P6: Radioactive Materials

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  • Created by: emmacram
  • Created on: 21-11-15 12:34

Atoms and Elements

All elements are made of atoms; each element contains only one type of atom. All atoms contain a nucleus and electrons. The nucleus is made from protons and neutrons with the one exception of hydrogen (the lightest element), which has no neutrons - just one proton and one electron.

Some elements give out ionising radiation all the time; we call these elements radioactive. Neither chemical reactions nor physical processes (e.g. heating) can change the radioactive behaviour of a radioactive substance.

An atom has a nucleus made of protons and neutrons. Every atom of a particular element always has the same number of protons (if it contained a different number of protons it would be a different element). For example...

  • hydrogen atoms have one proton
  • helium atoms have two protons
  • oxygen atoms have eight protons.

However, some atoms of the same element can have different number of neutrons - these are called isotopes.

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Nuclear Fusion

At the beginning of the 20th century, discoveries about the nature of the atom and nuclear processes began to help answer the mystery of where the Sun's energy comes from.

In 1911, there was a ground-breaking experiment - the Rutherford-Geiger-Marsden alpha particle scattering experiment. In this experiment, a thin gold foil was bombarded with alpha particles. The effect on the alpha particles was recorded and these observations provided the evidence for our current understanding of atoms.

Most alpha particles were seen to pass straight through the gold foil. This would indicate that gold atoms were composed of large amount of open space. However, some particles were deflected slightly and a few were even deflected back towards the source. This would indicate that the alpha particles passed close to something positively charged within the atom and were repelled by it.

These observations brought Rutherford and Marsden to conclude that: gold atoms, and therefore all atoms, consist of largely empty space with a small, positive region called the nucleus, the nucleus is positively charged and the elctrons are arranged around the nucleus with a great deal of space between them.

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Nuclear Fusion.

Rutherford's data allowed him to identify a small positive nucleus, but the limited data did not explain its structure. Later experiments revealed the presence of positive protons and neutral neutrons in the nucleus held together by the short-range strong nuclear force. Protons normally repel each other (because they have the same charge), but the strong nuclear force balances the repulsive electrostatic force between the protons.

The strong nuclear force between protons and neutrons can also cause hydrogen nuclei (or protons) that are close enough to each other to fuse into helium nuclei. This process releases large amounts of energy and is known as nuclear fusion. It is the same process that occurs inside the Sun.

One of Albert Einstein's greatest insights was to realise that matter and energy are really different forms of the same thing, shown in his famous equation: E=mc squared

The same equation can be used to calculate the energy released during nuclear fusion and fission.

If two hydrogen nuclei are forced together, one of the protons will change into a neutron, resulting in the formation of a deuterium nucleus which has a lower mass than the proton and neutron separately. The 'mass defect' is converted into energy and can be calculated with the equation.

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Radioactive Decay

The emission of ionising radiation occurs because the nucleus of an unstable atom is decaying. The type of decay depends on why the nucleus is unstable; the process of decay helps make the atom become more stable. During decay, the number of protons in the atom may change. If this happens, the element changes from one type to another.

The original atom decays by ejecting an alpha particle from the nucleus. This particle is a helium nucleus: a particle made up of two protons and two neutrons. With alpha decay a new atom is formed. This new atom has two protons and two neutrons fewer than the original.

The original atom decays by changing a neutron into a proton and an electron. This high-energy electron, which is now ejected from the nucleus, is a beta particle. With beta decay a new atom is formed. This new atom has one more proton and one less neutron than the original.

After alpha or beta decay, a nucleus a nucleus sometimes contains surplus energy. It emits this as gamma radiation, which is very high frequency electromagnetic radiation. During gamma decay, only energy is emitted. This does not change the type of atom.

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Background Radiation

Radioactive elements are found naturally in the environment. The radiation produced by these sources contributes to the overall background radiation.

There is nothing we can do to prevent ourselves from being irradiated and contaminated by background radiation, but the level of background radiation in most places is so low that is nothing to worry about. There is, however, a correlation between certain cancers and living for many years in areas where the underlying rock is granite.

(from biggest amount to smallest amount)

  • Radon gas - released at surface of ground from uranium in rocks and soil.
  • From food
  • Medical - mainly x-rays.
  • From rocks, soil and building materials.
  • Cosmic rays - from outer space and the Sun.
  • Nuclear industry.
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Half-life

The activity of a substance is a measure of the amount of radiation given out per second.

When a radioactive atom decays it becomes less radioactive and its activity drops. The half-life of a substance is the time it takes for the radiation emitted by a radioactive material to halve. The half-life of a radioactive material can range from a few seconds to millions of years.

All radioactive substances become less radioactive as time passes. A substance would be considered safe once its activity had dropped to the same level emitted as background radiation, which is a dose of around 2 millisieverts per year or 25 counts per minute with a standard detector.

Some substances decay quickly and could be safe in a very short time. Substances with a long half-life remain harmful for millions of years.

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Ionisation and Cell Damage

When radiation interacts with neutral atoms or molecules, the atoms or molecules may become charged due to electrons being knocked out of their structure. This alters their structure, leaving them as charged particles called ions. Alpha, beta and gamma radiation are therefore known as ionising radiation and can damage molecules in healthy cells, which results in the death of the cell

When living cells absorb radiation, damage can occur in different ways:

  • Ionising radiation can damage cells, causing ageing of the skin.
  • Ionising radiation can cause mutations in the nucleus of a cell, which can lead to cancer.
  • Different amounts of exposure can cause different effects, e.g. high-intensity ionising radiation can kill cells, leading to radiation poisoning.

Ionising radiation is able to break molecules into bits (called ions), which can then take part in other chemical reactions.

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Uses of Radiation

Although using ionising radiation can be dangerous, there are many beneficial uses:

Cancer Treatment - High-energy gamma rays can be used to kill cancer cells. However, ionising radiation can damage living cells too, so the radiation has to be carefully targeted from different angles to minimise the damage to healthy cells. Radioactive iodine can be used to target thyroid cancer. Iodine is needed by the thyroid, so it collects naturally in the thyroid, where it gives out beta radiation and kills the cancer cells.

In both of these examples there is a danger of damage to healthy cells, so the doctors need to carefully weigh the risks against the benefits. When deciding on the dose of radiation to use, the ALARA (as low as reasonably achievable) principle should be applied. This means that measures should be taken to make the risks as small as possible, while still providing the benefits and taking into account all the implications.

Sterilising Surgical Instruments - Surgical instruments must not have any bacteria on them. Bacteria are living cells, so are susceptible to damage from ionising radiation. Exposing surgical instruments in sealed bags to gamma radiation results in the death of the bacterial cells, and therefore sterilisation of the surgical instruments. The instruments remain sterile until they are ready to be used.

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Uses of Radiation.

Sterilising Food - Irradiating food kills any bacteria that would cause the food to go off. Radiation treatment is only allowed on a few foods in the UK and these have to carry a label stating that they have been treated with radiation.

Tracers in the Body - Doctors use radioactive chemicals called tracers to help detect damage to the internal organs. Once the tracer enters the body, it builds up in the damaged or diseased part of the body. Radiation detectors can then be used to detect where the problem is. These can be linked to computers, which produce an image showing the distribution of the radioactive chemical. Problem areas are highlighted by a high concentration.

Because it is being used inside the body, the radioactive tracer must be non-toxic. It also needs to have a short half-life so that it breaks down quickly after use. Gamma and beta sources are used because they pass out of the body easily. An alpha source is never used because it would be quickly absorbed and cause damage.

Technetium-99 is  gamma emitter often used for this purpose. It has a half-life of 6 hours. This gives doctors enough time to detect the problem, but ensures that the radiation decreases to a safe level quickly.

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Dangers of Radiation

New scientific advances often create an element of risk. The transportation and application of radioactive substances is carefully controlled by government rules and regulations to minimise the risk to the general public. However, people working in the nuclear industry, medical physics (x-rays etc.) and many other areas often have to work with radioactive materials. They can become irradiated or contaminated by the materials, which could lead to serious health problems or even death. Therefore, the exposure that these people are subjected to needs to be carefully monitored. The different types of radiation carry different risks:

  • Alpha is the most dangerous if the source is inside the body; all the radiation will be absorbed by cells in the body.
  • Beta is the most dangerous if the source is outside the body because, unlike alpha, it can penetrate the outer layer of skin and damage the internal organs.
  • Gamma can cause harm if it is absorbed by the cells in the body, but it is weakly ionising and can pass straight through the body, causing little damage.

The sievert is a measure of a radiation dose's potential to do harm to a person. It is based on both the type and amount of radiation absorbed. A 5 sievert dose is a 5 sievert dose regardless of the type of radiation absorbed. This makes the sievert a very useful unit for radiation safety measures.

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Radiation Protection

The transportation and application of radioactive substances is carefully controlled by government rules and regulations to minimise the risks to the general public. Authorised personnel who handle radioactive substances or operate machinery follow strict guidelines. These vary depending on the risk factor involved. Below is a list of some measures that can be taken to reduce/prevent exposure to different types of radiation.

External Exposure - Beta and Gamma

  • Minimise time of exposure.
  • Maximise distance from the source of radiation.
  • Wear protective clothing (and remove it before leaving a restricted area).
  • Avoid direct handling of radioactive materials, i.e. use implements like forceps and tongs.
  • Use protective shields, screens and containers.
  • Use instruments that can detect levels of radiation, e.g. Geiger-Muller counter or liquid scintillation counter.
  • Use materials that can provide a shield against radiation, e.g. lead, concrete and water.
  • Wear a film badge that monitors the degree of exposure to radiation.
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Radiation Protection.

Internal Exposure - Alpha, Beta and Gamma 

  • Wear chemical fume hoods and protective masks to prevent inhalation.
  • Never consume or store food and drink close to a radioactive source.
  • Wear protective clothing.
  • Ensure that any cuts or wounds are sealed up.
  • Minimise amount of radioactive material to be handled.

People who work in the nuclear industry, such as radiographers and nuclear power plant technicians, are regularly exposed to radiation. They often wear a badge to monitor their degree of exposure. The badge contains photographic film, which (after developing) becomes darker the more it is exposed to radiation. In this way, radiation exposure can be carefully monitored. Other physical barriers are used to protect people from ionising radiation:

  • Radiographers use lead screens to prevent exposure.
  • Nuclear reactors are encased in thick lead and concrete to prevent radiation escaping into the environment.
  • Nuclear technicians going into areas of high levels of radiation must wear a radiation suit made from materials that act as a shield against the radiation.
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Nuclear Fission

In a chemical reaction it is the electrons that bring about the change. The elements involved remain the same but join up in different ways.

A fission reaction takes place in the nucleus of the atom and different elements are formed. A neutron is absorbed by a large and unstable uranium or plutonium nucleus. This splits the nucleus into two, roughly equal-sized, smaller nuclei and releases energy and more neutrons.

A fission reaction releases far more energy than that released from a chemical reaction involving a smaller mass of material. Once fission has taken place the neutrons released can be absorbed by other nuclei and further fission reactions can take place. This is called a chain reaction.

A chain reaction occurs when there is enough fissile material to prevent too many neutrons escaping without being absorbed. This is called critical mass and ensures every reaction triggers at least one further reaction.

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The Nuclear Reactor

Nuclear power stations use fission reactions to generate the heat needed to boil water into steam. The reactor controls the chain reaction so that the energy is released at a steady rate.

Fission occurs in the fuel rods and causes them to become very hot.

The coolant is a liquid that is pumped through the reactor. The coolant heats up and is then used in the heat exchanger to turn water into steam.

Control rods, made of boron, absorb neutrons, preventing the chain reaction getting out of control. Moving the control rods in and out of the reactor core changes the amount of fission that takes place.

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Electricity

Electricity is called a secondary energy source because it is generated from another energy source e.g. coal, nuclear, wind, etc.

As part of the generation process some energy is always lost to the surroundings. This makes electricity less efficient than when using the primary resource directly.

However, the convenience of electricity makes it very useful. It can be easily transmitted over long distances and used in a variety of ways.

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Generating Electricity

To generate electricity, fuel (either fossil fuel or nuclear) is used to produce heat. The heat is used to boil water that produces steam, and the steam is then used to drive the turbines that power the generators. The electricity produced in the generators is sent to a transformer and then to the National Grid, from where we can access it in our homes.

In a nuclear power station, the energy is released due to changes in the nucleus of radioactive substance. Nuclear power stations do not produce carbon dioxide but they do produce radioactive waste. This nuclear waste is categorised into three types:

  • High-level waste (HLW) - very radioactive waste that has to be stored carefully. Fortunately, only small amounts are produced and its activity decreases quickly, so it is put into short-term storage.
  • Intermediate-level waste (ILW) - not as radioactive as HLW but it does remain radioactive for thousands of years. The amount produced is increasing; deciding how to store it safely and permanently is a problem. At the moment most ILW is mixed with concrete and stored in big containers, but this is not a permanent solution.
  • Low-level waste (LLW) - only slighly radioactive waste that is sealed and placed in landfills.
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