CANDU reactors, a pioneering nuclear power technology developed in Canada, offer a proven low-carbon pathway as nations accelerate the transition to sustainable energy systems. With unique design features including heavy-water moderation, pressure tube configuration, and the ability to operate on natural uranium, CANDU reactors deliver reliable baseload electricity without greenhouse gas emissions during operation. As countries seek to decarbonize electricity grids while maintaining energy security, these reactors stand as a mature solution with over 50 years of operational experience across seven countries. The technology’s inherent fuel flexibility and on-power refueling capability provide operational advantages that are increasingly valuable in grids with high shares of variable renewable energy. Understanding the technical, economic, and environmental dimensions of CANDU reactors is essential for policymakers, energy planners, and industry stakeholders working toward net-zero emissions by mid-century.

Understanding CANDU Reactor Technology

CANDU (CANada Deuterium Uranium) reactors use deuterium oxide (heavy water) as both moderator and coolant. This design choice enables the reactor to achieve criticality with natural uranium, eliminating the need for expensive enrichment facilities. The technology emerged from a partnership between Atomic Energy of Canada Limited, Ontario Hydro, and Canadian industry in the late 1950s, resulting in a distinctive reactor architecture with several key innovations that have proven remarkably durable and adaptable over decades of operation.

The Heavy Water Advantage

Heavy water’s low neutron absorption dramatically improves neutron economy, allowing CANDU reactors to run on unenriched natural uranium. While heavy water is costly to produce—typically several hundred dollars per kilogram—the savings from avoiding enrichment and the ability to utilize a wider range of fuel materials more than compensate over the reactor’s lifetime. The mechanical arrangement, which keeps the moderator at lower temperatures, further improves thermal neutron efficiency and fuel utilization. This design also reduces the required fissile inventory, which is a key factor in the reactor’s economic competitiveness. Heavy water production facilities, such as the Bruce Heavy Water Plant in Ontario, have historically been collocated with reactor sites to minimize transportation costs and supply chain risks.

Pressure Tube Design

Unlike light-water reactors with a single large pressure vessel, CANDU designs employ multiple horizontal pressure tubes housed within a larger, unpressurized calandria. This approach avoids the manufacturing challenges of large pressure vessels and enables the crucial capability of on-power refueling. Each pressure tube contains fuel bundles cooled by high-pressure heavy water, while the surrounding moderator in the calandria remains at lower temperatures and pressure. The pressure tubes are made of a zirconium‑niobium alloy that resists corrosion and radiation damage. This modular architecture also allows for incremental replacement of tubes during refurbishment, extending the plant’s operational life by decades. The calandria itself is a large, low-pressure vessel that holds the moderator at around 70 °C, which reduces the possibility of a violent steam‑water reaction during accident scenarios.

Fuel Bundle Configuration

Natural uranium dioxide pellets are packed into 37 fuel rods, bundled into cylindrical assemblies approximately 50 cm long and 10 cm in diameter. With only 0.7 % uranium‑235, the reactor also breeds plutonium‑239 from uranium‑238, which contributes roughly half of the energy output. This in‑situ breeding makes CANDU reactors remarkably fuel‑efficient, using 30–40 % less mined uranium per unit of electricity compared to light‑water reactors. The fuel bundles are designed for high reliability, with a very low defect rate in commercial operation. Advanced fuel designs, such as the 61‑element bundle, are under development to achieve higher burnup and further reduce waste volumes. The simplicity of the fuel fabrication process—no enrichment is required—makes it accessible to countries without advanced nuclear fuel cycle facilities.

Operational Advantages and Safety Features

CANDU reactors have accumulated more than 600 reactor‑years of operating experience worldwide, with a strong safety record and high capacity factors. The combination of on‑power refueling, inherent safety characteristics, and multiple engineered safety systems positions them as one of the most robust reactor designs for both baseload and flexible operation.

On-Power Refueling Capability

CANDU reactors can refuel at full power using robotic machines that insert fresh fuel bundles while removing spent ones from the opposite end of the pressure tube. This continuous operation maximizes plant availability—typically above 80 % capacity factor over decades—and allows removal of leaking fuel bundles as they are detected, reducing radiation levels in the primary cooling loop. The system operates at all reactor power levels and maintains optimal fuel burnup throughout the fuel cycle. This capability is particularly valuable for grids that require high capacity factors without extended refueling outages. For example, the Qinshan Phase III CANDU units in China have achieved world‑record continuous runs of over 700 days, demonstrating the reliability of this design.

Inherent Safety Characteristics

The reactor’s reactivity margins are low enough that the chain reaction can only function in the presence of heavy water. Flooding with light water instantly stops the nuclear reaction, providing a passive safety mechanism. As long as sufficient liquid water absorbs decay heat, fuel meltdown is physically impossible. In over 50 years of CANDU operation worldwide, the most serious accident with significant on‑site contamination was the 1952 NRX reactor incident, which was contained without radiological release to the environment. Additionally, the slow response of gamma‑generated neutrons from deuterium breakage gives operators extra time in emergencies. The large thermal inertia of the moderator also provides a significant heat sink that can mitigate station blackout scenarios.

Multiple Safety Systems

CANDU plants incorporate four special safety systems: two independent shutdown systems (gravity‑drop rods and liquid poison injection), an emergency core cooling system, and containment. The principle of defense in depth ensures multiple barriers between radioactive materials and the environment, with redundancy and diversity in safety functions. The shutdown systems are designed to be highly reliable, with automatic initiation on detection of any abnormality. The containment system includes a robust concrete structure with a pressure suppression pool that can condense steam from a postulated loss‑of‑coolant accident, preventing overpressure. Regular safety reviews and upgrades ensure that operating plants meet current regulatory standards, including post‑Fukushima requirements for extreme external events.

Fuel Flexibility and Resource Efficiency

The neutron‑efficient design of CANDU reactors gives them unparalleled fuel flexibility. This not only reduces uranium consumption but also opens pathways for recycling spent fuel from other reactor types and for utilizing alternative fuels such as thorium.

Natural Uranium Fuel Cycle

Operating on natural uranium eliminates enrichment costs and proliferation risks—no enrichment facilities are needed. CANDU reactors use 30–40 % less mined uranium than light‑water reactors per unit of electricity, and the fuel is much less expensive due to the lack of enrichment. This fuel cycle simplicity enhances energy security for countries without enrichment capabilities. The once‑through fuel cycle is relatively straightforward: fresh fuel bundles are delivered from the fabrication plant, used for about 12–18 months in the reactor, and then stored in spent fuel pools before being transferred to dry storage. The low enrichment level also reduces the attractiveness of the material for weapons proliferation, supporting nonproliferation goals.

Alternative Fuel Capabilities

CANDU reactors can use recovered uranium from light‑water reactor spent fuel (with 0.9 % uranium‑235 compared to natural uranium’s 0.7 %), extracting 30–40 % more energy. The DUPIC (Direct Use of spent PWR fuel in CANDU) process recycles spent fuel without reprocessing, reducing the volume of high‑level waste and making use of existing fissile resources. CANDU reactors can also breed fuel from abundant thorium, a pathway being actively explored by India, which has large thorium reserves. India’s Advanced Heavy Water Reactor (AHWR) is a pressure‑tube design derived from the CANDU concept that will use thorium‑uranium‑233 fuel. This neutron efficiency provides flexibility unmatched by other commercial reactor designs, and it positions CANDU technology as a key enabler for a more sustainable nuclear fuel cycle.

CANDU Reactors in the Global Energy Landscape

Twenty‑seven CANDU power reactors operate in seven countries. Their deployment history demonstrates the technology’s adaptability to different regulatory environments, grid conditions, and technological capabilities of host countries. The export model has also fostered long‑term partnerships and technology transfer.

International Deployment

Export sales have included units to South Korea (4), Romania (2), India (2), Pakistan (1), Argentina (1), and China (2). India has additionally built 16 reactors based on the CANDU design, called the Pressurised Heavy Water Reactor (PHWR) series. These projects transfer engineering expertise and support partner countries’ energy development goals. The CANDU 6 design, a 700 MWe class unit, has been the most widely exported variant. South Korea’s Wolsong units have performed exceptionally well, with lifetime capacity factors exceeding 85 %. Romania’s Cernavoda units have become the backbone of the country’s low‑carbon electricity generation, each providing about 10 % of national power demand.

Operational Performance

CANDU 6 units have achieved average lifetime capacity factors above 80 %. Unit 1 of Qinshan Phase III in China set a world record for CANDU‑6 reactors with 738 days of continuous operation. In Ontario, CANDU reactors supply over 60 % of the province’s 140 TWh annual electricity consumption, contributing to roughly 80 % non‑emitting electricity. The Darlington station, with four 881 MWe units, has been recognized for its outstanding performance, including achieving a capacity factor of 93 % in 2024. Ontario’s low‑carbon grid—among the cleanest in North America—is directly attributable to the province’s fleet of CANDU reactors.

Recent Developments and New Builds

Romania is building two new CANDU reactors at Cernavoda, the first new CANDU construction since 2007. The project, involving Candu Energy Inc. (an AtkinsRéalis company), Fluor, Ansaldo Nucleare, and Sargent & Lundy, represents renewed interest in the technology. Romania’s existing CANDU units have already prevented over 215 million tonnes of CO₂ emissions since 1996. In Canada, the government has committed up to CAD 304 million to support the development of the Candu Monark reactor, a 1000 MWe design that builds on the established CANDU technology base. This investment signals a strategic shift toward leveraging domestic nuclear technology for both domestic energy security and export opportunities.

CANDU’s Role in Low-Carbon Energy Transition

As the world accelerates toward net‑zero emissions, nuclear power is increasingly recognized as essential for providing reliable, low‑carbon baseload electricity and heat. CANDU reactors, with their unique features and extensive operational history, are well‑placed to contribute significantly to this transition.

Zero-Emission Baseload Power

CANDU reactors produce consistent, low‑carbon electricity that complements variable renewable sources like wind and solar. Their high capacity factors—often exceeding 90 %—provide the stable baseload that grid operators need to integrate intermittent generation without resorting to fossil fuel backup. This reliability is especially valuable during peak demand periods when renewable output may be low, such as winter evening hours in cold climates. In Ontario, CANDU reactors provide the foundational power that allows the province to operate one of the cleanest grids in the developed world, with over 90 % non‑emitting electricity. The ability to perform load‑following down to 50 % power in newer designs further enhances grid flexibility.

Supporting Climate Goals

Modernizing the CANDU reactor design strengthens energy security while supporting allies in transitioning to cleaner electricity. Government investments in next‑generation CANDU technology demonstrate commitment to nuclear energy as a climate solution. The intellectual property licensing agreement with Atomic Energy of Canada Limited reflects recognition of CANDU’s role in decarbonizing Canada and the world. An International Energy Agency analysis highlights that nuclear power must double by 2050 to meet net‑zero targets affordably; CANDU technology, with its proven performance and ability to use recycled fuel, can help achieve this goal without requiring new enrichment capacity.

Economic and Social Benefits

Beyond electricity, CANDU power reactors produce nearly the entire global supply of cobalt‑60 for medical sterilization and cancer treatment. Each nuclear facility employs hundreds of skilled workers and supports local supply chains, providing high‑quality jobs and economic stimulus. The construction of new CANDU units creates thousands of jobs in engineering, manufacturing, and construction. For example, the refurbishment of Ontario Power Generation’s Darlington station created over 14,000 person‑years of employment. The technology also supports a thriving export industry; Candu Energy Inc. (part of AtkinsRéalis) provides engineering services and reactor components to international customers. The Canadian Nuclear Association reports that the nuclear industry contributes over CAD 6 billion annually to Canada’s GDP.

Challenges, Innovations, and Future Development

No technology is without challenges, and CANDU reactors face specific technical and economic hurdles. However, ongoing innovation—from advanced fuel cycles to digital controls and small modular reactor designs—is addressing these issues and opening new opportunities.

Capital Cost and Reactor Size

The initial investment in heavy water inventory is significant, and CANDU cores are larger than comparable light‑water reactors due to natural uranium’s lower fissile content. However, these costs are offset by fuel savings and extended operating life of 60 years with mid‑life refurbishments. The pressure tube design, while enabling on‑power refueling, requires periodic fuel channel replacement—typically twice during the reactor’s design life. The heavy water itself is a valuable asset; if a reactor is permanently shut down, the heavy water can be recovered and reused in another unit, partially offsetting decommissioning costs. With proper maintenance, CANDU plants can operate economically for 60 years or more, as demonstrated by Ontario’s Pickering station, which operated reliably for over 45 years before entering refurbishment considerations.

Advanced CANDU Designs

AtkinsRéalis launched the Candu Monark design in 2023: a 1000 MWe reactor with 480 channels, passive safety features, and a two‑loop heat transport system. The Canadian government committed up to CAD 304 million to fund half of the design project. The Enhanced CANDU 6 (EC6) offers 740 MWe gross output, a 50+ year design life, and load‑following capability down to 50 % power, making it more compatible with grids with high renewable penetration. These new designs incorporate lessons from decades of operation, including improved materials that reduce corrosion, advanced control systems that enhance operator interfaces, and modular construction techniques that reduce build times. The Monark design also features a simplified heavy‑water inventory management system, reducing initial capital costs.

Small Modular Reactor Development

The CANDU SMR (CSMR) is a 300 MWe design that is the only SMR using natural, unenriched uranium—allowing fuel to be sourced from Canadian mines without international enrichment shipments. A public‑private enterprise aims to deploy the CSMR by 2028, supporting Canada’s net‑zero target by 2050. Beyond CANDU‑specific designs, Canada is also funding GE Hitachi BWRX‑300 SMRs at Darlington, demonstrating a diversified approach to advanced reactors. The CSMR uses the same pressure‑tube technology as larger CANDU units, leveraging established supply chains and regulatory knowledge. Its smaller size makes it suitable for smaller grids, remote communities, and industrial applications such as hydrogen production or process heat.

Refurbishment and Life Extension

Refurbishment programs replace fuel channels, steam generators, and ancillary systems, providing a new lease on life for aging reactors. Ontario Power Generation’s Darlington refurbishment completed work on four units ahead of schedule and under budget, with a 20‑year license renewal to 2045. Point Lepreau in New Brunswick was the first CANDU 6 to undergo full refurbishment, establishing procedures for future projects internationally. The refurbishment of these plants is a cost‑effective way to maintain clean capacity; the cost of refurbishment is typically one‑third to one‑half of the cost of new build, with zero fuel cost uncertainty. The Bruce Power refurbishment program, which includes eight units, is expected to sustain over 22,000 jobs annually during the project period and provide clean power for decades beyond.

Environmental Considerations and Waste Management

CANDU reactors avoid millions of tonnes of CO₂ emissions compared to fossil fuel generation. Qinshan III unit 1 alone reduced carbon dioxide emissions by 9.97 million tonnes over a single operating cycle. Spent fuel management uses dry storage systems that have proven safe and effective for decades. The higher fuel throughput produces larger volumes of spent fuel per unit of electricity, but with lower concentration of long‑lived actinides. Additionally, CANDU reactors produce tritium as a byproduct, which is recovered and potentially valuable for fusion research. The reduced actinide content in CANDU spent fuel makes it less challenging for final disposal in a deep geological repository, as the heat load per unit volume is lower. Canada’s Nuclear Waste Management Organization is progressing toward site selection for a deep geological repository, with CANDU spent fuel being the primary waste stream.

Integration with Renewable Energy Systems

As electricity grids incorporate increasing amounts of wind and solar, CANDU reactors provide the stable baseload foundation needed for reliability. Enhanced designs with load‑following capabilities allow output adjustment to accommodate renewable variability. Future hybrid systems could combine CANDU reactors with hydrogen production, industrial process heat, or district heating to maximize value and support decarbonization beyond the electricity sector. For example, Ontario Power Generation is exploring the use of CANDU heat for hydrogen production via high‑temperature electrolysis, which could provide a carbon‑free fuel for transportation and industrial processes. The ability to co‑generate hydrogen, heat, and electricity makes CANDU reactors a versatile cornerstone of a future integrated clean energy system.

Research, Development, and Innovation

Ongoing advances in CANDU fuel design include optimization for high burnup, use of graphite disks to lower fuel temperatures, advanced welding techniques, and 61‑element bundles for very high burnup applications. Digital technology integration—including advanced control systems, digital twins, and AI for predictive maintenance—enhances safety, performance, and operational efficiency while reducing costs. Research at Canadian universities and laboratories, such as the University of Ontario Institute of Technology and the Canadian Nuclear Laboratories, continues to improve materials science, thermal hydraulics, and reactor physics. The International Atomic Energy Agency actively supports collaborative research on heavy‑water reactor technology, including fuel cycle extensions and safety research. These innovations ensure that CANDU technology remains competitive and evolves to meet future energy demands.

Policy, Regulatory, and Market Position

Canada’s performance‑based licensing approach allows innovation while maintaining rigorous safety standards. Government support for nuclear R&D has spanned over 50 years, enabling world‑class technology and spin‑off industries. The global SMR market is estimated at $150 billion per year from 2030 to 2040, positioning Canada to benefit economically from decarbonization efforts. The World Nuclear Association consistently highlights CANDU technology as a proven option for countries seeking energy independence and low‑carbon power. The International Energy Agency includes nuclear power in all its net‑zero scenarios, and the unique features of CANDU reactors—fuel flexibility, on‑power refueling, and domestic supply chains—make them an attractive choice for many jurisdictions. The Canadian Nuclear Association works with policymakers to ensure that regulatory frameworks support both existing reactor life extension and new construction.

The Path Forward: CANDU in a Net-Zero Future

Ontario’s power demand is expected to increase 75 % by 2050, driven by electrification. CANDU reactors—both large‑scale and SMR variants—can meet growing demand while maintaining grid reliability. With proven technology, established supply chains, and strong operational experience, CANDU is well‑positioned for rapid deployment. The success of technology transfer programs, such as those in South Korea and India, demonstrates the potential for catalyzing broader nuclear industry development in partner countries. Looking ahead, the combination of large CANDU units for baseload and CSMRs for distributed generation can provide a comprehensive solution for decarbonization. International collaborations, such as the Candu‑India partnership and the Romania new‑build project, show that the technology is ready for global deployment. As the world strives for net‑zero emissions by 2050, CANDU reactors offer a reliable, proven, and increasingly flexible tool to achieve that goal while supporting economic growth and energy security.