“Now is the time” for more ATR capacity: A conversation with Lightbridge

May 28, 2021, 12:06PMNuclear News
A photo of a prototype Lightbridge fuel assembly. (Photo: Lightbridge)

Operators at the Advanced Test Reactor at Idaho National Laboratory have begun a nine-month outage to perform a core internals changeout. When the ATR is restarted in early 2022, the top head closure plate of the pressurized water test reactor will have new access points that could permit the irradiation of more fuel and material samples in the reactor’s high-flux neutron conditions.

Advanced Test Reactor core. (Photo: INL)

That’s good news for researchers and nuclear fuel developers. The neutrons produced by the aging ATR are in high demand, especially since the permanent shutdown of Norway’s Halden reactor in June 2018. New access points are a necessary first step to expand the ATR’s capacity, but more work will be required before one or more new test loops become operational.

Lightbridge Corporation president and chief executive officer Seth Grae is eager to see near-term federal investment to take advantage of the new access points and rapidly expand the ATR’s testing capacity to allow for accelerated testing of advanced fuels.

Nuclear News spoke with Grae and other Lightbridge representatives, including Aaron Totemeier, vice president of fuel cycle technology and fuel fabrication, and James Fornof, vice president of nuclear program management, about their plans and requirements for fuel testing.

It’s all in the timing: What makes this the right time to talk about funding, Grae said, “is the unique intersection between the federal funding cycle and the 10-year core internals changeout.” Looking for certainty about the Department of Energy’s plans to implement new test loops, Lightbridge is encouraging increased federal funding on the order of $35 million.

“Lightbridge and other nuclear vendors can make use of these potentially new capabilities in our R&D efforts almost immediately once available,” Grae said. “The lack of these added loops has become a choke point for advanced nuclear development in the United States for competition against Russia and China. Having something of a queue for who gets into the existing loop in the ATR and for how long and how much fuel, I think that it is very much in the nation’s interest where for $35 million we could triple the capacity of these flow loops. I really do think that now is the time.

“Our $35 million estimate comes from conversations we have had with numerous research reactor facilities, including internationally,” Grae added. “Of course, the ATR experts at INL will need to validate our estimate.”

Lightbridge’s three-lobed rod variant. (Graphic: Lightbridge)

Small modular reactors: Lightbridge predicts that meeting the global warming targets set by the Intergovernmental Panel on Climate Change could require as much as a quarter of future clean energy production to come from nuclear sources.

“We think a lot of that would come from light water reactor SMRs, so we’re adding a focus on SMRS in what we’re doing, because if they can be produced in factories and they can be produced at shipyards, they can be produced ultimately by the thousands and be usable in over one hundred countries,” Grae said. “I think that’s the only realistic way to get to nuclear having any significant climate change impact.”

Lightbridge fuel could be used in existing large water–cooled reactors and, on a smaller scale, in potential small modular LWR fleets in the future. Lightbridge announced on May 11 that it had successfully manufactured six-foot fuel rods using surrogate materials—sized for a small modular LWR such as those designed by GE Hitachi, NuScale Power, and Holtec.

“What we’re learning in our focus on SMRs is directly applicable to the large reactor fleet as well,” Fornof said.

Work underway: Lightbridge’s current work includes designing a drop-in capsule experiment for testing in the ATR, work that is supported by a voucher and a cooperative research and development agreement awarded through the Gateway for Accelerated Innovation in Nuclear. Design work is expected to be completed before the fourth quarter of 2021, and the test capsule could be inserted in late 2022 or early 2023. The experiment will yield information about the basic thermophysical properties and irradiation behavior of Lightbridge fuel, including microstructure evolution, thermal conductivity, and irradiation-induced swelling as a function of burnup.

“This type of test has limited heat removal from the experiment, and prototypic commercial reactor conditions can’t be realized,” Grae said. “While drop-in tests provide valuable data, developers need flow-loop tests with prototypic conditions to demonstrate fuel performance and form the technical basis for demonstration in commercial reactors.”

Go with the flow: “A flow loop experiment will tell us much more,” Grae said. “If we knew that space in a flow loop was available, we could probably be ready for an experiment in a couple of years with INL’s help. Ultimately, we will need flow loops that can simulate the conditions of the commercial fleet, including PWRs, BWRs, and the water-cooled SMRs under development.”

Totemeier explained that the underlying fuel behavior of advanced fuel technologies is going to be significantly different from pellets in cladding. “We need to rely on physical testing for a lot of that,” he said. “We can, of course, use advanced modeling and simulation to cover a lot of things that we couldn’t do 20 to 30 years ago, but the Nuclear Regulatory Commission and the industry still like to have some type of confirmatory physical testing for a lot of the call of the upper limits of what fuel would do. Having flow tests to be able to simulate those kinds of conditions is really the only way to do that.”

Fornof added, “It’s a sequential process. It’s a little bit of chicken and egg because we have to know that we can get into the flow loop before we can design for the flow loop. I think that's one of the reasons why we see this additional capacity at the ATR as being so important, because it takes away a lot of that uncertainty.”

According to INL spokesperson Joe Campbell, “At this time, one additional loop is planned, which is scheduled to come on line as early as 2023. This will enable ATR to provide even more capability and flexibility to meet the growing demand for irradiation test space caused by the Halden shutdown.” The planned loop would be able to simulate both pressurized water reactor and boiling water reactor conditions.

Fuel composition: Lightbridge’s fuel has three components that are bonded to form a helical multi-lobe fuel rod, which is about half zirconium and half uranium by weight. The displacer contains burnable poison alloys for neutronics control. The fuel core is uranium zirconium alloy with high thermal conductivity and low irradiation-induced swelling, and the metallurgically bonded barrier consists of corrosion-resistant zirconium-niobium alloy with variable thickness to increase protection at the lobe tips.

“The metal gets us the superior thermal conductivity,” Totemeier said. “It gets us a geometry where the fuel rods touch about once every 10 centimeters, with the rods next to them, eliminating the spacer grids. It gets us more surface area and a shorter path for heat to escape. None of this could be done in a uranium dioxide fuel.”

Four major U.S. nuclear utilities—Dominion Energy, Duke Energy, Exelon Generation, and Southern Nuclear—have representation on Lightbridge’s Nuclear Utility Fuel Advisory Board, and Lightbridge believes they could be potential customers for a fuel that permits uprates and/or increased time between outages for existing LWRs. New-build reactors could see even greater power uprates relative to today’s LWRs.

“I think that Lightbridge fuel was the only feasible way to go to the higher assays of HALEU,” Grae said. “In the current LWRs, we’re talking about 19.75 percent weight enrichment for our fuel.”

Before advanced fuels can be tested in real-world conditions, they need to be tested in neutron flux conditions. “We think that if these flow loops are added to the Advanced Test Reactor, it would help us with our schedule,” Grae said. “We would hope that in the late 2020s we would have fuel test assemblies in a reactor generating electricity.”

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