Fast-tracking efficiency in light water reactor fuels

Every breakthrough in nuclear energy starts with better data.

At the Department of Energy's Oak Ridge National Laboratory, researchers are gathering precise measurements by simulating design basis accidents under controlled conditions. This data aims to understand how nuclear fuels designed to enhance the performance of operating reactors will behave when it matters most.

This testing is part of a national effort to accelerate the licensing of accident tolerant and high-burnup fuels used in light water reactors designed to increase safety of the plant and promote greater operational efficiencies. By generating crucial data from accident testing, ORNL researchers are narrowing uncertainties to enable the nation's nuclear reactors to produce more power for longer stretches, ultimately increasing nuclear energy production without increasing cost.

"Improving light water reactor fuels offers a direct path toward producing abundant, affordable energy from the nation's 94 currently operating nuclear reactors," said Jason Harp, leader of the Nuclear Fuel Element Performance Group. "ORNL's testing and analysis on these fuels aims to safely increase the amount of electricity reactors produce, in addition to improving cost and operational efficiency."

Enabling energy production

Nuclear energy is a highly reliable and efficient form of electricity, powering nearly 20 percent of the country, 24 hours a day, seven days a week. This availability and dependability make nuclear a prime source of baseload power for homes, businesses and critical infrastructure, including artificial intelligence systems and data centers. Demand for this baseload power is soaring.

As new nuclear power plants are being considered to meet this demand, utilities across the country are looking to increase electricity production from currently operating light water reactors. This can be done through a "power uprate."

"A power uprate means increasing a plant's electrical output by optimizing its operational margins - and removing overly conservative guidelines where appropriate - so that the fuel can operate at higher power levels," said Nathan Capps, senior R&D staff in the Fuel Cladding and Core Internals group. "The Nuclear Regulatory Commission (NRC) has already approved 171 power uprates to date, adding the equivalent of eight new reactors to the grid without putting a shovel in the ground," said Capps.

Uprating a reactor's capacity can involve increasing linear heat generation rate (LHGR), or the energy produced from each fuel rod. By increasing this rate, fuel can operate at higher temperature, producing more power and in turn, extracting more energy from the same amount of fuel.

Coupling a power uprate with burnup extension - or how long fuel can produce power before being replaced - allows fuel to operate at a higher power for longer periods of time. Fuels with accident tolerant and higher burnup capabilities allow reactor operators to do both, effectively producing more power, more effectively, at a lower cost.

However, nuclear energy is a highly regulated industry. New light water reactor fuels, even those with incremental changes, must first be licensed and approved by the NRC, the industry's key regulator, before they can be used in a commercial reactor. The NRC also has to approve any operational changes, like burnup extensions and power uprates by approving amendments to a utility's operating license. As such, to operate longer cycles and increase their power output by using these new fuels, a reactor operator must provide sufficient documentation demonstrating safety.

This process requires comprehensive analysis with supporting experimental data demonstrating that fuel and other fuel components, like cladding, can safely operate for longer and at higher temperatures. Accident and transient testing are crucial to supporting licensing analyses and methodologies.

With its leading expertise and specialized facilities, ORNL is uniquely positioned to generate this data through advanced accident simulation and testing capabilities.

Comprehensive testing capabilities

Supported by the DOE Office of Nuclear Energy's Advanced Fuels Campaign, researchers at ORNL are gathering data on fuel behavior by simulating loss-of-coolant accidents, or what is known as LOCA testing.

This type of testing is relevant to a hypothetical scenario where the reactor's cooling water supply is suddenly removed from the core. To simulate these conditions, ORNL researchers developed the Severe Accident Testing Station (SATS), the lone domestic semi-integral capability for replicating accident scenarios to test commercially irradiated fuel and fuel cladding.

Capps, who leads high-burnup fuel qualification efforts for the Advanced Fuel Campaign within the DOE's Office of Nuclear Energy, works alongside a team of researchers performing LOCA tests. Yong Yan, a distinguished R&D staff scientist in the Nuclear Fuel Element Performance Group has been instrumental in establishing ORNL's accident testing capacity.

"The SATS capability allows us to simulate off-normal conditions, like a LOCA, by subjecting irradiated fuel to extremely high temperatures, various heating rates and internal pressures," said Yan. "Its primary advantage lies in its ability to conduct physical experiments on high-burnup fuel materials and use advanced, real-time diagnostics. Other facilities may not be able to handle these materials easily, or at all."

Simulating a LOCA event starts with a small segment of fuel ranging from a few inches to twelve inches. The segment is placed in SATS within a quartz tube and centered using two perforated spacer disks. The segment is then internally pressurized to conditions similar to full length rods at high burnup conditions and then heated to a specific maximum temperature at a specified heating rate using an infrared furnace.

Once reaching this temperature, the segment is cooled using water to bring the sample back to room temperature. This test typically takes from two to three minutes but can run longer depending on experiment conditions.

"SATS provides experimental data under realistic, severe accident scenarios, enabling ORNL to push the boundaries of nuclear safety and design, ensuring better performance and reliability of future nuclear energy systems," said Yan.

This data is then used by fuel vendors to develop reports justifying the safe use of new cladding and fuel materials. The NRC reviews these reports to ensure that all safety concerns are thoroughly addressed, and following an extensive evaluation, may approve the report.

Data that drives discovery

Designed and constructed at ORNL, the SATS capability is two complementary testing stations. One station sits in a traditional laboratory, where scientists can design experiments, adjust test conditions, and perform sensitivity studies prior to testing on irradiated material. The second station sits inside of a hot cell, where they apply those test conditions on commercially irradiated fuel and fuel rods.

Using these capabilities, researchers can recreate a wide range of "transient" events, or off normal conditions like LOCAs and other high temperature scenarios. Each SATS capability includes two types of furnaces: One furnace allows researchers to conduct LOCA tests to design basis accident conditions, or accidents that nuclear facilities must be designed to withstand. The second, high temperature furnace is used to perform tests under "Beyond Design Basis Conditions," or highly unlikely events that were not considered in the design of the facility.

By simulating these scenarios, researchers can capture data designed to provide detailed insight on how nuclear fuel and its protective cladding might respond. Testing across accident conditions provides vital data necessary for fuel and component licensing.

SATS can also be used to measure transient fission gas release, or how gases trapped within the fuel escape during rapid changes in temperature or pressure. Researchers have also studied fuel fragmentation, relocation and dispersal using SATS.

"Every experiment begins with a question necessitating data we don't yet have," said Yan. "With SATS, we can simulate conditions and collect a range of detailed measurements needed to provide more confidence in answering that question precisely and directly."

Recently, ORNL scientists have further expanded the station's capabilities by incorporating digital image correlation to visually capture and measure cladding deformation in real time, representing a first in LOCA testing.

"We machined a window in the furnace wall and installed a camera to capture precise optical images," said Mackenzie Ridley, an R&D associate in the Corrosion Science and Technology group. "These images were analyzed to calculate deformation data and are essential for improving understanding of the complex fuel cladding interactions under accident type conditions."

Partnerships powering progress

This range of data is imperative for nuclear reactor operators, as well as fuel manufacturers, as they seek NRC approval to insert enhanced fuels into operating reactors. ORNL has been a leading partner in collecting this data.

Working alongside Southern Nuclear Company and Constellation, and in collaboration with fuel manufacturers Westinghouse, Framatome and Global Nuclear Fuel, the ORNL team has conducted 16 LOCA tests in the last four years, with plans to nearly double testing in the coming years.

"The data from these experiments with fuel rods containing high-burnup, standard-burnup, and accident-tolerant fuels-all previously irradiated in commercial light water reactors, is leveraged to accelerate the review and approval process for novel fuels and associated codes and methods," said Zeses Karoutas, Chief Engineer for Nuclear Fuel at Westinghouse.

"As load growth continues to put more pressure on the grid, enhancing nuclear fuel performance has never been more important," said Johnathan Chavers, Director of Nuclear Fuels and Analysis at Southern Nuclear. "Innovative technologies that deliver the full benefit of nuclear fuel are essential to running plants more efficiently and getting more energy to the grid for customers, and Oak Ridge National Laboratory's testing and analytical expertise is essential in helping utilities optimize operations and unlock more value from innovative fuel technologies."

Looking ahead

With an increased reliance on nuclear energy to meet growing electricity demand, the strategic partnerships between ORNL and commercial reactor operators, as well as nuclear fuel vendors, will be pivotal to meeting the nation's energy goals. Capps recognizes ORNL's key role in providing the vital testing and analysis to accelerate progress.

"These tests show how innovation and safety can go hand in hand," said Capps. "ORNL is a trusted partner in delivering the important data that improves our confidence in innovative fuels. Every experiment helps industry partners operate more efficiently and strengthens America's position as a global leader in advanced nuclear technology."

ORNL is committed to supporting U.S. energy needs by pursuing strategic research that advances a wide variety of affordable, abundant and competitive nuclear technologies, and strengthens national security. The lab's scientific expertise and world-class facilities are often the first step in advancing nuclear energy innovations.

UT-Battelle manages ORNL for DOE's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. DOE's Office of Science is working to address some of the most pressing challenges of our time. For more information, visit energy.gov/science.