We are exposed to radiation every second of every day. This radiation comes from many sources, and depending on our jobs or where we live, can be higher or lower. However, our bodies have evolved to cope with this Background Radiation and can correct the damage it causes. The only time radiation becomes dangerous is when the amount being received is greater than what our bodies can repair.
There are many sources of background radiation, some natural and some man-made:-
As can be seen from the above pie chart, most of the background radiation people are exposed to comes from natural sources, with the majority coming from Radon gas which is found naturally in the ground, especially in regions with a large amount of Granite (eg Aberdeen).
The food and drink we eat also make up a large portion of the background radiation, mostly due to radioactive Carbon and Potassium isotopes. For example, Brazil Nuts are noticeably radioactive (though still safe to eat!) when measured with a Geiger-Muller tube, due their extensive root systems absorbing Radon gas.
The video below shows some examples of sources of background radiation:-
In previous sections, the function of a thermal power station (Coal, Oil or Gas fired) was discussed.
A Nuclear power station is also a thermal power station, the fuel heats water, which is converted to steam, which drives a turbine, turning a generator, generating Electricity. The only difference is how the water is heated.
Currently, although there are two processes that could be used for nuclear power, Fission and Fusion, only fission is currently used.
In a fossil fuel power station, the fuel is burnt to release heat energy to boil the water, but in a nuclear power station, the process is very different:-
Fission is process where a large atomic nucleus is split into two (or more) smaller nuclei, releasing several Neutrons and Energy.
The above diagram shows how energy is released during an Uncontrolled Fission Chain Reaction. Below is a step by step explanation of the process:-
1. A slow-moving Neutron collides with a Uranium-235 nucleus
2. The collision causes the nucleus to divide into two or more daughter elements (whose mass and proton numbers add to the original Uranium value).
3. This causes a large release of energy as well as 2-3 Neutrons.
4. Each of these Neutrons can then collide with further Uranium nuclei, causing a rapidly increasing release of Energy (a chain reaction).
Note - In the above process, the energy is released in the form of the Kinetic Energy of the daughter products. These will then collide with other materials and release the energy as heat.
Controlled Nuclear Fission
The above chain reaction releases a very large amount of energy in a very short time, which is effectively a nuclear explosion.
In order to use this energy safely within a power station, the Energy release must be kept at a constant level. The diagram below shows the core of a Fission Reactor, and can be used to show the control process:-
There are four main sections to the core of a Nuclear reactor :-
Radiation Shielding - The core is contained within a pressure vessel and surrounded by layers of lead and concrete to prevent the release of Radiation.
Fuel Rods - Long rods of very pure Uranium-235, the source of the Nuclear Fission.
Graphite Moderator - Graphite is used to slow down the Neutrons, increasing the efficiency of the reactor.
Control Rods - Made of Boron, the rods absorb Neutrons and by changing the depth of the control rods in the Reactor, the Energy output is controlled. In case of emergency, the rods can be dropped fully into the reactor, stopping the reaction completely.
As stated above, only fission reactions are used to generate power within nuclear power stations. This is because, at present, experimental fusion reactors do not give out a commercially viable amount of energy.
Fusion is the process by which two small nuclei combine under high temperatures and pressures to form a larger nucleus, releasing Energy.
The above diagram shows the process of nuclear fusion. Two isotopes of Hydrogen fuse together under intense Temperature and Pressure to form stable Helium, releasing a large amount of Energy.
It is this requirement for Intense temperature and pressure that means fusion can only occur naturally at the core of a star. The main focus of research into making nuclear fusion a viable power source is the development of systems to recreate these conditions.
In the above reaction, two isotopes of hydrogen underwent fusion. The most common isotope (more than 99.98%) is 1H, consisting of a single Proton. In order for fusion to give a stable end product of Helium, heavier isotopes of Hydrogen are required:-
1H - Hydrogen (or Protium) - 1 Proton.
2H - Deuterium - 1 Proton + 1 Neutron.
3H - Tritium - 1 Proton + 2 Neutrons.
When a Deuterium nucleus fuses with a Tritium nucleus a stable Helium nucleus is formed, as well as a free neutron.
The video below show a TEDTalk by nuclear physicist Michael Laberge about his work with nuclear fusion:-
The diagram below shows a summary of nuclear power:-