8.1 Reactor types
9 min read•august 21, 2024
Nuclear reactors are the heart of nuclear power generation and research. They come in various designs, each optimized for specific purposes like power production or scientific experiments. Understanding reactor types is key to grasping how nuclear is harnessed in real-world applications.
Thermal reactors dominate the industry, using moderators to slow neutrons for increased fission. Fast reactors, though less common, offer improved fuel efficiency. Both types have unique designs tailored to their neutron energy spectrum, with safety systems and containment structures ensuring safe operation.
Types of nuclear reactors
- Nuclear reactors serve as the cornerstone of nuclear power generation and research in applied nuclear physics
- Reactor designs vary based on factors like neutron energy spectrum, purpose, and coolant type
- Understanding different reactor types provides insight into the practical applications of nuclear fission and radiation interactions
Thermal vs fast reactors
Top images from around the web for Thermal vs fast reactors
Magnox - Wikipedia View original
Is this image relevant?
Transmutation and Nuclear Energy | Chemistry View original
Is this image relevant?
Sodium-cooled fast reactor - Wikipedia View original
Is this image relevant?
Magnox - Wikipedia View original
Is this image relevant?
Transmutation and Nuclear Energy | Chemistry View original
Is this image relevant?
1 of 3
Top images from around the web for Thermal vs fast reactors
Magnox - Wikipedia View original
Is this image relevant?
Transmutation and Nuclear Energy | Chemistry View original
Is this image relevant?
Sodium-cooled fast reactor - Wikipedia View original
Is this image relevant?
Magnox - Wikipedia View original
Is this image relevant?
Transmutation and Nuclear Energy | Chemistry View original
Is this image relevant?
1 of 3
- Thermal reactors moderate neutrons to thermal energies (~0.025 eV) for increased fission probability
- Fast reactors utilize high-energy neutrons (>1 MeV) to sustain chain reactions and potentially breed fuel
- Thermal reactors dominate commercial power generation due to simpler design and fuel requirements
- Fast reactors offer improved fuel utilization and waste reduction potential
Power vs research reactors
- Power reactors generate electricity for commercial or military applications
- Research reactors produce neutrons for scientific experiments, medical isotope production, and materials testing
- Power reactors typically operate at higher power levels (hundreds of MWe to GWe) compared to research reactors (kWt to tens of MWt)
- Research reactors often have specialized designs to maximize neutron flux or allow easy access to experimental facilities
Thermal reactor designs
- Thermal reactors form the backbone of the current nuclear power industry
- These designs rely on moderators to slow neutrons and increase fission probability in fuel
- Various coolant and moderator combinations have been developed to optimize performance and safety
Pressurized water reactors
- Most common reactor type worldwide for commercial power generation
- Use light water as both coolant and moderator
- Operate at high pressure (~15 MPa) to prevent coolant boiling in the reactor core
- Employ steam generators to transfer heat from primary to secondary coolant loop
- Fuel consists of low-enriched uranium dioxide pellets in zirconium alloy cladding
Boiling water reactors
- Second most common reactor type for commercial power generation
- Also use light water as coolant and moderator, but allow boiling in the reactor core
- Operate at lower pressure (~7 MPa) compared to PWRs
- Generate steam directly in the reactor vessel, eliminating need for steam generators
- Simplified design reduces capital costs but requires additional shielding for radioactive steam
Heavy water reactors
- Use deuterium oxide (D2O) as both coolant and moderator
- Excellent neutron economy allows use of natural uranium fuel
- CANDU (Canada Deuterium Uranium) reactors are the most common design
- On-line refueling capability increases capacity factor
- Higher heavy water inventory increases initial costs
Gas-cooled reactors
- Use graphite as moderator and gas (typically CO2 or helium) as coolant
- High temperature operation increases thermal efficiency
- Advanced gas-cooled reactors (AGRs) use enriched uranium fuel
- Pebble bed designs use spherical fuel elements for continuous refueling
- Lower power density compared to water-cooled reactors
Fast reactor designs
- Fast reactors operate with high-energy neutrons to improve fuel utilization
- These designs can potentially breed more fissile material than they consume
- Lack of moderator requires higher fissile content in fuel and presents unique cooling challenges
Sodium-cooled fast reactors
- Most developed fast reactor concept with operational experience
- Liquid sodium coolant offers excellent heat transfer properties
- Low pressure operation reduces stress on reactor vessel
- Sodium's chemical reactivity with air and water requires special safety precautions
- Pool-type designs immerse entire primary system in sodium for improved safety
Lead-cooled fast reactors
- Use liquid lead or lead-bismuth eutectic as coolant
- Higher boiling point of lead allows for higher operating temperatures
- Chemically inert coolant reduces fire risk compared to sodium
- High density of lead provides excellent natural circulation for decay heat removal
- Corrosion and erosion of structural materials pose challenges
Gas-cooled fast reactors
- Utilize helium gas as coolant in fast neutron spectrum
- High temperature operation increases thermal efficiency
- Direct Brayton cycle possible for improved plant simplicity
- Low coolant density requires high pressure operation and large core volume
- Fuel designs must accommodate high power density and fission gas release
Generation IV reactor concepts
- Advanced reactor designs aim to improve safety, sustainability, and economics
- These concepts incorporate innovative features and materials
- Many designs are still in research and development phases
Molten salt reactors
- Use liquid fuel dissolved in molten salt coolant
- Operate at low pressure with high temperature for improved efficiency
- Online fuel processing allows for actinide burning and reduced waste
- Passive safety features include freeze plugs for emergency fuel draining
- Corrosion of structural materials by hot salts presents a major challenge
Very high temperature reactors
- Gas-cooled reactors operating at temperatures above 900°C
- Utilize TRISO fuel particles for excellent fission product retention
- High outlet temperatures enable hydrogen production and industrial process heat
- Graphite moderator provides large thermal inertia for improved safety
- Material challenges at extreme temperatures limit near-term deployment
Supercritical water reactors
- Operate above the critical point of water (374°C, 22.1 MPa)
- Eliminate phase change for improved thermodynamic efficiency
- Compact design reduces capital costs
- Corrosion and materials issues at high temperature and pressure
- Both thermal and fast spectrum versions under development
Research reactor designs
- Research reactors provide neutron sources for various scientific applications
- Designs prioritize high neutron flux, experimental access, and operational flexibility
- Lower power levels and specialized safety features distinguish them from power reactors
Pool-type reactors
- Core submerged in open pool of water for cooling and shielding
- Simple design allows easy access to core for experiments
- Natural convection cooling possible at low power levels
- TRIGA (Training, Research, Isotopes, General Atomics) reactors are common pool-type design
- Inherent safety features include negative temperature coefficient of reactivity
Tank-type reactors
- Core contained in pressurized tank with forced cooling
- Higher power density possible compared to pool-type reactors
- Beam tubes penetrate tank for neutron extraction
- Often used for materials testing and isotope production
- May incorporate loops for fuel and materials irradiation studies
Pulsed reactors
- Designed to produce intense bursts of neutrons
- Rapid reactivity insertion creates brief supercritical state
- TRIGA reactors can operate in pulsed mode due to unique fuel properties
- Applications include weapons effects testing and fast neutron radiography
- Specialized control and safety systems required for pulsed operation
Reactor core configurations
- Core design impacts neutron economy, power distribution, and safety characteristics
- Fuel arrangement and composition determine reactor physics behavior
- Core configuration must balance performance, safety, and operational considerations
Homogeneous vs heterogeneous cores
- Homogeneous cores uniformly mix fuel and moderator (liquid-fueled reactors)
- Heterogeneous cores separate fuel and moderator into distinct regions
- Most power reactors use heterogeneous designs with discrete fuel assemblies
- Homogeneous cores offer simplified fuel management but face materials challenges
- Heterogeneous designs allow for local power shaping and improved neutron economy
Fuel assembly designs
- Fuel assemblies contain fuel rods, guide tubes, and structural components
- PWR assemblies typically have square lattice with 14x14 to 17x17 array
- BWR assemblies often use 10x10 arrays with water channels for improved moderation
- VVER (Russian PWR) designs use hexagonal fuel assemblies
- Advanced designs incorporate integral burnable poisons and axial enrichment zoning
Reactor coolant systems
- Coolant systems remove heat from the reactor core and transfer it to power conversion systems
- Design must ensure adequate cooling under normal and accident conditions
- Coolant choice impacts neutronics, thermal-hydraulics, and overall plant efficiency
Primary coolant loops
- Circulate coolant directly through the reactor core
- Contain radioactive materials produced during reactor operation
- Often use centrifugal pumps for forced circulation
- May incorporate natural circulation paths for passive safety
- Materials must withstand high temperatures, pressures, and radiation fields
Secondary coolant loops
- Separate from primary loop to prevent radioactive contamination
- Transfer heat from primary loop to power conversion system (turbines)
- Allow for isolation of reactor systems from balance of plant
- Often use water/steam as working fluid
- Optimize cycle efficiency through feedwater heating and moisture separation
Reactor control mechanisms
- Control systems adjust reactivity to maintain desired power level and distribution
- Must provide rapid shutdown capability for emergency situations
- Design integrates with instrumentation and protection systems
Control rods
- Neutron-absorbing materials (boron, hafnium, cadmium) inserted into core
- Mechanical drive mechanisms allow for precise positioning
- Gravity-driven insertion provides fail-safe shutdown capability
- Rod patterns optimized for power shaping and shutdown margin
- Some designs use followers to maintain moderation
Chemical shim control
- Soluble neutron absorber (typically boric acid) added to coolant
- Allows for long-term reactivity control without rod movement
- Reduces power peaking factors compared to rod-only control
- Requires chemical and volume control systems for concentration adjustment
- Potential for positive moderator temperature coefficient at end of cycle
Safety systems in reactors
- Multiple layers of protection ensure safe operation and accident mitigation
- Defense-in-depth approach incorporates diverse and redundant safety features
- Systems designed to maintain core cooling, control reactivity, and contain radioactive materials
Passive vs active safety systems
- Passive systems rely on natural phenomena (gravity, natural circulation) for operation
- Active systems require external power sources and mechanical components
- Passive systems enhance reliability but may have limited capacity
- Modern designs incorporate combination of passive and active features
- Examples include gravity-driven emergency core cooling and passive containment cooling
Emergency core cooling systems
- Provide core cooling in case of loss of coolant accident (LOCA)
- Multiple independent systems with diverse injection points
- High-pressure injection for small breaks
- Low-pressure injection for large breaks and long-term cooling
- Accumulators provide rapid initial injection without power
- Recirculation capability for extended cooling using containment sump water
Reactor containment structures
- Final barrier to release of radioactive materials to environment
- Design must withstand internal pressures and temperatures during accidents
- Provide shielding and controlled release paths for normal operation
Primary containment designs
- Steel or reinforced concrete structure immediately surrounding reactor vessel
- PWR containments typically large dry design or ice condenser type
- BWR containments use pressure suppression systems (drywell and wetwell)
- CANDU reactors use dousing system for pressure suppression
- Design pressure based on worst-case accident scenarios
Secondary containment features
- Surrounds primary containment to provide additional barrier
- Houses safety systems and spent fuel storage
- Maintains negative pressure to prevent uncontrolled releases
- Filtered ventilation systems reduce radioactive releases
- Some designs incorporate double containment for improved protection
Fuel cycle considerations
- Fuel cycle choices impact reactor design, performance, and waste management
- Reactor type determines fuel requirements and spent fuel characteristics
- Advanced fuel cycles aim to improve resource utilization and reduce waste volume
Once-through vs closed fuel cycles
- Once-through cycle uses fresh fuel and disposes of spent fuel directly
- Closed cycle reprocesses spent fuel to recover fissile materials
- Once-through simpler but less efficient use of uranium resources
- Closed cycle reduces waste volume but introduces proliferation concerns
- Some reactor designs (fast reactors) optimized for closed fuel cycle operation
Breeding ratios in fast reactors
- Breeding ratio defined as fissile material produced divided by fissile material consumed
- Ratios > 1 indicate net production of fissile material (breeder reactor)
- Fast neutron spectrum allows breeding from U-238 to Pu-239
- Breeding blankets of depleted uranium surround core to maximize production
- Higher breeding ratios improve fuel sustainability but may increase proliferation risk
Key Terms to Review (26)
Boiling water reactor: A boiling water reactor (BWR) is a type of nuclear reactor that uses water as both a coolant and a moderator, where the water boils inside the reactor core to produce steam, which then drives turbines to generate electricity. This design allows for a simpler system since the steam is produced directly in the reactor vessel, eliminating the need for separate steam generators found in other reactor types.
Chain reaction: A chain reaction is a series of nuclear fission events where the products of one reaction trigger additional reactions, leading to a rapid increase in energy release. This process is fundamental in both nuclear reactors and nuclear weapons, as it can be controlled for energy production or unleashed for explosive effects.
Containment structure: A containment structure is a critical safety feature in nuclear reactors designed to prevent the release of radioactive materials into the environment in case of an accident. These structures are robust and built to withstand extreme conditions, including pressure from internal explosions and external natural disasters. They play a vital role in reactor safety systems and ensure the integrity of the reactor core during normal operations and potential emergency situations.
Control Rod: A control rod is a crucial component in nuclear reactors used to regulate the fission process by absorbing neutrons. By adjusting the position of these rods within the reactor core, operators can control the rate of the nuclear reaction and maintain a stable output of energy. This ability to manage the reaction is essential for ensuring safety and efficiency in various types of reactors.
Criticality: Criticality refers to the condition in a nuclear reactor where a self-sustaining chain reaction occurs, enabling a controlled release of energy. Achieving criticality is essential for the operation of nuclear reactors, as it determines whether the reactor is in a subcritical, critical, or supercritical state, impacting the overall efficiency and safety of the reactor's function.
Emergency Core Cooling System: An emergency core cooling system (ECCS) is a safety mechanism designed to prevent the overheating of a nuclear reactor core during an accident or loss of coolant incident. This system is crucial in maintaining the integrity of the reactor by rapidly injecting coolant into the core to remove heat and ensure that the temperature remains within safe limits. The effectiveness of the ECCS is vital for reactor types that rely on water for cooling and is a key component in enhancing reactor safety systems.
Fast breeder reactor: A fast breeder reactor is a type of nuclear reactor that generates more fissile material than it consumes by using fast neutrons to convert fertile materials into fissile fuels. These reactors are significant because they can extend the fuel supply for nuclear power and help reduce radioactive waste through efficient use of resources. They operate on a closed fuel cycle, enabling the recycling of nuclear fuel and contributing to sustainability in energy production.
Fission: Fission is the process of splitting a heavy atomic nucleus into two or more lighter nuclei, accompanied by the release of a significant amount of energy. This phenomenon is critical in understanding various nuclear reactions, influencing reaction rates, and forming the basis of both nuclear power generation and nuclear weapon design.
Fuel rod: A fuel rod is a cylindrical tube that contains nuclear fuel, usually in the form of pellets, which is used in nuclear reactors to generate heat through fission. These rods are essential components of a reactor core, where they facilitate the nuclear reaction necessary for producing energy, and their design and material influence the reactor's efficiency and safety.
Gas-cooled fast reactor: A gas-cooled fast reactor (GCFR) is a type of nuclear reactor that uses helium or other gases as a coolant and operates with fast neutrons to sustain the fission process. This design allows for higher thermal efficiency and the potential for breeding fuel, which can improve fuel sustainability and minimize waste production.
Gas-cooled reactor: A gas-cooled reactor is a type of nuclear reactor that uses gas, typically carbon dioxide or helium, as its coolant instead of water. This design allows for higher operational temperatures and improved thermal efficiency, making it suitable for various applications such as electricity generation and hydrogen production.
Heavy water reactor: A heavy water reactor is a type of nuclear reactor that uses heavy water (D2O) as both a neutron moderator and coolant. This type of reactor allows for the use of natural uranium as fuel, making it distinct from light water reactors that require enriched uranium. Heavy water reactors have unique operational features that influence their efficiency and the type of isotopes produced.
Lead-cooled fast reactor: A lead-cooled fast reactor is a type of nuclear reactor that uses liquid lead or a lead-bismuth alloy as a coolant, allowing for fast neutron fission. This design provides several benefits, including enhanced safety features and efficient use of nuclear fuel, making it an important option in the realm of advanced reactor technologies.
Molten salt reactor: A molten salt reactor is a type of nuclear reactor that uses molten salt as both a coolant and a fuel solvent. This design allows for higher operating temperatures and improved thermal efficiency compared to traditional reactors, while also enabling a variety of fuel types, including thorium and uranium. The unique properties of molten salt reactors contribute to enhanced safety features and waste management options.
Neutron moderation: Neutron moderation is the process of slowing down fast neutrons to thermal energies, making them more likely to induce fission in fissile materials. This is crucial for sustaining a nuclear chain reaction in reactors, where the efficiency of fission depends on the ability of neutrons to interact with fuel nuclei. The choice of moderator affects reactor types, core design, and can even play a role in weapon design, influencing how efficiently nuclear reactions occur.
Nuclear Regulatory Commission: The Nuclear Regulatory Commission (NRC) is an independent agency of the United States government responsible for regulating civilian use of nuclear energy and materials. Its main goal is to ensure the safety and security of nuclear reactors, the handling of nuclear fuel, and the management of radioactive waste, ultimately protecting public health and the environment.
Plutonium-239: Plutonium-239 is a radioactive isotope of plutonium that is fissile, meaning it can sustain a nuclear fission chain reaction. This characteristic makes it an important fuel for nuclear reactors and a critical component in nuclear weapons, connecting it to various processes and technologies in nuclear physics.
Pool-type reactor: A pool-type reactor is a type of nuclear reactor where the core is submerged in a large pool of water, which serves as both a coolant and a radiation shield. This design allows for easy access to the reactor core for maintenance and research purposes, making it ideal for educational and experimental applications. The pool also helps to absorb radiation emitted from the reactor, providing enhanced safety features.
Pressurized Water Reactor: A pressurized water reactor (PWR) is a type of nuclear reactor where water is used as both a coolant and a neutron moderator, operating under high pressure to prevent boiling. This design allows for efficient heat transfer from the nuclear fission process to generate steam, which drives turbines for electricity production while maintaining a controlled environment for the fission process.
Pulsed Reactor: A pulsed reactor is a type of nuclear reactor that operates by releasing short bursts or pulses of neutron flux, allowing for precise control over the fission process. This method of operation enhances the ability to conduct experiments and achieve specific reactions, making it particularly useful in research applications and materials testing. Pulsed reactors can provide valuable data for understanding nuclear reactions, as well as developing advanced materials and nuclear technologies.
Safety Analysis Report: A Safety Analysis Report (SAR) is a comprehensive document that outlines the safety features and analysis of a nuclear facility, providing detailed evaluations of potential hazards, risk assessments, and safety measures in place. It serves as a critical tool for regulatory bodies to ensure that nuclear reactors, regardless of their type, meet stringent safety standards and can operate without posing undue risk to public health and the environment.
Sodium-cooled fast reactor: A sodium-cooled fast reactor is a type of nuclear reactor that uses liquid sodium as a coolant and operates with fast neutrons to sustain the nuclear fission process. This design allows for efficient energy generation and improved fuel utilization, making it a promising option in advanced nuclear technology, particularly in the context of sustainable energy solutions.
Supercritical water reactor: A supercritical water reactor (SCWR) is a type of nuclear reactor that uses supercritical water as both coolant and moderator. This design allows the reactor to operate at higher temperatures and pressures, leading to improved thermal efficiency compared to traditional reactors, and it has the potential to generate electricity more efficiently while minimizing waste.
Tank-type reactor: A tank-type reactor is a nuclear reactor design that features a large, cylindrical vessel where the nuclear fission process occurs. This type of reactor is often characterized by its large containment structure and is used primarily for research, training, and production of isotopes, offering controlled and accessible environments for various experimental purposes.
Uranium-235: Uranium-235 is a naturally occurring isotope of uranium that is crucial for nuclear fission, which is the process that releases energy used in nuclear reactors and atomic bombs. It constitutes about 0.7% of natural uranium and is significant in the context of atomic structure, neutron interactions, reactor design, and the nuclear fuel cycle, making it a vital element in both energy production and nuclear weapons.
Very High Temperature Reactor: A very high temperature reactor (VHTR) is a type of nuclear reactor that operates at temperatures exceeding 1000 degrees Celsius, designed to produce both electricity and high-temperature process heat for various applications. This reactor type utilizes helium as a coolant and has the capability to support hydrogen production and other industrial processes, making it an innovative solution for meeting future energy demands while minimizing carbon emissions.