The United States currently has 103 operating nuclear power plants producing over 100 GWe annually. These nuclear reactors are all light-water reactors (LWRs) and there are an additional six GWe of LWR capacity under construction. In the world today there are over 435 nuclear power plants, the preponderance of which are water-cooled reactors. There are also over 60 LWR nuclear plants under construction around the world including the United States. Given this situation and continued license renewals in the United States and in the world, it is safe to say that LWRs will be the dominant technology that is used to produce electricity from nuclear fission reactor plants for several decades.
There is a continued emphasis on improving the reliability and the safety of nuclear power plants. The accident at Fukushima reminded all of us of the need to stay vigilant and seek ways to improve the safety of both existing and new nuclear plants. Even though there were no fatalities from the accident, and its long-term health effects have been estimated to be far less than the tsunami, the economic impact has been huge and the release of radioactive materials off-site can have long-term environmental impacts in the region surrounding the site.
A direct way to improve the reliability and safety of current and future LWRs, is to focus on novel and transformative advancements in nuclear fuel and cladding design and development. This focus has the best chance of benefiting safety for current and future LWRs for decades.
It is important to consider strategic options, including time line and
budget considerations, for accelerated development and widespread deployment of advanced fuel designs (fuel and cladding) for use in existing (and new) pressurized water reactors (PWRs) and boiling water reactors (BWRs), such that fuel integrity can be maintained in the event of anticipated operational occurrences, and design-basis accidents and the fuel rod is robust under beyond-design-basis conditions. This strategy must look broadly at improving fuel performance and safety; e.g., reduction in the generation of combustible gases during fuel degradation. The strategy could consider the use of modern scientific computational tools that could reduce the experimental and development time line and the steps required for validation of such advanced models. The costs and risks of implementing a new fuel design should be weighed against the costs and risks associated with engineering and administrative changes to the existing systems that could achieve a similar reduction in risk.
The challenge is to create a coherent plan for developing a novel fuel that incorporates the complete discovery-to-product process: i.e., R&D plan, fuel demonstration, reliable fuel manufacture, acceptance testing, and performance. This can also provide the opportunity to develop advanced fuels that take the entire fuel-cycle implications into account (e.g., spent fuel disposition) to improve upon the current circumstance in which fresh fuels are developed without regard to the implications for the rest of the fuel cycle.
Develop a transformational fuel for light-water reactors (advanced and current) that:
• Improves the safety performance for the fuel under the range of operating conditions (anticipated operational occurrences and load following), design-basis accidents (e.g., LOCA and post-LOCA behavior as driven by stored energy and cladding-coolant compatibility), and special events considered in licensing (e.g., minimized fuel failure in extended station blackout or ATWS, which may be more limiting due to differential responses between fuel and clad);
• Can be produced at a cost competitive with the current generation of LWR fuel (as an example, the total cost of a single fuel rod today is about $2500/kg and it produces about 50 MWDth/kg of energy);
• As secondary benefits, improves fissile fuel utilization, enhances
disposal options, and improves safety performance and reduces costs for spent fuel disposition.”
IDR Team members are encouraged to explore ways fundamental advances in material science, nuclear fuels as well as computational modeling can help in these tasks. For example, there have been major advances in multiscale, multicomponent materials modeling that would allow for computational materials design and reduce trial-and-error experimentation. This could transform fuel and cladding design protocols and the novel fuel-clad system could improve behavior during operation and during more extreme environmental conditions.
Konings RJM, ed. Comprehensive nuclear materials, volume 3: Advanced fuels, fuel cladding and nuclear fuel performance modeling and simulation. Elsevier: Amsterdam, The Netherlands, 2012.
Nuclear Energy Agency. Nuclear fuel behaviour in loss-of-coolant accident (LOCA) conditions. State-of-the-art report. NEA-6846, 2009.
Nuclear Energy Agency. Nuclear fuel safety criteria. Technical review. NEA-7072, 2012. Zinkle SJ, Was GS. Materials challenges in nuclear energy. Acta Materialia 2013; 61:735-758.
Because of the popularity of this topic, two groups explored this subject. Please be sure to review the other write-up, which immediately follows this one.
• Michael J. Fluss, Lawrence Livermore National Laboratory
• John J. Gantnier, Bechtel Power Corporation
• Peter Hosemann, University of California, Berkeley
• Kevin J. Kramer, Lawrence Livermore National Laboratory
• Wei Lu, University of Michigan, Ann Arbor
• Digby D. Macdonald, University of California, Berkeley
• Brad Marston, Brown University
• Vikas Prakash, Case Western Reserve University
• Naveena Sadasivam, New York University
• Marius Stan, Argonne National Laboratory
• Brian D. Wirth, University of Tennessee
IDR TEAM SUMMARY—GROUP 2A
Naveena Sadasivam, NAKFI Science Writing Scholar New York University
Evolutionary Approach to a Transformational Fuel
IDR Team 2A was asked to formulate a coherent plan for developing a novel fuel for light-water reactors (LWR) that incorporates the complete process from discovering a fuel to producing it.
More than 80 percent of the 435 nuclear reactors worldwide fall into the category of LWRs. Known for their simplicity and comparatively low construction cost, LWRs are currently the most widely used type of nuclear reactor and are slated to remain the “go to” type of reactor in the coming years. In the United States, all of the 103 nuclear reactors are LWRs.
A majority of the technological advances that made the LWR cost-efficient, safe, and reliable took place in the 1970s and 1980s. Given the ubiquity of the LWRs and their slow technological development, it is befitting then that Team 2A worked to create a plan to develop a new fuel that is safer and more efficient.
Scope of Discussion
Team 2A agreed that it needed to suggest a solution that can be maintained in the event of anticipated as well as unanticipated accidents like those at Fukushima and Three Mile Island. While unanticipated events are rare, their consequences can be catastrophic and so it is important to factor them into design considerations. Therefore, the team concluded that because it is impractical to design fuel that can withstand the most catastrophic accidents, it could instead design fuel with a slightly different objective in mind—to provide the plant operator more response time.
Furthermore, the cost and practical implementation of introducing a new source of fuel to existing and new reactors must also be considered. If the nuclear reactor industry finds that the changes from the status quo are too drastic, it might reject the solution as a whole. An extremely safe solution might also be too expensive for the industry to adopt. Thus, the team will need to strike a balance between market feasibility and safety.
FIGURE 1 An annular fuel rod with two pathways for the flow of water will increase energy efficiency.
Annular Fuel Design
A typical fuel rod used in nuclear reactors consists of small pellets of enriched uranium in the form of uranium dioxide that is then placed into tubes of zirconium alloy. Bundles of 14 × 14 or 17 × 17 tubes of enriched uranium are assembled together with space in between for the coolant—water—to flow through and transfer heat.
1 Zhang, L. Evaluation of high power density annular fuel application in the Korean OPR-1000 reactor.” MS thesis, Massachusetts Institute of Technology, 2009; Morra, P., Design of annular fuel for high power density BWRs. MS thesis, Massachusetts Institute of Technology, 2004.
is a diagrammatic representation of the annular fuel rod that the Team designed. The inner diameter of the rod is not constant and is instead tapering outward. The water that enters at the bottom of the rod will be at a low temperature. As it moves through the inner pathway, it will absorb heat from the rod and exit at a higher temperature. Similarly, the water flowing along the outer surface of the rod will also experience a similar temperature increase. For pressurized water reactors, the flow rate of water can be adjusted so that it does not form steam. Since the coolant flows through the annulus and on its surface, it is expected that heat transfer will be faster and that it will increase energy efficiency.
The team also proposed that the gas-filled gaps that are usually present within the rod be replaced with porous graphite foam. By engineering the porosity of the pellets, channels can be formed for the fission products so that there is uniform accumulation of gas along the length of the rod. This will keep the geometry of the rod constant but change the pellets at a microscopic level. The team initially suggested that a fission gas vent be installed to deal with buildup of gas pressure, but because of a high risk of mechanical failure and drastic change to the entire reactor structure, the idea was discarded.
One other variation of the annular fuel rod design discussed was to have a matrix of four or six sets of fuel pebbles enclosed within an annular cladding. The team felt that this would provide sufficient thermodynamic stability but that the optimal number of fuel sets would need to be determined through simulation and experimentation.
Pathway to a Transformative Fuel
To develop a fuel that fundamentally transforms the nuclear industry by providing higher energy efficiency and better safety, the team believes that it will take several incremental steps over a long period of time for both innovation in nuclear engineering as well as development of new materials used in fuel rods. Short of an enormous political and societal will to mainstream nuclear reactors, the team believes that the series of iterative events outlined in Figure 2 are required to develop a new transformational fuel. These steps provide both the required time and allocation of resources required to complete the process.
On the engineering side, researchers need to first analyze and test the current LWR system and identify areas where optimization is feasible. At the same time, material scientists will need to begin by testing materials to
FIGURE 2 Steps to be taken in the engineering and materials research that will lead to the design of a new transformative fuel.
observe how they respond to different levels of radiation and then identify suitable substances that can be used. Once the initial research stage is complete, the annular fuel assembly design will need to be finalized and then submitted to regulators for licensing. To produce a truly revolutionary fuel, the cladding used in the system will also need to be improved. Hence, the cladding materials research will need to continue until a new type is developed. Once this is done, a lead test assembly will need to be conducted in order to identify implementation issues and assess the overall efficiency of the LWR.
The ideas listed above are by no means a complete list of work that needs to take place before a new fuel is on the market. The Team anticipates that the process will take several iterations and effective communication between the materials and engineering researchers to coalesce at a fundamentally transformative fuel for LWRs.
Research and Future Development
The IDR Team has also identified four key areas that require additional research work.
1. Multiple flow channels: Increasing the number of flow channels within the fuel assembly can help optimize power density, thermal hydraulics, and manufacturability.
2. Materials development: The current zirconium-based cladding needs to be replaced by advanced clads that have reduced oxidation kinetics.
3. Evaluation of alternative fuel forms: Inert matrix fuels such as TRISO and CerMet need to be analyzed. Micro-engineered porosity or engineered composite fuel forms are also alternatives to be considered.
4. Reactor-fuel-coolant concepts: There is a need for LWR concepts that reduce the high-power density and water use while also providing flexibility to meet the challenges with the back end of the fuel cycle, including the possibility of finding fuels that can be reprocessed in an economically viable way.
• Stephen T. Bell, Office of Naval Reactors
• Jeremy T. Busby, Oak Ridge National Laboratory
• Annalisa Manera, University of Michigan
• Martha L. Mecartney, University of California, Irvine
• Amit Misra, Los Alamos National Laboratory
• Jodi Murphy, University of Georgia
• David Petti, Idaho National Laboratory
• Mitra L. Taheri, Drexel University
• Bernhard R. Tittmann, Penn State University
• Steven J. Zinkle, Oak Ridge National Laboratory
IDR TEAM SUMMARY—GROUP 2B
Jodi Murphy, NAKFI Science Writing Scholar University of Georgia
IDR Team 2B was tasked with the challenge of developing a transformational fuel for light water reactors (LWRs)—advanced and current.
The 100 nuclear power plants operating in the United States are all light-water reactors. They produce more than 98 gigawatts of electric power
(GWe) annually. Six more GWe of LWR generation is under construction in the United States and will be available after current construction is completed. LWRs are likely to be the primary technology used to produce electricity from nuclear fission reactor plants in the coming decades. Most of the 436 nuclear power plants worldwide are water-cooled reactors. The number will soon grow, as more than 60 LWR nuclear plants are under construction throughout the world, including those being built in the United States.
The current widespread global use of LWRs for power production and the Fukushima accident in 2011 have led to many investigations, including tsunami tolerance and the ability to provide reliable backup power or tolerate a loss of site power. One area of investigation has been to consider the potential to develop an advanced fuel for LWRs that can better tolerate the high-temperature environment that can exist during an anticipated operational occurrence or an accident, without releasing large amounts of radioactivity from the fuel.
The IDR Team met to develop a conceptual approach for creating a novel fuel that incorporates the complete discovery-to-product process. This plan examines the R&D that would be needed, demonstration of the safety and effectiveness of a new fuel, reliable fuel manufacture, and performance. Industry acceptance would also be taken into account. A thorough and comprehensive plan would have to be created by a larger team over several months. A thorough analysis might also prompt the development of advanced fuels that take the entire fuel cycle implications into consideration in order to improve the current circumstance in which new fuels are developed. Notionally, a transformational fuel should have the following general characteristics to be successful:
• Decrease the risk of fuel failure and radioactivity release over the full range of operating conditions, including anticipated operational occurrences, load following, design-basis accidents (such as loss-of-coolant accidents [LOCAs]), and loss of decay heat removal accidents such as station blackout).
• Be cost-effective with current fuel in LWRs. For example, the total cost of a single fuel rod today is $2500/kg, and this rod produces approximately 50 Mw-d thermal/kg of energy.
• Should not significantly degrade fuel utilization and burnup efficiency (i.e., introduce excessive neutron absorbers) and should improve fissile fuel utilization if practical. It is desirable to improve reactor performance characteristics such as energy density or power density, increase options for
fuel disposal, and reduce operating costs. Early, comprehensive engineering design and business case assessment need to be done to ensure that candidate nuclear fuels have the promise to become practical and ecomomical reactor fuels.
The team considered ways to use fundamental advances in material science, including nuclear fuels and computational modeling, that can aid in materials design. Modeling methods exist to calculate some material properties, such as thermodynamic properties, and to predict the effects of material behavior on fuel element behavior or reactor performance. These methods can help reduce trial-and-error experimentation in developing new fuel materials and concepts. This may aid in designing alternative fuels with improved behavior during operation and during more extreme environmental conditions. Integrated behavior of new fuels will still have to be established by prototypic testing in, for example, a test reactor environment prior to operating a new fuel in a power reactor, since analytical techniques alone are not sufficient to describe the complex interactions of phenomena within nuclear fuels during operation.
In determining how to develop a transformational fuel for LWRs, the time line and budget must be handled strategically in order to accelerate the development and deployment of advanced fuel and cladding designs to be used in existing LWRs and new pressurized water reactors (PWRs) and boiling water reactors (BWRs). This foresight will enable fuel integrity to be maintained in the event of any breakdowns in operations. Safety must be a top consideration in the improvement and innovation schemes. Contemporary scientific computational tools may decrease the experimental and developmental time line, as well as the steps necessary to validate advanced models. The potential negative impact of instituting a new fuel design must be compared to the potential consequences of engineering and administrative changes to the existing systems that may achieve a similar decrease in case of a threat of danger.
The team incorporated perspectives of both mechanical engineers and materials engineers. The mechanical engineers considered the problem from the angle of assessing the how the reactor would function as a whole. Their opinions hinged on practicality and the effect of new materials on overall reactor design, operations, and maintenance from a systems perspective.
The materials engineers had aspirations of creating ceramic and/or metallic materials to coat the fuel pellets and/or cladding to improve the safety of the LWRs.
Another possibility that the team considered was the potential to replace LWRs entirely with a new type of reactor that is inherently more tolerant of accident conditions. The team acknowledged that this is a remote possibility for many reasons, such as the cost. For example, new clear reactor design projects are multi-billion-dollar programs executed over a decade or more, and construction costs can be several billion dollars per unit. The United States lacks the manufacturing infrastructure needed to make rapid changes needed to replace LWRs entirely and does not face the pressing lack of electricity that nations such as China and India do. In addition, there is widespread distrust and misunderstanding of nuclear power in the United States. It may be more likely that alternative reactor concepts will evolve in parallel with any efforts to upgrade the fuel in LWR plants. If practical, it would be desirable for development of a new fuel for LWRs to provide a springboard for development of advanced, more resilient nuclear power plants.
The team agreed on some characteristics of the ideal alternative fuel, which also addressed the ideal cladding system. It must be compatible with steam and liquid water over 280-1000 degrees Celsius. It must be hermetic to fission products and water. It must have high strain to failure and linear elastic behavior up to the stresses to about 18-20 ksi to allow for practical mechanical design. However, the materials must also be tolerant of changes in fuel or structures under neutron and gamma irradiation (e.g., fuel swelling and stress-free growth). A fuel cladding material must have substantial toughness because of inevitable manufacturing defects. It needs to be a nonneutron absorber. It must be manufactured with practical industrial and chemical processes. It should be corrosion resistant and in a form that lends itself to disposal. It must have fuel, clad, and structural compatibility and be compatible with the full fuel cycle.
Development Challenges and Constraints
Development of a new, accident-tolerant LWR fuel is a major, multidisciplinary effort with potentially high consequences if a new fuel is put into service and does not work as anticipated. The total costs of producing a new LWR fuel are likely on the order of $1 billion. Strong, central technical leadership would be essential to align and coordinate a diverse group
of scientists, engineers, manufacturers, regulatory interfaces, etc. There should be one person and organization responsible for providing technical leadership, coordinating all activities, making final technical decisions, and providing accountability to government regulatory agencies. The organization should be composed of professional R&D and materials test groups, major infrastructure operations, design engineering, fabrication vendors, regulatory review and standards (such as the Nuclear Regulatory Commission), and a commercial customer for an operational test in an existing commercial reactor.
Risk tolerance is necessary for the timely development of new materials. There must be a parallel test, design, and fabrication completed on a short time line, which requires decisive actions with limited information. The risk can be mitigated by simultaneously pursuing multiple options—in other words, having back-up plans. Rational risk mitigation would also involve conservative design and removal of the test assembly prototype.
The IDR Team devised an optimum time line in which a new fuel and cladding system could be developed, tested, and implemented. The project would require a huge financial investment up front that would need to be justified based on a compelling vision of improving operating reactor safety. It would be desirable to demonstrate the potential for long-range economic benefits from reducing power plant operating or maintenance costs.
The time line is representative of the likely course of events. In years 0-5, low-level lab assessment, including scoping studies, computational models, and early materials testing would occur. During years 2-20, the product would be licensed and energy companies interested in purchasing the new fuel and cladding system would be sought out. Years 6-12 involve finding investors and performing testing to garner more complete materials data needed to engineer a fuel assembly. In years 4-20, irradiations would be conducted. Years 4-7 are for the preconcept stage, 7-9 are for the concept stage, 8-10 are for the reference stage, and 9-20 are the final stage. In years 7-10, the first manufacturing trials would be conducted to scale up and determine the details of the design. During years 9-12, the fabrics used would be qualified, and during 10-13 the facilitization would be conducted. In the final 12-18 years, the fabrication and testing of a prototype set of fuel assemblies would go on.
The IDR Team estimates that if the time line were accelerated to oc-
cur at optimal speed, the entire process from concept to actualization and testing could happen in 12 years. This would require robust and stable funding to initiate and complete research, as well as increased tolerance for risk in government-funded research, so that activities could proceed more in parallel to shorten the development time line.
The alternative fuel and cladding system must have enhanced retention of fission products. To improve fuel containment of fission products, the IDR Team suggested minimizing fuel relocation and dispersion, lowering operating temperatures, inhibiting clad internal oxidation, and an increased fuel melting safety margin. This will also be accomplished through improved cladding, which will help maintain core cooling and retain fission products. Improved cladding could be created through improved high-temperature clad strength and fracture resistance, increased thermal shock resistance, greater high-temperature compatibility, and resistance to hydrogen embrittlement.
The IDR Team identified multiple concepts for a transformational fuel for LWRs. The materials-based solutions include enhanced confinement of fission products near their origin, perhaps microencapsulated dispersed fuel forms to provide more robust containment of radioactivity in the fuel. Improved cladding with high-temperature oxidation resistance in the stream and improved chemical reactivity were suggested. Other materials-based solutions included woven and engineered composite systems, made of either metals and/or ceramics interwoven for strength, ductility, and oxidation resistance. Other possibilities include fully ceramic, ductile nanograin materials. Engineering- and physics-based solutions included passive heat removal systems for severe accident conditions or modified fuel forms such as annular fuel that could provide more efficient heat transfer from the fuel to the coolant.