Nuclear Reactors

What are nuclear reactors? The 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 reactor examples

As of March 2022, there are currently 11 nuclear reactors in 5 different locations currently in operation in the UK. Nuclear power generated over 16% of the nation's energy requirements in 2020 and that percentage is rising due to recently introduced incentives and subsidies from the government. The list of active nuclear power plants in the UK includes Hinkley Point B, Hartlepool, Heysham 1, Heysham 2, Torness, and Sizewell B.

Nuclear fission reactor

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 \(3.2 \cdot 10^{-11} J\). This is insignificant compared to our modern energy requirements as the average UK home requires about 10¹⁰ joules 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.

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!

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).

You can see from the table above how a nuclear chain reaction could quickly get out of hand, releasing an enormous amount of energy in a very short time. This is actually how nuclear weapons work. An uncontrolled nuclear chain reaction that leads to a catastrophic explosion. Clearly, for our energy requirements, we would need to be able to regulate this reaction to control the amount of energy released.

Nuclear reactor diagram

To understand how to control a nuclear chain reaction for use in our power plants 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 Reactor

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.

The Nuclear Fusion Breakthrough

Nuclear fusion would solve many problems that humanity currently has regarding energy production. Unlike nuclear fission, nuclear fusion can produce very high amounts of energy without producing radioactive byproducts.

However, nuclear fission is easier to attain. Little effort needs to be made to split atoms compared to the temperatures and pressure required to fuse two atoms together, usually hydrogen atoms.

Therefore, until the end of 2022, researchers had to inject nuclear reactors with more energy than what was produced for a nuclear fusion reaction. However, in December 2022 in the National Ignition Facility at the Lawrence Livermore National Laboratory (LLNL) in California a breakthrough in nuclear fusion was achieved: researchers obtained more energy from the fusion reaction than what they had to inject into the reactor.

Even though this is a great achievement for humanity and the development of more sustainable energy resources, there is still a long path to travel until nuclear fusion becomes a feasible energy production system.