(Photo courtesy Southern Co. Nuclear)
Experts in the nuclear energy industry are making significant advancements in fuel technologies that will help today’s and tomorrow’s fleets of nuclear reactors operate efficiently, reliably and safely.
One area in which experts are rapidly making progress is with enhanced accident tolerant fuel (EATF). These technologies are designed to withstand the loss of active cooling in light water reactors for longer periods of time, providing operators with more time to respond. Additionally, they can improve fuel performance during normal operations.
Driving the Next Evolution of Nuclear Fuel with Two Advanced Technologies
Nuclear fuel typically has a long development process based on years of experience as experts work to deliver benefits to help preserve the existing fleet of reactors and support future technologies. These efforts build on the collective knowledge, skills and expertise of leaders from across the nuclear sector, including utilities, national laboratories, universities and the industry.
Framatome’s experts are currently developing two EATF designs, one near-term and one long-term, under the company’s PROtect program. The near-term design, known as PROtect Cr-Cr, includes chromia-enhanced pellets and chromium-coated cladding. Initially, Framatome is adding these combined features to its new GAIA fuel design for pressurized water reactors (PWRs). Chromia-enhanced pellets are currently provided as part of the Atrium 11 advanced boiling water reactor (BWR) fuel design and will be expanded to the PWR fleet.
The chromia-enhanced pellets have a larger grain structure and improved viscoplasticity. This leads to lower fission gas release under accident conditions, as well as during transient conditions when fluctuating temperatures or pressures change the reactor’s power output. Improved pellet clad interaction (PCI) characteristics also provide greater operational flexibility, such as potential load following capability.
Meanwhile, the chromium coating protects the cladding from debris damage and greatly reduces oxidation at higher temperatures. In accident-type tests, the cladding demonstrates reduced ballooning and rupture size, maintaining the coolable geometry of the fuel rods and reactor core for a longer period of time.
The longer-term EATF design, known as PROtect SiC, features chromia-enhanced pellets with a silicon carbide-based cladding. Silicon-carbide composites have been found to maintain their strength up to very high temperatures. Combined with low neutron absorption, this makes silicon-carbide composite materials ideal solutions for an accident tolerant cladding. Once technical challenges with the material are resolved, such as a need to improve the resistance to dissolution in water, silicon carbide-based cladding materials could provide even more time for operators to respond in the unlikely case of a severe event.
Framatome is researching unique solutions to these technical challenges. The company is currently irradiating samples in a European commercial reactor and plans to irradiate test rod segments in a research reactor in 2020 followed by lead test rods (LTR) in a commercial reactor in 2022.
Much of the work in the United States developing these EATF technologies is underway with support from the U.S. Department of Energy’s (DOE) EATF program. This includes an approximately $49 million, 28-month grant that Framatome received from DOE in November 2018, along with the ability to continue to use and receive support from its national laboratory facilities. This builds on a $10 million, two-year grant the company received in 2016. This funding and lab support accelerate the advancement of these technologies to the benefit of the industry and consumers. A continuation award is expected to support ongoing research, development and testing activities.
In addition to work with DOE, Framatome continues global research, development and testing activities with its partners that have collaborated for several years on some options of this fuel design. Partners include the French Alternative Energies and Atomic Energy Commission (AEC)-which initially explored and identified the suitable cladding coating process and developed SiC cladding-Électricité de France (EDF), the Goesgen Nuclear Power Plant in Switzerland and other leaders from across the nuclear sector. These collaborative efforts draw on each organization’s expertise in conducting advanced materials and manufacturing methods research.
Testing Activities Continue and Show Positive Results
Testing is well underway on Framatome’s EATF designs. After two irradiation cycles of the chromium-coated and silicon carbide-based cladding at the Goesgen Nuclear Power Plant, samples continue to show positive visual indications. This includes greatly reduced oxidation characteristics and no signs of coating delamination. Additional samples were removed after one and two cycles at Goesgen and are now undergoing further characterization and testing at the Paul Scherrer Institut, a research center in Switzerland. Goesgen is the first PWR in the world irradiating samples of EATF cladding, and this testing will continue through commercialization of accident tolerant fuel concepts.
Extensive out-of-pile testing continues at Framatome and CEA laboratories, where chromium coating technology development started in the past decade and shows promising results. This extensive test program has been built to create the in-depth databases of material properties and performance data necessary to model, license and implement chromium-coated cladding in commercial reactors worldwide. The out-of-pile test programs are complementary to irradiation test programs planned and underway on Framatome’s EATF concepts.
In June 2018, chromium-coated rodlets with chromia-enhanced pellets were inserted for testing at the Idaho National Laboratory’s (INL) Advanced Test Reactor (ATR). These rodlets are the first complete combined (cladding and pellets together) EATF concept in the world to be irradiated under PWR conditions. A total of 26 rodlets are being tested in a special loop that mimics the coolant conditions of a commercial light water reactor. The data from this testing will be used to help qualify the fuel design with the U.S. Nuclear Regulatory Commission (NRC).
Following irradiation, the rodlets are scheduled to undergo transient testing at INL’s Transient Reactor Test Facility (TREAT). The results from this testing will also support work to qualify the fuel with the NRC. The Framatome team is working with Oak Ridge National Laboratory (ORNL) to test chromium-coated cladding test specimens planned for irradiation in the High Flux Isotope Reactor (HFIR) test facility in 2019.
Upgrading and Qualifying Manufacturing Processes to Support EATF Testing and Deployment
While testing on these EATF technologies is underway, Framatome is also developing and qualifying the manufacturing process. For the rods, development has advanced to the stage of small-batch, full-length rod coating capability, which supports lead test assembly and test rod production. In March 2018, the team produced its first full-length fuel cladding coated with chromium. The coating was performed on prototype equipment in France.
With the additional support recently awarded by the DOE, Framatome will embark on the next phase of scaling up these advanced manufacturing processes. Framatome’s fuel manufacturing facility in Richland, Washington, has been upgraded and qualified to begin production of chromia-enhanced pellets for BWR reload quantities and to support lead test rod fabrication for PWRs.
With the lessons learned from the small-batch prototype, the company will accelerate a pilot program to develop the large-batch capability required to support the high volume of rods needed for the lead fuel assembly (LFA) program and the initial reload batch. While a fuel assembly can contain more than 200 rods, depending on the design, one reload-batch size quantity could require 14,000 to approximately 17,000 rods depending on fuel design and batch size. While much progress has been made with promising results, more work must be done to manufacture this quantity of rods.
Based on the promising results obtained using its advanced physical vapor deposition (PVD) coating process, Framatome will expand its research into the applicability of this process and beneficial coatings for the BWR market.
Furthering Accident Tolerant Control Components
As industry implements accident tolerant fuels and cladding materials, Framatome is also developing advanced technology for reactor control components. These components allow operators to control the nuclear reaction and power levels in the core. While accident tolerant fuel solutions tolerate higher temperatures and provide operators with more response time in the unlikely event of an accident, accident tolerant control component materials allow reactor operators to control the reactors even at those higher temperatures.
Framatome is independently developing an accident tolerant control rod comprising ceramic pellets. These pellets exhibit extremely high temperature tolerance–higher than silver-indium-cadmium (AIC) rods and comparable to uranium dioxide or chromia-enhanced pellets. This means that the reactor control system is better able to maintain a fully shut down condition during the unlikely case of a severe transient and allow more time for the emergency core cooling systems to inject borated water.
Ceramic pellets exhibit another significant benefit to normal operations: substantially less swelling than AIC control rods as a function of fluence. This allows the control rod to be operated over a longer lifetime and inserted into the core during power operation without developing issues due to swelling. As a result, these control rods can be replaced less frequently and be used in plants that need power maneuvering capabilities to support load following, also referred to as flexible operations.
(Photo courtesy Southern Co. Nuclear)
Supporting the Reactor Fleet in Fully Realizing EATF Benefits
Research, development and testing work on EATF designs is the result of decades of experience and expertise. In early 2019, a team of experts from Framatome, Southern Company and Georgia Power placed full-length (not segmented) lead fuel rods with chromia-enhanced fuel pellets and chromium-coated cladding into Unit 2, a Westinghouse reactor design, at Georgia Power’s Alvin W. Vogtle Electric Generating Plant. These were the first complete, full-length and fueled, ATF concepts loaded into a commercial reactor anywhere in the world. Framatome began producing these lead fuel rods in October 2018. Production of the assemblies was completed in December 2018. In January 2019, the LFAs were delivered to Plant Vogtle where they were inserted during the outage completed in March 2019. Southern Company and Georgia Power have been key supporters of the EATF program and instrumental in the success achieved with this delivery.
In the summer of 2019, it is planned to load two LFAs with 10 full-length Cr-Cr rods each in the Goesgen commercial reactor in Europe. These rods have already been produced and are ready for bundle assembly in Framatome’s European manufacturing facility.
In the fall of 2019, Framatome will load fuel assemblies that have chromium-coated rods into Unit 1 at Entergy’s Arkansas Nuclear One. These LFAs will provide important data and insight on the fuel’s performance in B&W design commercial reactors. Additionally, Framatome signed a contract with Exelon to deliver two full fuel assemblies with the company’s EATF solution to be loaded in early 2021 at the Calvert Cliffs Nuclear Power Plant. All fuel rods in these assemblies will be chromium coated and will contain chromia-enhanced pellets.
Fuel technology development is an intensive process that requires robust expertise and experience. With support from the DOE and domestic and international partners, Framatome continues to achieve significant milestones in its journey to deliver the next evolution of nuclear fuel.
About the author: Jeff Reed is program director of the Advanced Fuel Development program for Framatome’s Fuel Business Unit in North America. In this role, Jeff is responsible for leading Framatome’s participation in the U.S. Department of Energy’s Accident Tolerant Fuel Program and the fuel design program for NuScale small modular reactors. He also serves as the licensing and U.S. project manager for Enfission, a joint venture between Framatome and Lightbridge that aims to develop, license and manufacture fuel assemblies incorporating the innovative metallic Lightbridge Fuel™ technology. A graduate of Baker University, Jeff holds a bachelor’s degree in business, with a strong technical background in mechanical engineering.