Nuclear reactors come in a wide variety of designs, each built around a different combination of fuel, coolant, and moderator. The most familiar type worldwide is the light water reactor, which accounts for the vast majority of commercial plants operating today, but the full landscape includes everything from graphite-moderated channels to liquid-metal-cooled fast breeders to experimental concepts that dissolve their fuel directly in molten salt. How these designs differ, and why new ones keep appearing, comes down to tradeoffs in safety, efficiency, waste, and what you want the reactor to do.
Four Generations of Reactor Design
The nuclear industry conventionally divides reactor development into four generations. The first generation, spanning roughly the 1950s through 1970, consisted of early prototypes exploring many different approaches. The second generation, from about 1970 to 1995, brought commercially viable power plants, mostly light water reactors that proved reliable and economically competitive. The third generation, emerging from the mid-1990s onward, refined those light water designs with improved safety and efficiency. A fourth generation, expected to reach commercial deployment around 2030 or later, encompasses designs considered revolutionary because they break substantially from the light water paradigm that has dominated for half a century.1Energy Policy. Generation IV nuclear reactors: Current status and future prospects
This generational framework is useful shorthand, but it can be misleading. Not every reactor type fits neatly into one generation. Some fourth-generation concepts, like molten salt reactors, were actually tested in the 1960s but never commercialized. And some third-generation designs incorporate features so advanced they blur the line with Generation IV goals. The generations describe a rough timeline of deployment more than a strict hierarchy of technology.
Light Water Reactors
Light water reactors use ordinary water as both their coolant and their neutron moderator, the substance that slows neutrons down enough to sustain a chain reaction. They come in two main flavors. Pressurized water reactors keep their primary cooling water under high enough pressure that it never boils inside the reactor vessel. That hot, pressurized water then transfers its heat to a separate loop of water in a steam generator, and the steam from that secondary loop spins the turbines. Boiling water reactors, by contrast, let the water boil directly inside the reactor core and send the resulting steam straight to the turbines, skipping the intermediate heat exchanger.
The practical difference matters for plant design. Pressurized water reactors add complexity with their two separate water loops, but this keeps the turbine-side equipment free from direct contact with water that has passed through the core. Boiling water reactors are simpler in that respect, since they eliminate the steam generator entirely, but their turbines handle steam that was in direct contact with fuel assemblies, which creates different maintenance considerations. Both types have compiled strong safety records over decades of commercial operation, and together they make up the overwhelming majority of the roughly 440 power reactors currently running worldwide.
Heavy Water and Graphite-Moderated Designs
Not every reactor uses ordinary water as its moderator. Heavy water reactors, most famously the Canadian CANDU design, use deuterium oxide instead. Heavy water is a more efficient moderator than light water, which gives CANDU reactors a distinctive advantage: they can run on natural uranium without enrichment. That sidesteps the need for uranium enrichment facilities entirely, which was a major appeal for countries that wanted nuclear power without building enrichment infrastructure.
Graphite-moderated reactors take a different route. Instead of any kind of water moderator, they stack blocks of graphite around fuel channels. The Soviet-designed RBMK is the most widely known graphite-moderated design, and also the most infamous because of the Chernobyl disaster. The RBMK-1500, for instance, is a multichannel boiling water reactor with a core built from 2,488 graphite columns made of rectangular blocks.2Nuclear Engineering and Design. Modeling of void, fast power and graphite temperature reactivity coefficients measurements for the validation of RELAP5-3D RBMK-1500 reactor model The combination of graphite moderation with boiling water coolant created certain physics characteristics, particularly a positive void coefficient, that contributed to the 1986 accident. Modern graphite-moderated designs have moved well beyond the RBMK concept, incorporating fundamentally different safety approaches.
Gas-Cooled Reactors
Gas-cooled reactors replace liquid coolants with a gas, typically helium or carbon dioxide. The United Kingdom operated carbon dioxide-cooled Magnox and Advanced Gas-Cooled Reactors for decades. The more modern approach is the High Temperature Gas-Cooled Reactor, or HTGR, which is classified as a Generation IV technology. These reactors use graphite as their moderator, helium as their coolant, and a distinctive fuel form called TRISO (tristructural isotropic) particles, tiny spheres of uranium coated in multiple protective ceramic layers.3Journal of Physics: Conference Series. Study on the Effects of Enrichment and Fraction of Coated Fuel Particles on Fissile Utilization of 100 MWt Prismatic-type of High Temperature Gas Reactor
The TRISO fuel is one of the most compelling features of HTGRs. Each particle is essentially its own tiny containment system, with layers designed to retain fission products even at very high temperatures. This means the fuel itself serves as a safety barrier, and the reactor can tolerate a loss of coolant without the fuel melting down in the way conventional reactor fuel might. The high operating temperatures also open the door to industrial process heat applications, not just electricity generation, since the helium coolant exits the core hot enough to drive chemical processes like hydrogen production.
Fast Reactors
All the reactor types discussed so far rely on slowing neutrons down with a moderator. Fast reactors take the opposite approach: they operate with fast, unmoderated neutrons. This changes what the reactor can do with its fuel. Fast neutrons are more effective at splitting certain isotopes and can convert non-fissile material like uranium-238 or thorium into usable fuel, which is why many fast reactor designs are called “breeders” since they can produce more fissile material than they consume.
The catch is that fast reactors cannot use water as a coolant, because water would moderate the neutrons and defeat the purpose. Instead, they use liquid metals. Sodium has been the traditional choice, and several sodium-cooled fast reactors have operated over the decades. The liquid-metal-cooled fast breeder reactor concept uses sodium to carry heat from the core to steam generators.4International Journal of Heat and Mass Transfer. Transient flow and heat transfer of liquid sodium coolant in the outlet plenum of a fast nuclear reactor Sodium is an excellent heat transfer fluid and does not slow neutrons significantly, but it reacts violently with water and burns on contact with air, which creates its own engineering challenges.
Lead and lead-bismuth alloys are alternative liquid-metal coolants that avoid sodium’s chemical reactivity problems. Lead-cooled fast reactors are a major area of Generation IV research. They also have a waste management advantage: fast reactors can burn certain long-lived radioactive waste products from conventional reactors. A 600-megawatt lead-cooled fast reactor configured as a self-breeder without blankets can burn roughly 67 kilograms of minor actinides per year, while a similarly sized sodium-cooled fast reactor with blankets loaded with minor actinides can burn even more, around 131 kilograms annually, albeit with a larger swing in reactivity that must be managed.5Progress in Nuclear Energy. Comparative study of minor actinide transmutation in sodium and lead-cooled fast reactor cores
Molten Salt Reactors
Molten salt reactors dissolve their nuclear fuel directly into a hot liquid salt, or in some designs use solid fuel with a molten salt coolant. The concept was first tested at Oak Ridge National Laboratory in the 1960s, but it was shelved in favor of the light water path. Interest has surged in recent years because molten salt reactors offer some unusual advantages: the fuel is already liquid, so a meltdown in the traditional sense is physically impossible, and the chemistry can be adjusted during operation.
One of the most distinctive engineering features of molten salt reactors is the freeze valve, sometimes called a freeze plug. Because no mechanical valve has yet been demonstrated that can withstand the corrosive, high-temperature conditions inside these reactors, designers use a plug of frozen salt to block a drain line during normal operations. If the reactor overheats, the plug melts and the fuel salt drains by gravity into a passively cooled tank.6Nuclear Engineering and Design. A unique molten salt reactor feature – The freeze valve system: Design, operating experience, and reliability The elegance of this approach is that no power, no operator action, and no moving parts are needed to activate it. Getting the freeze plug to form reliably and melt predictably requires careful thermal engineering, and research has shown that plug shape and stability depend on factors like sub-cooling temperature and the distance from the core.7Annals of Nuclear Energy. Preliminary investigation on the melting behavior of a freeze-valve for the Molten Salt Fast Reactor
Small Modular Reactors and Microreactors
The most talked-about trend in nuclear energy right now is the small modular reactor, or SMR. Instead of building a single enormous plant generating over a gigawatt of electricity, SMR designs aim for outputs in the range of a few tens to a few hundred megawatts, with components manufactured in factories and shipped to the site for assembly. The appeal is lower upfront capital cost, shorter construction timelines, and the ability to add capacity in increments.
Some SMR concepts push the envelope on where a reactor can be placed. One design study explored a pressurized SMR housed in a horizontal cylindrical hull placed on the floor of a sea or an artificial lake. The surrounding water acts as a permanent heat sink, and simulations of a station blackout scenario showed that natural circulation could effectively transfer heat from the fuel all the way to the external water with no active systems required. The design inherently prevents the kind of combined loss-of-power and loss-of-heat-sink scenario that caused the Fukushima disaster.8Progress in Nuclear Energy. Preliminary analysis of an integral Small Modular Reactor operating in a submerged containment
Microreactors are an even smaller class, typically under 30 megawatts of electricity and 100 megawatts of thermal output. They target different markets entirely: remote communities, mining operations, military bases, or disaster-relief scenarios where grid power is unavailable. One prominent concept uses heat pipes built directly into a solid metal core block. Each sealed pipe contains a small amount of working fluid and operates passively at pressures below one atmosphere, removing heat without any pumps.9Annals of Nuclear Energy. The technology of micro heat pipe cooled reactor: A review The idea of a reactor with no moving parts in its cooling system has obvious reliability appeal for locations where maintenance crews are scarce.
Passive Safety Across Designs
A common thread running through many of the newer reactor types is a reliance on passive safety, meaning systems that work without electrical power, operator action, or mechanical components. Natural circulation of coolant, driven purely by the tendency of hot fluid to rise and cool fluid to sink, is one of the most important passive mechanisms. A wide range of next-generation reactor concepts across multiple classes, including pressurized, boiling, supercritical, and liquid-metal fast reactors, have been specifically designed to operate with natural circulation as the primary mode of coolant flow.10Progress in Nuclear Energy. Review Innovations and challenges in natural circulation for advanced nuclear reactors: A comprehensive review of passive safety and future research opportunities
This represents a genuine philosophical shift from earlier generations. In a typical second-generation plant, safety systems rely on active components: pumps that must start, valves that must open, diesel generators that must supply emergency power. Each of those introduces a potential failure point. Passive designs aim to let physics do the work. The freeze valve in molten salt reactors, the heat pipes in microreactors, and the natural-circulation cooling in submerged SMRs all follow this principle. Passive does not mean foolproof, since the physics still has to be right, but it removes the dependence on hardware and human response time that has historically been the weak link in reactor safety.
Thorium and Alternative Fuel Cycles
Most operating reactors run on uranium fuel, but thorium has attracted sustained interest as an alternative. Thorium-232 is not fissile on its own, but when it absorbs a neutron it transforms into uranium-233, which is fissile and can sustain a chain reaction.11Renewable and Sustainable Energy Reviews. Viability of thorium-based nuclear fuel cycle for the next generation nuclear reactor: Issues and prospects Thorium is roughly three to four times more abundant in Earth’s crust than uranium, and a thorium fuel cycle produces less of the long-lived transuranic waste that makes spent uranium fuel so problematic to store.
Several reactor types are well suited to thorium fuel. Molten salt reactors were originally conceived in part to use thorium, since the liquid fuel makes it easier to process and reuse the bred uranium-233. Heavy water reactors like the CANDU can also accommodate thorium. India, which has vast thorium reserves but limited uranium, has pursued a long-term three-stage nuclear program built around eventually running a thorium-based fleet. Despite the theoretical appeal, thorium fuel cycles remain largely undeployed at commercial scale because the reprocessing chemistry is more complex than for uranium, and the existing global fuel infrastructure is built around uranium enrichment.
Waste Reduction Through Transmutation
One of the strongest arguments for fast reactors is their ability to transmute long-lived radioactive waste into shorter-lived or stable isotopes. Minor actinides, the elements that make spent nuclear fuel dangerous for tens of thousands of years, can be loaded into a fast reactor core and fissioned. Loading appropriate amounts of minor actinides into a lead-cooled fast reactor does not significantly disturb the neutron flux or power distribution in the core, which is an advantage over trying to do the same thing in a conventional pressurized water reactor.12Progress in Nuclear Energy. Minor actinide transmutation in the lead-cooled fast reactor
Both sodium-cooled and lead-cooled fast reactors can preferentially consume minor actinides without burning plutonium, either by mixing actinides directly into the core fuel or by placing them in separate target assemblies in blankets surrounding the core.13Progress in Nuclear Energy. Comparative study of minor actinide transmutation in sodium and lead-cooled fast reactor cores This does not eliminate the need for a geological repository, but it could dramatically reduce the volume of waste that needs to be stored for geological timescales, and shorten the period during which that waste remains hazardous.
Fuel Supply Challenges for Advanced Reactors
Many advanced reactor designs require fuel enriched to higher levels than what conventional light water reactors use. Standard reactor fuel is enriched to about 3 to 5 percent uranium-235. Several Generation IV and SMR designs call for high-assay low-enriched uranium, or HALEU, enriched to between 5 and 20 percent. Transitioning to HALEU-fueled reactors alters the material requirements of the entire fuel cycle, including both the mass of enriched uranium needed and the enrichment capacity to produce it.14EPJ Nuclear Sciences & Technologies. Enrichment dynamics for advanced reactor HALEU support
This is not just an academic concern. As of now, the global supply chain for HALEU is extremely limited. Russia has been one of the few commercial suppliers, which creates geopolitical complications for Western nations trying to deploy advanced reactors. The United States and others are investing in domestic HALEU production capacity, but scaling it up will take years. For any advanced reactor that requires HALEU, the fuel supply question is just as consequential as the reactor engineering itself.
Cogeneration and Non-Electricity Applications
Reactors are increasingly being evaluated not just for electricity but for process heat, hydrogen production, and desalination. A study modeling the integration of NuScale SMRs with both hydrogen production through alkaline electrolysis and seawater desalination through multi-stage flash distillation found that a cogeneration configuration achieved a global energy efficiency of about 35 percent, outperforming configurations dedicated to only one product.15Progress in Nuclear Energy. Thermo-technical assessment of hydrogen production and seawater desalination coupled with the 4×160 MWth NuScale SMR project The main sources of energy loss were heat exchangers and condensers in the steam cycle, not the nuclear or chemical processes themselves.
High-temperature reactors like HTGRs are particularly attractive for hydrogen production because their outlet temperatures are high enough to drive thermochemical water-splitting cycles, which are more efficient than electrolysis. District heating, industrial steam supply, and even synthetic fuel production are other potential applications. The versatility of nuclear heat is one reason that advanced reactor developers are pitching their products to industrial customers, not just electric utilities.
Reactor Pressure Vessel Aging
One constraint on the lifespan of any reactor is the condition of its pressure vessel, the thick steel container that holds the core. Over decades of operation, the neutron bombardment from the core gradually damages the steel’s crystal structure, making it more brittle. This embrittlement is a critical safety concern because a brittle vessel is more likely to crack under the thermal stresses of rapid temperature changes during transient events.16PubMed Central. Evaluation of the Embrittlement in Reactor Pressure-Vessel Steels Using a Hybrid Nondestructive Electromagnetic Testing and Evaluation Approach
Monitoring embrittlement is essential for deciding whether an aging reactor can safely continue operating, and it is one of the key factors in license-extension decisions. Traditional methods involve placing steel samples inside the reactor and periodically removing them for destructive testing, but newer nondestructive electromagnetic approaches aim to assess the vessel’s condition without cutting into it. For reactor designs that eliminate the traditional pressure vessel altogether, such as some molten salt concepts that operate at near-atmospheric pressure, this particular aging problem simply disappears.
Fusion Reactors
Every reactor type discussed so far is a fission reactor, splitting heavy atoms to release energy. Fusion reactors would do the opposite: fuse light atoms, typically isotopes of hydrogen, at extreme temperatures. The two leading approaches to containing a fusion plasma are the tokamak and the stellarator, both of which use powerful magnetic fields to confine superheated gas. Tokamaks are doughnut-shaped devices with relatively compact proportions, while stellarators use more complex, twisted magnetic coils and tend to have larger aspect ratios.17Matter and Radiation at Extremes. A general comparison between tokamak and stellarator plasmas
Fusion has been “30 years away” for the better part of a century, and that running joke contains a real frustration. The physics of sustaining a self-heating plasma are enormously difficult. But the recent surge in private fusion ventures, combined with advances in high-temperature superconducting magnets, has brought a level of investment and engineering progress that is genuinely new. Even optimistic timelines put commercial fusion power plants well into the 2040s or beyond, meaning fission reactors of the types described above will remain the backbone of nuclear energy for decades to come. When fusion does arrive, it will likely complement rather than replace fission, occupying different roles in an energy system that will need every low-carbon option it can get.