Nuclear Reactor
Overview
A nuclear reactor (원자로, 原子爐) is a device that artificially controls the chain reaction of fissile materials such as uranium-235 to obtain thermal energy. When the enormous energy released during fission is converted into steam to drive a turbine, electricity is produced, and this is the core principle of nuclear power generation. Nuclear reactors are used not only for power generation but also for submarine and aircraft carrier propulsion, research neutron sources, medical isotope production, and seawater desalination, among other purposes.
Main Content
Nuclear Fission and Chain Reactions
When a uranium-235 nucleus absorbs a slow neutron, it splits, releasing two fragments, 2–3 neutrons, and about 200 MeV of energy. The neutrons produced in this process then split other nuclei, which is the chain reaction. A reactor uses control rods (boron, cadmium, hafnium, etc.), a moderator, and a coolant to regulate the number of neutrons, thereby maintaining the reaction rate at a critical state. The fact that 1 g of fission yields energy equivalent to about 3 tons of coal underpins the economic viability of nuclear power.
Major Components
- Nuclear fuel: Typically in the form of fuel rods, in which enriched uranium (3–5% U-235) is made into uranium dioxide (UO₂) sintered pellets and placed in zirconium alloy cladding tubes.
- Moderator: Slows fast neutrons into thermal neutrons. Light water (ordinary water), heavy water, graphite, and the like are used.
- Coolant: Carries the heat generated in the core to the turbine side.
- Control rods: Absorb neutrons to regulate or shut down output.
- Shielding: Blocks radiation with concrete and steel plates.
- Containment vessel: The last line of defense preventing the release of radioactive materials in the event of an accident.
Types of Nuclear Reactors
A pressurized water reactor (PWR) keeps the coolant under high pressure so that it does not boil, and transfers the heat of the primary system to the secondary system through a steam generator. Most of South Korea's Gori, Hanbit, and Hanul nuclear power plants use this method, and it is the most widely used in the world. A boiling water reactor (BWR) has a simpler structure in which water is boiled directly in the core to produce steam that is sent to the turbine, but radioactive steam passes through the turbine. A heavy water reactor (CANDU) uses heavy water as both moderator and coolant, allowing natural uranium to be used as fuel and enabling refueling during operation. The Wolseong nuclear power plant is a representative example.
A fast breeder reactor (FBR) operates with fast neutrons without a moderator, uses plutonium as fuel, and converts uranium-238 into new fuel. A small modular reactor (SMR) is a modular reactor with an output of 300 MWe or less, and because it can be factory-manufactured and assembled on site, it has improved safety and economics. Generation IV reactors under development include the very high temperature reactor (HTGR), the sodium-cooled fast reactor (SFR), and the molten salt reactor (MSR).
Safety Systems
Nuclear reactors are designed according to the concept of defense in depth. The shutdown system, residual heat removal system, emergency core cooling system (ECCS), and containment building operate in stages. After the 2011 Fukushima nuclear accident, stress tests assuming extreme situations such as loss of power and loss of cooling water, hydrogen explosion prevention facilities, and response systems such as the deployment of mobile power vehicles and pumps were strengthened. Recent designs actively adopt passive safety systems such as natural circulation and gravity.
History
In 1942, at the University of Chicago, the team led by Enrico Fermi brought the world's first nuclear reactor, "Chicago Pile-1 (CP-1)," to criticality. In 1954, the Soviet Union's Obninsk plant was the first in the world to be connected to the power grid, and in 1957 the U.S. Shippingport plant began commercial operation. Since the Gori Unit 1 began commercial operation in 1978, South Korea has steadily increased the share of nuclear power generation.
Latest Trends
In 2024–2025, the nuclear reactor industry entered a phase of "revival." As electricity demand surged due to the spread of AI data centers and electric vehicles, big tech companies such as Microsoft, Google, and Amazon signed a series of SMR power purchase agreements (PPAs). NuScale Power and Kairos Power in the United States, Rolls-Royce SMR in the United Kingdom, and South Korea's i-SMR entered the licensing stage, and construction of multiple SMRs is expected to begin in 2025.
In addition, uranium prices renewed their record highs in 2024, and governments around the world are expanding policies that classify nuclear power as "green energy" in order to achieve their 2050 carbon neutrality goals. A representative example is that at COP28 held in Dubai in 2023, more than 20 countries signed a declaration to triple nuclear power generation capacity by 2050.
Progress also continued in the field of nuclear fusion. After achieving net energy gain (ignition) for the first time at the end of 2022, the U.S. Lawrence Livermore National Laboratory succeeded in repeated experiments in 2024–2025, and ITER and South Korea's KSTAR are also setting new records for plasma confinement time. However, the prevailing view is that nuclear fusion still requires decades before commercialization.
On the safety regulation side, the ocean discharge of treated water from Fukushima (begun in 2023), the expansion of dry storage facilities for spent nuclear fuel, and the issue of selecting a site for a high-level radioactive waste disposal facility remain major pending issues. In South Korea, the Special Act on the Management of High-Level Radioactive Waste entered the enactment process after National Assembly discussions in 2024–2025.
Related Topics
- [[Nuclear fission]]
- [[Nuclear power generation]]
- [[Small modular reactor]]
- [[Nuclear fusion]]
- [[Radioactive waste]]
- [[Uranium]]
- [[Fukushima nuclear accident]]