Argonne National Lab: Making nuclear research reactors more secure

This conversion effort has been in process for decades. The Reduced Enrichment for Research and Test Reactors (RERTR) program was first initiated by the Department of Energy in 1978. Working with partner countries and institutions, the DOE’s National Nuclear Security Administration has converted more than 70 research reactors from HEU to LEU, and more than 30 other HEU-fueled reactors have been confirmed as shut down.

The conversion task is a complex one that requires a blend of rigorous science and engineering, collaboration, and perseverance. Using LEU makes the world's research reactors safer and more secure, but there is no room for compromise when it comes to reactor performance.

Fuels for a more secure global future

The drive to convert research reactors to LEU has its origins in the Nuclear Non-Proliferation Act, which was signed into law during the Carter administration. U-235, the fissionable isotope of uranium, makes up HEU—but also has the potential to be misused as a weapon rather than an energy source. In contrast, LEU fuels contain less than 20 percent U-235 (HEU fuels contain 90–93 percent U-235).

Reactors currently operating on HEU fuel need the same amount of U-235 to maintain their research missions—just at a lower percentage of the total uranium. That means a lot more U-238 (an isotope that absorbs neutrons better at higher temperatures than does U-235 and helps stabilize a reactor’s temperature) needs to be packed into the fuel.

Nuclear engineers must figure out a way to achieve this balancing act without fundamentally changing a reactor’s layout. Argonne National Laboratory is part of this effort to upgrade research reactors to run on LEU fuels. Scientists at Argonne and other national labs are working to develop, test, and deploy new fuels that can bolster global security while maintaining the integrity of reactors’ research missions.

"Reactor facilities don't want to fully redesign their entire core. They just want to swap out the fuel," said Laura Jamison, a principal nuclear engineer at Argonne who works on fuels for global reactor conversion efforts. "As we try to convert reactors from HEU to LEU, we basically have to cram more uranium into the same space."

In the United States and Europe, high-power research reactors like the University of Missouri Research Reactor (MURR) represent a significant opportunity—and a significant challenge. Such reactors are some of the biggest users of HEU, so switching them to LEU would take a lot of high-risk material out of circulation. At the same time, however, their large power requirements make the reactors more difficult to retrofit, because they need lots of uranium packed into a similar fuel design—the fuel density issue Jamison referenced.

Four high-performance research reactors in the United States (MURR, the Massachusetts Institute of Technology Nuclear Research Reactor, the Advanced Test Reactor at Idaho National Laboratory and the National Bureau of Standards Reactor) are working toward conversion to a uranium-molybdenum fuel that has the highest uranium density of any fuel to date for research reactors. A fifth, the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory, is exploring the use of a uranium-silicide fuel.

While LEU fuels fall within broad classes, every reactor conversion is a bespoke project. "Each [reactor] has a slightly different geometry in terms of the size and thickness of the plates and the exact form of the overall fuel element itself," Jamison says. "Research reactors are fun and challenging in that aspect. They are all unique."

Pushing the boundaries of nuclear fuel research

Argonne’s Advanced Photon Source is undergoing a comprehensive upgrade to replace its original electron storage ring with a new, state-of-the-art accelerator.

Argonne can examine at the atomic level the changes fuel undergoes in the extreme environment of a nuclear reactor. To produce and inspect irradiated samples of fuels, Jamison and her fellow engineers use the Argonne Tandem Linac Accelerator System (ATLAS) and the Advanced Photon Source (APS), DOE Office of Science user facilities with a fundamental research mission.

With ATLAS, scientists can irradiate fuel with ions to simulate what occurs in a reactor, rather than doing neutron radiation tests (which are often necessary but can take several years and cost millions of dollars). Ion irradiation at ATLAS mimics neutron radiation by simulating the damage resulting from fission. When a uranium atom splits, most of the resulting energy is imparted into fission fragments that inflict ballistic damage as they interact at high speed with surrounding atoms in the fuel.

"If you're trying to narrow down fabrication processes or look at single effects on a fuel, ion irradiation can be used," Jamison said.

With the APS, powerful X-ray beams allow scientists to see how irradiated fuel performs and responds to the imparted damage.

"The APS is great because we can take a fuel sample and nondestructively create a 3D view of what the fuel looks like on the interior," Jamison said. "If we want to go back and take different measurements, we can do that, and the sample is still intact."

Technicians working on the upgrade of the Advanced Photon Source. When complete, the upgraded facility will enable more powerful X-ray beams.

The upgrade to the APS, which was begun in July, will enable more-powerful X-ray beams and includes the addition of the Activated Materials Lab. Now being built adjacent to the APS, the Activated Materials Lab will make it possible to study radioactive materials and fuels on site, rather than sending them to a separate location if they need to be repositioned for an experiment, for example.

"With the upgrade, we'll be able to look at larger pieces of material and get an even broader view of the structure," Jamison said.

Both uranium-molybdenum and uranium-silicide fuels have been or are currently in operation. In particular, uranium-silicide LEU has been used in the conversion of research reactors since the late 1980s. Ongoing development has advanced these fuel systems in terms of fuel loading and power level. Most uranium-molybdenum fuel in use today is an HEU form. Researchers are working to develop an LEU version of that fuel. Jamison's group at Argonne is conducting research that will allow for safer and more secure operations of existing research reactors and will enable the next generation to run on effective, powerful LEU fuels.

"With these high-performance research reactors, we are pushing the boundaries of where these fuel forms have been used previously or developing new ones," Jamison said. "That opens the design space for future reactor builds as well."

Christina Nunez is an independent writer who specializes in science, climate, and innovation. She writes and edits content for four U.S. national laboratories under a contract with the Department of Energy.