An accident in February has shut down a diminutive nuclear reactor at the National Institute of Standards and Technology (NIST)—and has cost the United States, at least temporarily, almost half of its capacity to study materials with beams of neutrons. In the incident, a fuel rod in the 52-year-old reactor at the NIST Center for Neutron Research (NCNR) in Gaithersburg, Maryland, overheated and partially melted, releasing a small amount of radiation. The public was never in danger, NIST says. But the reactor won’t restart until April 2022 at the earliest, leaving thousands of users scrambling to find beamtime elsewhere.
“We fully intend to restart the reactor,” says Robert Dimeo, director of NCNR. “We’re only going to do it when we’re confident that it is safe to do so.”
In the meantime, the shutdown is “a huge problem,” says Robert Birgeneau, a condensed matter physicist at the University of California (UC), Berkeley, whose team has used the neutron source to study exotic iron-based superconductors. Michael Hore, a polymer physicist at Case Western Reserve University, says the shutdown will set one of his projects back 1 or 2 years. “It’s not like we can go somewhere else,” he says. “There are only so many instruments and they’re all oversubscribed as it is.”
Liberated when atomic nuclei split, neutrons can probe materials in ways x-rays cannot. Whereas x-rays interact with both the electrons and the atomic nuclei in a sample, uncharged neutrons bounce off only the nuclei, providing a complementary probe of a material’s atomic-scale structure. They can also penetrate materials that x-rays cannot, enabling researchers to image the interiors of big objects such as a running engine or a steel girder. Because neutrons act like little magnets, they can reveal the atomic-scale patterns of magnetism within materials.
Hore uses neutrons to probe the dynamics of polymers, the chainlike molecules in plastics and many biological materials. By replacing hydrogen in a specific part of a molecule with deuterium—which neutrons are more likely to bounce off—researchers can track that bit of the molecule in a jumble of similar material, Hore says. Neutrons also do less damage to delicate samples than x-rays, says Tonya Kuhl, a chemical engineer at UC Davis. “You can just blast biological samples with neutrons and it’s not an issue,” she says.
The NCNR reactor generates just 20 megawatts of heat—less than 1% as much as a typical power reactor—and is the smallest of the three main neutron sources in the United States. The other two are the 85-megawatt High Flux Isotope Reactor (HFIR) at the Department of Energy’s (DOE’s) Oak Ridge National Laboratory and the $1.4 billion Spallation Neutron Source (SNS), also at Oak Ridge, which fires protons from an accelerator into a target to blast out pulses of neutrons.
Nevertheless, NCNR’s 29 spectrometers, imagers, and other instruments nearly equal the total at HFIR and SNS together, and the lab serves more than 2600 researchers each year. Scientists credit the lab’s staff of 194 with its success. “In some odd way, having a lower power reactor has forced them to be more creative,” Birgeneau says.
On 3 February, research at NCNR came to an abrupt halt. At 8 a.m. that Wednesday, operators began to restart the reactor after a refueling stop, according to a report NIST submitted to the Nuclear Regulatory Commission (NRC) last month. At 9:07 a.m., the reactor’s power plummeted from 15 megawatts to 7 megawatts. Within 1 minute, monitors sensed radiation in the reactor’s reinforced concrete confinement building. At 9:09 a.m., automated systems shut down the reactor and sealed the building.
Recognizing that the reactor’s uranium fuel may have been damaged, the operator immediately issued an alert, the most urgent alarm a research facility can sound, says Scott Burnell, an NRC spokesperson. Melting fuel is rare at research reactors, says Dale Klein, a nuclear engineer at the University of Texas, Austin, who chaired NRC from 2006 to 2009. “I used to be the director of our research reactor,” he says, “and my worst nightmare was having a fuel failure.”
Still, the safety systems worked. The 10 staff then in the confinement building received a radiation dose roughly equal to that from a computerized tomography scan, NIST’s website says. Only trace amounts of three radioactive isotopes escaped the building, and radiation at the boundary of NIST’s 2.34-square-kilometer campus never increased above background levels, according to the website.
NIST traced the accident’s roots to a mistake in refueling the reactor a month earlier. Typically, operators replace the oldest four of 30 rodlike fuel elements and rearrange the others. Working by feel, they use a special tool to lock each element in place by twisting it into a spring-loaded latching mechanism, and an inexperienced crew failed to secure one element. Circulating cooling water then lifted it out of position, impeding the flow around it. When the reactor restarted, the element overheated and part of its aluminum cladding melted.
Staff turnover and institutional culture played a role in the accident, according to the NIST report. In 2011, nine of NCNR’s 21 reactor operators had more than 20 years of experience. Now, just three of 22 do. NIST also relied too much on hands-on training and too little on explicit procedures to guide operators, the report says. “We didn’t make that transition effectively from a skills-based workforce to a knowledge-based workforce,” Dimeo says.
The reactor suffered no damage beyond the overheated fuel element, Dimeo says. Workers have removed all but three elements and now plan to clear debris from the core and purify the deuterated cooling water, he says, which means the reactor cannot restart before April. However, NCNR needs NRC’s explicit permission to restart it, and lab officials must convince the commission they have eliminated the causes of the accident, which could take longer, Burnell says. Still, he says, “The agency absolutely understands the importance of the facility, and we are going to do the most prompt and thorough review we can.”
A radiation release led to the loss of another neutron source 2 decades ago. In 1996, researchers found tritium in the groundwater near the reactor-based source at DOE’s Brookhaven National Laboratory. Although the leak was small and confined to the Brookhaven campus, public outcry led DOE to permanently shutter the reactor in 1999.
NIST hopes openness will forestall a similar outcry. It held a virtual meeting with residents of the suburbs surrounding the lab on 10 February, says Jennifer Huergo, a NIST spokesperson. NIST is also posting its communications with NRC to its website and updating residents by email, Huergo says. “As soon as we get questions, we respond as quickly as we can.” The shutdown comes as the United States has lost the lead in neutron resources. DOE shut down an accelerator-based neutron source at Argonne National Laboratory in 2008 and, in 2015, stopped supporting basic research at an accelerator-based source still running at Los Alamos National Laboratory. “Europe has us beat by a factor of three, Asia has us beat by a factor of two,” Kuhl says. “It’d be catastrophic to lose more capabilities.”
Even before the accident, researchers fretted about the reactor-based neutron sources. Both NCNR’s reactor and 55-year-old HFIR run on “highly enriched” fuel, in which more than 90% of the uranium is the fissile isotope uranium-235. In principle, such fuel could be fashioned into a nuclear bomb. As part of U.S. antiproliferation efforts, both reactors are supposed to be retrofitted to use a fuel containing less than 20% uranium-235 when it becomes available, perhaps in the 2030s. In recent years, expert panels have also advised NIST and DOE to plan to replace the aging reactors.
Planning needs to start now because it will take 10 to 20 years to build a new reactor, Birgeneau says. NCNR’s reactor is licensed through 2029, and Dimeo envisions renewing its license even as NIST explores replacing it. “From our perspective,” he says, “neutrons are here to stay at NIST.”
Source: Science Mag