Nuclear Power Production and Technology
Reactor theory
Nuclear reactors operate a controlled chain reaction, with key components including uranium fuel, moderators (to slow down neurons), control rods (to absorb neurons), cooling systems (to transfer heat), generators (to convert thermal energy to electrical energy), and containment (to prevent radiation escape). Over the years, different reactor types have been developed with various features, advantages, and safety mechanisms.
Chain reactions
Chain reactions are at the core of every nuclear reactor. The process to induce a nuclear reaction involves bombarding the fissile uranium-235 isotope with a neuron. Because that neuron is not a charged particle, it does not experience electrostatic repulsion and can therefore penetrate a target nucleus. The bombardment of the U-235 atom nuclei by a neuron produces new nuclei, energy, and additional neutrons. This scenario can lead to three outcomes:
If all the released neutrons are lost, either by escaping the core or being absorbed by non-fissile elements like U-238, the chain reaction ceases.
If two or more neutrons trigger further fissions in other U-235 atoms, an exponential increase in atom splitting occurs, initiating what's known as a runaway chain reaction.
If precisely one neutron continues the process with another U-235 atom, it sustains a controlled chain reaction.
This is the goal of nuclear power production. The energy released can be found from mass differences and final speeds of particles released from conservation of momentum. Many different radioactive isotopes are produced in the fission process, and more neutrons are released than are used, which makes possible a chain reaction.
Reactor components
A nuclear power plant employs technology to safely capture energy from a sustained chain reaction, using the heat generated from the splitting of uranium atoms to produce steam and generate electricity.
Fuel
Uranium is enriched to increase the U-235 to U-238 ratio and processed into uranium oxide (UO2) pellets. These pellets are small ceramic cylinders, approximately 3/8-inch in diameter and 5/8-inch in length,² and are arranged in tubes to form fuel rods. Some types of reactors may contain around 50,000 fuel rods, holding around 18 million pellets.³ These rods are then assembled into clusters within the reactor core, called an assembly.
Moderator, control mechanisms, and coolant
Surrounding these assemblies is the moderator, typically water, heavy water, or graphite, which slows down neutrons released during fission to promote more sustained reactions. Control mechanisms within the reactor include rods made of neutron-absorbing materials such as cadmium, hafnium, or boron; which can be adjusted to control or halt the nuclear reaction.³ The reactor's heat is managed by a coolant - fluid that circulates through the core to transfer heat away from it. This system is enclosed within a pressure vessel or series of pressure tubes, generally made from sturdy steel, that contain the core and the moderator/coolant.⁵
Steam turbine and generator
Turbines and generators work together to convert the thermal energy produced by fission into mechanical energy with steam from the reactor turning the turbines. This drives the generators to produce electrical energy. Reactor types differ in their methodology. For example, Pressurised Water Reactors use steam generators for a secondary loop; Boiling Water Reactors directly produce steam in the core to drive turbines; and Gas-Cooled Reactors utilise gas turbines for power generation.³
Containment
The containment structure, usually made of concrete and steel about a metre thick, encloses the reactor and associated steam generators to shield the interior from external threats and protect the external environment from radiation in the event of a serious malfunction.³
Cooling
To optimise energy conversion, the steam from the turbine is condensed upon exit. This cooling can occur directly through sea or river water, or via cooling towers. A prominent structure in many power plants features a hyperboloid concrete shell about 100 metres high. The tower's design promotes a natural draft that pulls air upward, effectively cooling the water.⁶
Reactor types
Nuclear reactor designs vary, including Pressurised and Boiling Water Reactors, Gas-Cooled, Heavy Water, and Fast Breeder Reactors, each using different cooling and moderation methods.
Pressurised Water Reactors (PWR)
Pressurised Water Reactors use water both as a coolant and a moderator under high pressure to prevent boiling. They are the most common type in use, with about 300 operable reactors for power generation and several hundred more in naval propulsion.³ The design features a primary cooling circuit that circulates under high pressure through the reactor core and a secondary circuit where steam is produced to power the turbine. In the reactor core, water reaches approximately 325°C and is maintained at roughly 150 times atmospheric pressure to keep it from boiling.⁵
Boiling Water Reactor (BWR)
Boiling Water Reactors allow water to boil in the reactor core, which produces steam to drive turbines directly. The design is very similar to a PWR, except that there is only a single circuit in which low pressure water boils at about 285°C.³ The efficiency gained in the single system energy transfer process is balanced by the complexities in shielding system components from radiation and providing protection during maintenance.
Gas-Cooled Reactor (GCR)
Gas-Cooled Reactors use gas, such as CO2 or helium, as the coolant in the inner cycle of the reactor to transfer thermal energy to a heat exchanger to evaporate water. Advanced Gas Reactors (AGRs) represent the second generation of British reactors and are moderated by graphite.
GCRs operate with lower fuel enrichment levels than many other reactor types, which can reduce fuel costs. These reactors achieve higher efficiency due to elevated core temperatures. However, their designs require larger cores and extensive use of graphite, resulting in higher initial capital costs.⁷ Despite these potential operational savings, the UK's GCR fleet has not performed as well as other designs, leading to the decision against planning future AGRs.⁸
Light Water Graphite-Moderated Reactor (LWGR)
The Light Water Graphite-Moderated Reactor, or the RBMK (Reaktor Bolshoy Moshchnosty Kanalny), is a Soviet-designed reactor, which operates with graphite moderation and water cooling, allowing water to boil directly in the fuel channels, creating steam for turbine power generation. Despite its higher efficiency through elevated core temperatures, the RBMK’s design flaws were highlighted by the 1986 Chernobyl disaster. Following the disaster, significant safety enhancements were implemented in remaining RBMK reactors, but no new RBMK reactors are planned.⁹
Fast Neutron Reactor (FNR)
Fast Neutron Reactors represent an advanced stage in nuclear reactor technology, generating power from plutonium produced by the more prevalent U-238 isotope. They are expensive to build but generate more than 60-times as much energy from the original uranium when compared to conventional reactors. Approximately 20 FNRs have been operational since the 1950s, and the majority of Generation IV reactor designs are FNRs due to their efficiency.
Footnotes
Chemistry LibreTexts, "Types of Radioactivity," accessed November 2024, https://chem.libretexts.org/Courses/Grand_Rapids_Community_College/CHM_120_-_Survey_of_General_Chemistry%28Neils%29/2%3A_Atomic_Structure/2.04%3A_Types_of_Radioactivity.
U.S. Nuclear Regulatory Commission, "Pellet, Fuel," accessed November 2024, https://www.nrc.gov/reading-rm/basic-ref/glossary/pellet-fuel.html.
World Nuclear Association, "Nuclear Power Reactors," updated August 2024, accessed November 2024, https://world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-power-reactors/nuclear-power-reactors.
Deep Isolation, "What Is Spent Nuclear Fuel?" accessed November 2024, https://www.deepisolation.com/about-nuclear-waste/what-is-spent-nuclear-fuel/.
Raymond L. Murray and Keith E. Holbert, Nuclear Energy: An Introduction to the Concepts, Systems, and Applications of Nuclear Processes, 8th ed. (Amsterdam: Butterworth-Heinemann, 2019).
Energy Encyclopedia, "The Nuclear Power Plant: How It Works," accessed November 2024, https://www.energyencyclopedia.com/en/nuclear-energy/the-nuclear-reactors/the-nuclear-power-plant-how-it-works.
International Atomic Energy Agency, "General Design and Principles of the Advanced Gas-Cooled Reactor (AGR)," IAEA Graphite Knowledge Base, accessed November 2024, https://nucleus-qa.iaea.org/sites/graphiteknowledgebase/wiki/Guide_to_Graphite/General%20Design%20and%20Principles%20of%20the%20Advanced%20Gas-Cooled%20Reactor%20(AGR).aspx.
ScienceDirect, "Gas-Cooled Reactor," accessed November 2024, https://www.sciencedirect.com/topics/engineering/gas-cooled-reactor#:~:text=Gas%2Dcooled%20reactor%20uses%20gas,moderator%20and%20control%20the%20reaction.
World Nuclear Association, "RBMK Reactors," accessed November 2024, https://world-nuclear.org/information-library/appendices/rbmk-reactors.
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