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CANDU reactors, a pioneering type of nuclear power plant developed in Canada, represent a critical technology in the global transition to low-carbon energy systems. As nations worldwide intensify their efforts to combat climate change and reduce greenhouse gas emissions, these innovative reactors offer a proven, reliable pathway toward sustainable energy production. With their unique design characteristics, exceptional safety record, and ability to operate on natural uranium fuel, CANDU reactors stand as a testament to Canadian engineering excellence and continue to play an increasingly important role in meeting the world’s growing demand for clean, baseload electricity.
Understanding CANDU Reactor Technology
CANDU is an acronym for CANada Deuterium Uranium, representing a unique design that uses deuterium oxide (heavy water) as moderator and natural uranium as fuel. CANDU reactors were first developed in the late 1950s and 1960s by a partnership between Atomic Energy of Canada Limited (AECL), the Hydro-Electric Power Commission of Ontario, Canadian General Electric, and other companies. This collaborative effort resulted in a reactor design that would distinguish itself from other nuclear technologies through several innovative features.
The Heavy Water Advantage
The defining characteristic of CANDU reactors is their use of heavy water (deuterium oxide, D₂O) as both a moderator and coolant. While heavy water is very expensive to isolate from ordinary water, its low absorption of neutrons greatly increases the neutron economy of the reactor, avoiding the need for enriched fuel. This fundamental design choice has profound implications for the reactor’s operation and fuel cycle.
Heavy water’s extra neutron decreases its ability to absorb excess neutrons, resulting in a better neutron economy, which allows CANDU to run on unenriched natural uranium, or uranium mixed with a wide variety of other materials such as plutonium and thorium. This neutron efficiency is what enables CANDU reactors to achieve criticality with natural uranium, a feat impossible with light water reactors.
The mechanical arrangement of the PHWR, which places most of the moderator at lower temperatures, is particularly efficient because the resulting thermal neutrons have lower energies than in traditional designs, and the neutron cross section for fission is higher in uranium-235 the lower the neutron temperature is. This thermal efficiency translates directly into improved fuel utilization and reactor performance.
Pressure Tube Design Innovation
Unlike conventional light water reactors that use a single large pressure vessel, CANDU reactors employ a unique pressure tube design. The fuel bundles are contained in pressure tubes within a larger vessel containing additional heavy water acting as a moderator, and this larger vessel, known as a calandria, is not pressurized and remains at lower temperatures, making it easier to fabricate.
This design innovation emerged from practical necessity. Building a pressure vessel of the required size is a significant challenge, and at the time of the CANDU’s design, Canada’s heavy industry lacked the requisite experience and capability to cast and machine reactor pressure vessels of the required size, a problem amplified by natural uranium fuel’s lower fissile density, which requires a larger reactor core.
Pressure tubes running horizontally through the reactor contain the fuel, with high-pressure heavy water coolant passing through the pressure tubes, while the calandria is about 6 meters long and 7 meters across and is not a pressure vessel. This horizontal configuration allows for one of CANDU’s most significant operational advantages: on-power refueling.
Fuel Bundle Configuration
In CANDU reactors, fuel in the form of uranium oxide powder is packed into pellets and placed in fuel rods, with thirty-seven fuel rods bunched together to complete a cylindrical fuel bundle that is approximately 50 cm in length and 10 cm in diameter. This compact bundle design facilitates efficient heat transfer and allows for the unique refueling system that sets CANDU apart from other reactor types.
Natural uranium is composed of about 0.7% uranium-235, and the remaining 99.3% is mostly uranium-238 which cannot directly be used in a fission process to obtain energy, but uranium-238 is fertile and can absorb high speed neutrons and convert to plutonium-239 which is fissile and then undergoes fission, a process that accounts for around half of the energy produced within the reactor. This in-situ breeding and burning of plutonium contributes significantly to the reactor’s fuel efficiency.
Operational Advantages and Safety Features
On-Power Refueling Capability
One of the most distinctive features of CANDU technology is its ability to refuel while operating at full power. On-line refueling in a CANDU reactor is a unique trait that is a major advantage over other reactors such as the pressurized water reactors and boiling water reactors. This capability provides significant operational and economic benefits.
In conventional pressurized water reactors, refuelling the system requires shutting down the core and opening the pressure vessel, but in CANDU reactors, the tube being refuelled remains pressurized, which allows the CANDU system to be continually refuelled without shutting down. This continuous operation maximizes electricity generation and plant availability.
In modern systems, two robotic machines attach to the reactor faces and open the end caps of a pressure tube, with one machine pushing in the new fuel whereby the depleted fuel is pushed out and collected at the other end, and a significant operational advantage is that a failed or leaking fuel bundle can be removed from the core once it has been located, thus reducing the radiation levels in the primary cooling loop.
The fuel handling system refuels the reactor with new fuel bundles without interruption of normal reactor operation and is designed to operate at all reactor power levels. This continuous refueling approach ensures optimal fuel burnup and maintains consistent reactor performance throughout the operating cycle.
Inherent Safety Characteristics
CANDU reactors incorporate multiple layers of safety features that have contributed to their exceptional safety record. A hidden benefit of CANDU reactors is that their reactivity margins are relatively low, and the nuclear chain reaction can only function in the presence of heavy water, making CANDU reactors very safe as compared to other power reactor technologies, and if a CANDU reactor is flooded with light water the nuclear reaction stops, and as long as there is sufficient liquid water to absorb nuclear decay heat fuel meltdown is physically impossible.
This inherent safety feature provides a fundamental physical barrier against severe accidents. The reactor’s dependence on heavy water for maintaining criticality means that any loss of heavy water or contamination with light water automatically reduces reactivity, providing a passive safety mechanism that requires no operator intervention.
In more than 50 years of CANDU power reactor operation in many countries the most serious CANDU reactor accident only caused “a puddle on the floor”. This remarkable safety record demonstrates the effectiveness of the CANDU design philosophy and its multiple, redundant safety systems.
A further unique feature of heavy-water moderation is the greater stability of the chain reaction due to the relatively low binding energy of the deuterium nucleus leading to some energetic neutrons and gamma rays breaking the deuterium nuclei apart to produce extra neutrons, and the slow response of these gamma-generated neutrons delays the response of the reactor and gives the operators extra time in case of an emergency. This delayed neutron effect enhances reactor controllability and provides additional safety margins.
Multiple Safety Systems
Four special safety systems (shutdown system number 1, shutdown system number 2, the emergency core cooling system) are provided to minimize and mitigate the impact of any postulated failure in the principal nuclear steam plant systems. These redundant systems ensure that the reactor can be safely shut down and cooled under all credible accident scenarios.
The Principle of defense in depth is the cornerstone of CANDU safety philosophy, with SDS 1 consisting of shut off rods that fall under gravity and SDS 2 consisting of liquid poison injection. This multi-layered approach to safety ensures that multiple independent barriers exist between radioactive materials and the environment.
Fuel Flexibility and Resource Efficiency
Natural Uranium Fuel Cycle
The ability to operate on natural uranium fuel represents one of CANDU’s most significant advantages. Operating on natural uranium removes the cost of enrichment, and this also presents an advantage in nuclear proliferation terms, as there is no need for enrichment facilities, which might also be used for weapons. This characteristic makes CANDU technology particularly attractive for countries seeking energy independence without developing sensitive fuel cycle facilities.
CANDU reactor has been designed for the use of natural Uranium, and natural Uranium fuel eliminates the need for enrichment facilities. This simplification of the fuel cycle reduces both capital costs and operational complexity while enhancing energy security.
Overall, CANDU reactors use 30–40% less mined uranium than light-water reactors per unit of electricity produced, which is a major advantage of the heavy-water design as it not only requires less fuel, but as the fuel does not have to be enriched, it is much less expensive as well. This superior uranium utilization translates directly into lower fuel costs and reduced environmental impact from uranium mining.
Alternative Fuel Capabilities
Beyond natural uranium, CANDU reactors demonstrate remarkable fuel flexibility. A heavy-water design can sustain a chain reaction with a lower concentration of fissile atoms than light-water reactors, allowing it to use some alternative fuels such as recovered uranium from used LWR fuel, and CANDU was designed for natural uranium with only 0.7% uranium-235, so reprocessed uranium with 0.9% uranium-235 is a comparatively rich fuel, which extracts a further 30–40% energy from the uranium.
The Qinshan CANDU reactor in China has used recovered uranium, and the DUPIC (Direct Use of spent PWR fuel in CANDU) process under development can recycle it even without reprocessing, with the fuel being sintered in air then in hydrogen to break it into a powder, which is then formed into CANDU fuel pellets. This capability to utilize spent fuel from other reactor types represents a significant contribution to nuclear waste management and resource conservation.
CANDU reactors can also breed fuel from the more abundant thorium, which is being investigated by India to take advantage of its natural thorium reserves. This thorium fuel cycle option provides a pathway to vastly expanded nuclear fuel resources, potentially extending nuclear energy availability for thousands of years.
The requirement to be able to use natural-uranium fuel has resulted in the CANDU reactor being the most neutron-efficient commercial reactor, achieved through the use of heavy water for both coolant and moderator, the use of low-neutron-absorbing structural materials in the fuel and core, and on-line refuelling, and high neutron economy provides a flexibility in fuel use that is not available in other reactors, including the ability to utilize low-grade nuclear fuels, such as recovered uranium from conventional reprocessing of used LWR fuel which can be used as-is in CANDU reactors.
CANDU Reactors in the Global Energy Landscape
International Deployment
According to World Nuclear Association information, today there are some 27 Candu power reactors operating in seven countries. This global deployment demonstrates the technology’s versatility and adaptability to different national contexts and regulatory environments.
There have been two major types of CANDU reactors, the original design of around 500 MWe that was intended to be used in multi-reactor installations in large plants, and the optimized CANDU 6 in the 600 MWe class that is designed to be used in single stand-alone units or in small multi-unit plants, with CANDU 6 units built in Quebec and New Brunswick, as well as Pakistan, Argentina, South Korea, Romania, and China.
Export sales of 12 Candu units have been made to South Korea (4), Romania (2), India (2), Pakistan (1), Argentina (1) and China (2), along with the engineering expertise to build and operate them. These international projects have transferred valuable nuclear technology and expertise to partner countries, supporting their energy development goals.
Canada has exported CANDU reactors to Argentina, China, India, Pakistan, Romania and South Korea, with 30 CANDU reactors in operation globally, and India developed the design and built 16 reactors that are based on the CANDU design. This technology transfer has enabled India to develop a substantial indigenous nuclear power program.
Operational Performance
CANDU reactors have demonstrated excellent operational performance worldwide. Over the past 10 years, the four operating CANDU 6 units have achieved average lifetime capacity factors higher than 80%. This high availability reflects the reliability of the design and the effectiveness of on-power refueling in maintaining continuous operation.
Unit 1 of the Qinshan Phase III nuclear power plant near Shanghai in China’s Zhejiang province was taken offline on 1 May after 738 days of continuous operation, setting a new record for the longest uninterrupted operation of a power reactor in China as well as a world record for an operating run for a Candu-6 reactor. Such performance records demonstrate the maturity and reliability of CANDU technology.
Currently CANDU reactors dependably supply at a competitive cost over 60% of the 140 TWh per year of electricity consumed in the Province of Ontario. This substantial contribution to Ontario’s electricity supply underscores the technology’s importance to Canada’s energy security and its role in providing clean, reliable baseload power.
Recent Developments and New Builds
After a hiatus in new construction, CANDU technology is experiencing renewed interest. Candu Energy Inc., an AtkinsRéalis company, in a joint venture with Fluor Corporation, Ansaldo Nucleare and Sargent & Lundy, has been awarded a contract from EnergoNuclear S.A., to build two new CANDU reactors at the Cernavoda Nuclear Generating Station in Romania, the first in the world since 2007, and these two new units for Romania signal a new era in the construction of large reactors in support of the nuclear super cycle in response to the increasing global demand for energy.
Canada is proud to stand at the forefront of the global clean energy transition, with CANDU nuclear technology as a pillar of innovation and reliability, and this collaboration in Romania highlights Canada’s leadership in nuclear energy, providing clean, resilient solutions that enhance energy security, reduce emissions, and support economic growth, while creating good, high-paying jobs and advancing sustainable growth and reinforcing Canada’s commitment to a low-carbon future.
Romania has already prevented the emission of over 215Mt of CO2 since the initial reactors became operational in 1996 and 2007, and the expansion of Candu technology is seen as a pivotal step in Romania’s commitment to decarbonisation and energy stability, including the transition away from coal-powered energy.
CANDU’s Role in Low-Carbon Energy Transition
Zero-Emission Baseload Power
As countries worldwide commit to reducing greenhouse gas emissions, nuclear power’s role as a low-carbon energy source becomes increasingly critical. CANDU reactors provide consistent, reliable baseload electricity without producing greenhouse gases during operation. This characteristic makes them an essential complement to variable renewable energy sources.
By producing low-carbon electricity, CANDU reactors help reduce greenhouse gas emissions in the region, and CANDU reactors are different from most other sources of low-carbon electricity generation because the output is consistent, unlike variable generating sources such as wind and solar, which helps maintain a balanced electrical grid.
It provides consistent, low-carbon electricity and about 30 per cent of the province’s annual electricity demand, part of the province’s roughly 80 per cent non-emitting supply of electricity. This substantial contribution to clean electricity generation demonstrates nuclear power’s importance in achieving decarbonization goals.
The stability and reliability of CANDU reactors make them particularly valuable for grid management. Unlike intermittent renewable sources that depend on weather conditions, CANDU reactors can operate continuously at high capacity factors, providing the stable foundation needed for modern electricity grids while other sources handle variable demand and supply.
Supporting Climate Goals
The intellectual property licensing agreement dates back to 2011, and at that time, transitioning to a low-carbon world was not a consideration, but the expanded agreement will reflect the changing priorities and the organisations’ belief in the role that Candu technology will play in decarbonising Canada and the world. This evolution reflects the growing recognition of nuclear power’s essential role in climate change mitigation.
Modernising the CANDU reactor will strengthen Canada’s energy security while supporting its allies in transitioning to cleaner electricity generation. International cooperation on CANDU technology development and deployment can accelerate the global transition to low-carbon energy systems.
Canada accelerates clean energy leadership with a major investment in modernising its homegrown CANDU reactor technology, and modernising the CANDU reactor will strengthen Canada’s energy security while supporting its allies in transitioning to cleaner electricity generation. These investments demonstrate governmental commitment to nuclear energy as a climate solution.
Economic and Social Benefits
Beyond environmental benefits, CANDU reactors provide significant economic advantages. PLNGS is an important economic contributor to New Brunswick as it employees approximately 900 employees and supports local supply chains. Nuclear facilities create high-quality employment opportunities and stimulate regional economic development.
The construction and maintenance of CANDU reactors support a wide range of local businesses, suppliers, and service providers, helping grow the regional economy. This economic multiplier effect extends throughout the supply chain, creating value far beyond the plant site itself.
As well as their use for electricity, Candu power reactors produce almost all the world’s supply of the cobalt-60 radioisotope for medical and sterilization use, and today, there are 27 Candu power reactors in seven countries, as well as 17 ‘Candu derivative’ reactors in India. This medical isotope production represents an important additional benefit of CANDU technology, supporting healthcare systems worldwide.
Challenges and Limitations
Capital Cost Considerations
Despite their many advantages, CANDU reactors face certain challenges. Heavy water generally costs hundreds of dollars per kilogram, though this is a trade-off against reduced fuel costs. The initial investment in heavy water inventory represents a significant capital cost that must be balanced against long-term fuel savings.
The disadvantages of heavy water reactors lie in the rather important investments which are needed and in the complexity of operational problems. These higher upfront costs can make CANDU reactors less attractive in markets where capital availability is limited or where financing costs are high.
The pressure tube design, while offering advantages in terms of fabrication and on-power refueling, also introduces unique maintenance requirements. The design working life of a properly maintained CANDU reactor is 60 years, and typically there are two fuel and moderator channel changes during the reactor design working life. These mid-life refurbishments, while extending reactor life, represent significant planned maintenance activities.
Reactor Size and Footprint
The core of a CANDU reactor needs to be larger than comparable light water reactors however if it wants to achieve the same output capacity, and this due to the CANDU’s use of natural uranium. The lower fissile content of natural uranium necessitates a larger core volume to achieve the same power output as enriched uranium reactors.
A lot of heavy water is needed to slow neutrons down, which makes a heavy water moderated reactor large, and a large pressure vessel is difficult to build and very expensive. While the pressure tube design addresses the pressure vessel challenge, the overall reactor size remains larger than comparable light water reactors.
Operational Complexity
The reduced energy content of natural uranium as compared to enriched uranium necessitates more frequent replacement of fuel, which is normally accomplished by use of an on-power refueling system, and the increased rate of fuel movement through the reactor also results in higher volumes of spent fuel than in LWRs employing enriched uranium. This higher fuel throughput requires robust fuel handling systems and larger spent fuel storage facilities.
The complexity of operating with heavy water systems requires specialized training and expertise. Maintaining heavy water purity, managing tritium production, and operating the unique fuel handling systems all demand skilled personnel and sophisticated procedures. However, decades of operational experience have developed comprehensive training programs and operational best practices that effectively address these challenges.
Advanced CANDU Designs and Future Development
CANDU Monark and Next-Generation Designs
In late 2023, AtkinsRéalis (formerly SNC-Lavalin) announced its new Candu Monark design, with several passive safety features, a 1000 MWe design that is a heavy water cooled and moderated Candu reactor core of 480 channels, with a two-loop ‘figure of eight’ heat transport system, similar to the design used for the Darlington plant, using natural uranium in 37-element bundles, and the conceptual design was completed in September 2024, with AtkinsRéalis signing a service agreement with the CNSC for familiarization and planning of a future pre-licensing design review in October 2024.
Jonathan Wilkinson, Minister of Energy and Natural Resources, recently announced a preliminary agreement with AtkinsRéalis to support the development of a new large-scale CANDU nuclear reactor, and as part of this agreement, the Government of Canada will provide up to $304m over four years to finance half of AtkinsRéalis’ design project for the next-generation CANDU reactor, known as MONARK, and this effort will also engage Atomic Energy of Canada Limited (AECL), which owns the CANDU intellectual property, along with operators and the broader Canadian supply chain.
These new designs incorporate lessons learned from decades of CANDU operation while integrating modern safety features and digital technologies. The focus on passive safety systems reflects industry-wide trends toward inherently safe reactor designs that rely on natural physical phenomena rather than active systems requiring operator intervention or external power.
Small Modular Reactor Development
In 2017, a consultation with industry led Natural Resources Canada to establish a “SMR Roadmap” targeting the development of small modular reactors (SMRs), and in response, SNC-Lavalin developed a 300 MWe SMR version of the CANDU, the CANDU SMR, which it began to highlight on its website, and SNC-Lavalin is still looking at marketing a 300 MW SMR in part due to projected demand due to climate change mitigation.
SNC-Lavalin is engaging Canada’s prominent nuclear engineering, supplier and construction community to bring together a truly Team Canada approach to deploy the CANDU Small Modular Reactor (CSMR), and through the construct of a Public-Private Enterprise, they can support Canada’s SMR roadmap by building a grid-scale SMR that is online before the end of the decade, employing Canada’s own scientists, universities, laboratories, utilities, engineering firms, manufacturers and construction companies to deliver an all Canadian SMR to start powering the country in 2028, and based on proven Canadian reactor technology with over 75 years of safe operations, the CSMR project will play a significant role in helping Canada achieve NetZero by 2050.
The CSMR is the only SMR technology to use natural, unenriched uranium, which allows the fuel to be sourced from highly productive mines in Saskatchewan and be processed in Canada, avoiding the need for shipments of enriched uranium fuel into and across Canada. This fuel independence represents a significant advantage for the CANDU SMR concept, maintaining the proliferation resistance and energy security benefits of the larger CANDU designs.
Beyond CANDU reactor advancements, Canada is also investing in emerging nuclear technologies like small modular reactors (SMRs), with Minister Wilkinson announcing $55m in funding from the Future Electricity Fund to support Ontario Power Generation’s Darlington New Nuclear Project, and this project will advance three GE Hitachi BWRX-300 SMRs, each capable of generating 300 megawatts of zero-emission electricity – enough to power approximately 900,000 homes.
Enhanced CANDU 6 Design
Many of the operational design changes were also applied to the existing CANDU 6 to produce the Enhanced CANDU 6, also known as CANDU 6e or EC 6, an evolutionary upgrade of the CANDU 6 design with a gross output of 740 MWe per unit, with reactors designed with a lifetime of over 50 years and a projected average annual capacity factor of more than 90%, and improvements to construction techniques including modular, open-top assembly decrease construction costs, while the CANDU 6e is designed to operate at power settings as low as 50%, allowing them to adjust to load demand much better than the previous designs.
This load-following capability addresses one of the traditional limitations of nuclear power plants, which typically operate most economically at constant full power. The ability to adjust output in response to grid demand makes Enhanced CANDU 6 reactors more compatible with electricity systems incorporating variable renewable generation.
Refurbishment and Life Extension Programs
Major Component Replacement
To meet current and future electricity needs, provincial governments and power companies have made the decision to extend the operating lifetime of a number of reactors by refurbishing them, and refurbishing Candu units consists of such steps as replacing fuel channels and steam generators and upgrading ancillary systems to current standards. These refurbishment programs effectively provide a new lease on life for aging reactors.
Work on unit 1 began in February 2022 and the unit was reconnected to the grid in November 2024, ahead of schedule, and refurbishment of unit 4 commenced in July 2023, and the unit was reconnected to the grid in March 2026, completing the C$12.8 billion programme C$150 million under budget and four months ahead of schedule, marking the end of the major refurbishment project that began in 2016, and earlier in September 2025 the CNSC granted a 20-year licence renewal for the station, to November 2045.
These successful refurbishment projects demonstrate the technical feasibility and economic viability of extending CANDU reactor operating lives. The ability to replace major components while maintaining the basic reactor structure provides a cost-effective alternative to new construction while incorporating modern safety and performance improvements.
International Refurbishment Experience
In mid-2005, the decision was made to refurbish New Brunswick Power’s 635 MWe Point Lepreau reactor, which provides one-quarter of the province’s power, and it was the first Candu 6 type in commercial operation and was the first Candu-6 reactor to undergo full refurbishment, including replacement of all calandria tubes as well as steam generators. This pioneering refurbishment project established procedures and techniques subsequently applied to other CANDU refurbishments.
The two reactors are now approaching the end of their initial 30-year design life, and operator Third Qinshan Nuclear Power Company is undertaking a programme to refurbish the reactors and associated fuel channels, and the refurbishment will allow the Candu units to continue generating power for a further 30 years. International refurbishment projects demonstrate the global applicability of life extension strategies developed in Canada.
CANDU Technology and Energy Security
Fuel Supply Independence
The ability to operate on natural uranium provides CANDU operators with significant energy security advantages. Candu reactors’ ability to use unenriched uranium is beneficial for energy security by reducing dependency on foreign fuel refinement processes. Countries without uranium enrichment capabilities can operate CANDU reactors using domestically mined uranium or uranium purchased on international markets without requiring enrichment services.
This was a major goal of the CANDU design; by operating on natural uranium the cost of enrichment is removed, and this also presents an advantage in nuclear proliferation terms, as there is no need for enrichment facilities, which might also be used for weapons. This proliferation resistance makes CANDU technology particularly suitable for countries seeking to develop nuclear power programs under international safeguards.
Canada’s position as a major uranium producer further enhances the energy security proposition for CANDU technology. With an abundance of uranium reserves and an exceptional cadre of skilled professionals, Canada has a vibrant nuclear industry, bringing several advantages to the country, and Canada exports about 85 per cent of uranium mined in the country. This domestic fuel supply chain provides additional security and economic benefits.
Grid Reliability and Resilience
The CANDU reactor fleet is a critical component of Ontario electricity generation, especially in the mid to late summer when renewable electricity production is low due to both reduced river flow and reduced wind generation while the grid load is maximum due to use of air conditioning to combat hot humid summer conditions that have been aggravated by global warming. This reliability during peak demand periods demonstrates nuclear power’s value for grid stability.
The high capacity factors achieved by CANDU reactors ensure consistent electricity supply regardless of weather conditions or seasonal variations. This predictability allows grid operators to plan with confidence and reduces the need for expensive backup generation capacity. As electricity grids incorporate increasing amounts of variable renewable generation, the stable baseload provided by CANDU reactors becomes even more valuable.
Environmental Considerations and Waste Management
Greenhouse Gas Emissions Avoidance
The primary environmental benefit of CANDU reactors is their contribution to greenhouse gas emissions reduction. By generating electricity without combustion, CANDU reactors avoid the carbon dioxide emissions associated with fossil fuel power generation. Over their operating lifetimes, CANDU reactors prevent millions of tonnes of CO₂ emissions compared to equivalent fossil fuel generation.
During the latest operating cycle, which began on 24 April 2023, Qinshan III unit 1 has generated more than 12.5 billion kilowatt-hours of electricity, equivalent to reducing standard coal consumption by 3.8 million tonnes and reducing carbon dioxide emissions by 9.97 million tonnes. These substantial emissions reductions demonstrate nuclear power’s climate change mitigation potential.
Spent Fuel Management
While CANDU reactors produce spent fuel that requires long-term management, the volume and characteristics of this waste are well understood. Dry storage of used CANDU fuel is now widespread, and these dry-storage systems provide economic, simple, and easy-to-implement used-fuel storage for decades. Interim storage solutions have proven safe and effective while permanent disposal solutions are developed.
The higher fuel throughput of CANDU reactors compared to enriched uranium reactors results in larger volumes of spent fuel per unit of electricity generated. However, this spent fuel contains lower concentrations of long-lived actinides due to the lower burnup, which may simplify long-term disposal. Additionally, the fuel flexibility of CANDU reactors offers potential waste management solutions through the use of spent fuel from other reactor types.
Tritium Production and Management
Between 1.5 to 2.1 kilograms of tritium were recovered annually at the Darlington separation facility by 2003, of which a minor fraction was sold, and consequently, the Canadian Nuclear Laboratories in 2024 announced a decades-long program to refurbish existing CANDU plants and equip them with tritium breeding facilities. Tritium production, while requiring careful management, also represents a valuable byproduct with applications in fusion research and other technologies.
Heavy water reactors naturally produce tritium through neutron activation of deuterium. While this requires careful monitoring and control to prevent environmental releases, established procedures and technologies effectively manage tritium in CANDU facilities. The potential value of tritium for future fusion reactors adds another dimension to CANDU’s contribution to clean energy development.
Economic Competitiveness and Market Position
Levelized Cost of Electricity
The economic competitiveness of CANDU reactors depends on various factors including capital costs, fuel costs, operating costs, and capacity factors. The higher capital costs associated with heavy water inventory and larger core size must be balanced against lower fuel costs from using natural uranium and high capacity factors from on-power refueling.
The high cost of the heavy water is offset by the lowered cost of using natural uranium and/or alternative fuel cycles. Over the reactor’s operating lifetime, fuel cost savings can compensate for higher initial capital investment, particularly in scenarios where uranium enrichment costs are high or natural uranium prices are favorable.
The ability to achieve high capacity factors through on-power refueling improves the economic performance of CANDU reactors by maximizing electricity generation from the capital investment. Extended operating cycles between major maintenance outages further enhance economic competitiveness by reducing downtime and associated costs.
Export Market Opportunities
The CANDU technology has been a Canadian success story with a track record of excellent performance in export markets, most recently in Romania with the construction of the second unit on schedule and on budget, with the lifetime capacity factors of the CANDU reactors abroad averaging over 90% compared to around 80 percent in Canada, and the prospects for new nuclear power reactors abroad are quite promising according to recent outlooks, and Canada, as one of the few countries in the world offering reactor technology and related services, is well positioned to benefit from the renewed global interest in nuclear energy.
A conservative estimate of the export potential of SMRs is $150 billion per year globally from 2030 to 2040, including applications for electricity generation, remote mine sites, island nations and off-grid communities, and the world market could be much larger if countries adopt SMRs as part of their fight against climate change by reducing or eliminating their use of fossil fuels to generate electricity. This substantial market opportunity positions Canada to benefit economically from global decarbonization efforts.
Integration with Renewable Energy Systems
Complementary Generation Mix
As electricity systems worldwide transition toward higher penetrations of renewable energy, the role of dispatchable, low-carbon generation becomes increasingly important. CANDU reactors provide the stable baseload generation that complements variable renewable sources, helping to maintain grid stability and reliability.
The consistent output from CANDU reactors allows renewable generation to be integrated more effectively by providing a stable foundation for the grid. When wind and solar generation are high, CANDU reactors continue operating at steady output, and when renewable generation drops, the nuclear baseload ensures continued electricity supply without resorting to fossil fuel backup generation.
Enhanced CANDU designs with improved load-following capabilities can provide additional flexibility to accommodate renewable generation variability. The ability to adjust output while maintaining safe, stable operation allows CANDU reactors to play an even more active role in supporting renewable energy integration.
Hybrid Energy Systems
Future energy systems may incorporate hybrid configurations combining CANDU reactors with renewable generation and energy storage. The reliable heat and electricity output from CANDU reactors could support industrial processes, hydrogen production, or district heating systems while simultaneously providing grid electricity. These multi-purpose applications maximize the value of nuclear generation and support broader decarbonization objectives beyond the electricity sector.
The thermal energy from CANDU reactors could potentially support industrial applications requiring process heat, reducing fossil fuel consumption in manufacturing and other sectors. Integration with hydrogen production facilities could create clean fuel for transportation and industry while providing grid balancing services through flexible hydrogen production rates.
Research, Development, and Innovation
Ongoing Technology Improvements
Design work continued throughout, and new design concepts were introduced that dramatically improved safety, capital costs, economics and overall performance. Continuous improvement efforts have enhanced CANDU technology over decades of development and operation, incorporating lessons learned and advancing the state of the art.
Technological sustainability is assured by ongoing advances in CANDU fuel design to meet the needs of future fuels and fuel cycles, and features that can be employed in future fuel designs include optimization of the internal element design for high-burnup applications, the use of graphite disks between pellets to lower fuel temperatures, advanced welding techniques, improved CANLUB coatings for high-burnup applications, further bundle sub-division such as a 61-element bundle for very high-burnup applications, tailoring the reactivity coefficients, and further enhancements in the thermalhydraulic margins.
These ongoing research and development efforts ensure that CANDU technology continues to evolve and improve, maintaining competitiveness with other reactor designs and addressing emerging challenges and opportunities in the nuclear energy sector.
Digital Technology Integration
Modern CANDU designs incorporate advanced digital control systems, instrumentation, and monitoring technologies that enhance safety, performance, and operational efficiency. Digital twins and advanced simulation capabilities support operator training, maintenance planning, and performance optimization. These digital technologies represent a significant advancement over the analog systems in earlier CANDU designs.
Artificial intelligence and machine learning applications offer potential for predictive maintenance, anomaly detection, and operational optimization. As these technologies mature, their integration into CANDU operations could further enhance safety margins, reduce operating costs, and extend component lifetimes through better-informed maintenance strategies.
Policy and Regulatory Considerations
Licensing and Regulatory Framework
In the licensing process responsibility for meeting the requirements is on the designer, with regulators performing an auditing role to ensure compliance with the regulations, and the Canadian licensing process is not a prescriptive approach and allows innovation by the designer to improve the plant safety and the performance. This performance-based regulatory approach has facilitated continuous improvement in CANDU designs while maintaining rigorous safety standards.
International deployment of CANDU technology requires navigation of different national regulatory frameworks. The successful licensing of CANDU reactors in multiple countries demonstrates the technology’s ability to meet diverse regulatory requirements and safety standards. Harmonization efforts and mutual recognition of safety assessments can facilitate future international deployment.
Government Support and Investment
The Federal government has funded nuclear research and development for over 50 years, and the Government’s support has enabled Canada to develop its own nuclear power technology and other related technologies, and as a result, Canada has developed a world-class indigenous technology and various spin-off nuclear technologies which have made a major contribution to our economy and society over and above energy benefits.
Continued government support for CANDU development, including funding for advanced designs, SMR development, and international deployment, will be essential for maintaining Canada’s position as a nuclear technology leader. Public-private partnerships can leverage government funding to accelerate technology development and commercialization while managing risks and sharing benefits.
The Path Forward: CANDU in a Net-Zero Future
Meeting Growing Electricity Demand
Ontario’s power demand is expected to surge by 75% by 2050 and we’ll need all sources of cleaner power to meet the need, including large and small nuclear reactor technology. This dramatic increase in electricity demand, driven by electrification of transportation, heating, and industry, will require substantial new generation capacity, much of which must be low-carbon to meet climate goals.
CANDU reactors, both large-scale and SMR variants, can contribute significantly to meeting this growing demand while maintaining grid reliability and minimizing environmental impact. The proven technology, established supply chains, and operational experience position CANDU well to support rapid deployment of new nuclear capacity where needed.
International Climate Commitments
As countries strengthen their climate commitments and pursue net-zero emissions targets, the need for reliable, scalable low-carbon energy technologies becomes more urgent. CANDU reactors offer a proven solution that can be deployed in diverse national contexts, supporting global decarbonization efforts while enhancing energy security and economic development.
Candu technology is the only home-grown Canadian power reactor technology that competes on a global stage, providing energy security, creating export opportunities and thousands of well-paying, highly skilled jobs, while also supporting the local supply chain and creating economic value to Canada, and the deployment of the Candu Monark will provide safe, carbon free, base load power to the country and the world.
Technology Transfer and Capacity Building
International deployment of CANDU technology includes comprehensive technology transfer and capacity building programs that enable partner countries to develop indigenous nuclear capabilities. This approach supports sustainable development by creating local expertise, employment, and industrial capacity while ensuring safe, effective reactor operation.
The success of technology transfer programs in countries like South Korea, which developed substantial indigenous nuclear capabilities based on initial CANDU imports, demonstrates the potential for CANDU deployment to catalyze broader nuclear industry development in partner countries.
Conclusion: CANDU’s Essential Role in Energy Transition
CANDU reactors represent a mature, proven technology with unique characteristics that make them particularly well-suited to supporting the global transition to low-carbon energy systems. Their ability to operate on natural uranium, exceptional safety record, fuel flexibility, and high capacity factors position them as a valuable tool in the fight against climate change.
As the world confronts the dual challenges of meeting growing energy demand while dramatically reducing greenhouse gas emissions, CANDU technology offers a reliable pathway forward. The ongoing development of advanced CANDU designs and small modular reactors ensures that this Canadian innovation will continue to evolve and adapt to changing energy system needs.
With renewed international interest in nuclear power as a climate solution, significant new build projects underway, and continued investment in technology development, CANDU reactors are poised to play an increasingly important role in global energy systems. Their contribution to energy security, economic development, and environmental protection makes them an essential component of sustainable, low-carbon energy futures.
The success of CANDU technology over more than five decades of operation, across multiple countries and diverse applications, provides confidence in its continued relevance and value. As nations worldwide accelerate their clean energy transitions, CANDU reactors stand ready to deliver the reliable, low-carbon electricity needed to power a sustainable future while supporting economic prosperity and energy independence.
For more information on nuclear energy and climate change, visit the International Atomic Energy Agency’s climate change resources. To learn more about Canada’s nuclear industry and CANDU technology, explore the Canadian Nuclear Association website. Additional technical information about CANDU reactors can be found through World Nuclear Association. For insights into small modular reactor development in Canada, see Canada’s SMR Action Plan. Those interested in the role of nuclear power in achieving net-zero emissions can review resources from the International Energy Agency.