The Advanced Fuel CANDU Reactor is designed to use derivatives of recycled uranium and thorium-based fuels. Now that the conceptual design has been completed, the AFCR is ready for implementation in the Chinese market. By Fabricia Pineiro, Dan Popov, Ziping Li, Mustapha Boubcher, Catherine M. Cottrell and Sermet Kuran.
Candu Energy’s Advanced Fuel CANDU® Reactor (AFCRTM) is capable of efficiently using recycled uranium- (RU) and thorium-based fuels. It is the only generation III reactor design that can do so, that is available now, and that meets post- Fukushima requirements.
The AFCR was developed by Candu Energy, in co-operation with China National Nuclear Corporation’s Third Qinshan Nuclear Power Corporation (TQNPC), China North Nuclear Fuel Company, and the Nuclear Power Institute of China, as part of a strategic plan to reduce the dependency of countries like China on natural uranium (NU), while providing significant performance and economic advantages.
The development of the AFCR followsthe recent successful test irradiation and licensing submission for full-core implementation of natural uranium equivalent (NUE) fuel in China’s operating Qinshan CANDU reactors (See also ).
The AFCR design is the next step in implementing advanced fuels in CANDU reactors. This approach offers the lowest technological risks to nuclear utilities and their various stakeholders.
The AFCR is a 740 MWe heavy water- moderated, heavy water-cooled pressure tube reactor. It has evolved from the CANDU 6 plants licensed and operating in five countries (four continents) with more than 150 reactor years of safe operation. In recent years, this global CANDU 6 reactor fleet has repeatedly ranked in the world’s top performing reactors.
The AFCR is designed to use advanced fuel cycles: derivatives of recycled uranium (DRU) and low-enriched uranium/thorium (LEU/Th) fuels. DRU fuel is a mixture of dysprosium and recycled uranium (reprocessed from LWR spent fuel) with slightly higher U-235 content than NU fuel, and higher burnup. These fuels reduce dependence on NU and offer the opportunity to use previously-unused LWR uranium feedstock (that is reprocessed) efficiently and economically in a reactor design that has already been commercially proven.
The AFCR design is suitable for all electricity grids. It has enhanced seismic capacity of 0.25g peak ground acceleration to meet site requirements, and the reactor can be built with greater than 70% localization.
While retaining proven features of the CANDU design, the AFCR incorporates innovative features and improvements that enhance safety, operation and performance, including:
- A robust plant design based on proven engineering practices to ensure fundamental safety functions are achieved for all operational states and for all accident conditions
- Additional, defence-in-depth safety enhancements for accident prevention and mitigation
- Increased building robustness to improve defence against malevolent acts
- Improved solid, liquid and gas waste management systems to reduce emissions
- Improved plant operability and maintainability with advanced control room design
- Significant fire protection system improvements per CSA N293 and Chinese standards
- Incorporation of modern digital distributed control, plant display and safety monitoring systems using a network of modular, programmable digital controllers with reliable, high-security data transmission methods, enabling automation to a level requiring minimal operator actions
- Reduced operation and maintenance costs by hardware and process improvements
- Emergency power supply upgrades for automation of start-up and station blackout.
Numerous additional AFCR enhancements are also incorporated based on operational feedback, client requirements and post- Fukushima improvements.
The bigger picture
The AFCR design can operate in synergy with other reactor types, such as light water reactors, and use alternative fuels to produce more
energy out of recycled resources (nominally four LWRs can fuel one reactor).
The main driver of the AFCR programme is to increase the sustainability and availability of fuel resources in China, using DRU and LEU/
Th fuels. Using DRU fuel in the AFCR results in a front-end fuel cycle cost ~32% lower than the traditional NU-fuelled reactor designs and
~128% lower than representative advanced LWR designs. The front-end fuel cycle cost of LEU/Th fuel is expected to be comparable to
This superior utilisation capability is particularly appealing to countries with limited uranium resources. The utilisation of abundant indigenous thorium sources reduces dependency on foreign uranium fuel.
Additionally, the successful demonstration of LEU/Th fuel implementation creates a path for more advanced thorium fuel applications, such
as plutonium/thorium fuel, which have a larger potential for greater economic and resource utilisation benefits.
Towards closing the nuclear fuel cycle
It is anticipated that the path forward set in motion by the use of NUE, DRU and LEU/Th fuels will culminate in additional use of alternative fuels and infrastructure associated with recycling capability. Recycled spent fuel from uranium- and thorium-based fuels,
respectively, will return the fissile plutonium and U-233 to the nuclear fuel cycle. Successfully completing this process would be a major step
in closing the fuel cycle.
With increasing operational and technology experience obtained from the AFCR, the energy content from thorium will be increased
to near-100% recycling of spent nuclear fuel. This evolution would encourage both the further reuse of spent fuel in existing plants and
the development of larger AFCR plants. In turn, these plants would drastically reduce waste streams and enable full energy independence
from NU fuel.
Reactor core and fuel design
The AFCR uses the CANFLEX fuel bundle as a fuel carrier for both DRU and LEU/Th fuels. The bundle consists of 43 elements arranged in three concentric rings and a centre element (see figure). The centre element and the seven elements in the inner ring have a larger
diameter than the 35 elements in the two outer rings.
The DRU fuel bundle consists of 42 outer elements containing RU with a nominal fissile content of 0.95 percent by weight U-235 and a centre element containing a mixture of RU and dysprosium oxide (Dy2O3). The LEU/Th fuel bundle consists of 35 outer elements containing LEU and eight inner elements containing thorium oxide (ThO2). These configurations provide significant improvements relative to the traditional features of the 37-element NU fuel bundle in terms of fuel utilisation, thermal-hydraulic performance, fuel integrity and overall safety.
The AFCR design can use DRU or LEU/ Th bundle designs, with fuel utilisations of 10,000 MWd/tHE and 20,000 MWd/ tHE, respectively. These designs offer major improvements of ~40% to ~180%, respectively, relative to fuel utilisations of 37-element NU designs.
The CANFLEX bundles successfully fulfil safety requirements for the AFCR by providing reduced fuel-element linear power and enhanced critical heat flux (CHF) compared to 37-element NU designs. These features ensure reduced risks for commercial delivery of the design.
The fuel channel layout and geometry of the AFCR core are identical to those of the CANDU 6 reactor. The AFCR assembly consists of a horizontal, cylindrical, low- pressure calandria and end-shield assembly. This calandria assembly contains the heavy water moderator, 380 fuel channel assemblies containing 12 fuel bundles each, and vertically-oriented reactivity mechanisms. The thickness of the pressure tubes in the fuel channel assemblies is increased relative to the CANDU 6 reactor to achieve a reactor lifetime of 60 years. The calandria is supported within a concrete, light water- filled calandria vault. Fuel channels that pass through the calandria and the end-shield assembly allow for on-power refuelling of the AFCR via an automated fuelling machine.
The fuel channel refuelling patterns for the AFCR are a bi-directional four-bundle shift and a bi-directional two-bundle shift for DRU and LEU/Th fuels respectively. (The CANDU 6 features an eight-bundle shift). This refuelling pattern, in combination with an optimised arrangement of adjuster rods, results in favourable axial power profiles and low refuelling perturbations, which increase the design safety margins and simplify reactor control.
The two fuelling machines are coordinated to perform reactor refuelling operations for DRU or LEU/Th fuel. Despite the changes to either a four-bundle or two-bundle shift, the current NDU6 fuelling machine design is expected to operate safely at a >90% capacity factor, and meets the 60-year plant operating life.
The use of advanced fuels in the AFCR requires a reduced number of bundles for normal operation and decreases spent fuel production.
Heat transport system & moderator
The main component design of the AFCR heat transport system (HTS) remains similar to that of the CANDU 6. The HTS circulates pressurised D2O through the 380 reactor fuel channels to remove heat produced by the fuel. The heat is transported to steam generators where it is transferred to light water to generate steam, which subsequently drives the steam turbine generators. The HTS is comprised of two loops. The main differences in the AFCR HTS design are:
- Minor flow distribution changes to increase power and improve design safety margins
- Radiation field activity changes associated with the use of DRU and LEU/Th fuels.
The AFCR moderator system design is also similar to that of the CANDU 6. The system is a closed, low-pressure, low-temperature, cooled D2O circuit. It slows high-energy fission neutrons in the core to promote further nuclear fission.
The AFCR meets and significantly exceeds the following generation III quantitative safety goals, in accordance with International Atomic Energy Agency (IAEA):
- The core damage frequency (CDF) is less than 10-5 events per operating reactor year.
- The large release frequency (LRF) is less than 10-6 events per operating reactor year.
Generally, the CANDU reactor design consists of inherent active and passive attributes that aim to prevent accidents, limit the consequences if they occur, or provide protection and mitigation in case of severe accidents.
The AFCR is equipped with two physically and functionally separate shutdown systems: SDS1 and SDS2. SDS1 drops shutoff rods from the top of the core, while SDS2 injects a neutron absorber to the moderator from the side of the reactor core to achieve shutdown. A rod-based guaranteed shutdown state capability is incorporated into the design, greatly reducing the time to enter and exit from a major maintenance outage, and reducing occupational dose and moderator cleanup burden.
The AFCR is a robust design with multiple layers of defence to prevent and mitigate the consequences of both design-basis and beyond-design-basis accidents. Numerous redundant feedwater supplies from active and passive sources provide a heat sink and give operations staff ample time (>72 hours) to deploy alternative measures.
The AFCR maintains the design of the CANDU 6 emergency core cooling (ECC) system that removes residual and decay heat from the reactor by re-filling the HTS following a postulated loss of coolant accident (LOCA). This system includes the emergency coolant injection function and supporting functions of HTS loops isolation and steam generator crash cooldown. The ECC system is initiated automatically and designed to supply emergency coolant to the reactor in three stages.
The dousing system, which is similar to the proven CANDU 6 design, is designed to operate during postulated accident scenarios to effectively reduce containment peak pressure, including during a main steam line break (MSLB).
The AFCR emergency water supply (EWS) is a safety support system provided to ensure that an adequate heat sink is available to remove decay heat if the normal modes of heat removal are lost during postulated accident scenarios.
A new severe accident recovery and heat removal system (SARHRS) is incorporated into the AFCR design to remove decay heat from the core and to provide a containment heat sink for the long term during postulated beyond- design-basis accidents (BDBAs). This system can obtain its water source either from the EWS reservoir, or by recovering recirculating and cooling water from the reactor building basement. The water can be used as make-up for the calandria vessel and the calandria vault, preventing a severe accident or stopping a severe accident progression and mitigating accident consequences.
The AFCR design includes numerous flow paths to the ultimate heat sink (local body of water and/or atmosphere) for normal operation, design basis events, and beyond- design-basis accidents.
The spent fuel bay in the AFCR is located outside the reactor building at grade elevation to prevent interaction with events inside containment. In postulated events, there is significant time (on the order of days) for the operator to take corrective actions, and a small make-up rate of cooling water is provided, sufficient to support evaporative cooling of the fuel in the storage bay.
CANDU reactors in general have low postulated design basis accident containment pressures in comparison to other major reactor types. The AFCR design has still greater safety advantages, including large quantities of stored cooling water in the containment building, and the inherentprevention of high-pressure core melting scenarios which plague other reactor designs. Also, the AFCR containment design has been modified to protect against external events (for example, aircraft crash), to incorporate a steel liner to reduce containment leakage, and to include an enhanced hydrogen control system based on igniters, passive autocatalytic recombiners (PARs) and a dedicated hydrogen monitoring system, as well as other improvements.
The AFCR is synergistic with current and planned LWR reprocessing technologies around the globe. It uses an undesired by- product (in the form of RU), to both produce power and provide a near-term path forward for implementing advanced and sustainable alternative fuel technologies, such as LEU/Th, CANMOXTM and Pu/Th fuels.
The AFCR design is based on the proven CANDU reactor design and incorporates numerous technological enhancements in terms of safety features, design margins and fuel utilisation. It meets the latest safety codes and standards, generation III and post- Fukushima requirements.
The AFCR is optimised for DRU and LEU/ Th advanced fuel cycles, requiring only minor modifications to the proven CANDU 6 reactor core, control mechanisms, heat transport and fuel handling systems, mainly due to the physics characteristics of the fuels.
Building on the recent success with NUE in China, Candu Energy, with its Chinese partners, has successfully completed the AFCR detailed conceptual design. It is ready for implementation in the Chinese market, and in other jurisdictions with a desire to use DRU and LEU/Th advanced fuels.
About the author
Fabricia Pineiro, Dan Popov, Ziping Li, Mustapha Boubcher, Catherine M. Cottrell and Sermet Kuran, Candu Energy Inc.
The authors wish to acknowledge colleagues at CNNC (TQNPC, CNNFC, NPIC), Candu Energy, and Atomic Energy of Canada Limited (AECL) for their contributions to the AFCR project.