.Other nuclei can become more stable by emitting a betaparticle. These particles come from the nucleus of the atom. ␥ ␣ ␤ It is the nucleus of an atom that makes it radioactive and emits the radiation.The emission of either an alpha or a beta particle from anunstable nucleus produces a nucleus of a different element,called a ‘daughter product’ or ‘decay product’. The daughterproduct may itself be unstable. There may be a series ofchanges, but eventually a stable end-element is formed. Questions 1 Look at these nuclei: A carbon-11 B boron-11 C carbon-12 D nitrogen-12 a Which two are the same element? Summary box b Which ones have the same number of particles in DDAlpha or beta the nucleus? particles are emitted c Do any of them have identical nuclei? by an unstable 2 Put in order of size with the biggest first proton, atom, nucleus. DDThis produces a nucleus, molecule, pinhead. nucleus of a 3 What part of the atom does the radiation come from? different element. C: Inside the Atom 261
  9. 9. D Using radioactive isotopes Radioactive isotopes have many uses, but they are quite Find out about rare in Nature – because most of them have decayed – so DDalpha, beta and radioactive isotopes are made in nuclear reactors for use in gamma radiation laboratories and hospitals around the country. DDsterilisation using ionising radiation Alpha, beta or gamma? To decide which radiation to use, scientists consider these properties. Alpha radiation Alpha particles are much heavier than beta particles, and they quickly collide with air molecules and slow down. This means that they are the least penetrating, but also the most strongly ionising radiation. They are stopped most easily. invisible nuclear radiation Beta radiation Beta particles move very fast. They are much smaller than alpha particles so less alpha likely to collide with other particles. beta lead gamma This means they travel further in aluminium air and other materials and are less ionising. paper Uses of ionising radiations are linked to their properties. Gamma radiation Sometimes, the protons and neutrons in the nucleus just rearrange themselves to become more stable. Question When this happens the nucleus emits a photon of electromagnetic radiation called a gamma ray. 1 Which type of radiation: a is the most penetrating? This does not cause a change of element. b is the most ionising? The photons have more energy than most X-ray photons and rarely collide with particles, so the c has the longest radiation is very penetrating. It has only a range in air? very weak ionising effect.262
  10. 10. P6: Radioactive mateRialsProperties of ionising radiationradiation range in air stopped by ionisation chargealpha a few cm paper / dead strong ϩ skin cells The logo shows that thebeta 10–15 cm thin aluminium weak Ϫ herbs and spices have been irradiated with gammagamma many thick lead or very weak no charge radiation from Cobalt–60. metres several metres Gamma rays pass through of concrete the glass and kill any bacteria in the jar. Cobalt–60 does not pass in to the jar– thereSterilisation is no contamination.Ionising radiations can kill bacteria. Gamma radiation is used for sterilising surgical instruments and some hygiene products such as tampons. • The products are first sealed from the air and then exposed to the radiation. uses of ionising radiation are• This passes through the sealed packet and kills the bacteria linked to their properties. inside. Food can be treated in the same way. Irradiating food kills bacteria so the food lasts longer. Since 2010, irradiation is permitted in the UK for herbs and spices. The label must show that they have been treated with ionising radiation. This can be better than heating or drying, because it does not affect the taste. Questions 2 a Why seal the packets of surgical instruments before gamma rays kill the bacteria sterilising them? on and inside these test tubes. b does the gamma radiation make them radioactive? explain your answer. 3 smoke detectors used in homes contain a source that Summary box D Alpha, beta, and emits alpha particles. gamma radiations a explain why these are not dangerous in normal use. have the different b What might make them dangerous? properties described above. D Gamma radiation is used to sterilise food and surgical products. D: using Radioac tive isotoPes 263
  11. 11. E Radiation all around Radiation sources Find out about If you switch on a Geiger counter, you will hear it click. It is DDbackground picking up background radiation, which is all around you. radiation Most background radiation comes from natural sources. DDa radioactive gas Natural 84% called radon DDradiation dose 50% radon gas and risk from the ground 9.5% from food and drink 13% gamma rays from 12% 15% the ground and cosmic rays medical buildings How different sources contribute to the average radiation dose in the UK. Source HSE 0.1% nuclear discharges 0.1% products 0.2% fallout 0.2% occupational The UK average annual dose is Artificial 16% 2.5 mSv. Radiation dose Questions Radiation dose measures the possible harm the radiation could do to the body. It is measured in millisieverts (mSv). 1 a In what units is • The UK average dose is 2.5 mSv a year. radiation dose • With a dose of 1000 mSv (400 times larger) three out of a measured? hundred people, on average, develop a cancer. b What is the average radiation dose per Ionising radiation from outer space is called cosmic radiation. year in the UK? • Flying to Australia gives you a dose of 0.1mSv, from cosmic c What percentage of rays. That’s not much if you go on holiday, but it soon adds the average up for flight crews. radiation dose in the UK comes from food and drink? What affects radiation dose? The dose measures the possible harm done by the radiation. It 2 a How big a dose of depends on: radiation do you get • the amount of radiation reaching the body by catching a flight • the type of radiation. Alpha is the most ionising of the three to Australia? radiations. So it can cause the most damage to a cell. The b Where do cosmic same amount of alpha radiation gives a bigger dose than rays come from? beta or gamma radiation.264
  12. 12. P6: Radioactive mateRialsThe damage to the body depends on the type of tissue affected. Lung tissue is easily damaged. Radon gas is dangerous because it emits alpha particles. If it is breathed into the lungs then the alpha radiation will be absorbed in the lung tissue.Is there a safe dose?There is no such thing as a safe dose. Just one radon atom might cause a cancer. This is like a person being knocked down by a bus the first time they cross a road. The chance of it happening is low, but it still exists. The lower the dose, the lower the risk. But the risk is never zero.IrradiationIrradiation is when you are exposed to a radiation source outside your body. Alpha irradiation presents a very low risk because alpha particles:• only travel a few centimetres in air• are easily absorbed.Your clothes will stop alpha particles. So will the outer layer of dead cells on your skin. in the 1970s, alice stewart’s research suggested that radiation is more harmfulIrradiation by beta particles is more risky as they to children and to elderly people. she waspenetrate a few centimetres into the body. attacked for her ideas.Most gamma rays pass straight through the body. They have high energy, so if they are absorbed they are dangerous.ContaminationContamination is when a radiation source enters your body, or gets on your skin or clothes. You become Summary boxcontaminated. If you swallow or breathe in any D Radiation dose isradioactive material, your organs will be exposed to affected by:continuous radiation. Sources that emit alpha particles  amount ofare the most dangerous because alpha particles are the radiation  type of radiation.most ionising. Contamination by gamma sources is the D Radiation dose isleast dangerous as most gamma rays will pass straight measured inout of the body. millisieverts (mSv). E: Radiation all aRound 265
  13. 13. Radiation protection Health physicists study radiation hazards and give advice to protect against them. They also keep a close eye on people who work with radioactive materials, for example, in hospitals, and industry. These people are called ‘radiation workers’. Employers must ensure that radiation workers receive a radiation dose ‘as low as reasonably achievable’. For example, if better equipment would reduce the risk, and the cost is reasonable, they must buy it. There is guidance to protect hospital patients who receive radiation treatment too. If one hospital uses smaller doses but its results are just as good, then all hospitals are asked to copy them. Working with radiation Manisha is a nuclear medicine technician in a hospital. She prepares radioactive isotope doses and may be exposed to radiation. To keep her dose low she: • uses protective clothing and screens • wears gloves and an apron • wears a personal radiation monitor whenever she is working. staff handle radioactive sources Questions with gloves and forceps. 3 on what two factors does radiation dose depend? 4 explain the difference between irradiation and contamination. 5 explain how each of the precautions manisha takes helps her to keep her radiation dose as low as possible. People working with radiation wear a personal radiation monitor to keep track of their dose.266
  14. 14. Living with radon FRadon gasOver 400 years ago, a doctor wrote about the high death rate Find out aboutamongst German silver miners. He thought they were being D radon gaskilled by dust, causing disease. D radiation doseWe now know that radon gas is harmful because it is and riskradioactive. It produces ionising radiation that can damage cells. The silver miners were dying of lung cancer. Radon gas escapes from rocks.Radon and lung cancer Radon is breathed in. The miners are contaminated.Radon seeps into houses in some areas of the UK, as described in the leaflet on the following page. Scientists have done lots of studies to see if a low dose over a long time increases the risk of lung cancer.Scientists measure radon levels in the homes of people with lung cancer and compare them with levels in homes of people silver mines were contaminatedwho have not got lung cancer. One study: with radon gas. the miners• chose women who had lived in the same homes for 20 years breathed it in and suffered.• compared 413 women with lung cancer with 614 without lung cancer• showed a link between radon exposure and lung cancer. the pipe runs beneath the floorSome studies have not shown a link. This may be because: of the house and a small fan sucks the radon from the building.• they had a smaller sample size• it is difficult to measure radon exposure over time, especially if people move. Question 1 a What was the correlation that the doctor observed? b What was i) the factor and ii) the outcome in what happened to the silver miners? F: living With Radon 267
  15. 15. A hazard at home Radon gas builds up in enclosed spaces. In some parts of the UK, it seeps into houses. LIVING WITH RADON GOVERNMENT INFORMATION LEAFLET There is radon all around you. It is radioactive and Radon information can be hazardous – especially in high doses. Percentage of homes where Visit 7 radon levels are too high. or write to: 0–1 5–10 Radon Survey, NRPD Radon gives out a type of ionising radiation called 1–3 10–30 Chilton greater DIDCOT 3–5 alpha radiation. Like all ionising radiations, alpha than 30 OX11 0RQ 6 radiation can damage cells and might start a cancerous growth. Radon is a gas that can build up in enclosed 5 spaces. Some homes are more likely to be contaminated with radon. 4 What about my home? 3 You and your family are at risk if you inhale radon- contaminated air. The map shows the areas where 2 there is most contamination. If you live in one of these areas, get your 1 house tested for radon gas. 4 5 6 2 3 What if the test shows radon? Radon-affected areas in england and Wales. Based Radon comes from the rocks underneath some on measurements made in over 400 000 homes. buildings. It seeps into unprotected houses through the floorboards. If your house Not adapted is contaminated, get it protected. An Adapted approved builder will put in: • a concrete seal to keep the radon radon gas dispersed under your floorboards and • a pump to remove it safely. radon gas in house The risk is real: put in a seal. fan wooden floor wooden floor concrete pipe hardcore sump Radon gas can build up inside your home. sealing the floor and pumping soil out the gas is an effective cure.268
  16. 16. P6: Radioactive mateRialsRadon and riskThe risk to miners was high because radon can build up in Questionsenclosed spaces, such as mines. In the atmosphere, the radon 2 there is a risk fromspreads out. In mines the rocks keep producing the gas and it radon gas building upcannot escape. So the radon concentration can be 30 000 times in houses. Which ofhigher than in the atmosphere. these are good ways to reduce the risk?There is a lower concentration in a house. And it is much lower • stop breathingif the windows are open or there are draughts. • wear a special gasOn average, radon makes up half the UK annual radiation maskdose. About 1100 people die each year from its effects. That is • move houseabout 1 in every 50 000 people. Radon is only one hazard. There • adapt the house.are risks with driving to school, sunbathing, swimming, and 3 choose three causes ofeven eating. death from the table on the left. Write downMany risky activities have a benefit. You need to decide a way of reducing thewhether to take the risk. risk from each one.The table shows how the risk of cancer from radon compares 4 Write a letter to a friendwith some other common risks. living in a high-radon area to persuade themCause of death Average number of deaths per year to get their housecancer caused by radon 1100 checked for radon.cancer among workers 4000caused by asbestos Summary boxskin cancer caused by 1400ultraviolet radiation D Ionising radiation can damage livingroad deaths 2500 cells. This may causecancer caused by smoking 35 000 cancer. D Radioactive materialscJd 98 may irradiate orhouse fire 360 contaminate people. D The risk depends onall causes 510 000 the radiation dose. estimated deaths per year in the uK population of 60 million (2008). F: living With Radon 269
  17. 17. G Half-life Radioactive decay is random. You can never tell which nucleus Find out about will decay next. Scientists can’t predict whether a particular DDthe half-life of nucleus will decay today or in a thousand years time. But in a radioactive sample of radioactive material there are billions of atoms, so materials they can see a pattern in the decay. The pattern of radioactive decay The amount of radiation from a radioactive material is called its activity. This decreases with time. • At first there are a lot of radioactive atoms. • Each atom gives out radiation as it decays to become more stable. • The activity of the material falls because fewer and fewer radioactive atoms remain. Half-life Technetium-99m is a radioactive element used as a medical tracer. The diagram shows what happens when it is injected into a patient at 9:00 am. am t9 r ta a St 6 hours 6 hours 6 hours later later later Another half have Another half gone This is the sample half of the nuclei decayed – only a have decayed ​ 1 now only __ ​ left 8 of radioactive quarter are left nuclei injected 1 Every six hours about half of the nuclei decay. We say the half-life of technetium-99m is six hours. The half-life is the 1 time it takes for the activity to drop by half. 2 1 4 1 Question 8 0 0 6 12 18 24 1 What fraction of a radioactive material is left after: time (h) a one half-life? b  two half-lives? The decay curve for Technetium-99m.270
  18. 18. P6: Radioactive materialsTime Hours since Number of Fraction of original injection half-lives sample remaining9.00 am 0 0 13.00 pm 6 1 ​  1 __ ​ 2 19.00 pm 12 2 ​ __ ​ 4 13.00 am 18 3 ​ __ ​ 8 19.00 am 24 4 ​ __ ​ 16   The radioactive decay of technetium-99m. 100The six-hour half-life makes it a useful medical tracer. It % of radon-220 activitylasts long enough for doctors to get some scans of the decay,but it has almost all gone in a few days. 50Different half-lives 25 12.5All radioactive materials show the same pattern but they can 0 0 1 2 3have different half-lives. The graph on the right shows the time (min)pattern of radioactive decay for radon. The decay curve for radon-220.There is no way of slowing down or speeding up the rate at Isotope Half-lifewhich radioactive materials decay. No chemical reaction ofphysical change makes any difference. Some decay slowly over Iridium-192 74 daysthousands of millions of years. Others decay in milliseconds – Strontium-81 22 minutesless than the blink of an eye. Uranium-235 710 million yearsThe shorter the half-life, the greater the activity for the sameamount of material. Of the four radioactive isotopes listed in Neon-17 0.1 secondsthe table on the right, neon-17 is the most active. Half-lives can be short or long. Question 2 Iodine-123 is used to investigate problems with the Summary box thyroid gland. It is a gamma emitter. DDHalf-life is the time a Explain why it is useful that iodine-123 gives out it takes for the activity of a gamma radiation. radioactive material b Iodine-123 has a half-life of 13 hours. Why would it be to drop by half. a problem if the half-life was: DDRadioactive materials i a lot shorter? have a wide range of ii a lot longer? half-lives. G: Half-life 271
  19. 19. H Medical imaging and treatment Radioactive materials can cause cancer. But they can also be Find out about used to diagnose and cure many health problems. D different uses of radiation Medical imaging D types of radiation Jo has been feeling unusually tired for some time. Her doctors D benefits and risks decide to investigate whether an infection may have damaged of using radioactive her kidneys when she was younger. materials They plan to give her an injection of DMSA. This is a chemical D limiting radiation that is taken up by normal kidney cells. dose The DMSA has been labelled as radioactive. This means its molecules contain an atom of technetium-99m (Tc-99m), which is radioactive. The Tc-99m gives out its gamma radiation from within the kidneys. Gamma radiation is very penetrating, so nearly all of it escapes from Jo’s body and is picked up by a gamma camera. Jo’s scan shows that she has only a small area of damage. The doctors will take no further action. Glowing in the dark Jo was temporarily contaminated by radioactive technetium. For the next few hours, until her body got rid of the technetium, she was told to: this gamma scan shows • flush the toilet a few times after using it correctly functioning kidneys – the top two white areas. • wash her hands thoroughly • void close physical contact with friends and family. a Is it worth it? There was a small chance that some gamma radiation would damage Jo’s healthy cells. Before the treatment, her mum had to sign a consent form, and the doctors checked that Jo was not pregnant. Exposure to gamma Put mind at rest radiation GAMMA Can treat problem SCAN Jo’s mum weighed the risk against the benefit and felt the investigation was worth it. RISKS BENEFITS 272
  20. 20. P6: Radioactive materialsJo’s mum said ‘We felt the risk was very small. It was worthit to find out what was wrong. Even with ordinary medicines,there can be risks. You have to weigh these things up. Nothingis completely safe.’Treatment for thyroid cancer thyroid glandAlf has thyroid cancer. First, he will have surgery to removethe tumour. Then he must have radiotherapy, to kill anycancer cells that may remain. The thyroid gland is located in the front of the neck, below the voice box.A hospital leaflet describes what will happen. Radioiodine treatment You will have to come in to hospital for a few days. You will stay in a single room. You will be given a capsule to swallow, which contains iodine-131. This form of iodine is radioactive. You cannot eat or drink anything else for a couple of hours. • The radioiodine is absorbed in your body. • Radioiodine naturally collects in your thyroid, because this gland uses iodine to make its hormone. Summary box • The radioiodine gives out beta radiation, which is DDRadioactive materials can be absorbed in the thyroid. used to diagnose • Any remaining cancer cells should be killed by the radiation. and treat medical problems. You will have to stay in your room and take some precautions DDThere are benefits for the safety of visitors and staff. You will remain in hospital and risks when using for a few days, until the amount of radioactivity in your body radioactive has fallen sufficiently. materials. Questions 1 Look at the precautions that Jo has to 3 What are the risks and the benefits to Jo take after the scan. Write a few sentences of having the treatment? explaining to Jo why she has to do each 4 Suggest how the risk to Alf’s family and of them. other patients is kept as low as possible. 2 It would be safe to stand next to Jo but 5 Explain why a half-life of eight days is not to kiss her. Use the words ‘irradiation’ more suitable than: and ‘contamination’ to explain why. a eight minutes b eight years H: Medical imaging and treatment 273
  21. 21. I Nuclear power Nuclear fission Find out about Radioactive atoms have an unstable nucleus. Some nuclei can D energy from nuclear be made so unstable that they split in two. This process is fission called nuclear fission. D nuclear power smaller nucleus stations ENERGY neutron U-235 nucleus splitting the nucleus of an atom. When the nucleus breaks apart a small amount of the mass of the nucleus is converted to a huge amount of energy. So the products of nuclear fission have a lot of kinetic energy. Each fission reaction produces roughly a million times more energy than when a molecule changes during a chemical reaction. Nuclear weapons during the second World War there was a race to ‘split the atom’ and use the energy in a bomb. on 16 July 1945, in a desert in the usa, a group of scientists tested ‘the gadget’. some thought it would not work. others worried that it might destroy the atmosphere. at 5.29 a.m., it was detonated and filled the skies with light. the bomb vaporized the metal tower supporting it. all desert sand within a distance of 700 m was turned into glass. some of the scientists were worried about the power of the bomb and wanted the project stopped. a few weeks later, the devastating power of a the americans dropped two nuclear bombs on Japan. nuclear weapon. 274
  22. 22. P6: Radioactive mateRialsGenerating electricityA nuclear power station uses a nuclear reactor. This is designed to release the energy of at a slow and steady rate. The fission takes place in the nuclear fuel. This makes them extremely hot.A fluid, called a coolant, is pumped through the reactor. The hot fuel rods heat the coolant to around 500 ˚C. It then flows through a heat exchanger in the boiler, turning water into steam. the reactor core is sealed andThe steam is used in the same way as in a coal- or gas-fired shielded. very little radiationpower station. One reason for building nuclear power stations gets to reduce the need for fossil fuels.Nuclear weaponsCountries sometimes build nuclear reactors to make nuclear material for weapons. Nuclear weapons inspectors try to ensure that nuclear power stations are very secure, account for all their waste and are not operated in unstable countries. Questions 1 explain why these countries might decide to build, or not to build, nuclear power stations. a no reserves of gas or coal. b a neighbouring country that has nuclear weapons. c a small land area, and a large population. d a population worried about climate change. e a history of nuclear accidents. When the fuel rods are being 2 suggest how gamma radiation from a nuclear reactor is used they become more contained, so that living things are not irradiated. radioactive because of the waste products. 3 Write down two risks and two benefits of living in a country with nuclear power stations. Summary box D In nuclear fission a nucleus splits, releasing energy. The energy released is much greater than in a chemical reaction. I: nucleaR PoWeR 275
  23. 23. J Nuclear waste Nuclear waste in the UK Find out about The Nuclear Decommissioning Agency (NDA) is responsible for D the UK’s nuclear cleaning up nuclear waste. Most of the radioactive waste comes waste from power stations. The rest comes from medical uses, industry D the half-life of and scientific research. In addition to this ‘everyday’ waste, radioactive when power stations are too old to be used anymore the waste materials radioactive materials must be taken away to be stored. The D possible methods of waste is called the UK’s ‘nuclear legacy’. disposal A long-term hazard Radioactive waste has very little effect on the UK’s average background radiation. But it is still hazardous. This is because of contamination. Imagine that some waste leaks into the water supply. This could be taken up by food, which you eat. The radioactive material is now in your stomach, where it can irradiate your internal organs. Some radioactive materials last for thousands of years. They must be kept safe and secure for all that time. high-level radioactive waste is Types of waste hot, so it is stored the nuclear industry deals with three underwater. types of nuclear waste. • High-level waste (hlW). this is ‘spent’ fuel rods. hlW gets hot because it is so radioactive. it has to be stored carefully but it doesn’t last long. and there isn’t very much of it. all the uK’s hlW is kept in a pool of water at sellafield. • Intermediate-level waste (ilW). this is less radioactive than hlW. But the amount of ilW is increasing, as hlW decays to become ilW. • Low-level waste (llW). Protective clothing and medical equipment can be slightly radioactive. it is packed in the control room at a nuclear waste storage drums and dumped in a landfill site plant. People monitor the waste continuously. that has been lined to prevent leaks. 276
  24. 24. P6: Radioactive materialsType of waste Volume (m3) Radioactivity 100% remaining in high-level waste percentage of radioactivityLLW 196 000 weakILW 92 500 strong 50%HLW 1730 extremely strong The amount of nuclear waste in store (2007). The problem of what to do with it remains unsolved. 0 50 100 10000Sellafield time in years (note changing scale)Sellafield, in Cumbria, is the biggest nuclear site in High Level Waste decays quickly at first. When its activity falls, itthe UK. Thousands of people work there. Sellafield processes becomes Intermediate Levelnuclear waste and stores it ready for permanent disposal. Waste. ILW stays radioactive for thousands of years.Keeping risks low is very important at Sellafield. They haveplans to maintain production and safety if anything goes wrong.Intermediate-level waste is the biggest technical challenge, When will it be ‘safe’?because it is very long-lived. Currently it is chopped up, mixed We are exposed to somewith concrete, and stored in thousands of large stainless-steel radiation all the timecontainers. This is secure but not permanent. The long-term (background radiation).solution has to be secure and permanent. When the nuclear waste only emits very low levelsThe work of the NDA of radiation, similar to theManaging waste is very expensive. In 2010 the NDA spent background radiation,£28 billion. There have been a number of public consultations it poses little risk. Theabout what to do with the waste. At the time of writing these longer the half-life ofare still going on. The preferred plan at the moment is to store the radioactive material, the longer it will take toit until a safe site can be found to bury it. become ‘safe’. Questions 1 Explain why disposing of ILW needs to be both a secure and b permanent 2 What are the advantages and disadvantages of keeping all the waste together above ground rather than burying it in a deep shaft and sealing it? Summary box DDNuclear waste is 3 A small amount of nuclear fuel produces a lot of energy, radioactive and so in the 1950s scientists thought this would be a cheap must be safely way of generating electricity. Explain why the real cost is stored for thousands much greater than realised at the time. of years. J: Nuclear waste 277
  25. 25. K Nuclear fusion In nuclear fusion the nuclei of two small atoms join together Find out about and energy is released. D nuclear fusion The diagram below shows one possible hydrogen fusion D the iter project reaction. Two hydrogen nuclei fuse to make a helium nucleus, and there is a neutron left over. left-over neutron Hydrogen Helium Fusion Hydrogen fusion of two hydrogen nuclei gives helium. The two hydrogen nuclei in the diagram have different numbers of neutrons, but you can tell they are both hydrogen because they have just one proton in the nucleus. Helium is formed. It has two protons. Helium is an unreactive gas. Positive charges repel It is difficult to get the two hydrogen nuclei close enough to fuse, because both nuclei have a positive electric charge. Similar charges repel. The hydrogen nuclei must have enough energy to overcome this force and collide, so that they can fuse. When the nuclei do fuse the new nucleus is more stable, and energy is released. The amount of energy released is very large. hydrogen is fused to helium in The quest for fusion power stations the sun. This is what scientists want to do: • fuse the hydrogen nuclei from water to give helium nuclei • use the energy released by this fusion reaction to generate electricity in power stations.278
  26. 26. P6: Radioactive mateRialsThe advantages are:• the fuel is water, so there is plenty and it is cheap The H bomb• nuclear fusion does not produce as much radioactive waste hydrogen bombs, which as nuclear fission. fuse hydrogen, releaseOver the past 70 years there has been a lot of research. hundreds of times more energy than atomicScientists can give hydrogen nuclei enough energy to (fission) bombs. they areovercome the repulsive force. The problem is controlling the triggered using an atomicreaction and keeping it going. bomb to compress theWhen hydrogen is heated to a very high temperature, the hydrogen so that it fuses.atoms lose their electrons and form a cloud of charged particles called a plasma. This is kept from touching the sides of the container by using magnetic fields. The JET project in the UK has researched fusion for many years. So far no reactor has produced more energy than it used.The ITER projectThis is a joint project between China, the European Atomic Energy Community, India, Japan, Korea, Russia and the USA. ITER means ‘the way’ in Latin. Fusion research is very expensive so these countries have joined together to build a research reactor in France. Construction has begun. It will take 10 years to build, and be used for research for 20 years. ITER will investigate how plasmas behave during the hydrogen fusion reaction at 150 million ЊC. One day they hope the planned iteR fusionto build a nuclear fusion power station. reactor. fusion will take place in the doughnut-shaped hole. Questions 1 When two hydrogen nuclei fuse what element is formed? 2 a What is the electric charge on a hydrogen nucleus? b explain whether two hydrogen nuclei attract or repel. 3 What are the advantages of countries working together on the iteR project? Summary box D Nuclear fusion 4 iteR is very expensive. Write a letter to persuade the releases energy. government eitheR: D Hydrogen nuclei a to stay part of iteR or fuse to form helium b to leave iteR to save money. nuclei. K: nucleaR fusion 279
  27. 27. ScienceExplanationsOur understanding of radioactivity and the structure of the atom has enabledmany applications, such as nuclear power stations and cancer treatment, to bedeveloped. Knowledge of the way ionising radiation behaves is essential forworking safely and making good risk assessments. ow: You should kn heliu m why some materials are radioactive and emit ionising radiation all the time hydrog how ionising radiation can damage living cells en that atoms have a nucleus made of protons and neutrons and is surrounded by electrons plutonium about the alpha scattering experiment and how it showed that the atom has a small, massive, positively charged nucleus that there are alpha and beta particles and gamma radiation uranium about the different penetration properties of alpha, beta, and gamma radiations  that there is background radiation all around us, mostly from natural sources what radiation dose measures, and what factors affect it foo d the difference between contamination and irradiation how to interpret data on risk related to radiation dose surg ical in struments that radioactive materials randomly emit ionising radiation all the time and that the rate of decay cannot be changed by physical or chemical changes ti o n i r ra d i a that the activity of a radioactive source decreases over time io n what is meant by the half-life of a radioactive isotope m i n at that radioactive sources have a wide range of half-life values nta co about how the half-life of a radioactive source affects the time it takes to become safe about uses of ionising radiation from radioactive materials and the people who work with them that nuclear fuels release energy when the nucleus changes during nuclear fission about the three categories of radioactive waste, and the different methods of disposal nuc lear that hydrogen nuclei can fuse together to form helium if they are brought was te close enough together and this releases energy that the energy released in a nuclear reaction is much greater than that released in a chemical reaction such as burning a similar mass of fuel.280