Atoms and Elements
All elements are made of atoms; each element contains only one type of atom. All atoms contain nucleus and electrons.
The nucleus is made from protons and neutrons. Hydrogen (the lightest element) is the one exception; it has no neutrons, just one proton and one electron.
Radioactive elements emit ionising radiation all the time. Neither chemical reactions nor physical processes (e.g. smelting) can change the radioactive behaviour of a substance.
Every atom of a particular element always has the same number of proton. If it contained a different number of protons it would be a different element. For example:
- hydrogen atoms have 1 protons
- helium atoms have 2 protons
- oxygen atoms have 8 protons.
But some atoms of the same element can have different numbers of neutrons - these are called isotopes.
Radioactive materials can give out three types of of ionising radiation:
Different radioactive materials will give out any one, or a combination, of these radiations.
The different types of radiation have different penetrating powers.
Ionising radiation is emitted when the nucleus of an unstable atom decays. The type of radioactive decay depends on why the nucleus is unstable, the process of decay helps the atom become more stable. During decay the number of protons may change. If this happens the element changes to another type.
Radioactive elements are found naturally in the environment and contribute to background radiation. Nothing can stop us being irradiated and contaminated by background radiation, but generally the levels are so low it's nothing to worry about. However, there is a correlation between certain cancers and living in particular areas, especially among people who have lived in granite buildings for many years.
As a radioactive atom decays, it's activity drops. This means that radioactivity decreases over time.
The half-life of a substance is the time it takes for its radioactivity to halve.
Different substances have different half lives, ranging from a few seconds to thousands of years.
Half life and safety.
A substance is considered safe once its activity drops to the same level as background radiation. This is a dose of around 2 milli-sieverts per year or 25 counts per minute with a standard detector.
Some substances decay quickly and could be safe in a very short time. Those with a long half-life remain harmful for thousands of years.
Half life calculations
The half-life can be used to calculate how old a radioactive substance is, or how long it will take to become safe.
For example, if a sample has an activity of 800 counts per minute and a half life of two hours, you need to find out how many half lives if takes for the background rate to be 25 counts per minutes.
It takes 5 half lives to reach a count of 25 and each half life is 2 hours.
So, it takes 5 times 2 hours = 10 hours.
Dangers of radiation
Ionising radiation can break molecules into ions. These ions can harm living cells.
Ions are very reactive and can take part in other chemical reactions.
Many jobs involve radioactive materials. People can become irradiated or contaminated so their exposure needs to be carefully monitored.
Different types of radiation carry out 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. Unlike alpha, it can penetrate the outer layer of skin and damage internal organs.
- Gamma - can cause harm if it's absorbed by the cells, but it is weakly ionising and can pass straight through the body causing no damage at all.
The sievert is a measure of a radiations dose's potential harm to a person. It's based on both the type and amount of radiation absorbed.
Uses of radiation
Although using ionising radiation can be dangerous, there are many beneficial uses.
High energy gamma rays in cancer treatment can destroy cancer cells but damage healthy cells too. The radiation has to be carefully targeted from different angles to minimise the damage.
Risks must be assessed and the A L A R A rule applied.
Radiation is also used to sterilise surgical instruments and to sterilise food. This kills bacteria.
The precautionary principle is applied if the risks are unknown e.g. only a few foods are allowed radiation treatment and they must carry a label stating this. This priority is to protect the public.
Electricity is a secondary energy source. This means it generated from another energy source, e.g. coal, nuclear power etc.
Electricity is a very useful energy source as it can be easily transmitted over long distances and used in many ways.
To generate electricity, fuel is burned to produce heat:
- the heat is used to boil water; which produces steam.
- the steam drives the turbines, which power the generators.
- electricity produced in the generators is sent to a transformer and then to the national grid from where you can access it in your home.
Power stations which burn fossil fuels like coal produce carbon dioxide, a greenhouse gas.
Nuclear power stations release energy due to changes in the nucleus of radioactive substances. They don't produce carbon dioxide but they do produce radio active waste.
Nuclear waste is categorised into three types:
- high level waste - very radioactive waste that has to be stored carefully. Fortunately, only small amounts are produced and it doesn't remain radioactive for long, so it's put into short term storage.
- intermediate level waste - not as radioactive as high level waste but it remains radioactive for thousands of years. Increasing amounts are produced; deciding on how to store it is a problem. At the moment most is mixed with concrete and stored in big containers, but this isn't a permanent solution.
- Low level waste - only slightly radioactive waste that is sealed and placed in landfills.
Energy is lost at every stage of the process of electricity generation.
San-key diagrams can be used to show the generation and distribution of electricity. Including the efficiency of energy transfers.
The San-key diagram shows that from the energy put in the power station, almost half is lost to the surroundings (mostly as heat) before the electricity even reaches the home.
Further energy is lost during energy transfers in the home when electricity is used.
Conventional energy supplies are running out and both nuclear and fossil fuels cause environmental damage. This means that alternative energy sources are becoming important.
Alternative ways to generate electricity include wind, water and solar power. These are renewable energy sources so will not run out like fossil fuels.
An example of a renewable energy source is wind turbines. The force of the wind turns the blades of the wind turbine, which provides power to a generator. The amount of electricity produced is small. It would need hundreds on wind turbines to replace a conventional power station. However, once built they provide free energy, as long as the wind is blowing.
Hydro-electric dams are another example of a renewable energy source. Water stored in the reservoir flows down pipes and turns the turbines, which powers the generators and produces electricity.
Large areas of land may need to be flooded to build hydro-electric stations. However, once built they provide large amounts of reliable, fairly cheap energy.
When comparing energy sources for generating electricity, the factors used to assess which source is the most favourable are efficiency, cost and environmental damage.
Power output and lifetime can also be assessed when comparing energy sources.
In a chemical reaction it is the electrons that cause the change. The elements involved stay the same but join up in different ways.
Nuclear fission takes place in the nucleus of an atom and different elements are formed:
- A neutron is absorbed by a large and unstable uranium nucleus. This splits into two roughly equal-sized smaller nuclei. This releases energy and more neutrons.
- A fission reaction releases far more energy than even the most exothermic chemical reactions. Once fission has taken place the neutrons can be absorbed by other nuclei and further fission reactions can take place. This is a chain reaction.
- A chain reaction occurs when there's 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.
The nuclear reactor
Nuclear power stations use fission reactions to generate the heat needed to produce steam. The nuclear reactor controls the chain reaction so that the energy is steadily released.
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 which takes place.