Nuclear fission
The nuclei of atoms contain a large amount of energy. Releasing this energy would free the world from having to use fossil fuels. There are two methods of doing this: fission and fusion.
Nuclear fission
Nuclear fission is the splitting of a large atomic nucleus into smaller nuclei.
In a nuclear reactor, a slow-moving neutron is absorbed into a nucleus (typically uranium-235). This causes the nucleus to become uranium-236, which is unstable.
The entire nucleus splits into two large fragments called ‘daughter nuclei’. In addition to the ‘daughter’ products, two or three neutrons also explode out of the fission reaction and these can collide with other uranium nuclei to cause further fission reactions. This is known as a chain reaction.
The fast moving neutrons carry most of the energy from the reaction with them (99%) but before the neutrons can collide with fresh uranium nuclei, they need to be slowed down.
Their energy is passed on to other components in the nuclear reactor, which is used to heat water to drive the turbines that turn the generators.
A fission reactor contains a number of different parts:
- Nuclear fuel – the uranium or plutonium isotope that will split when triggered by an incoming neutron. The fuel is held in rods so that the neutrons released will fly out and cause nuclear fission in other rods.
- Moderator – graphite core – a graphite core, for example, slows the neutrons down so that they are more likely to be absorbed into a nearby fuel rod.
- Control rods – these are raised and lowered to stop neutrons from travelling between fuel rods and therefore change the speed of the chain reaction.
- Coolant – this is heated up by the energy released from the fission reactions and is used to boil water to drive turbines in the power station.
- Concrete shield – the daughter products of the fission reaction are radioactive and can be a hazard.
Many of the features of the reactor are designed to control the speed of the reaction and the temperature inside the shielding. An uncontrolled fission reaction is the basis of an atomic bomb.
Advantages and disadvantages of nuclear power stations
Generating electricity using nuclear reactors carries high risk but offers large rewards. In operation, a very small amount of nuclear fuel will consistently generate a very large amount of electricity and generate very little polluting material. However, the financial costs of building and decommissioning a nuclear power station are very large, and the waste produced will remain radioactive – hazardous to humans and the environment – for thousands of years.
| Advantages | Produces no polluting gases. |
|---|---|
| Disadvantages | Waste is radioactive and safe disposal is very difficult and expensive. |
| Advantages | Does not contribute to global warming. |
|---|---|
| Disadvantages | Local thermal pollution from wastewater affects marine life. |
| Advantages | Very low fuel costs. |
|---|---|
| Disadvantages | Large-scale accidents can be catastrophic. |
| Advantages | Low fuel quantity reduces mining and transportation effects on environment. |
|---|---|
| Disadvantages | Public perception of nuclear power is negative. |
| Advantages | High technology research required benefits other industries. |
|---|---|
| Disadvantages | Costs of building and safely decommissioning are very high. |
| Advantages | Power station has very long lifetime. |
|---|---|
| Disadvantages | Cannot react quickly to changes in electricity demand. |
Nuclear fusion
Nuclear fusion is when two small, light nuclei join together to make one heavier nucleus. Fusion reactions occur in stars where, for example, two hydrogen nuclei fuse together under high temperatures and pressure to form a nucleus of a helium isotope.
There are a number of different nuclear fusion reactions happening in the Sun. The simplest is when four hydrogen nuclei become one helium nucleus.
The combined mass of four hydrogen nuclei is 6.693 × 10-27 kilograms (kg). The mass of one helium nucleus is 6.645 × 10-27 kg. This means that there is a missing amount of mass equalling 0.048 × 10-27 kg.
The missing mass is converted to energy, which radiates away. This is seen happening in the Sun.
In all nuclear reactions, a small amount of the mass changes to energy. This may not seem like a lot of energy but this energy is a result of the fusion of only four hydrogen nuclei.
A 250 millilitre (ml) glass of water will contain around 1.6 × 1025 hydrogen atoms. Complete fusion of all these hydrogen nuclei would release about 17,200,000,000,000 joules (J) of energy. It is estimated that the sun releases 3.8 × 1026 joules of energy every second.
However, the issue with fusion is that it requires the fusing of nuclei, which are positive particles. As two nuclei approach each other, they will repel because they have the same charge. The fusion of the nuclei has to happen under intense pressure and very high temperatures in order to force the nuclei together and overcome this electrostatic repulsion.
This need for a very high temperature and pressure makes it very difficult to build a practical and economic fusion power station. For fusion to occur at the lower pressures in a reactor on Earth, the temperature would need to be between 100 and 200 million degrees. Fusion has been successfully achieved by scientists at the JET experiment near Oxford but so far they have been unable to create a financially viable reactor.
