An overview of how different nuclear power plants work; or how to get electricity from rocks.
Nuclear energy accounts for about 9% of the world's electricity production. As technology adapted from making powerful weapons, nuclear energy may be a technology that can help decarbonize the world's energy systems. Let's dive into how to get electricity from radioactive rocks.
Nuclear Energy Working Principles
Nuclear energy operates on the principle of releasing and harnessing the immense energy stored in atomic bonds of radioactive elements, transforming it into a source of electricity for human use. Nuclear reactors use nuclear fission, the process of splitting heavy atomic nuclei, usually uranium, into smaller ones. The total mass of the fission products and emitted particles is slightly less than the original nucleus and neutron used to split it. This missing mass, or mass deficit, is converted into energy according to Einstein’s equation, E=mc2. The small loss of mass produces a large amount of energy due to the speed of light squared being a very large number.
The process begins with the mining of uranium ore, which contains uranium-238 and the fissile isotope uranium-235. After extraction, the uranium undergoes milling, where it is processed into uranium oxide concentrate, often called "yellowcake." This is then enriched to increase the proportion of uranium-235 from its natural 0.7% to around 3-5%, the level required for most reactors. The enriched uranium is formed into ceramic pellets and stacked inside long metal tubes made of zirconium alloy, forming fuel rods.
The fuel rods sit in a reactor core where the uranium atoms absorb neutrons, causing them to become unstable, break apart, release energy, and release more neutrons. The reactions are carefully controlled by absorbing some neutrons to maintain "criticality," where the number of neutrons produced by fission matches the number of neutrons lost through absorption or leakage. This control of neutrons and the reaction rate is what distinguishes a nuclear power plant from a nuclear bomb—which is a supercritical, uncontrolled, runaway chain reaction—where one nuclear fission causes several others to occur rapidly.
The reactor core is designed to carefully control this chain reaction. Control rods, usually made of cadmium, absorb neutrons and can be inserted or withdrawn to regulate the reaction rate or shut it down entirely. Water, often serving as both a coolant and a moderator, slows down neutrons to increase the likelihood of fission. The continuous nuclear fissions heat up water flowing around the reactor core.
The hot water produces steam, which is used to drive turbines that are connected to electrical generators. And voila, we have electricity from radioactive rocks.
Generations of Nuclear Reactor Design
Nuclear reactors are categorized into four generations, each representing advancements in technology, safety, and efficiency.
The first generation, Gen I, included early commercial reactors developed from the 1950s to the 1970s. These designs, such as the Magnox reactors in the UK and the Shippingport Atomic Power Station in the U.S., focused on demonstrating the feasibility of nuclear power. While functional, these reactors had limited efficiency, minimal safety features, and short operational lifespans. Chernobyl was a Soviet-designed RBMK (reaktor bolshoy moshchnosty kanalny) water-cooled reactor with individual fuel channels using graphite as its moderator. That reactor design was inherently unstable and, therefore, vulnerable to human error or equipment failure—both of which contributed to its catastrophic meltdown. Many of these Gen I reactors were decommissioned by the 1980s as newer designs emerged.
Generation II reactors followed in the 1970s and dominated through the 1990s. These reactors improved safety, standardization, and efficiency, setting the foundation for modern nuclear energy. They included widely used designs like Pressurized Water Reactors (PWRs), the most common reactor type worldwide, and Boiling Water Reactors (BWRs), the second most prevalent. Other examples include CANDU reactors, which use heavy water as a moderator, and VVER reactors, a Russian variant of PWRs. Generation II reactors were designed with lifespans of 30 to 40 years and introduced basic safety features that made them reliable and scalable. Today, these designs still make up the majority of the global nuclear fleet.
The introduction of Generation III and III+ reactors in the 1990s brought significant advancements. These reactors feature passive safety systems, enhanced fuel efficiency, and extended lifespans of 60 years or more. Designs like the AP1000, the European Pressurized Reactor (EPR), and the Advanced Boiling Water Reactor (ABWR) incorporate lessons learned from nuclear incidents, such as Chernobyl and Fukushima. While they remain a minority of operational reactors, they are gradually being adopted as older reactors are retired.
Generation IV reactors represent the future of nuclear energy. Conceptualized in the early 2000s, these designs prioritize sustainability, minimal waste, and resistance to proliferation risks. Examples include Molten Salt Reactors (MSRs) and Sodium-Cooled Fast Reactors (SFRs), which offer innovative features like liquid fuel and closed fuel cycles. These reactors are still in development, with the first deployments expected in the 2030s.
Currently, about 80% of operational reactors are PWRs, with BWRs accounting for another 17%. While Generation II reactors remain dominant, the shift toward advanced Generation III and IV designs reflects the ongoing effort to make nuclear energy safer, more efficient, and more sustainable. Ultimately, a nuclear reactor design must be so stable that if all the nuclear power plant operators suddenly dropped dead, the reactor would be able to slow the reaction rate, adequately cool the reactor core, and completely power down safely.
How Light Water Reactors Work
Light water reactors (LWRs) use regular ole water with hydrogen atoms that have no neutrons (protium, or hydrogen-1). They are the most common type of nuclear reactor used to generate electricity through nuclear fission. They rely on three main systems: the reactor core, the primary loop, and the secondary loop.
The reactor core contains fuel assemblies of enriched uranium pellets within zirconium alloy cladding. When neutrons strike uranium nuclei, they split, releasing heat and more neutrons, sustaining a chain reaction. However, not all neutrons will interact with other uranium nuclei. Fast neutrons actually have a lower probability of reacting and causing further fissions, so they need to be slowed down by a moderator to increase their chance of causing more reactions. In light water reactors, light water is the moderator that acts to slow the neutrons.
Two methods of controlling the reaction rate to maintain criticality are used. Control rods made of cadmium absorb neutrons and regulate the reaction. These control rods fit in between the uranium fuel rods and can be lowered into the reactor to slow the reaction rate, or can be pulled out in different increments to increase reactivity. Boron can be added to the water around the reactor core to help absorb neutrons as well. Higher concentrations of boron absorb more neutrons, decreasing the reactivity of the core.
The primary loop of the reactor circulates water under extremely high pressure (about 15.5 MPa) to prevent boiling, even at temperatures around 315°C (600°F). This superheated water transfers heat to a secondary loop. Importantly, the primary loop is a closed system, keeping radioactive materials isolated.
In the secondary loop, water is turned into steam by the heat from the primary loop. The steam drives a turbine connected to an electrical generator, producing electricity. The efficiency of the heat engine steam cycle is about 30-35%. Afterward, the steam is condensed and recirculated.
The key difference between a PWR and a Boiling Water Reactor (BWR) is how they handle water and steam. In a BWR, the reactor core heats water, which directly boils into steam within the reactor vessel. This steam drives the turbine without a secondary loop. BWRs can be a little more thermally efficient and smaller, but parts of the turbine system can be exposed to radiation.
By contrast, the PWR uses a two-loop system, keeping radioactive materials confined to the primary loop, making it safer for maintenance and operation. This dual-loop design and high-pressure water system make the PWR effective, and these make up 80% of the world's nuclear power plants. BWRs are used in 17% of nuclear power plants, with only 3% being other types.
Heavy Water Reactors
Heavy water reactors are another type of reactor that use heavy water as both a moderator and a coolant. Heavy water has its normal hydrogen atoms (protium) replaced with deuterium (hydrogen with a neutron). The heavy water slows down neutrons without absorbing them, so these reactors can use natural uranium as fuel, eliminating the need for enrichment. This made heavy water reactors attractive to countries like Canada and India, which had uranium mines but initially lacked enrichment capabilities.
Fast Reactors
Fast nuclear reactors are a type of nuclear reactor that operates using fast neutrons instead of the slow thermal neutrons used in most conventional reactors. Unlike traditional reactors, fast reactors do not require a moderator to slow down neutrons, allowing them to sustain a chain reaction with fast-moving neutrons. They are typically cooled with substances like liquid sodium or lead, which do not moderate the neutrons. Fast reactors are highly efficient at utilizing uranium, as they can use both fissile and unenriched fuels.
Fast Breeder Reactors
A fast breeder reactor (FBR) is a specific type of fast reactor designed to "breed" more fissile material than it consumes. It does this by converting natural isotopes like uranium-238 or thorium-232 into fissile isotopes such as plutonium-239 or uranium-233. This process occurs when fast neutrons interact with the natural material, transmuting it into a fissile isotope via neutron capture and subsequent radioactive decay. The bred fissile material can then be extracted and reused as nuclear fuel, significantly extending the supply of nuclear resources. Fast breeder reactors often use mixed oxide fuel (MOX), which combines plutonium and uranium oxides, or recycled nuclear fuel. Fast reactors are used to enhance fuel efficiency, reduce nuclear waste, and generate additional fissile material.
While breeder reactors have these advantages, they are more complex and expensive to produce. But perhaps the biggest concern with breeder reactors is the production of material suitable for nuclear weapons. In fact, Gen 1 reactors of various designs were intended to breed fuel for nuclear weapons during the Cold War, particularly Plutonium-239 and highly enriched uranium (HEU). This led to a huge stockpile of nuclear warheads, estimated at over 70,000 worldwide in 1986.
The fast breeder reactor was believed to be the future of nuclear power, but the process of nuclear disarmament has resulted in a cheap supply of fissile U-235 and Pu-239. Some of this material could be used in nuclear reactors. For example, under the U.S.-Russia Megatons to Megawatts Program (1993–2013), HEU from over 20,000 Russian warheads was down-blended to fuel commercial reactors. This fissile fuel supply made the fast breeder reactor uneconomical compared to newer Gen III and IV designs.
A key consideration for modern reactor designs is to avoid producing weapons-grade fissile material.
Thorium as a Fuel
Thorium-based nuclear reactors present a promising alternative to conventional uranium-fueled reactors, offering potential advantages in safety, fuel availability, and waste management. Thorium-232 is an abundant, naturally occurring element that differs significantly from uranium. Unlike uranium-235, thorium is not fissile on its own but is fertile, meaning it can absorb a neutron and transmute into the fissile isotope uranium-233 through neutron capture and decay.
Despite its promise, thorium reactors remain largely experimental. While concepts like the Molten-Salt Reactor Experiment were tested in the mid-20th century, no commercial thorium reactors are operational today. Countries such as India and China are actively pursuing thorium research to leverage their large reserves and growing energy needs. However, technical and economic barriers have delayed widespread adoption.
Thorium reactors have several advantages. Thorium is more abundant than uranium, with global reserves estimated to be three to four times greater. Thorium fuel cycles produce fewer long-lived transuranic elements, reducing the long-term toxicity of nuclear waste. They are also more resistant to nuclear proliferation, as thorium does not directly produce weapons-grade materials like uranium or plutonium. Additionally, thorium reactors can achieve higher theoretical fuel utilization rates than uranium reactors.
However, there are significant challenges. Thorium reactors require advanced technology, particularly molten salt reactors, which are considered the most viable for thorium fuel cycles but remain underdeveloped. Thorium requires an external neutron source, often uranium or plutonium, to start the reaction, complicating reactor operation. While thorium produces less long-lived waste, it still generates radioactive isotopes such as U-233 and Pa-231, which require careful handling. Moreover, retrofitting existing uranium-based reactors or developing new thorium-specific designs is costly and time-intensive.
Compared to the uranium fuel cycle, thorium offers benefits such as reduced waste and improved safety margins. However, the uranium fuel cycle’s maturity, established supply chain, and lower costs make it more practical for immediate use. Thorium reactors hold great potential for the future of nuclear energy, but technological and economic hurdles must be addressed before they become viable alternatives.
Radioactive Waste
Once the fuel rods in a nuclear reactor have undergone enough fissions such that much of the fissile isotopes are used up, the fuel rods need to be changed. The spent nuclear fuel is composed mostly of uranium isotopes, approximately 94% uranium-238, with a small fraction of uranium-235. About 5% is comprised of fission products, notably strontium-90 and cesium-137, which account for most of the heat and penetrating radiation in high-level waste.
Some uranium atoms absorb neutrons released during fission, forming heavier elements. Plutonium isotopes (Pu-239 and Pu-240) account for about 1% of waste, and 0.1% are other actinides such as neptunium-237 and americium-241. These elements, known as transuranics because they are heavier than uranium, generate far less heat and penetrating radiation than fission products, but they decay much slower and pose a significant long-term hazard. After 1,000 years, transuranic wastes constitute the majority of the remaining radioactive hazard in high-level nuclear waste.
Isotopes and their half-lives:
Uranium-238 (U-238): Half-life of approximately 4.5 billion years.
Uranium-235 (U-235): Half-life of about 700 million years.
Plutonium-239 (Pu-239): Half-life of 24,100 years.
Plutonium-240 (Pu-240): Half-life of 6,560 years.
Neptunium-237 (Np-237): Half-life of 2.14 million years.
Americium-241 (Am-241): Half-life of 432 years.
Strontium-90 (Sr-90): Half-life of 28.8 years.
Cesium-137 (Cs-137): Half-life of 30.2 years.
Radioactive waste returns the radioactivity level of the originally mined uranium ore in about 10,000 years. An average 1 GW nuclear power plant produces about 20 metric tons of spent nuclear fuel per year.
Spent nuclear fuel is initially stored in cooling pools at the reactor site for several years to dissipate heat and reduce radiation levels. Once sufficiently cooled, the fuel is transferred to dry cask storage, where it is encased in steel and concrete for long-term containment. Some countries reprocess spent fuel to extract usable isotopes like uranium and plutonium for reuse. However, most spent fuel remains in interim storage, awaiting permanent disposal in geological repositories.
Small Modular Reactors
Small Modular Reactors (SMRs) are a new generation of nuclear technology designed for flexibility, scalability, and safety. Generating up to 300 megawatts of electricity per unit, SMRs are factory-built and transported to sites, reducing construction costs and time. Countries like Russia, the United States, Canada, and China are leading SMR development. Russia’s Akademik Lomonosov, operational since 2020, is the first floating SMR, while the U.S.-based NuScale Power is working on deploying its VOYGR design by the late 2020s.
Most SMRs use the uranium fuel cycle, relying on low-enriched uranium (LEU), with some exploring alternative fuels like thorium or high-assay low-enriched uranium (HALEU). They produce radioactive waste similar to conventional reactors, including high-level waste (spent nuclear fuel) and lower-level operational waste. SMRs offer advantages like incremental scalability, advanced passive safety features, reduced land use, and potential deployment in remote locations or industrial settings. However, challenges include high R&D costs, limited economies of scale, regulatory hurdles, and public acceptance concerns. Waste management remains a factor, as SMRs still produce long-lived radioactive materials.
It's important to note that it's not possible to make very small nuclear reactors because for any given fissile fuel, there's a critical mass required to sustain the nuclear fissions. The size of the critical mass depends on the fissile isotope and the shape. A sphere of pure U-235, for example, has a critical mass of 52 kg. Anything smaller than this will not be able to maintain the chain reaction. 52 kg of Uranium-235 contains roughly the equivalent energy used by 111,200 American homes in one year (that's not the amount of usable electricity), so homes and businesses will not have their own small nuclear reactors anytime soon.
Nuclear Fusion
Rather than splitting heavy nuclei, nuclear fusion power seeks to replicate the energy production of stars by fusing light atomic nuclei, hydrogen isotopes, into heavier ones, like helium. Fusion promises abundant fuel from hydrogen isotopes like deuterium and tritium, minimal long-lived radioactive waste, and no carbon emissions.
Despite its potential, fusion remains experimental. Several approaches exist, each aiming to sustain the extreme temperatures and pressures needed for the reaction. Magnetic Confinement Fusion (MCF) uses strong magnetic fields to stabilize plasma, with devices like tokamaks (e.g., ITER) and stellarators leading the field. Inertial Confinement Fusion (ICF) compresses small fuel pellets using lasers or particle beams, as seen at the U.S. National Ignition Facility. A hybrid approach, Magneto-Inertial Fusion (MIF), combines magnetic fields with compression techniques. Emerging concepts like Direct Fusion Drive (DFD) explore plasma jets for propulsion. Magnetic confinement remains the most advanced, while inertial confinement shows promising breakthroughs.
While creating nuclear fusion is possible—and not that hard—commercial fusion power is currently nowhere near ready. The central challenge remains to sustain a nuclear fusion reaction and extract more electricity than is put in. While progress is accelerating, fusion is still in the research phase, requiring breakthroughs to become a viable energy source.
An article in the near future will cover the pros and cons of nuclear energy, explore alternatives, barriers to construction, and more.
Questions for you:
Do you think humans should pursue nuclear energy further? Why or why not?
Are Small Modular Reactors (SMRs) a game-changer for nuclear energy, or are their challenges too great for widespread adoption?
After learning how nuclear power plants work, what questions or concerns do you still have about this technology?
Resources:
Claudio Tuniz. Radioactivity : A Very Short Introduction. Oxford, United Kingdom, Oxford University Press, 2012.
Higginbotham, Adam. MIDNIGHT in CHERNOBYL : The Untold Story of the World’s Greatest Nuclear Disaster. S.L., Simon & Schuster, 2020.
Irvine, J M. Nuclear Power : A Very Short Introduction. Oxford ; New York, Oxford University Press, 2011.
Tucker, Colin. How to Drive a Nuclear Reactor. Cham, Switzerland, Springer Praxis, 2019.
United States Nuclear Regulatory Commission. “NRC: Backgrounder on Radioactive Waste.” Nrc.gov, United States Nuclear Regulatory Commission, 26 Jan. 2024, www.nrc.gov/reading-rm/doc-collections/fact-sheets/radwaste.html.
World Nuclear Association. “Radioactive Waste – Myths and Realities - World Nuclear Association.” World-Nuclear.org, 12 Aug. 2024, world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-waste/radioactive-wastes-myths-and-realities.
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