Part 2 

Introduction:

There is no doubt that energy has been driving and will drive the technological progress of human civilization. It is a vital component for economic development and growth, and thus our modern way of life. Researchers project that the world energy demand will almost double by 2040 (based on energy usage)^[3], which must be met by utilizing energy sources other than fossil fuels such as coal and oil. Fossil fuel power generation contributes to significant greenhouse gas emissions into the atmosphere and influences the climate change trend. Although several research and development programs (for example, carbon sequestration and ultra-supercritical steam turbine programs) have been initiated to make fossil power generation much cleaner, more is needed to fend off the bigger problem. Therefore, many countries worldwide have recognized the importance of clean (i.e., emission-free) nuclear energy, and there are proven technologies that are more than ready for deployment. Nuclear power can solve the energy trilemma of supplying clean and affordable base-load power.

The use of nuclear energy for power generation varies widely in different parts of the world.  About 454 nuclear power reactors currently supply more than 10% of the world’s electricity, operating at a high capacity factor of 81% (world average)^[1]. Thirty-one countries use nuclear power plants, with 70% of the world’s nuclear electricity generated in five countries – the USA, France, China, Russia, and South Korea. As such, many other countries have tremendous opportunities for nuclear energy growth. Today the average age of the operational power reactors stands at 30 years, with over 60% of all nuclear power plants having operated for more than 31 years^[1]. Hence, nuclear power reactors are an essential energy source that has been providing electricity to millions of people worldwide. Despite the controversy surrounding nuclear power due to the risk of radiation exposure, nuclear reactors have proven to be a reliable, efficient, and emission-free source of electricity generation. Nuclear reactors have been built for the primary purpose of electricity production, although they are used for other applications such as desalination, radioisotope production, rocket propulsion, and much more. A view inside the Olkiluoto 2 nuclear reactor vessel is shown in Figure 2.

What is a nuclear reactor, and how does it work?

Even though a nuclear power reactor can be defined in different ways, almost all reactors except fusion reactors (commercially non-existent) can be described as, “a device designed to produce and sustain a long-term, controlled fission chain reaction, and made with a carefully selected and strategically placed collection of various materials”. Generally, nuclear reactors are considered the heart of a nuclear power plant, because they contain and control the nuclear chain reactions that produce heat through a physical process called fission. Nuclear fission is the process of splitting an atom’s nucleus into two smaller nuclei, releasing a vast amount of heat in the process. This heat is used to generate steam that spins a turbine to generate electricity.

A nuclear power plant can typically be divided into nuclear and conventional parts. Fission energy in the nuclear domain converts into heat, producing steam. In the conventional part, this steam runs the turbine connected to the generator. The conventional section of a nuclear power plant is very similar to a traditional thermal power plant from the boiler onwards. The nuclear part is also called the nuclear steam supply system. A nuclear reactor is its main component.

Main components of nuclear reactors:

There are various types of nuclear reactor technologies available. Nonetheless, these nuclear reactors have several components in common such as the fuel, moderator, control rods, and coolant, as shown in Figure 3.

Fuel: Nuclear fuel element is the material used in nuclear reactors that tend to undergo fission and generate energy. Uranium is the primary fuel. Usually, uranium oxide (UO₂) pellets are arranged in tubes to form fuel rods. Engineers organize the rods into fuel assemblies in the reactor core.

Moderator: It is the material that is present in the reactor core, which slows down the neutrons released from fission so that they cause more fission. It is usually water but may be heavy water or graphite.

Control rods: These are made with neutron-absorbing material such as cadmium, hafnium, or boron and are inserted or withdrawn from the core to control the rate of reaction, or to halt it.

Coolant: A fluid circulating through the core to transfer the heat from it to the steam generator to produce steam, which drives a turbine to generate electricity.

Concrete shield: It is the structure that surrounds the nuclear reactor. The prime function of the shield is to protect the device from external contaminant particles or radiations. The shield of a nuclear reactor is built with the help of concrete and steel. The concrete safeguard walls are about one meter thick.

Pressure vessels: Pressure vessels are used by the nuclear reactors to hold the fuel and to provide a passage to the coolant that enables it to travel freely through the moderator assembly. The material that is typically preferred to manufacture pressure vessels is steel.

Nuclear reactors use uranium to process tiny ceramic pellets and construct them jointly into fuel rods. A group of over 200 fuel rods can form a fuel assembly. Assemblies can usually fabricate a reactor core (Figure 3) through these assemblies based on the power level. In the vessel of a nuclear reactor, the fuel rods are placed within the water so that they can act as a coolant and moderator to assist while reducing the speed of the neutrons. Fission can generate these neutrons to maintain the chain reaction. After that, one can place control rods into the reactor core to reduce the reaction rate or to halt it. The heat generated through the fission process can make the water into steam to rotate a turbine to generate carbon-free electricity.

Now that we understand what a nuclear reactor is, why it is used, and how it works, we will take a deeper dive into the different types of nuclear reactors, their benefits and limitations, and strategies to design and model nuclear reactor cycles using AxSTREAM System Simulation. Stay tuned for part two of this blog coming next week!

Part 2 

References

1. World Nuclear Industry Status Report [Internet]. 2018 Nov 1. Available from: https://www.worldnuclearreport.org/
2. https://www.researchgate.net/publication/331692630_A_review_on_the_development_of_nuclear_power_reactors
3. https://application.wiley-vch.de/books/sample/3527407677_c01.pdf
4. https://3dprintingindustry.com/news/nordic-nuclear-power-plants-break-new-ground-with-3d-printed-debris-filters-210897/
5. https://www.open.edu/openlearn/mod/oucontent/view.php?id=26802&section=1.1
6. Power Reactor Information System, IAEA [Internet]. 2018 Nov 1. Available from: https://pris.iaea.org/PRIS/home.aspx
7. https://gnssn.iaea.org/main/bptc/BPTC%20Module%20Documents/Module04%20Design%20of%20a%20nuclear%20reactor.pdf