P6- Radioactive materials

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Atomic structure and radiation

Atoms contain three sub-atomic particles called protons, neutrons and electrons. The protons and neutrons are found in the nucleus at the centre of the atom. The protons and neutrons are held together in the nucleus by a strong force which balances the repulsive electrostatic force between the protons. The electrons are outside the nucleus.

Radioactive elements give out ionising radiation from their nuclei. This happens all the time, whatever is done to the substance. For example, radiation is still given out if the substance is cooled in a freezer or involved in a chemical reaction.

Radioactive elements are found naturally in the environment. The radiation they give off is called background radiation

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Nuclear fusion

If hydrogen nuclei are brought close enough together then they have the ability to fuse. This process is called nuclear fusion. It takes a lot of work to force nuclei together but a lot of energy is released when they fuse.

[2/1]H + [3/1]H -> [4/2]He + [1/0]n (http://www.bbc.co.uk/staticarchive/afb9bb535a90660f212b31da0963ed2cef301b2e.gif)

The equation above is an example of nuclear fusion. Two isotopes of hydrogen have fused to create helium.

Isotopes are atoms of an element that have different mass numbers because they have different numbers of neutrons in the nucleus. The following equation can be used to calculate the energy produced during nuclear fusion (and fission) reactions: E = mc2

Where: E = is the energy produced

  • M = change in mass
  • C = speed of light
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Penetrating properties of radiation

Radiation can be absorbed by substances in its path. Alpha radiation travels only a few centimetres in air, beta travels tens of centimetres, while gamma radiation travels many metres. All types of radiation become less intense the further they are from the radioactive material, as the particles or rays become more spread out.

The thicker the substance, the more the radiation is absorbed. But the three types of radiation penetrate materials in different ways

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Types of Radiation

Alpha radiation is the least penetrating. It can be stopped, or absorbed, by just a sheet of paper.

Beta radiation is able to penetrate air and paper. It can be stopped by a thin sheet of aluminium.

Gamma radiation is the most penetrating. Even small levels can penetrate air, paper or thin metal. Higher levels can only be stopped by many centimetres of lead, or many metres of concrete.

alpha radiation cannot pass through humans, beta is stopped by aluminium, gamma by lead  (http://www.bbc.co.uk/staticarchive/c80f7f1afb8f57f82b4e3c799af680b12b24f856.gif)

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Nuclear equations

When an unstable nucleus decays, there are three ways that it can do so. It may give out:

  • An alpha particle- Two protons and two neutrons – the same as a helium nucleus
  • A beta particle- Fast-moving electron
  • A gamma ray- High energy electromagnetic radiation

The nucleus of an atom can be represented as:

Shows X representing the chemical symbol. In front of X, A (at the top) represents atomic mass and Z (at the bottom) represents atomic number (http://www.bbc.co.uk/staticarchive/e977174f8a2a2a25e08cbb5c64ec4640c354a625.gif)

Where:

  • A = atomic mass (number of protons + neutrons)

  • Z = atomic number (number of protons)

  • X = chemical symbol (as shown on the periodic table)

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Alpha decay

When an alpha particle is emitted from the nucleus, the nucleus loses two protons and two neutrons. This means the atomic mass number decreases by four and the atomic number decreases by two. A new element is formed that is two places lower in the Periodic Table than the original element.

Radon decays into polonium when it emits an alpha particle. Here is the equation for that radioactive decay.

219 / 86 Rn -> 215 / 84 Po + 4 / 2 He (http://www.bbc.co.uk/staticarchive/8ee333f1783d861b6561384678cda56b9b24bc2d.gif)

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Beta decay

In Beta decay a neutron changes into a proton plus an electron. The proton stays in the nucleus and the electron leaves the atom with high energy, and we call it a beta particle.

When a beta particle is emitted from the nucleus, the nucleus has one more proton and one less neutron. This means the atomic mass number remains unchanged and the atomic number increases by 1.

Carbon-14 is a radioactive isotope of carbon. (It's a carbon atom with 8 neutrons instead of the usual 6.) Here is the equation for the beta decay of carbon-14 into nitrogen.

14 / 6 C leads to 14 / 7 N + 0 / -1 e - (http://www.bbc.co.uk/staticarchive/32e0b33c55ac253fce174523732497c7d5244393.gif)

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Half-life

The nuclei of radioactive atoms are unstable. They break down and change into a completely different type of atom. This process is called radioactive decay. For example, carbon-14 decays to nitrogen-14 when it emits beta radiation.

Over time, as the unstable atoms in a source of radiation change, the activity of the source goes down because there are fewer unstable atoms present to decay.

It is not possible to predict when an individual atom might decay, but you can measure how long it takes for half the atoms to decay. This is called the half-life of this type of radioactive atom.

There are two definitions of a half-life, but they mean essentially the same thing:

  1. The time it takes for the number of atoms in a sample to halve
  2. The time it takes for the activity of a source of radiation to fall to half its starting level

Different radioactive isotopes and types of radioactive material have different half-lives. For example, the half-life of carbon-14 is nearly 6000 years, but the half-life of radon-222 is under four days.

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Graphs to illustrate a half-life

It is possible to find out the half-life of a radioactive substance from a graph showing the count rate against time. The graph shows the decay curve for a radioactive substance. The count rate drops from 80 to 40 counts per minute in two days. So the half-life is two days. In the next two days, it drops from 40 to 20, ie it halves. In the two days after that, it drops from 20 to 10, ie it halves again, and so on.

counts per minute drops from 80 to 5 in 10 days (http://www.bbc.co.uk/staticarchive/36722163c4eb8ddcd1812c9e2fdc04621cb1728c.gif)

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Isotopes

All the atoms of a given element have the same number of protons. The number of neutrons can vary. Atoms of the same element that have different numbers of neutrons are called isotopes of that element.

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Changing elements

When an atom emits alpha or beta radiation, its nucleus changes. It becomes the nucleus of a different element. This is because the number of protons in the nucleus determines which element the atom belongs to.

These are the changes that occur to the number of protons in an unstable nucleus when it emits a radioactive particle:

  • Alpha particle - the number of protons goes down by two
  • Beta particle - the number of protons increases by one

In each case, a different element is left behind.

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Ionising radiation and living cells

The radiations from radioactive materials – alpha, beta and gamma radiation – are all ionising radiations which can damage living cells.

This happens because ionising radiation can break molecules into bits called ions. These ions can then take part in other chemical reactions in the living cells.

This may result in the living cells dying, or becoming cancerous.

yellow circle with 3 black pie shaped segments inside (http://www.bbc.co.uk/staticarchive/1f3a8158c606e99bf8b7b1ad96eaf6ef41d39c66.gif)

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Background radiation- sources

Radiation is all around us. It comes from radioactive substances including the ground, air, building materials and food. Radiation is also found in cosmic rays from space.

Cosmic rays- Radiation that reaches the Earth from outer space

Animals- Emit natural levels of radiation

Rocks- Some rocks give off radioactive radon gas

Soil and plants-  Radioactive materials from rocks in the ground are absorbed by the soil and hence passed on to plants

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Radon gas

Some rocks contain radioactive substances that produce a radioactive gas called radon. The left-hand pie chart shows the average contribution of these different sources to natural background radiation.

50% radon gas from the ground, 12% buildings and the ground, 12% food and drink, 12% cosmic rays, 14% artificial sources - mainly cosmic rays, small amount of nuclear power and weapons test (http://www.bbc.co.uk/staticarchive/297db7543d9bf4f4bef75de80eb0f2816d3c05b5.gif)

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Hazards from radioactive materials

Radioactive materials in the environment, whether natural or artificial, do expose people to risks.

This can happen in two ways:

  • The radiation from the material can damage the cells of the person directly. This is damage by irradiation.
  • Some of the radioactive material can be swallowed or breathed in. While inside the body, the radiation it emits can produce damage. This is damage by contamination.
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Length of time for radioactive materials to become

Radioactive materials, such as the radioactive waste from power stations, can be a real health hazard.

However, they do not stay radioactive forever. Each radioactive material has a half-life, and after this time it will have only half the activity it had before. Another half-life, and it's down to a quarter. Eventually, the activity will be similar to background radiation, and the material will be safe. For some sources, this could be thousands of years.

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Radiotherapy

Although ionising radiation can cause cancer, high doses can be directed at cancerous cells to kill them. This is called radiotherapy. About 40 per cent of people with cancer undergo radiotherapy as part of their treatment. It is administered in two main ways:

  • From outside the body using x-rays or the radiation from radioactive cobalt
  • From inside the body by putting radioactive materials into the tumour, or close to it

Some normal cells are also damaged by the radiation, but they can repair themselves better than the cancer cells are able to.

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Radioactive tracers

Radioactive molecules of an atom can be used as radioactive tracers. These tracers emit gamma radiation that will almost all penetrate out of the body to be detected by a gamma camera. This camera traces where the radioactive molecule goes in the body. Images can then be formed to allow doctors to see areas of damage to assess what treatment is needed.

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Sterilising surgical instruments

Surgical instruments are sterilised using high doses of gamma radiation from radioactive cobalt. Food can also be sterilised by gamma radiation in the same way. The radiation kills microorganisms, preserving the food for longer. But in the UK only irradiated herbs, spices or vegetable seasonings, with the correct labels, are allowed. Major supermarkets say they will not stock irradiated foods because people are reluctant to buy them, even though the food itself is not radioactive.

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Monitoring radiation

The human senses cannot detect radiation, so we need equipment to do this. Photographic film goes darker when it absorbs radiation, like when it absorbs visible light. The more radiation the film absorbs, the darker it is when developed.

People who work with radiation wear film badges which are checked regularly to monitor the levels of radiation absorbed. The diagram shows the inside of a typical radiation badge when it is closed and opened.

photographic film is sealed in thin plastic (http://www.bbc.co.uk/staticarchive/ef1363b91ca0ba080dfe7716f4e4494b952aafaf.gif)

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Continued

There is a light-proof packet of photographic film inside the badge. The more radiation this absorbs, the darker it becomes when it is developed. To get an accurate measure of the dose received, the badge contains different materials that the radiation must penetrate to reach the film. These may include aluminium, copper, lead-tin alloy and plastic. There is also an open area at the centre of the badge.

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Being exposed to radiation

People who may be exposed to radiation regularly include:

  • Medical staff such as radiographers
  • Workers at nuclear power stations
  • Research scientists
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Nuclear power stations

The main nuclear fuels are uranium and plutonium, both of which are radioactive metals. Nuclear fuels are not burned to release energy. Instead, heat is released from changes in the nucleus.

Just as with power stations burning fossil fuels, the heat energy is used to boil water. The kinetic energy in the expanding steam spins turbines, which drive generators to produce electricity. Nuclear waste is given different categories.

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Nuclear fission

Nuclear power stations use the heat released by nuclear reactions to boil water to make steam.

The type of nuclear reaction used is called nuclear fission. In nuclear fission:

  1. A neutron collides with a uranium nucleus. A uranium nucleus is large and unstable
  2. The uranium nucleus splits into two similar-sized smaller nuclei
  3. More neutrons are released
  4. These neutrons can then collide with more uranium nuclei

These processes are repeated continuously, forming a chain reaction.

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Rate of energy released

In a nuclear reactor, the reaction is controlled so that energy is released at a steady rate.

The energy released in nuclear fission is far greater than the energy released in a chemical reaction, such as burning fuel. This means that the power output of a nuclear power station is large. The lifetime of a nuclear power station is about 20 years.

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Advantages and disadvantages

In considering the subject of nuclear power, it is important to weigh up the advantages and the disadvantages. These are some of the advantages:

  • No carbon dioxide is produced when the station is operating
  • There is a high power output
  • A small amount of fuel is needed, when compared with coal or gas

These are some of the disadvantages:

  • Hazardous radioactive waste is produced
  • Building the power stations is quite expensive
  • Decommissioning, ie taking apart, the power stations at the end of their lifetime is very costly.
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Diagram

(http://www.bbc.co.uk/staticarchive/b0df4d70e21905aa3d9af7af8ce9f9b64c400eed.gif)

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Nuclear

The nuclear fuel, usually uranium oxide, is held in metal containers called fuel rods. These are lowered into the reactor core. A coolant, usually water or carbon dioxide, is circulated through the reactor core to remove the heat. Control rods are also lowered into the core. These absorb neutrons and control the rate of the chain reaction. They are raised to speed it up, or lowered to slow it down.

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Ideas about science - risk

Scientific or technological developments often introduce new risks.

  • The development of radioactive materials in the early 20th century led to the deaths of many workers. As the materials were new, no one realised they could be dangerous.

Risk can sometimes be assessed by measuring its chance of occurring in a large sample.

  • The safe dose that people may receive has been based on the rate of cancer in workers exposed to radiation over many years.

It is important to be able to assess the size of risks in any activity. No activity is completely safe.

  • The likelihood of dying from a nuclear accident has been calculated, and it is very low. Cycling is much more dangerous.

For most risky activities, there are benefits as well as risks.

  • A gamma scan gives doctors valuable information to help cure a patient. This benefit outweighs the slight risk from the gamma radiation itself.
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Making a judgement

To make a judgement about a possible bad outcome you need to consider two factors:

  1. What is the chance of the outcome happening?
    • If the dose (in sieverts (Sv)) someone has received is known, the chance of them getting a cancer is also known.
  2. What is the consequence of that outcome?
    • Cancers are serious conditions but, if diagnosed early, treatment is now very successful for most cancers. This makes the consequence less severe.

People's perception of a particular risk can often differ from the statistically calculated risk. The risk of unfamiliar things (such as skydiving) and things that have an invisible effect (like ionising radiation) is often overestimated by people. Attempts are often made to assess the chance of something happening and the consequences if it did.

  • People who are worried about working with radioactive materials may turn down a job in any situation where radioactive materials are used
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Continued

The real risk may be very different from the perceived risk ie the risk that you think is there.

  • Nuclear radiation is invisible, and sounds threatening to many people. This makes the risk seem worse than something you can see, and which is more familiar.
  • Many people do not realise that nuclear radiation has always been part of our environment.
  • People are afraid that irradiated food is itself radioactive, even though this is not true.

When making decisions about science and technology a number of social and economic factors must be considered. This usually applies to an organisation which is responsible for its employees.

  • Government regulations set a limit on the amount of radiation any worker may receive
  • Radiation workers must wear radiation badges to monitor the dose they receive to make sure they do not exceed the limit
  • The employers check the workers' doses regularly and arrange regular medical checks for them
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