Scientists will probably never have enough neutrons available to satisfy their basic research needs. But they have had a lot fewer to work with since the closure of the NIST Center for Neutron Research (NCNR) in Gaithersburg, Maryland, nine months ago. And it’s likely to be several more months before the research reactor that is the source of its neutrons can resume operations.
In the meantime, most researchers whose experiments were in the queue when the NCNR shut down have had their wait for beam time extended. “To say that it’s been a tremendous loss to the neutron user community is putting it mildly,” says Robert Dimeo, NCNR director. “We account for about 40% of US publication output in neutron science, and more than 300 publications from 3000 researchers annually.”
The NCNR is one of just three major neutron user facilities in the US. The two others are located at Oak Ridge National Laboratory. The High Flux Isotope Reactor (HFIR), like the NCNR, is a fission source. The Spallation Neutron Source (SNS) is a proton accelerator that produces pulses of neutrons from a heavy-metal target. Even before the NCNR outage, each of the three was oversubscribed by a factor of two or more, says Dimeo.
The NCNR shutdown occurred on 3 February during a reactor restart, when an operator failed to properly latch a fuel element in the core. The reactor shut down automatically, but elevated temperatures created by the accident produced radiation that caused minor exposure of six control room operators and contamination of the reactor’s primary cooling system. Monitoring of radiation levels outside the building showed that the public received no appreciable dose, the Nuclear Regulatory Commission (NRC) confirmed.
But the cleanup process is on hold until a shipment of special filters arrives, which should be this month. “Then we are hopeful we’ll be able to clean up the primary coolant system relatively quickly, although that is an unknown until we actually do it,” Dimeo says. But NIST also needs to complete a formal incident analysis and develop a corrective action plan for the NRC. “We’re developing a comprehensive set of actions to make sure it never happens again,” Dimeo says.
Neither Dimeo nor the NRC could provide a definitive date for resumption. In a statement, an NRC spokesperson said that in addition to the two reports required, NIST must make a formal request to the commission for authorization to restart the reactor. “We expect the root cause report soon, with the other documents to follow,” the spokesperson said. “The agency continues its inspection of the event, and the findings from that inspection will also factor into the restart decision process.”
The NRC staff has authority to approve restart without having the full commission consider it. Dimeo is hopeful that the NCNR will be operational within a few months.
An unsatiated appetite
Starved of neutrons, some researchers have been analyzing data from previous experiments. Others have switched to using x rays or other probes. “The biggest interruptions are probably going to be for students who require neutron scattering data for their dissertations and theses,” says University of Maryland chemist Efrain Rodriguez, who grew crystals and produced samples for neutron scientists while a postdoc at the NCNR. “In cases where we don’t have neutron data, we’ve had to just move on and see what we can figure out with synchrotron x-ray data.”
Despina Louca, a University of Virginia condensed-matter physicist, says her students are primarily working with in-house instruments, including x-ray machines in her lab and a nearby single-crystal diffractometer. “The basic stuff we can do here. We can’t just sit here waiting; we would be out of business.”
Still, Louca says, it will be difficult for her students to complete their theses without neutrons. “We do a lot of research on magnetic materials, and the neutron is the best probe when it comes to magnetism because of its intrinsic magnetic moment and the very high resolution that neutron techniques provide us with. Because they are highly penetrative, neutrons are also best for determining structures and dynamics in specialized environments, such as those involving magnetic and electric fields.”
Collin Broholm, a condensed-matter physicist at Johns Hopkins University, is within commuting distance of the NCNR. He and his students were frequent users. He notes that neutrons are indispensable for probing a broad range of energy scales, from nanoelectron to electron volts. “You can’t do that even with inelastic x rays. We gravitate toward other tools that access other materials properties and we’ve been able to make some progress, but some things we lack completely until NCNR runs again.”
Broholm says it’s not likely that he’ll hold PhD students back. “We really try to have them graduate at the end of their fifth or sixth year. But they will leave with less hands-on experience,” he says. They will also miss out on the camaraderie and the “buzz” that occurs at the NCNR when scientists rub shoulders and discuss their respective experiments.
The outage has prolonged the hiatus in experimentation that was created by the COVID pandemic. Travel restrictions have prevented US-based scientists from visiting neutron sources abroad. Experiments can be conducted remotely at other neutron sources by sending samples to be handled by local facility scientists and technicians. But remote access has its limitations. “It’s definitely not the same,” says Rodriguez, “because you just can’t run the experiments you want to run.” Adds Louca, “Many experiments are too complex and require the experimenter to be on-site.”
Some of Broholm’s students have managed to get remote beam time at other sources. “We had a very successful experiment at ISIS in the UK a month ago. We sent samples there, they put them on the instruments, and we talked online. We just finished an experiment at SNS for one of my grad students. It was a very important part of his thesis.” The Paul Scherrer Institute in Switzerland also provided some remote-access beam time to his lab. Broholm estimated the success rate for beam-time requests to be similar to the chances of getting an NSF grant. To the extent that they can, the other US sources have been trying to accommodate postdocs whose research must be completed within a couple of years, he notes.
A spokesperson for the Institut Laue–Langevin, Europe’s largest neutron source, says access is limited mostly to the 14 European countries that contribute to its operation; only 5% of beam time is awarded to outside researchers, including US users, through the lab director’s discretionary time allotment. Some NCNR users have had their samples characterized at the University of Missouri’s high-performance research reactor, which has some neutron scattering capability, and at the Australian Nuclear Science and Technology Organisation’s research reactor.
A unique facility
NIST excels at generating and exploiting cold neutrons. They are particularly relevant for structural biologists and researchers who study soft matter, such as polymers and membranes. (See the article by David Hoogerheide, Trevor Forsyth, and Katherine Brown, Physics Today, June 2020, page 36.) Cold neutrons are produced by directing them through a cryogenic gas or liquid to reduce their energy.
Alan Hurd, former director of neutron scattering at the Los Alamos Neutron Science Center (LANSCE), rates NIST as the premier neutron scattering facility in the US and perhaps the world. LANSCE had been a neutron user facility until the Department of Energy’s Office of Science cut off funding for that purpose in 2012. It currently operates principally for nuclear weapons R&D, though some nondefense collaborations take place between lab and outside scientists. In addition to an outstanding radiography setup, Hurd says, the NCNR’s liquid reflectometer can probe thin films and monolayers of amphiphilic molecules that are of interest to drug development companies.
Funded over years primarily by NSF, the NCNR’s experimental hall has been packed with instruments used for soft-matter and bioscience studies. “It’s where NCNR shines as a leader in the US and the world,” says Rodriguez.
“NIST has a long history of embracing cutting-edge ideas for spectrometers to do condensed matter,” says Hurd. “They’ve broken a lot of ground on things like spin-echo techniques, which is like [nuclear magnetic resonance] on the fly.”
The current outage is especially dismaying because experiments at the NCNR will halt again in 2023, when the reactor is scheduled for a year-long closure for installation of a new liquid-deuterium cold neutron source. Dimeo says the upgrade, which is expected to double the flux of long-wavelength neutrons, required years of planning. “It’s a very cost-effective way to increase facility performance,” he says. Several of the neutron guides—optical elements that transport neutrons from the cold source to the instruments—will be upgraded during the 2023 outage to further enhance instrument performance.
Dimeo says NIST plans to stick to the closure date, even if it means the NCNR operates for just a few months next year. “We are absolutely interested in delivering the maximum to the users and believe that will best be done by getting that outage over with.” The installation can’t be moved up to the current outage because of the long lead times required for acquiring some components.
The installation shutdown has backing from users. “It’s really essential that the facility is always marching forward in its capabilities,” says Broholm. Continuous improvements to the source, the guide network, and instrumentation “have kept NCNR fresh and moving into new areas of science.”
The loss of the NCNR highlights the ongoing dearth of neutron facilities, says Louca. “There’s a zero-sum game in the neutron world. When they build a new source, they shut down another.” She points to the closures of the Intense Pulsed Neutron Source at Argonne National Laboratory in 2008 and the loss of LANSCE, which were offset by the SNS, the Oak Ridge facility that opened in 2007. (See the article by Thomas Mason, Physics Today, May 2006, page 44.) A second target station for SNS has been green-lighted for construction, but it won’t be completed for another decade.
Two other North American neutron user facilities were closed in the past several decades. The High Flux Beam Reactor at Brookhaven National Laboratory was shuttered in 1999 because of opposition from the surrounding community that was provoked by a tritium leak. Canada’s neutron user facility was closed in 2018 when the National Research Universal reactor in Chalk River, Ontario, was retired.
A 2018 assessment of the NCNR by the National Academies of Sciences, Engineering, and Medicine (NASEM) said the US has fallen behind Europe and China in the “neutron enterprise” and warned that the US position will erode further as new facilities come on line in those regions and elsewhere. The assessment noted that the NCNR and the HFIR are both more than 50 years old and lack planned successors. Even if there were plans, it would take 15 years or more to build one.
According to the NASEM report, “Closure of either facility would have a major and instantaneous negative impact on U.S. capabilities for developing advanced materials that drive future innovation, as well as important research on fundamental properties of the neutron, like its lifetime or an upper limit on its electric dipole moment.”
An aging source
The NASEM assessment called for planning to begin on a replacement for the NCNR reactor, which was commissioned in 1967. Dimeo says that the reactor’s age played no role in the accident and that the NRC relicensed its operation for 20 years in 2009. Most importantly for users, he says, the reactor has continued to perform reliably, and the recent accident notwithstanding, safety hasn’t been an issue. “When the reliability starts to diminish, we really need to think about a new source.” Nonetheless, NIST has begun a rough preconceptual design for a new reactor, and both an upgrade of the current facility or one built from the ground up would be considered when the time comes.
The NCNR and the HFIR are among the five remaining US reactors that are fueled with highly enriched weapons-grade uranium (HEU). All other US research reactors, and all but a handful of foreign reactors that were supplied with US weapons-grade material, have been converted to low-enriched uranium (LEU), which doesn’t present a proliferation concern. (See Physics Today, April 2016, page 28.) The American Physical Society’s Panel on Public Affairs in 2018 urged the timely conversion of the NCNR and the others, but it added that “any transition from HEU to LEU reactor fuel must not compromise neutron research and engineering capabilities, especially those that cannot be duplicated using spallation sources.”
Alan Kuperman, coordinator of the Nuclear Proliferation Prevention Project at the University of Texas at Austin, says NIST has committed to conversion but has maintained that it will require an ultra-high-density uranium–molybdenum fuel, the development of which has been delayed for decades because of technological challenges. Other high-performance research reactors—including the HFIR, which officials had once said required uranium–molybdenum fuel—have since committed to converting to high-density silicide LEU. That fuel, says Kuperman, has been available since the 1980s.