Researchers use one-of-a-kind expertise and capabilities to test fuels of tomorrow

Instead of clamping down, Maupin maneuvers the clawlike tongs within millimeters of the fuel rod and hovers there. Another operator, Rob Cox, cuts through the fuel rod with a slow-speed diamond saw. The moment Cox finishes the cut, the section of the fuel rod drops away and Maupin snags it in mid-air with the manipulator tongs.

“Nice catch,” Cox says.

Over the coming months, Cox, Maupin, and their colleagues will repeat this operation dozens of times as they prepare next-generation fuel rods—designed and built by Westinghouse and irradiated in a commercial reactor—for postirradiation examination and testing.

Engineers designed the fuel to be accident tolerant and achieve high burnup.

Accident tolerant characteristics offer improved safety performance during normal operations, power spikes, and reactor accidents. These same characteristics can enable the fuel rods to achieve higher burnup (e.g., increased use of the fissionable uranium-235 in the fuel pellet) that could increase the electricity output of power plants and extend operating cycles—think increased horsepower and better fuel economy.

The stakes are high. These accident tolerant fuels (ATFs) could eventually power the entire U.S. reactor fleet.

Together, these technologies could save electricity ratepayers millions of dollars per year while increasing a nuclear power plant’s resilience under potential (though highly unlikely) accident conditions.

But first, industry and the Nuclear Regulatory Commission need terabytes of data on how those fuels perform. INL is one of the few places in the world with the capabilities and experts to quantify how nuclear fuel behaves during normal operating and accident conditions.

The chance to put high-burnup and ATFs through their paces is the opportunity of a lifetime for the scores of INL experts involved in the tests. Now, after years of preparation, the tests are finally underway.

A long time coming

Discussions of a Westinghouse fuel shipment to INL began roughly a decade ago, but the real history of the fuel rods began in the wake of the 2011 Tōhoku earthquake and subsequent tsunami. These combined events ravaged the Japanese coastline and severely damaged several reactors at the Fukushima Daiichi nuclear power plant. The plant power supplies and cooling systems for three of the four reactors were disabled, ultimately leading to significant core damage and the release of radioactive materials. This outcome might have been prevented if emergency personnel had more time to navigate washed up debris to deliver emergency electrical power or additional cooling water.

ATFs could provide plant operators with this increased margin for safety during abnormal operating conditions and, during an accident, could provide additional time to implement safety measures.

An added benefit of ATFs is that they can power a reactor longer, a feature known as high burnup. This feature is the nuclear industry’s immediate solution to provide the energy necessary to drive the modern economy, especially emerging needs for data centers driving artificial intelligence applications. Such fuels are designed to produce more energy, burning longer inside conventional light water reactors.

To answer those dual needs, Westinghouse is one of several companies that, under cooperative agreements with the U.S. Department of Energy, developed fuel with accident-tolerant and high-burnup characteristics. After three rounds of irradiation testing at INL, the Westinghouse fuel was inserted in the commercial reactor in 2019.

Oak Ridge National Laboratory received a shipment of the Westinghouse fuel in 2021 for examination and testing in its Severe Accident Test Station, and INL received its first shipment of the fuel in December 2023.

Examining the fuel

Shielded by the four-foot thick, multipaned windows at INL’s Hot Fuels Examination Facility, operators rely on an odd mixture of high and low tech to perform nondestructive and destructive examinations of the fuel and prepare it for irradiation tests. Alongside lasers and gamma spectrometers, there are rows of open-ended wrenches hanging on the wall.

Before Cox and Maupin make the fine cut to the fuel rods with their diamond saw, operators use a regular pipe cutter—the kind a plumber might use—to make a rough cut. The cutting locations are marked using a Sharpie wrapped in tape, which makes the marker easier to grip with the clawlike remote manipulators.

A thorough examination of the fuel rods, which have already sustained 54 months of neutron damage in the commercial reactor, establishes baseline data. This data will help researchers better understand how next-generation fuels might behave in a reactor accident.

“We perform extensive nondestructive examination so we can gather as much information as possible before starting destructive examinations,” said Aaron Colldeweih, lead investigator for the postirradiation examination of the Westinghouse fuel. “We’ll do visual exams, gamma spectrometry, and we’ll closely measure the diameter of the rods. —That will give us an idea of how the cladding and the fuel interacted with each other during irradiation.”

As the U-235 in the fuel undergoes fission in the reactor, various radionuclides are created. Gamma spectrometry is used to detect the gamma radiation field being emitted from the rod at incremental steps along its entire length.

“We’re measuring gamma emissions at a very high resolution along the length of the rod,” Colldeweih said. “That allows us to define the location of the physical features that were part of the fuel assembly, like spacer grids or, inside the rod, like individual fuel pellets. This information helps guide us on where we want to cut the fuel rod for destructive examinations.”

Because each radionuclide emits a unique signature, the data also provide a glimpse into the irradiation history of the rod and the chemical behavior of the specific fission product species created by the fission process.

Another examination called profilometry closely measures surface dimensions of the rod. When those data are compared with the fuel rod’s original measurements, researchers can calculate how much the cladding dimensions changed over the lifetime of the fuel, where the cladding initially shrinks down around the fuel pellets and then expands as the fuel swells (primarily due to internal gas pressure). The internal pressure increases throughout irradiation as gaseous fission products accumulate inside the fuel rod. This measurement is used to estimate the remaining margin to failure.

Segmentation and destructive tests

Now the fuel rod is ready for segmentation and destructive tests, including irradiation tests, in the Advanced Test Reactor (ATR) or in the Transient Reactor Test (TREAT) Facility at INL’s Materials and Fuels Complex.

Using a laser, researchers make a tiny puncture in the cladding to sample the fission gases that formed during irradiation and directly measure the gas pressure and the free volume within the rod. Samples of the gas are collected to perform compositional analysis.

“We start with13-foot-long fuel rods, and we need to get them into segments of about 10 inches for future TREAT tests,” Colldeweih said. “It involves fine cutting with very high accuracy. We also take a number of different samples from each fuel rod for isotopic and hydrogen measurements that ultimately support additional experiments required to understand material behavior and properties relevant to future fuel rod design.”

Experts then cut away sections of the cladding and fuel and prepare the samples for examination under conventional microscopes and electron microscopes at INL.

The microscopes allow researchers to see damage and changes that occurred during irradiation, such as cracks and pores in the fuel. The microscopes can also help identify where hydrogen-bearing phases, called hydrides, have precipitated in the cladding. These hydrides, which mostly arise from corrosion of the cladding in the reactor, can cause the cladding to become embrittled. Measuring that hydrogen provides important clues to understand damage incurred in the reactor core.

“There are interesting regions in the shape of rings to indicate how the fuel’s microstructure has evolved,” Colldeweih said. “It’s what happens to the fuel after it has been irradiated for a length of time, and it’s important for understanding how the fuel will behave in an accident scenario—and for understanding what’s going to happen in TREAT.”

Other samples of the cladding are subjected to mechanical tests that push, pull, or expand the sample to its breaking point to test its strength. “We will mill out different dog-bone shapes from the cladding wall, and we can test the sample strength in different directions,” Colldeweih said. “We perform axial-tension tests and hoop-tension tests, which allow us to measure the mechanical response of the material.”

Researchers also take density measurements of the fuel to learn how fission gases—which cause tiny bubbles and create porosity—and thermal expansion have changed the fuel’s density. “We want to know if the density changed from its nominal value and how that affects its performance and accident tolerance,” Colldeweih said.

The ultimate test: Accident conditions in TREAT

Much like the automobile industry, safety features must be demonstrated under extreme conditions to validate their performance. The hot cell examinations ultimately culminate in the nuclear equivalent of ramming several sports cars into a concrete wall.

In this case, sports cars are advanced fuel rods and the concrete wall is TREAT: a nuclear test reactor designed to deliver a short, high-intensity burst of radiation to push nuclear fuels and materials to their limit and beyond. In other words, TREAT is designed to break things under accident scenarios, much like crash testing a car.

“We want to know if a fuel will fail in a reactor and how it will fail,” said Jordan Argyle, one of the researchers involved in refabricating the fuel rods for the tests. “The only way to find out is to do the testing.”

“This may be the coolest thing happening at [INL’s Materials and Fuels Complex],” Argyle continued.

Fabricating rodlets for accident testing

Once experts have segmented the 13-foot fuel rods and taken samples of the fuel and the cladding, each segment must be reassembled into a sealed rodlet for insertion into the transient water irradiation system (TWIST) capsule—an experiment capsule designed for specialized testing. The process requires a high degree of precision for the rodlet to perform like a normal fuel rod.

“The purpose of refabrication is to get it as prototypical as possible to a real fuel rod,” Argyle said.

Completing each rodlet takes a team of dedicated experts. “There are probably 26 people, from receipt to welding, who are integral to this process,” Argyle said. “If you took any one of those people away, we wouldn’t be able to do this.”

Once the rodlet is assembled and tested in the rodlet holder, the TWIST device is assembled, and the rodlet is loaded inside. This integrated configuration—the TWIST capsule—is then ready for the experiment.

To ensure safety, once the TWIST module is assembled, it is placed inside an irradiation system known as Big-BUSTER (broad use specimen transient experiment rig). The 9-foot-long zirconium-niobium alloy pipe provides the safety pressure boundary used for irradiation testing in TREAT.

The experiment, housed in Big-BUSTER, is transferred to TREAT via a shielded cask. The experiment is then inserted into TREAT and all instrumentation is checked out to determine functionality after shipping and handling. This work could be completed as early as this month.

INL technicians will fabricate three more rodlets in the next fiscal year, with two to six rodlets fabricated each year thereafter for the foreseeable future.

“The sheer volume of data we’re able to collect has a lot of people interested,” Argyle said.

Mimicking a LOCA

One type of test performed in the TWIST device is a loss-of-coolant accident (LOCA), a type of postulated accident in a commercial power plant where the water drains out of reactor core due to a highly unlikely severe disruption of its mechanical systems. In this scenario, the core could heat up, resulting in several potential consequences, including a release of radioactivity due to fuel damage (popularly referred to as a meltdown). Next-generation nuclear fuels like the ATF rods from Westinghouse are designed to minimize the chance and severity of these consequences.

TWIST looks a little like a barbell tipped on its end and features two chambers. The top chamber—the TWIST capsule—holds nuclear fuel, water, and instrumentation.

During a LOCA test, TREAT reactor operators bombard the fuel, surrounded by water, in the TWIST capsule with an intense burst of radiation for around 30 seconds. This burst of radiation establishes thermal conditions like a normal operating fuel pin in a commercial reactor. Then, the accident event simulation starts by draining the water into an empty tank at the bottom chamber of the TWIST capsule.

Operators then subject the fuel rod, minus the water, to a lower-level radiation burst for 100 additional seconds, simulating the fuel rods residual heat generation during an accident. Both the irradiation times and TREAT power levels can be easily modified by the experimenter and are adjusted based on the test’s specific objectives.

Normally, the rodlet increases in temperature until the specimen fails, said Klint Anderson, an experiment design engineer at INL. “In other words, the rod gets really hot,” he said. “This increase in temperature, combined with the pressure differential between the pressurized rod and lower pressure capsule, can cause the cladding to balloon and burst.”

While a loss-of-coolant condition (known as an undercooling scenario) is the more restrictive safety event, the system can also be used to explore other types of events, such as reactivity insertion accidents (known as overpower scenarios). In a reactivity insertion accident test, TREAT operators bombard the submerged fuel rod with a pulse of neutron radiation that simulates sudden removal of a control rod from a commercial reactor core. During this test, the water never drains from the TWIST capsule.

“The water remains at the same water level the whole time,” Anderson said. “We put it in TREAT with a higher power pulse to look at things like the pellet cladding mechanical interaction or sudden pressure spikes from the water quickly boiling.”

Once the experiments are completed in TREAT, the rodlets will be transferred back to the Hot Fuel Examination Facility for disassembly and post-transient examinations, which will occur over the summer and fall of 2025.

Why these tests are important

In the end, “crash testing” these high-burnup and ATF rods will help industry license these rods’ designs for reactors operating across the country. The result is safer, higher-performing nuclear reactors.

The advanced fuel is special because it marks a new era of innovation for commercial LWRs, said Dan Wachs, national technical director of the DOE’s Advanced Fuels Campaign at INL.

“Innovation in the LWR community stagnated for several decades,” he said. “The start of the Accident Tolerant Fuels program really started a reawakening of the desire for innovation in the commercial nuclear reactor sector.”

It took about 10 years to get the initial ideas for advanced fuels to the point where researchers could perform demonstrations in commercial reactors.

“But to really assess them properly, we need to do examinations in our hot cells,” Wachs said.

INL is one of the best and, in some cases, the only place for companies to get data about their fuels to verify their performance and ultimately get approval from the NRC. With the success of the initial experiments in TREAT, the nuclear energy industry will gain confidence that its innovations can be commercialized.

“By examining and testing fuels from Westinghouse and other manufacturers, INL researchers are providing industry with a clear pathway to get their fuels to market,” Wachs said. “Success just generates an appetite for more innovation. Our performance in this area will ultimately increase investment in new nuclear fuels.”

Cory Hatch is a science writer for Idaho National Laboratory.

About Idaho National Laboratory

Battelle Energy Alliance manages INL for the U.S. Department of Energy’s Office of Nuclear Energy. INL is the nation’s center for nuclear energy research and development and also performs research in each of DOE’s strategic goal areas: energy, national security, science and the environment. For more information, visit inl.gov. Follow us on social media: Facebook, Instagram, LinkedIn, and X.