Understanding the CANDU Fuel Cycle

The CANDU reactor—short for CANada Deuterium Uranium—stands apart from the global light-water reactor fleet due to its unique design philosophy. Its defining characteristics center on the use of natural uranium fuel combined with heavy water (deuterium oxide) serving as both moderator and coolant. This design choice eliminates the expensive and strategically sensitive step of uranium enrichment, fundamentally altering the fuel cycle’s front-end requirements and the waste profile at the back end. The complete fuel cycle encompasses every stage from uranium mining and milling, through fuel bundle fabrication and in-core irradiation, to the long-term management of spent fuel assemblies. Strategic management of this cycle represents not merely a technical necessity but a powerful opportunity to minimize environmental impact, extract greater energy from each kilogram of mined uranium, and reclaim valuable fissile materials that would otherwise be consigned to permanent disposal.

The defining operational feature of the CANDU fuel cycle is its on-power refueling capability. Unlike pressurized water reactors (PWRs) or boiling water reactors (BWRs) that must shut down for batch refueling every 12 to 24 months, CANDU units are refueled continuously by robotic refueling machines that insert fresh fuel bundles into pressure tubes while simultaneously removing spent bundles without reducing reactor power. This continuous process provides operators with exceptionally fine-grained control over burnup distribution throughout the core, reduces the excess reactivity margin that must be managed by control absorbers, and enables fuel cycle flexibility that is difficult to replicate in other reactor designs. Natural uranium dioxide pellets, sintered and ground to precise dimensions, are sealed in thin-walled Zircaloy-2 or Zircaloy-4 cladding tubes and assembled into short half-meter-long bundles. A typical 700 MWe CANDU 6 unit contains approximately 4,560 fuel bundles distributed across 380 fuel channels, with roughly 14 fresh bundles introduced each full-power day to maintain criticality and flatten the core power distribution.

The spent fuel discharged from a CANDU reactor contains roughly 0.2 to 0.3 percent plutonium and 0.2 percent uranium-235, alongside fission products, minor actinides, and activation products. While the overall radiotoxicity and decay heat load remain formidable challenges that dictate disposal requirements, the material also represents a substantial resource of partially burned fuel. Thoughtful recycling strategies can transform what is conventionally labeled as waste into a valuable feedstock for current or future reactor operations, reducing the volume of material requiring deep geological isolation. This article examines the leading technical approaches to waste minimization and recycling that are tailored to the CANDU fuel cycle, assessing both proven techniques that have reached commercial maturity and emerging technologies still under development.

The global CANDU fleet, including operating units in Canada, South Korea, Romania, Argentina, China, and India, generates tens of thousands of tonnes of spent fuel annually, with the cumulative inventory growing steadily. Each country operates under its own regulatory framework, ranging from Canada’s commitment to adaptive phased management leading to a deep geological repository to South Korea’s active exploration of recycling options. Understanding these national contexts is critical for developing strategies that are technically sound, economically viable, and politically feasible. The evolution of the CANDU fuel cycle over the coming decades will depend on continued innovation in materials science, reactor physics, process engineering, and international cooperation.

Waste Minimization at the Front End

Reducing the volume and radiotoxicity of spent fuel begins long before fuel bundles enter the reactor core. Optimizing the front end of the cycle—encompassing fuel element design, burnup strategy, and reactor physics management—directly limits the quantity of high-level waste generated per unit of electricity produced. Every gram of fissile material that undergoes complete fission reduces the mass of residual actinides requiring disposal and the associated long-term radiotoxicity burden.

Advanced Fuel Bundle Designs

The standard 37-element natural uranium bundle has served CANDU reactors reliably for decades, demonstrating excellent performance and defect resistance. However, advanced bundle geometries like the CANFLEX (CANdu FLEXible) bundle push performance boundaries further. The CANFLEX design uses 43 smaller-diameter elements with modified uranium dioxide pellets that incorporate a central graphite coating to reduce pellet-cladding interaction. This configuration lowers the linear element heat rating by approximately 15 percent compared to standard bundles, improving thermal margins and allowing the bundle to achieve higher discharge burnups without compromising safety margins. Higher burnup means extracting more thermal energy from the same initial mass of uranium, thereby reducing the number of spent bundles generated per terawatt-hour of electricity produced. In operational terms, a CANDU unit transitioning from a natural uranium burnup of roughly 7,500 MWd per tonne to a slightly enriched uranium cycle using CANFLEX bundles could push discharge burnup toward 10,000 to 12,000 MWd per tonne, directly cutting waste arisings by 20 to 30 percent over the reactor’s operating lifetime.

Beyond the CANFLEX design, fuel researchers are exploring variable axial loading patterns within individual bundles, where uranium dioxide pellets of different densities or enrichments are stacked axially to flatten the power profile along the element length. This technique reduces the peak-to-average power ratio, allowing higher overall discharge burnup without exceeding cladding temperature limits at bundle mid-planes. Another promising concept involves replacing zirconium alloy cladding with silicon carbide composite cladding, which offers superior corrosion resistance under both normal and accident conditions, improved high-temperature strength, and reduced hydrogen generation during steam oxidation events. Extended cladding lifetime directly supports higher discharge burnup and reduces the rate of spent fuel accumulation.

On-Power Refueling and Flux Shaping Optimization

On-power refueling is not merely an operational convenience that improves capacity factor; it is a powerful waste minimization tool that enables continuous optimization of core reactivity. Reactor physicists can continuously adjust the core neutron flux distribution by selecting which specific fuel channels to refuel and at what intervals, effectively flattening the radial flux profile and reducing the peak-to-average fuel burnup ratio across the core. This operational strategy avoids the premature discharge of fuel bundles that still contain significant fissile content, ensuring that each bundle delivers maximum energy extraction before removal. Computerized fuel management codes, such as the industry-standard RFSP (Reactor Fuelling Simulation Program), optimize the daily refueling sequence to maximize each bundle’s energy extraction while respecting bundle power limits, channel power limits, and rate-of-change constraints. This operational fine-tuning can squeeze up to five percent more energy from the same initial uranium load compared to simpler refueling schemes, directly translating to reduced spent fuel volumes over the reactor lifetime.

Modern fuel management tools increasingly incorporate machine learning algorithms that analyze historical refueling data to predict optimal channel selection sequences. Ontario Power Generation has employed adaptive optimization techniques at the Darlington Nuclear Generating Station to reduce the standard deviation of bundle burnup across the core, effectively raising the average discharge burnup by approximately three percent without requiring any physical changes to the fuel. These methods are gradually being standardized across the CANDU fleet as core monitoring instrumentation upgrades, including in-core flux detectors and delayed neutron monitoring systems, become available for broader deployment.

Using Slightly Enriched Uranium (SEU)

Even modest enrichment levels of uranium-235—between 0.9 and 1.2 weight percent compared to natural uranium’s 0.711 percent—can dramatically increase CANDU fuel discharge burnup. SEU fuel can more than double the discharge burnup of natural uranium, slashing spent fuel volumes by at least half while delivering more energy per bundle. Because the heavy water moderator remains highly efficient at thermalizing neutrons due to its low neutron absorption cross section, the use of slightly enriched fuel does not demand fundamental changes to the core physics or control system design. Canadian Nuclear Laboratories, in partnership with utilities, has conducted successful test irradiations of SEU fuel bundles in research reactors and operating CANDU units, confirming the viability of the concept in existing hardware without requiring major modifications.

There is growing interest in using recovered uranium (RU) from reprocessed light-water reactor fuel as a feedstock for SEU cycles. RU typically contains 1.0 to 1.5 percent uranium-235, well within the SEU range and significantly higher than natural uranium. Using RU avoids the environmental impacts associated with new uranium mining and the proliferation and economic concerns of new enrichment capacity, effectively creating a partially closed fuel cycle between the LWR and CANDU fleets. Economic analyses conducted by the OECD Nuclear Energy Agency show that SEU cycles become cost-competitive with natural uranium when avoided waste management costs and potential future carbon pricing are factored into the lifecycle comparison. At current uranium prices near $100 per kilogram, SEU fuel cycles add approximately five to ten percent to fuel costs but reduce back-end waste management costs by a proportionally larger margin.

Thorium as a Resource Extender and Waste Reducer

Thorium has long been recognized as a complementary fertile fuel for CANDU reactors due to the system’s excellent neutron economy. Thorium-232 is a fertile isotope that, when placed in a neutron flux, absorbs a neutron and undergoes two beta decays to become uranium-233—an excellent fissile isotope with a high neutron yield per absorption. The CANDU reactor’s low neutron losses through heavy water moderation allow it to sustain a thorium-uranium fuel cycle that can achieve conversion ratios approaching or exceeding unity, meaning the reactor breeds nearly as much new fissile material as it consumes. Initial physics studies demonstrate that a thorium-based seed-blanket arrangement—where a central driver region containing enriched uranium or plutonium supplies neutrons to a surrounding thorium blanket—could achieve breeding ratios of 0.9 to 1.0. By leveraging thorium, which is three to four times more abundant in the Earth’s crust than uranium, operators can stretch fissile resources and reduce the volume of spent fuel requiring disposal by up to 30 percent per unit of energy produced.

Recent experimental work at Canadian Nuclear Laboratories has focused on thorium-plutonium MOX bundles designed to simultaneously burn excess weapons-grade or reactor-grade plutonium while generating uranium-233 in the thorium matrix. Such co-processing arrangements could reduce both civil and military plutonium stockpiles, contributing to nuclear non-proliferation goals while producing useful energy. The radiotoxicity of thorium-derived spent fuel is also significantly lower after approximately 300 years compared to typical uranium fuel cycles due to the absence of plutonium and the shorter half-lives of thorium-series fission products. This is a key advantage for long-term waste management and reduces the isolation time required for geological disposal.

Waste Minimization Through Spent Fuel Management

Even with aggressive front-end measures, spent fuel will continue to be discharged from operating reactors. Minimizing the long-term hazard of this material relies on strategic management approaches that either concentrate and isolate the most dangerous constituents or return usable material to the fuel cycle for additional energy extraction.

Dry Storage Optimization

After spending six to ten years in water-filled cooling pools to remove decay heat and allow short-lived fission products to decay, CANDU spent fuel is typically transferred to dry storage containers made of reinforced concrete and steel. These containers provide passive cooling through natural air convection while maintaining radiation shielding and criticality safety over their design life, which can extend to 50 years or more. Ontario Power Generation’s Pickering and Darlington stations use large modular dry storage systems that accommodate decades of spent fuel at each site, with individual modules holding up to 10,000 fuel bundles. Continued advances in container design, including higher-density storage layouts that optimize basket geometry and improved computational fluid dynamic modelling of natural convection heat transfer, allow interim storage footprints to shrink without compromising safety margins. While dry storage is not a recycling step in itself, efficient interim storage reduces environmental disturbance and defers the need for final repository space, buying valuable time for recycling technologies to mature and for regulatory frameworks to adapt.

The Nuclear Waste Management Organization (NWMO) has developed a Mark III dry storage container that utilizes a modular concrete cylinder with an inner steel liner designed to maintain leak-tightness over extended storage periods. Cooling is provided by natural air convection through inlet and outlet vents, with thermal performance verified through extensive testing and modelling. These containers have demonstrated the ability to maintain peak cladding temperatures below 80 degrees Celsius for CANDU spent fuel over a 50-year storage period with minimal maintenance requirements. Future container designs may incorporate advanced neutron absorber materials such as boron carbide or gadolinium oxide in the basket structure, allowing tighter packing of fuel bundles without increasing criticality risk and further reducing the storage footprint per unit of spent fuel.

Direct Reuse in CANDU Through DUPIC

One of the most technically elegant and proliferation-resistant concepts for CANDU waste minimization is the DUPIC (Direct Use of spent Pressurized water reactor fuel In CANDU) fuel cycle. Instead of chemically separating plutonium and uranium from spent LWR fuel—a process that raises proliferation concerns due to the production of separated plutonium—DUPIC takes intact spent fuel from light-water reactors and mechanically re-fabricates it into CANDU-compatible fuel pellets without separating the constituent elements. The process involves mechanical decladding of the spent fuel rods, followed by an oxidation-reduction powder treatment that converts the uranium oxide to a reactive powder, which is then pressed and sintered into new fuel pellets. Because LWR spent fuel typically contains about 1.5 to 2.0 percent fissile content (a mixture of uranium-235 and plutonium isotopes), it can drive a CANDU reactor efficiently without additional enrichment. A single irradiation pass in a CANDU reactor can extract an additional 50 to 60 percent of the energy originally produced in the LWR, effectively doubling the useful energy yield from the original uranium and halving the final waste volume destined for a repository.

The DUPIC concept has been extensively validated through an international research program involving the Korea Atomic Energy Research Institute, Atomic Energy of Canada Limited, and the International Atomic Energy Agency. Korean researchers at KAERI have operated a pilot-scale DUPIC fuel fabrication line and produced test pellets that have been irradiated in research reactors, confirming acceptable performance characteristics. The proliferation resistance of DUPIC is exceptional because the process never separates plutonium from other actinides; the recycled fuel contains all the transuranic elements present in the original spent fuel, making it unattractive for weapons diversion while still providing a viable reactor fuel. Challenges remain in minimizing radioactive dust generation during the mechanical decladding step and ensuring uniform powder reactivity across batches, but these are engineering challenges rather than fundamental obstacles.

Recycling and Reprocessing Techniques for CANDU Fuel

Full recycling closes the nuclear fuel cycle by chemically separating usable actinides from true fission product waste, allowing the recovered materials to be fabricated into new fuel. While the CANDU community has historically favored the once-through cycle due to the low cost of natural uranium and the relative simplicity of direct disposal, interest in recycling grows as uranium prices fluctuate and environmental pressures mount. Several distinct reprocessing approaches offer different trade-offs between recovery efficiency, proliferation resistance, and economic viability.

Aqueous Reprocessing: The PUREX Legacy and Advanced Variants

The PUREX (Plutonium and Uranium Recovery by Extraction) process is the industrial standard for reprocessing spent oxide fuel, with commercial plants operating in France, the United Kingdom, Japan, and Russia. In the PUREX flow sheet, chopped fuel bundles are dissolved in hot nitric acid, and a tributyl phosphate solvent selectively extracts uranium and plutonium from the fission product-laden aqueous phase. Recovered uranium can be recycled directly into CANDU fuel fabrication, either after re-enrichment if its fissile content is too low for direct reuse, or used as a natural uranium equivalent if residual uranium-235 remains sufficient. Separated plutonium can be blended with depleted uranium to fabricate mixed oxide (MOX) fuel for both LWR and CANDU applications. PUREX is well-established with decades of operational experience, but it has historically been considered financially unattractive for CANDU fuel because the plutonium content in natural-uranium spent fuel is relatively modest—typically only 0.2 to 0.3 percent of the heavy metal mass. However, if CANDU units shift toward SEU fuel or receive LWR spent fuel through DUPIC, the plutonium inventory in the spent fuel rises substantially, improving reprocessing economics and making PUREX-based recycling more viable.

Advanced PUREX variants such as COEX (co-extraction) and UREX+ are designed to co-process uranium and plutonium without producing a pure plutonium stream, enhancing proliferation resistance while maintaining high recovery efficiency. The UREX+ process, developed at the U.S. Department of Energy’s Savannah River National Laboratory, also separates technetium and cesium early in the flow sheet, reducing the volume and heat load of the high-level waste requiring vitrification and geological disposal. Adapting these advanced flow sheets to handle CANDU fuel, with its aluminum-tin alloy cladding materials and unique fission product inventory resulting from lower burnup and distinct neutron spectrum conditions, is an active area of research and development that could yield significant benefits for the CANDU fleet.

MOX Fuel Fabrication and Irradiation in CANDU

Mixed oxide fuel originally developed for light-water reactors can be adapted for CANDU cores with relatively modest modifications. The heavy water moderator’s high neutron economy makes the reactor surprisingly tolerant of plutonium-based fuel, even at relatively low plutonium oxide concentrations. CANDU MOX fuel bundles would typically contain a blend of 1 to 3 percent plutonium oxide dispersed in a depleted uranium oxide matrix, providing both energy production and a means of disposing of plutonium from weapons dismantlement or reprocessing. Test irradiations performed at Chalk River Laboratories in the 1990s demonstrated that MOX assemblies can achieve burnup targets comparable to natural uranium bundles without exceeding power peaking limits or causing adverse fuel performance issues. Recycling plutonium into CANDU MOX fuel reduces the plutonium stockpile and extracts additional energy from material that would otherwise be classified as waste, effectively reducing the volume of high-level waste per unit of electricity generated.

The CANDU MOX demonstration program conducted jointly by Atomic Energy of Canada Limited and Ontario Hydro successfully loaded several full-scale MOX bundles into the Nuclear Power Demonstration reactor and later into commercial units at the Bruce A and Pickering stations. Results from these demonstrations showed that MOX fuel exhibits in-reactor behavior very similar to natural uranium fuel, with only minor differences in fission gas release rates and pellet-cladding interaction characteristics. Advanced MOX designs now under development incorporate gadolinia as a burnable poison to further flatten axial power profiles, allowing higher initial plutonium content without exceeding safety limits and enabling discharge burnups comparable to SEU fuel cycles.

Pyroprocessing and Advanced Dry Separation Methods

Aqueous reprocessing generates secondary liquid waste streams that themselves require treatment and immobilization, increasing the cost and complexity of the overall fuel cycle. Pyroprocessing—electrochemical separation conducted in molten salt media at elevated temperatures—offers a more compact, proliferation-resistant alternative that produces minimal liquid waste. In the pyroprocessing flow sheet, spent fuel is dissolved in a molten lithium chloride-potassium chloride eutectic salt at approximately 500 degrees Celsius, and electrolytic reduction deposits uranium and transuranic elements on solid or liquid cathodes with high selectivity. Because pyroprocessing does not produce a pure plutonium stream but rather a grouped actinide product containing plutonium together with other transuranics, it is inherently more resistant to weapons diversion than aqueous methods. For CANDU fuel, pyroprocessing can separate uranium with greater than 99 percent purity while leaving plutonium and minor actinides together as a group, suitable for recycling into fast reactors or advanced thermal reactor fuel forms.

South Korean researchers at KAERI have demonstrated a pyroprocessing pilot facility capable of handling up to 10 tonnes of spent fuel per year, with a target recovery rate exceeding 99.5 percent for uranium. The process leaves a fission product-laden salt waste that is immobilized by vitrification into borosilicate glass or ceramic waste forms. For CANDU fuel specifically, the relatively low burnup and correspondingly low fission product content simplify the electrorefiner operation, reducing the number of process steps required and lowering capital costs. The PRIDE facility in Korea has conducted integrated demonstration campaigns using simulated CANDU fuel to validate the complete pyroprocessing flow sheet from head-end handling through electrorefining to waste treatment, providing the technical basis for commercial-scale deployment.

Minor Actinide Partitioning and Transmutation

True reduction of the long-term radiotoxicity of spent fuel requires addressing the minor actinides—neptunium, americium, and curium—that dominate the radiotoxicity inventory after approximately 300 years of cooling. Partitioning and transmutation (P&T) strategies separate these elements from the main fuel stream and then fission them inside a reactor or accelerator-driven system, converting them into shorter-lived fission products. CANDU reactors, particularly if operated in a slightly modified neutron spectrum, can transmute minor actinides effectively due to the high thermal neutron flux and favorable fission-to-capture ratios for americium and neptunium. In advanced fuel cycle scenarios, minor actinides recovered from reprocessing could be loaded into inert matrix fuel rods alongside plutonium, drastically shortening the decay time of the final waste from hundreds of thousands of years to a few centuries. While P&T remains largely at the research and development stage, its potential to reduce repository requirements is profound and continues to motivate international research programs.

The EUROTRANS program and subsequent initiatives have studied the feasibility of using CANDU-type reactors for minor actinide transmutation. Because CANDU’s heavy water moderator produces a well-thermalized neutron spectrum, the fission-to-capture ratio for minor actinides is generally favorable compared to fast reactor spectra, although the transmutation rates are lower due to the lower neutron flux. Core design changes such as reduced moderator-to-fuel ratio or addition of neutron-absorbing materials are required to achieve effective transmutation rates while maintaining reactor safety. A concept called the DCTR (Deep-moderated CANDU Transmutation Reactor) has been proposed, using a heterogeneous seed-blanket arrangement with thorium and minor actinides to simultaneously transmute waste and breed uranium-233, producing net positive energy output while reducing the long-term radiotoxicity of the final waste stream.

Challenges and Considerations for Implementation

Implementing recycling and waste minimization strategies on a commercial scale is not without substantial challenges. Technical feasibility, economic viability, regulatory acceptance, and public support must all align before these approaches can move from demonstration programs to commercial practice. The challenges span multiple domains and require coordinated solutions.

Proliferation Resistance and Safeguards

Any process that separates or concentrates plutonium raises legitimate proliferation concerns that must be addressed through rigorous international safeguards. The International Atomic Energy Agency’s safeguards framework imposes strict monitoring, material accounting, and physical protection requirements on reprocessing and fuel fabrication facilities. CANDU’s on-power refueling and frequent bundle movements already demand rigorous fuel tracking systems; adding recycling loops compounds that complexity significantly. The DUPIC and pyroprocessing routes are attractive precisely because they avoid producing a pure plutonium stream, making them more acceptable under contemporary non-proliferation norms than traditional PUREX reprocessing. The IAEA continues to refine its safeguards guidelines for advanced fuel cycles through the International Project on Innovative Nuclear Reactors and Fuel Cycles.

Advanced material protection, control, and accounting (MPC&A) systems using real-time neutron and gamma spectroscopy can now monitor actinide concentrations in process streams continuously, providing near-real-time material balances that enhance transparency and reduce the risk of undetected diversion. These systems, combined with remote authentication technologies and tamper-indicating devices, allow safeguards authorities to verify fuel composition and material flows without interfering with process operations. The development of spent fuel signatures unique to each reactor type and fuel cycle further strengthens accountability and provides independent verification of declared operations.

Economic Factors and Lifecycle Costs

Recycling and reprocessing require substantial capital investment in hot cells, remote handling equipment, chemical processing plants, and associated infrastructure. For CANDU operators who have historically fueled on low-cost natural uranium available from stable mining operations in Canada and Australia, the economic incentive to recycle has been weak. However, as deeper decarbonization targets raise the cost of carbon emissions and uranium mining faces increasing social and environmental scrutiny, the lifecycle cost equation may shift in favor of recycling. Government policy support in the form of tax incentives, carbon credits, and assumption of long-term waste management liabilities all influence the business case for recycling investments. Recent lifecycle cost analyses indicate that recycling CANDU fuel via DUPIC could become cost-competitive with direct disposal when the avoided repository costs are internalized, particularly for countries with limited availability of suitable geological disposal sites.

Regulatory and Licensing Hurdles

For many countries operating CANDU reactors, used fuel has been classified as waste destined for direct geological disposal. Shifting to a recycling paradigm requires a national strategic decision supported by a comprehensive regulatory framework covering reprocessing operations, fuel re-fabrication, transport of recycled materials, and waste management for process effluents. In Canada, the Canadian Nuclear Safety Commission would need to develop new regulatory documents and conduct environmental assessments for any recycling facility, a process that typically requires five to ten years. Public acceptance, informed by transparent consultation processes and clear communication about the benefits and risks of recycling, is equally critical for project success.

Technical Readiness and Demonstration Status

Several of the advanced techniques described, notably DUPIC fueling and pyroprocessing, have been proven at laboratory and pilot scale but have not yet been deployed commercially. The remote fabrication of DUPIC pellets demands extraordinary precision and radiation shielding, raising production costs above those for conventional fuel fabrication. Similarly, pyroprocessing must demonstrate consistent fission product removal efficiency, long-term corrosion resistance of electrorefiner vessels, and reliable salt waste immobilization before commercial deployment can proceed. International research collaborations under the Generation IV International Forum and other frameworks are actively addressing these technical gaps, with several programs targeting pilot-scale demonstrations by the early 2030s.

Future Directions and Emerging Technologies

The CANDU fuel cycle is not static. Emerging reactor concepts and deeper integration with the global nuclear enterprise promise further waste minimization and recycling breakthroughs that could fundamentally change the economics and environmental profile of nuclear power.

CANDU and Small Modular Reactors (SMRs)

Small modular reactor developers are exploring heavy water and even molten salt variants that can consume spent nuclear fuel as feed material. A fleet of CANDU-based SMRs could operate on a mix of recycled uranium, plutonium, and thorium, forming a symbiotic fuel cycle with larger light-water reactors. The flexible, smaller-core architecture of SMRs makes it easier to test new fuel types under controlled conditions, accelerating the deployment of advanced recycling strategies and providing a pathway for incremental innovation rather than requiring a single large-scale commitment to a new fuel cycle.

The Integral CANDU SMR concept under development by Canadian engineering firms leverages a reduced core size to allow modular construction and factory fabrication of fuel bundles, potentially reducing capital costs and construction times. This design could incorporate a fuel cycle service center located at the reactor site, where spent fuel from multiple SMR units is processed and re-fabricated using compact pyroprocessing equipment. Such a distributed recycling model reduces transportation risks and provides cradle-to-cradle fuel management for remote mining operations or isolated communities, expanding the potential market for nuclear energy beyond traditional grid-connected applications.

Deep Geological Repositories and Retrievability Concepts

Canada’s plan for a deep geological repository, managed by the NWMO and currently targeting a site selection decision, is designed as a permanent disposal solution for the existing inventory of CANDU spent fuel. Yet even within this framework, retrievability provisions have been discussed to leave the door open for future generations to access the energy remaining in the spent fuel. Standardized dry storage containers can be designed for future retrieval and reprocessing should technology and economic conditions change. This retrievable disposal concept bridges the present once-through reality with a potential future circular fuel cycle, providing flexibility without requiring immediate decisions about reprocessing infrastructure.

The NWMO’s Adaptive Phased Management approach includes a period of at least 50 to 100 years of retrievability after repository closure, during which the spent fuel containers remain accessible for removal if desired. During this period, advanced sensing technologies such as muon tomography for non-invasive monitoring of fuel condition could confirm the integrity of stored fuel while maintaining the option for future recovery. International consensus on retrievability standards and best practices is being developed under the auspices of the OECD Nuclear Energy Agency, with input from repository programs in Finland, Sweden, France, and Canada.

International Collaboration and Shared Infrastructure

CANDU operators in Canada, South Korea, Romania, Argentina, and China each bring unique operational experience and waste management strategies to the global nuclear community. South Korea’s extensive work on DUPIC recycling, Romania’s Cernavoda station’s interest in SEU fuel cycles, and Canada’s national laboratories’ expertise in pyroprocessing and fuel development are complementary and mutually reinforcing. Multinational programs, perhaps coordinated under IAEA auspices, could pool resources to construct a shared pilot-scale recycling facility open to all CANDU operators, lowering individual costs and accelerating the licensing process through shared regulatory experience.

A CANDU Fuel Cycle Collaboration Forum established in 2024 between the nuclear research institutes of Canada, South Korea, and Romania has begun coordinating advanced fuel testing activities. Initial collaborative projects include irradiation testing of thorium-SEU hybrid bundles and fabrication of DUPIC pellets using an automated pelletizing line provided by Canadian Nuclear Laboratories. The sharing of legacy data from past CANDU MOX demonstration programs is being digitized and consolidated to support new license applications and reduce the need for duplicate testing, accelerating the timeline for commercial deployment of advanced fuel cycles.

The Path Toward a Sustainable CANDU Fuel Cycle

Waste minimization and recycling in the CANDU fuel cycle are more than environmental aspirations—they represent essential pillars of a truly sustainable nuclear energy system capable of operating for centuries. By optimizing fuel bundle design, adopting higher burnup cycles, and integrating spent LWR fuel through DUPIC processing, the mass of waste destined for a geological repository can be substantially reduced. Recycling technologies, whether aqueous or dry, can recover uranium, plutonium, and potentially minor actinides, transforming liabilities into valuable resources and extracting maximum energy from each tonne of mined uranium. The challenges of cost, proliferation risk, and regulatory complexity are real but surmountable given sustained political will, appropriate investment, and effective international cooperation.

Ultimately, the CANDU reactor’s unique combination of heavy water moderation, on-power refueling capability, and exceptional neutron economy places it at the center of any serious discussion about closing the nuclear fuel cycle. By investing today in the enabling technologies, regulatory frameworks, and institutional arrangements necessary for commercial-scale recycling, the global CANDU fleet can lead the transition toward a truly circular nuclear fuel economy—one that extracts maximum value from every atom while shrinking the environmental footprint for generations to come. The next decade will be pivotal as demonstration projects transition to commercial reality, guided by rigorous safety analysis, transparent public engagement, and a shared commitment to sustainable nuclear energy.