The 40-year effort to make research reactors safer and more secure has led to the conversion of 71 reactors worldwide from HEU fuel to LEU.
In late 2018, Nigeria’s sole operating nuclear research reactor, NIRR-1, switched to a safer uranium fuel. Coming just 18 months on the heels of a celebrated conversion in Ghana, the NIRR-1 reboot passed without much fanfare. However, the switch marked an important global milestone: NIRR-1 was the last of Africa’s 11 operating research reactors to run on high-enriched uranium fuel.
The 40-year effort to make research reactors safer and more secure by replacing HEU fuel with low-enriched uranium is marked by a succession of quiet but immeasurably significant milestones like these. Before Africa, a team of engineers from many organizations, including the U.S. Department of Energy’s Argonne National Laboratory, concluded its conversion work in South America and Australia. Worldwide, 71 reactors in nearly 40 countries have undergone conversions to LEU, defined as less than 20 percent uranium-235. Another 31 research reactors have been permanently shut down.
Although they vary widely in both size and mission, the world’s operating nuclear research reactors are indispensable for a variety of reasons. In Africa, they might help detect soil contamination from mining operations or support the development of nuclear power, a potential way to meet growing energy demand without planet-warming carbon emissions. Larger reactors in Australia, Europe, South Africa, and the United States produce essential radioisotopes used in medicines and diagnostic scans. Research reactors even enabled hybrid electric cars, playing a key role in making the high-power silicon semiconductors that transfer power to their motors.
HEU fueled the early development of many research reactors into potent resources for society, but today, LEU can support most of their requirements without the risk of the material being weaponized. Since the 1970s, a coordinated international effort to remove HEU from research reactors has brought together scientists, policymakers, regulators, and industry.
On the technical side, Argonne engineers, along with those at several other DOE facilities, have been tackling some of the thorniest issues related to these complex projects. And as the effort enters its fifth decade, they are gearing up to meet some of their biggest challenges yet.
Small, but mighty
Nigeria’s NIRR-1 is one of seven operating miniature neutron source reactors (MNSR) worldwide. Every research reactor is unique, but they do exist in families. The MNSRs are one family; another is the TRIGA class of reactors, all of which have been fully converted to LEU.
“Within each family,” said John Stevens, a nuclear engineer who leads Argonne’s reactor material management programs, “we can square away all the technical, logistical, and partnership matters, and then proceed through their conversions as a group—when the stars stay aligned.” The compact, Chinese-built MNSRs operate at low power, with fuel rods less than one foot high. That makes them especially attractive candidates for LEU conversion. The International Atomic Energy Agency began analyzing MNSR conversion in 2006.
In 2016, China’s prototype MNSR became the first of its type to convert to LEU, followed by the reactors in Ghana and Nigeria in 2017 and 2018, respectively. The remaining unconverted MNSRs are operating in China, Pakistan, Syria, and Iran. As with every other international reactor conversion, a host of factors—from political climate to technical challenges to funding—need to align before the conversion projects can begin. All of these factors have an impact on the likelihood and timing of any given reactor’s switch to LEU, since each reactor is unique.
“One issue is whether or not the responsible states will work to develop the fuels necessary to convert the reactors,” Stevens said. In Russia, for example, several HEU reactors require fuel development that has not yet begun.
Serving “peaceful pursuits”
The United States’ effort to convert the world’s HEU reactors began in 1978 with the Reduced Enrichment for Research and Test Reactors (RERTR) program. Four years earlier, India had conducted its first nuclear bomb test, and Pakistan accelerated the development of its own nuclear weapons program in response. The need to stop the spread of nuclear weapons was clear.
Less clear was what to do about the world’s research reactors, most of them too scientifically valuable to shut down. Hundreds of them had been built through the 1950s and 1960s after U.S. President Dwight Eisenhower’s famous “Atoms for Peace” speech to the United Nations in 1953 in which he proposed that fissionable material “be allocated to serve the peaceful pursuits of mankind.”
Research and test reactors were subsequently built with uranium and expertise from the United States, Russia, and China, scattered across universities and research institutions in more than 40 countries.
“People were not focused on the fact that the highly enriched material could, in principle, be used for weapons,” Stevens said. “They instead focused on the fact that the HEU-fueled systems, when compared to the LEU fuels available at that time, were more efficient as scientific machines.”
The missions for such reactors quickly expanded: energy production, space exploration, new materials, cancer treatments, medical diagnostic scans, explosives detection in airports—these and other activities all now rely on their capabilities. Their HEU fuel use ranges from about 1 kilogram for 20 years of MNSR operation to more than 100 kilograms each year in larger, more powerful reactors.
In the first two decades of the RERTR program, a total of 30 reactors were converted from HEU to LEU, 10 in the United States as a matter of leading by example.
The global interest in conversion changed in 2001. The September 11 attacks lent a new urgency to anti-terrorism efforts, and three years later, the RERTR program became part of the Global Threat Reduction Initiative (GTRI), a comprehensive plan to secure, remove, relocate, or dispose of radioactive materials as expeditiously as possible. Conversion schedules were accelerated, and so was fuel development for reactors that could not be converted with existing fuels.
The initiative also made it possible for low-power reactors at universities, for example, to give up their long-lasting and expensive HEU fuel supplies by compensating them for the fuel, making conversion to LEU economically feasible. Since the DOE’s National Nuclear Security Administration established GTRI and its successor programs, now housed in the NNSA’s Office of Material Management and Minimization, 41 reactors have been converted, and 31 reactors have been shut down. The progress of 72 metrics in the recent decades of the effort occurred at twice the pace of the first two decades.
A process with many players
Once the partners have been identified and funding for a reactor conversion has been secured, the technical problem-solving begins. The reactor’s new LEU fuel and reactor core must be designed, keeping the research mission requirements and safety in mind. Test elements must be fabricated, demonstrated, and analyzed post-irradiation. Fuel specifications are completed, and the fuel is qualified. The licensing basis documents, including the safety analysis report, must be revised. All the while, regulators must be involved for interim and final approval.
Whether the conversion is international or domestic, any one of those steps can take years, which is why the entire process can take more than a decade to complete. And as the reactor conversion program clears more and more reactors of HEU, the degree of difficulty increases. Technical support is provided by a team of U.S. national laboratories, including Argonne, Pacific Northwest, Idaho, Savannah River, Brookhaven, Sandia, Los Alamos, and Oak Ridge. The Y-12 National Security Complex has also been involved, providing uranium for fuel fabrication and coordinating fresh fuel transportation.
“Our job at the national laboratories, along with our international partners, is to make sure that we have the technical solutions necessary for reactor conversion whenever the political opportunities arise,” Stevens said. “The partnerships, both at home and abroad, are fundamental to our success.”
To make up for the fact that the content of U-235 in LEU is low, researchers needed to find a way to have a higher fuel density overall in the same space. To do that, they changed the uranium alloy in order to achieve a higher U-235 density, even with low enrichment. Argonne and Oak Ridge National Laboratory developed a uranium silicide (U3Si2) dispersion fuel that became the standard for those early reactor conversions.
“The early conversions were machines that were less optimized, so the ability to maintain their mission was more straightforward,” Stevens said.
By the end of the 1990s, most of the reactors capable of switching to the qualified U3Si2 fuel had been converted. But many were unable to use that fuel without unacceptable compromises in performance. Fuel development for those reactors has since focused on uranium molybdenum (U-Mo) fuels that could provide the necessary density.
“We’ve been working for decades on the next high-density fuels,” Stevens said. The fundamental challenge in this effort is to change as little as possible about the fuel and reactor geometry while still allowing the reactor to fulfill its mission with LEU. When the fuel changes, so does the power distribution and the way heat is removed.
“The more optimized the system was originally, the harder it is to go in and change the fuel,” Stevens said. “Fortunately, the global team has been effective in finding solutions to such technical challenges.”
In addition to the performance challenge, engineers must ensure the reactor’s safety. Years of experience, along with advanced computing techniques, have made it possible to create safety cases that are both less conservative and more robust. The safety assessment is also a learning opportunity for everyone involved.
“The process of upgrading the safety basis is the point where designers understand the most details about how the machine really works and what its capabilities are,” Stevens said. “We can make sure we’re addressing the real way accidents could be initiated and unfold, and how to mitigate any consequences.”
Currently, engineers at Argonne and other national laboratories are working on more than 20 active conversion projects, including reactors in Japan and the IVG reactor in Kazakhstan, which stand to make the switch to LEU within the next couple of years.
The latter reactor is the third to be converted in Kazakhstan, after the VVR-K and the associated critical facility at the Institute of Nuclear Physics in 2016. Despite the two successfully completed projects in Kazakhstan, the oft-repeated truth, “every reactor is different,” still applies.
“The VVR-K and IVG reactors are both cooled with water,” said Patrick Garner, an Argonne engineer who has racked up many miles traveling to Kazakhstan while working on multiple conversions. “That might be about where the similarities end.”
The VVR-K reactor core has a donut-like hexagon of fuel plates, whereas the IVG reactor core consists of about 15,000 fuel rods that resemble pieces of twisted linguine, each about one-tenth of an inch wide. As a result, the two reactors required completely different design analysis methods. Working with a company in Russia and Idaho National Laboratory, Garner and team fabricated two fuel assemblies that were then put into the IVG reactor and tested off and on for two years. That testing period ended last October, and the fuel is now undergoing post-irradiation inspections.
Fuel testing at the IVG reactor got off to a slow start. Plans required the assemblies to be irradiated 30 times at 6 megawatts, six hours at a time. But IVG had only one cooling system, and it required a one-month cooling period after each cycle. To move the project along, a secondary cooling system was installed to allow testing every week, rather than every month.
“It’s a modification the U.S. paid for in order to get the work done faster,” Garner said. It also provides a benefit, he added, since the reactor now has boosted operational capacity.
Despite the benefit of such modifications, the conversion teams are sometimes met with reluctance from operators who are dealing with a reactor that was designed and analyzed elsewhere—in this case, Russia.
“They know their reactor, they know what it can do,” Garner said. “They’re a little bit unprepared for making changes, because they didn’t design the reactor.”
Garner, who specializes in thermal hydraulics, works with another Argonne engineer, a neutronics expert, on the Kazakhstan projects. They visit three to four times a year, holding meetings with the help of an interpreter.
“We accomplish a lot by being there and sitting across the table,” Garner said. “Going forward, they want to be self-sufficient in being able to analyze their reactor. A lot of our role is helping them get there.”
Training the next generation
In Kazakhstan and other countries with foreign-built reactors, such as Ghana and Nigeria, converting to LEU is a priceless opportunity both to gain that self-sufficiency and train a new generation of nuclear engineers.
“The safety case for every research reactor is owned by the research reactor—nobody else can come in and declare what to do to keep things safe,” Stevens said. “They can get a lot of help, but in the end, they have to own it.”
That means conversion projects often involve multiple training sessions on design philosophy and safety analysis. For the conversions of Ghana and Nigeria’s Chinese MNSRs, operators traveled to the Zero Power Test Facility in Beijing to learn how to load and handle fuel. Other training sessions included dry runs for extracting the old core.
Argonne also hosts extended training courses during which reactor operators and regulators spend several months at a time learning how to analyze the cores for neutronics and thermal hydraulics—key skills for being able to monitor safety and for reactor operators to interact with regulators.
In the case of NIRR-1, the training in Beijing and at Argonne was a large benefit of converting to LEU because of the understanding local operators gained. “We need to own that understanding and know every aspect of the reactor in terms of the material composition and even the dimensions,” said Sunday Jonah, a research professor at Ahmadu Bello University, in Nigeria, where the reactor is located. “This has given us the ability to develop expertise with respect to reactor physics calculations.”
Taking responsibility for their own research reactors also strengthens countries’ pathways toward energy development. “Both Ghana and Nigeria are looking forward to eventually having nuclear power plants in their countries,” said Jim Morman, a lead nuclear engineer at Argonne who worked on the conversions in Africa. “The MNSRs are ideal training grounds, not only for future operators but for their regulators as well.”
On to the “crown jewels”
In addition to low-power reactors like those in Kazakhstan and Japan, scientists are also working on the “crown jewels” of the research reactor fleet, as Stevens calls them: five U.S. high-performance research reactors (HPRRs) and four European high-flux reactors (HFRs). From medical isotope production to materials testing to fundamental research through neutron scattering, these high-power reactors are essential to science and medicine. They are also fuel hungry.
“As the crown jewels, they consume hundreds of kilograms of HEU per year,” Stevens said. “To be able to convert those will be really important in the goal to remove HEU from civilian applications.” All of them are projected to convert by the mid-2030s.
The monolithic U-10Mo fuel that is being deployed for four of the five U.S. HPRRs has the highest density of any fuel yet developed for application in research reactors—a “revolutionary” product, Stevens said. Europe plans a more “evolutionary” approach based on U-Mo dispersion fuels for its HFRs because those reactors do not need as high a density.
Two European HFRs—Belgium’s BR2 and France’s RHF—currently operate using HEU from the United States. That creates a clear path for conversion because of the 1992 “Schumer amendment” to the Energy Policy Act of 1992, which allows HEU exports only to countries that are actively pursuing LEU conversion. The BR2, in particular, serves a critical mission as a producer of molybdenum-99, an isotope used in more than 40,000 medical procedures daily in the United States alone.
“Where these reactors are lacking technical expertise or staff capacity, Argonne can come in and fill that role,” said Jeremy Licht, who leads Argonne’s contributions on HFRs in the European Union. “And where they do have that expertise, we can also perform verification calculations for safety analysis and keep that momentum going forward.”
Other European reactors with fuel supplies from other countries will take a different path. The FRM II reactor near Munich, Germany, for example, currently receives uranium from Russia. The reactor, licensed in 2003, has a curved fuel plate design similar to that of ORNL’s high-performance High Flux Isotope Reactor.
“FRM II is extremely challenging because it’s already a high-density HEU fuel and a very compact core,” Licht said. “So it’s impossible to convert this one to LEU without making changes to the fuel element design.” In such cases, he said, one possible strategy is to accept a drop in the reactor’s performance by 5 or 10 percent but give it a longer life cycle. The reactor might then run a few days longer with each cycle and be able to perform its mission without significant upset to its operations.
In the United States, preliminary reports on three HPRRs have been provided to the Nuclear Regulatory Commission. At the same time, INL and partners, including Argonne, have completed and provided to the NRC a research report on uranium-molybdenum monolithic fuel for the reactors.
“These high-performance research reactors have some of the highest known power densities of any engineered system, and they’ve been highly optimized to perform their mission,” said Argonne’s Erik Wilson, who is leading the U.S. HPRR conversions. “To redesign them is a significant undertaking.”
The high-density, high-performance U-10Mo fuel has performed well in irradiation testing done over the past 10 years, and fuel fabricator BWX Technologies has installed and commissioned a pilot line to produce it. Irradiation testing of commercially produced small-size fuel plates is under way at Idaho’s Advanced Test Reactor.
More milestones ahead
Recently, the reactor conversion program saw yet another major milestone: the production of experimental fuel for Japan’s KUCA reactor. In February of this year, representatives from Kyoto University formally inspected and accepted the LEU fuel at the Framatome plant in France. Next, the fuel gets shipped to the reactor for a series of characterization tests. Production-scale fabrication of the remaining fuel is expected to begin in the months ahead.
With each successful conversion, U.S. national labs add to a growing body of knowledge about LEU that can support the world’s research reactors far into the future as they carry out their crucial missions.
“We’ve now got a lot of experience in terms of what we can do with low-enriched fuel and how we can tailor the spectra,” said Morman, who is working on the KUCA project. “We know how to design cores using LEU to give them a neutron flux to match what they had and not let them lose any capability. We can apply that to new reactor designs as well.”
For the more difficult conversions, more hurdles most certainly lie ahead, but so do more solutions and milestones. For many nuclear engineers, that is a satisfying place to be. Licht remembers his previous work on advanced reactors, where it seemed like he was doing paper study after paper study because nothing was taking off.
“Here, we’re working on real reactors and minimizing usage of HEU around the world,” Licht said. “We have a goal in mind, and we have to just execute the process. That’s something that has been rare in nuclear engineering.”
Christina Nunez is a freelance writer based near Washington, D.C.