The Unique Waste Profile of CANDU Reactors

Canada’s CANDU reactor design represents a distinctive engineering achievement in the nuclear energy landscape. Unlike conventional light-water reactors that dominate global nuclear fleets, the CANDU system uses heavy water—deuterium oxide—as both its moderator and coolant. This design choice eliminates the need for uranium enrichment, allows natural uranium fuel to sustain fission, and enables on-power refueling without reactor shutdown. These operational characteristics directly influence the composition, volume, and management requirements of the radioactive waste produced at every CANDU facility across Canada and abroad.

Every unit of electricity generated by a CANDU station carries with it the responsibility of managing radioactive by-products that must remain isolated from the environment for periods ranging from decades to hundreds of thousands of years. Understanding the specific waste streams, storage strategies, and disposal pathways associated with CANDU reactors is essential for policymakers, energy professionals, and the public engaged in nuclear energy decisions.

The waste management approach for CANDU reactors follows a multi-layered strategy that begins with careful classification, moves through on-site interim storage, and culminates in permanent geological disposal. Each stage involves engineered systems designed to protect human health and the environment while accommodating the unique physical and radiological characteristics of heavy-water reactor waste.

Classification of CANDU Radioactive Waste Streams

CANDU reactors generate the same broad categories of radioactive waste as other fission reactors—low-level, intermediate-level, and high-level waste—but the heavy-water fuel cycle imparts a distinct isotopic fingerprint to each category. Proper classification determines the handling protocols, storage conditions, and ultimate disposal pathway for each waste type.

Low-Level Waste Characteristics and Management

Low-level waste from CANDU operations includes lightly contaminated items such as protective clothing, cleaning materials, tools, air filters, and construction debris from radiological control areas. The presence of tritium sets CANDU low-level waste apart from that of light-water reactors. Tritium forms when deuterium atoms in the heavy-water moderator and coolant absorb neutrons during reactor operation. As a weak beta emitter with a 12.3-year half-life, tritium poses minimal external radiation hazard but requires careful internal dose controls when handling contaminated materials.

Canadian nuclear operators manage low-level waste through compaction, incineration, and engineered near-surface storage facilities. These facilities are designed to contain radioactivity until it decays to background levels—typically within a few hundred years. The volume of low-level waste produced annually by a typical CANDU station is significant but manageable, with most materials suitable for volume reduction techniques before final emplacement. Operators also use segregation to minimize the amount of material that crosses the low-level threshold, reducing disposal costs and extending the capacity of near-surface storage.

Intermediate-Level Waste from Reactor Systems

Intermediate-level waste carries higher radionuclide concentrations than low-level waste but generates insufficient heat to require active cooling. For CANDU operators, this category includes spent ion-exchange resins used to purify the heavy-water and light-water circuits, filtration media from primary coolant systems, and irradiated reactor components removed during refurbishment projects. These components—pressure tubes, calandria tubes, and feeder pipes—become activated by decades of neutron exposure, accumulating long-lived isotopes including nickel-59 with a 76,000-year half-life and niobium-94 with a 20,300-year half-life.

The resins and filters from CANDU systems often contain carbon-14, a weak beta emitter with a 5,730-year half-life that forms when nitrogen impurities in the coolant absorb neutrons. This isotope presents unique challenges because carbon can migrate through biological systems if released. CANDU operators condition intermediate-level waste through cementation or polymer solidification, creating stable waste forms that are then stored in shielded vaults pending geological disposal. The volume of intermediate-level waste increases significantly during major refurbishment projects, which typically occur every 25 to 30 years at CANDU stations. At Ontario Power Generation’s Darlington site, the refurbishment of four units will produce thousands of cubic meters of intermediate-level waste, requiring expanded on-site storage capacity and careful sequencing with existing operations.

High-Level Waste: Spent Fuel from Natural Uranium

The most challenging waste stream from any thermal reactor is irradiated nuclear fuel, classified as high-level waste due to its intense radioactivity and decay heat generation. A CANDU fuel bundle consists of approximately 500 grams of uranium dioxide pellets sealed within Zircaloy tubes. Each bundle measures roughly 50 centimeters in length and 10 centimeters in diameter—a compact geometry that proves advantageous for both storage and eventual disposal.

When a fuel bundle exits the reactor core, it contains a complex mixture of fission products, plutonium isotopes, and minor actinides that emit penetrating gamma radiation and significant decay heat. A single 740 megawatt-electric CANDU unit discharges approximately 4,000 bundles annually. Because natural uranium fuel achieves a relatively low burnup—around 7,500 megawatt-days per metric tonne compared with 45,000 for pressurized-water reactors—the volume of spent fuel per unit of electricity is higher for CANDU reactors. However, the radiotoxicity per metric tonne is somewhat lower initially, and the short bundle length lends itself well to efficient packaging in storage and disposal containers.

Canada’s national policy designates used nuclear fuel as a waste product destined for direct disposal rather than reprocessing. This once-through fuel cycle simplifies the waste management pathway but places the full responsibility for long-term isolation on the engineered and natural barriers of a deep geological repository. The CANDU’s ability to operate on mixed-oxide fuel or thorium cycles has attracted international interest, as these fuel cycles could reduce the volume and radiotoxicity of high-level waste requiring permanent disposal. India, a major operator of CANDU-derived pressurized heavy-water reactors, reprocesses spent fuel to recover plutonium for its fast breeder program—a different policy choice that influences waste management across its reactor fleet.

On-Site Storage Infrastructure and Operations

Immediately after discharge from the reactor, spent fuel bundles embark on a cooling and storage journey that begins inside the reactor site and can extend for decades. The initial phase is critically important because freshly discharged fuel generates sufficient decay heat to cause melting if not actively cooled.

Spent Fuel Pool Design and Function

Every CANDU station maintains a spent fuel pool—a reinforced concrete basin lined with stainless steel and filled with demineralized water. The water serves dual purposes: it removes decay heat through continuous circulation and provides radiation shielding that reduces gamma and neutron fields to harmless levels at the pool boundaries. CANDU fuel bundles are stored vertically in high-density racks engineered to maintain safe subcritical geometry while maximizing storage capacity. Heat exchangers transfer the thermal load to a secondary cooling system, and water purification systems prevent corrosion of the Zircaloy cladding that could compromise fuel integrity.

Bundles typically remain in the spent fuel pool for seven to ten years, during which time the decay heat decreases by more than 99 percent. At major CANDU facilities such as Ontario Power Generation’s Darlington station and Bruce Power’s Bruce Nuclear Generating Station, the pools have undergone expansion and re-racking projects to accommodate decades of continuous operational discharge. The pools represent the first line of defense in the waste management chain, providing a safe and proven environment for initial cooling and short-term storage. Operators monitor pool water chemistry, temperature, and radionuclide levels continuously to ensure conditions remain within design specifications.

Dry Cask Storage Systems for CANDU Fuel

After sufficient cooling in the pool, fuel bundles are transferred to dry cask storage systems, which have become the dominant interim storage method across Canada and internationally. In the CANDU context, this process involves loading individual bundles into sealed, stainless-steel canisters. These canisters are backfilled with inert gas to prevent oxidation of the fuel cladding and are welded shut to create a leak-tight containment boundary. The canisters are then placed inside robust concrete or steel overpacks that provide structural protection and radiation shielding.

The dry cask systems operate on passive cooling principles: natural convection draws air through intake ducts, across the canister surfaces, and out through exhaust vents, dissipating residual heat without requiring pumps, fans, or electrical power. Ontario Power Generation and Bruce Power have deployed thousands of dry storage casks at their Great Lakes sites. These casks carry design lives of 50 to 100 years, extendable through periodic inspection and maintenance programs. The casks sit on purpose-built concrete pads engineered to withstand seismic events, tornado impacts, and extreme weather conditions, ensuring continuous protection of the surrounding communities and environment. In addition, the cask designs must accommodate the specific geometry and heat load of CANDU fuel, which has led to several proprietary systems qualified by the Canadian Nuclear Safety Commission.

Centralized Interim Storage Concepts

While on-site dry cask storage provides safe and secure containment, it places a long-term operational burden on individual reactor sites and limits the flexibility of national waste management strategies. Centralized interim storage facilities have emerged internationally as a transitional solution, consolidating waste from multiple sites into a single, purpose-built facility where it can be managed more efficiently while a permanent repository is developed.

Sweden’s Centralt mellanlager för använt kärnbränsle facility, known as Clab, has stored spent fuel in a rock cavern beneath the Baltic Sea since 1985. Similar concepts exist in the Netherlands and Switzerland. In Canada, the Nuclear Waste Management Organization (NWMO) is pursuing the Adaptive Phased Management approach, which emphasizes that used fuel will ultimately move to a single deep geological repository. The interim period continues to rely on site-based storage systems, but the NWMO has developed a comprehensive transportation and logistics framework to safely move fuel when the repository becomes operational. The phased plan envisions the repository site beginning as an underground characterization and demonstration facility with surface storage modules before transitioning to full-scale waste emplacement operations. The NWMO maintains detailed information on its approach at its official website: https://www.nwmo.ca/.

Deep Geological Repositories as the Permanent Solution

The international scientific consensus, endorsed by national academies, regulatory bodies, and the International Atomic Energy Agency, holds that deep geological disposal provides the most responsible method for isolating high-level radioactive waste from the biosphere. The approach relies on a multi-barrier system that combines engineered containment with natural geological isolation to prevent radionuclide migration for hundreds of thousands of years.

The Multi-Barrier System Architecture

In a typical deep geological repository design, used fuel bundles are first placed in a durable inner container. Copper and carbon steel are the most common container materials, selected for their corrosion resistance under the chemically reducing conditions found deep underground. The container is surrounded by a buffer layer of compacted bentonite clay, which swells upon contact with groundwater—sealing any cracks, crevices, or gaps in the emplacement zone and retarding the movement of any radionuclides that might escape the primary container.

This engineered barrier system is emplaced in boreholes or tunnels hundreds of meters below the surface, within a host rock formation chosen for its long-term stability and minimal groundwater flow. The final barrier is the geology itself: ancient, stable formations such as crystalline granite or sedimentary clay that have remained undisturbed for billions of years and are expected to provide isolation well into the future. CANDU used fuel, with its short bundle geometry and robust Zircaloy cladding, is particularly well suited to this containment approach, allowing relatively slender disposal canisters that accommodate efficient emplacement in a variety of repository layouts. Repository designers can load multiple bundles per canister, optimizing space while maintaining thermal limits.

Canada’s Adaptive Phased Management Program

Canada’s deep geological repository program is led by the Nuclear Waste Management Organization, a not-for-profit entity established by the nuclear electricity producers under federal legislation. The NWMO’s Adaptive Phased Management process received government approval in 2007 following extensive public consultation and technical review. The phased approach proceeds through site selection and characterization, construction of an underground demonstration facility, a monitored retrievable storage period, and finally full repository operation with eventual closure and decommissioning.

As of 2024, the site selection process has narrowed to two consent-based candidate areas in Ontario: the Wabigoon Lake Ojibway Nation–Ignace area and the Saugeen Ojibway Nation–South Bruce area. Both communities have participated in extensive learning and engagement activities over many years, and the final site selection requires support from both the municipal government and the affected Indigenous communities. This consent-based, partnership-driven model has received international recognition for its emphasis on social license and community decision-making. The NWMO provides ongoing updates on its site selection process through its official publications and community bulletins.

International Progress in Geological Disposal

While Canada advances its repository program, Finland has already constructed the world’s first operational deep geological repository for spent nuclear fuel. The Onkalo facility, developed by the waste management organization Posiva, is excavated within 2-billion-year-old granite near the Olkiluoto nuclear power plant. Construction began in 2004, and final disposal operations are expected to commence in the mid-2020s. Sweden’s Forsmark repository is proceeding on a similar timeline, with the Swedish Nuclear Fuel and Waste Management Company receiving a construction license in 2022 for a repository in 1.9-billion-year-old granite.

Both the Finnish and Swedish projects employ copper canisters with bentonite buffer systems, creating a compelling technical reference for the international community. France is developing the Cigéo repository in clay formations, while Switzerland, Belgium, and Japan are investigating their own host rock options. These projects demonstrate that the technical challenges of geological disposal are surmountable and that CANDU fuel inventories can be accommodated within proven repository concepts. The IAEA coordinates research and publishes safety standards that guide member states in developing their own waste management programs.

Alternative Waste Treatment and Disposal Technologies

Beyond the direct disposal of spent fuel, several technological pathways could reduce the volume, heat load, or radiotoxicity of CANDU waste requiring geological isolation. While Canada’s current policy does not include reprocessing, other nations with CANDU-derived reactors have integrated fuel recycling into their nuclear strategies.

The closed fuel cycle via reprocessing involves chemical separation of spent fuel to recover plutonium and uranium for reuse. India, which operates a large fleet of pressurized heavy-water reactors based on the CANDU design, reprocesses spent fuel as part of its three-stage nuclear program. The recovered plutonium fuels fast breeder reactors, which in turn can breed more fuel from abundant thorium reserves. Because CANDU reactors can accept a wide variety of fissile materials—including mixed-oxide fuel and fuels recycled from light-water reactors—they could serve as the thermal component of a symbiotic fuel cycle that dramatically reduces the long-lived actinide inventory destined for disposal.

Advanced partitioning and transmutation concepts aim to separate minor actinides from spent fuel and burn them in dedicated fast reactors or accelerator-driven systems. Research suggests these techniques could reduce the radiotoxicity timeline for high-level waste from approximately 100,000 years to a few hundred years. While partitioning and transmutation remain at the research and demonstration stage, international programs in the European Union, Japan, and the United States continue to investigate their technical and economic feasibility.

Deep borehole disposal offers an alternative to mined repositories, involving drilling holes three to five kilometers into crystalline bedrock and emplacing waste canisters in the lower portion. The deep continental basement exhibits very sluggish groundwater movement and high salinity that discourages radionuclide migration. Proponents argue that borehole disposal could offer a simpler and less expensive alternative for certain waste forms, including spent CANDU fuel. The small-diameter boreholes would accommodate CANDU-sized canisters, and conceptual designs suggest technical feasibility. However, borehole disposal has not been selected for any national program and remains under investigation through government and private-sector research initiatives.

Transportation Infrastructure for Used Nuclear Fuel

Moving used nuclear fuel from reactor sites to a future centralized facility requires a transportation system that can withstand extreme accident conditions without releasing radioactive material. The international standard for such shipments is the Type B package—a robust shipping cask engineered to survive a demanding sequence of impact, puncture, fire, and water immersion tests specified by the IAEA and enforced in Canada by the Canadian Nuclear Safety Commission.

A typical CANDU transport cask contains multiple fuel bundles sealed inside a stainless-steel inner container. This container is surrounded by layers of depleted uranium or other dense shielding materials, all housed within a rugged outer steel shell. The total weight of a loaded cask can exceed 100 tonnes, providing structural resilience that makes it impervious to rail or road collisions. Shipments travel primarily by road or barge, using routes that avoid major urban centers and are coordinated with provincial and local emergency response agencies. Canada has safely transported thousands of radioactive packages over decades of operation—including used fuel and decommissioning waste—using these established protocols and equipment. The CNSC licenses each transport cask design and verifies that operators adhere to the regulations outlined in the Packaging and Transport of Nuclear Substances Regulations.

Regulatory Oversight and Public Engagement

Radioactive waste management in Canada operates under a comprehensive regulatory framework administered by the Canadian Nuclear Safety Commission. The CNSC licenses every facility involved in storage and disposal and sets stringent safety requirements for design, operation, and decommissioning. Repository projects must pass a multi-year environmental assessment—now conducted under the Impact Assessment Act—that examines long-term radiological and non-radiological impacts, including effects on surface water, ecosystems, and Indigenous rights.

Public hearings, community liaison committees, and ongoing engagement are mandated at every stage of the licensing process. The NWMO’s Adaptive Phased Management approach places community willingness at the center of decision-making. Both candidate communities in Ontario are conducting formal referenda and municipal council votes before committing to host the repository. This degree of public involvement, coupled with scientific rigor, aims to build trust and ensure that decisions of such generational consequence are made openly and democratically. The CNSC also publishes its technical reviews and regulatory decisions online, allowing stakeholders to track the progress of waste management projects.

Ongoing Challenges in Long-Term Waste Management

While the technical pathway for CANDU waste disposal is well established, significant challenges persist. Demonstrating that a repository will perform safely over a million-year timeframe requires reliance on natural analogues, advanced computational modeling, and long-term monitoring programs that extend beyond human experience. The cost of a deep geological repository in Canada is estimated in the tens of billions of dollars over the project’s century-long lifetime, with funding drawn from a segregated trust financed by the nuclear electricity producers.

Maintaining public and political support across decades and across shifting government administrations presents an enduring challenge. Some communities welcome the economic benefits and employment opportunities associated with hosting a repository, while others express concerns about environmental risks and potential stigmatization. The international technical community continues to refine disposal concepts, test new engineered barrier materials, and share data to build the confidence needed to proceed with permanent disposal operations. The NWMO has established a robust public engagement program that includes newsletters, open houses, and independent research projects to address these concerns.

Future Developments and Innovation Pathways

Research into CANDU-specific waste solutions continues to evolve. Advanced fuel cycles that incorporate thorium in heavy-water reactors could produce significantly less long-lived waste compared with conventional uranium fuel cycles. Canada’s extensive thorium reserves have prompted investigations into once-through thorium cycles in CANDU-type cores, which could reduce the actinide content of spent fuel and simplify disposal requirements.

At the engineering level, the industry is developing improved canister alloys with enhanced corrosion resistance, self-healing backfill materials that can seal cracks in the engineered barrier system, and wireless sensors capable of monitoring underground repositories for centuries without human intervention. The Canadian small modular reactor program may introduce new waste forms that overlap with CANDU legacy waste, fostering a broader and more integrated long-term management approach that addresses multiple reactor technologies within a unified strategy. Organizations like the Canadian Nuclear Laboratories are collaborating with international partners to test these innovations at laboratory scale and in underground research laboratories, such as the proposed underground characterization facility associated with the future repository.

As the international community witnesses the commissioning of Finland’s Onkalo repository, the vision of a permanent, passive solution for used CANDU fuel continues to move toward reality. This vision is grounded in sound science, sustained by meaningful public engagement, and carried forward by a collective commitment to protect people and the environment for generations far into the future. The CANDU reactor’s unique contributions to clean energy generation carry with them an equal commitment to responsible waste stewardship—a commitment that Canada and its international partners are actively fulfilling through technical innovation, regulatory rigor, and community partnership.