Nuclear Reactors

An immense amount of energy is contained within the nucleus of atoms. For example, one kilogram of uranium-235 can provide between two and three million times the amount of energy as an equivalent kilogram of coal! If the public's concerns over safety could be alleviated and the technology perfected, then nuclear power would free us from reliance on our limited reserves of polluting fossil fuels.

Nuclear power has several advantages over non-renewable energy sources. While the waste produced by nuclear reactors is toxic, the radioactive waste will eventually decay into harmless substances such as lead, even though this might take up to 1 million years. Also, nuclear fuels such as uranium are far denser than traditional fossil fuels, so much less waste is physically produced. Lastly, green renewable energy sources such as solar or wind power can't produce nearly as much energy as Nuclear and are usually dependent on weather conditions.

Nuclear Reactor Explanation and Fundamentals

The nuclear reactor is the heart of any nuclear power plant. However, outside of the main reactor, a nuclear power plant generates electricity in a surprisingly similar way to a coal power plant. Ultimately, the energy released from the nuclear reactions inside a reactor is simply used to heat and boil water. The steam then produces mechanical work to spin a turbine to generate electricity. The steam is later cooled inside the condenser to be reused in the reactor. Nuclear power plants are an example of a heat engine.

Diagram of how electricity is generated in a nuclear powerplant, Flickr CC BY-NC-SA 2.0

There are two methods of heating the water by utilising nuclear reactions. The first is nuclear fission, where a parent nucleus is split into two daughter nuclei. The mass of the two daughter nuclei is always less than the parent nucleus, this missing mass is released as energy.

The second possible method of heating is through nuclear fusion, where two light atomic nuclei are forced together and merged into a single nucleus. Similarly to fission, the resultant nucleus from a fusion reaction has a lower mass than the two original nuclei. The leftover mass is released as energy.

Nuclear Fission

In a nuclear fission reaction, a parent nucleus is split to produce two daughter nuclei. The difference in mass before and after the reaction is converted directly into energy. The most common type of nuclear fuel used in fission reactors is Uranium-235. Unfortunately, the energy released from splitting just one uranium-235 atom is only. This is insignificant compared to our modern energy requirements as the average UK home requires aboutjoules of energy per year. Thankfully, within just a single kilogram of Uranium exist an unfathomably large number of Uranium-235 atoms. So how do we split more than one atomic nucleus at once? The answer is nuclear chain reactions.

The fission of Uranium-235 into its daughter products, adapted from image by Wikimedia Commons CC-BY-SA-3.0

Inside a nuclear fission reactor, when a neutron is absorbed by a Uranium-235 isotope, it briefly becomes Uranium-236. U-236 is extremely unstable and will quickly decay into two daughter nuclei, Caesium-140 and Rubidium-92 while releasing energy. However, the two daughter nuclei are not the only products of nuclear fission. Two or three neutrons are also emitted. If the uranium fuel source is dense enough, then these neutrons can then be absorbed by other U-235 isotopes, causing more nuclei to split in further nuclear fission reactions, releasing more energy!

A diagram of a nuclear chain reaction, flickr CC BY 2.0

In the diagram above you can see that the fission of the example nucleus above produces 3 new neutrons, which in turn are absorbed by 3 more atomic nuclei. Those nuclei will split too, emitting 9 new neutrons in total! So if every instance of fission produces 3 new neutrons, then the number of fission reactions will triple in each new generation (assuming all emitted neutrons actually collide with an atomic nucleus).

Nuclear Fission Reactor Diagram

To understand how to control a nuclear chain reaction for use in our powerplants we should study the design of a nuclear fission reactor. A fission reactor has mechanisms engineered to moderate a chain reaction so that we can extract the exact amount of energy desired. This is particularly useful as the UK's energy demands on the national grid change based on many different factors, including the time of day, weather, season and so on.

The primary components of a nuclear fission reactor, Wikimedia Commons CC-BY-2.5

A nuclear fission reactor contains many important parts. The nuclear fuel source (uranium, plutonium, thorium etc.) is held in fuel rods which are encased with a graphite moderator. The graphite between the fuel rods slows down any emitted neutrons, which makes them more likely to be absorbed by the nuclear fuel in another rod which will induce a higher rate of nuclear fission.

The main mechanism that controls the rate of the nuclear chain reaction inside the fission reactor is the control rods. They are typically made out of elements such as silver or boron, which can readily absorb neutrons without splitting themselves. A nuclear chain reaction can therefore be controlled by lowering or raising these control rods. You can slow the reaction rate by lowering the control rods further into the core. Oppositely, you can increase the rate of the reaction by steadily removing the control rods. With multiple control rods, it is simple to maintain real-time control of the fission process.

Radiation shielding (typically made of concrete) is used to protect the external environment from the radioactive and harmful daughter products of the fission reactions. The energy generated by nuclear fission in the reactor is used to heat water, so the steam can perform useful work by turning a steam turbine, which is ultimately used to generate electricity.

Nuclear Fusion

In a nuclear fusion reaction, two atomic nuclei are forced together and combined into one nucleus. The difference in mass before and after a fusion reaction is converted directly into energy. Nuclear fusion powers our Sun, where a near-countless number of nuclear fusion reactions occur every second. The mass difference is then radiated away as energy.

Nuclear fusion can create immense amounts of energy, several times greater than fission. The fuel used in fusion is extremely abundant and cheap, unlike heavier, radioactive elements used in fission. Also, none of the products of fusion are themselves radioactive, so a nuclear fusion power plant would be a green and renewable energy source. Finally, a nuclear fusion reactor would be incapable of having a nuclear meltdown even with human error, so they would be much safer.

It is obvious then that a lot of energy can be extracted through nuclear fusion. It may shock you then that there are currently zero nuclear fusion reactors in the world to help generate our electricity! For fusion to occur you need to overcome the repelling force between two positively charged atomic nuclei. The two nuclei must be close enough that the nuclear force will be strong enough to induce nuclear fusion. An environment with extremely high temperature and pressure is needed to accomplish this, like that found inside a star.

The force repelling two positively charged nuclei that must be overcome for nuclear fusion, Wikimedia Commons CC-BY-2.5

Unfortunately, the amount of energy needed to artificially create this environment takes more energy to sustain than we receive from the fusion itself. Scientists and engineers have been making steady progress on this problem over the past few decades, but currently, nuclear fusion reactors exist as experimental technology only.