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NIST Center for Neutron Research.

The drought of experimental neutrons in the US nears its end

22 March 2023

The NIST research reactor will resume operations this spring before shutting down again in about a year for upgrades.

NIST Center for Neutron Research.
The NIST Center for Neutron Research in Gaithersburg, Maryland, has been shut down since an accident in February 2021. Credit: NIST

The Nuclear Regulatory Commission (NRC) has authorized the restart of the NIST Center for Neutron Research (NCNR), more than two years after an accident forced the shutdown of what had been the nation’s most productive source of experimental neutrons. Located in Gaithersburg, Maryland, the NCNR began low-power operation in mid March and is expected to open for experiments in May or June, depending on how calibrations and tests go, says center director Robert Dimeo.

The reactor was shut down in early February 2021 when an operator error during a routine refueling caused one of the 30 fuel elements to become dislodged from its flow of primary coolant and partially melt down (see Physics Today, October 2021, page 26). The incident led to the 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 NRC confirmed.

Since then, the NRC has worked with NCNR officials to establish the root causes of the accident and to verify implementation of the 23 corrective actions that were developed. Among the findings of the regulatory agency was that the facility had inadequate operator training, a lack of management oversight during the refueling process, “a culture of complacency,” written procedures that did not capture all the necessary steps to secure fuel elements, and insufficient enforcement of procedures. The tools and equipment used to determine when fuel elements are properly locked into place were also inadequate, according to the NRC. NIST’s corrective actions for each cause were verified by the NRC before restart authorization. The commission will also enhance its surveillance of reactor operations for an unspecified period.

“We’ve reached this decision after extensive review of the event, NIST’s corrective actions, and additional work the facility has done to ensure safe operation,” said Andrea Veil, director of the NRC’s Office of Nuclear Reactor Regulation, in a 10 March statement. “We’re satisfied this research facility’s important systems and components are ready to go, and we’re satisfied the reactor staff are ready to carry out improved procedures for maintaining safety.”

During the hiatus, scientists have had to cope with the loss of a device that accommodated about 40% of the nation’s neutron-scattering experiments. Efrain Rodriguez, a University of Maryland physicist who explores the crystal and magnetic structures of inorganic materials he makes in his laboratory, says he is excited about the restart. During the shutdown, his students had to move many experiments to the High Flux Isotope Reactor at Oak Ridge National Laboratory in Tennessee. “We were able to get most of what we needed at Oak Ridge,” he says. But because he usually accompanied his students to the NCNR, “an educational component was lost” during the closure.

Since graduate students and postdocs have only a limited time to perform their experiments, many have had to travel to European neutron sources, such as France’s Institut Laue-Langevin and Germany’s FRM-II. Rodriguez says he has heard of some researchers who have gone as far as the Australian Centre for Neutron Scattering to obtain access to neutrons.

The reactor will be operational for about a year to clear a backlog of more than 250 experiments selected in the call for proposals that NIST issued just before the shutdown, Dimeo says. Some proposals also were left over from the suspension of NCNR operations during the start of the COVID pandemic.

After the NCNR works through the backlog, staff will close the facility again to install a new cold neutron source that should nearly double the flux of low-energy neutrons. The installation process is expected to last about a year. The new cold source will enable measurements on small biological samples and viruses as well as time-resolved images that show kinetics, Dimeo says. “Neutron scattering is a flux-starved technique, and any opportunity you have to get more beam on the sample really allows researchers to do better things.”

During the closure, upgrades will also be made to neutron guides, which transport neutrons from the reactor core to the instruments used for experiments. After the upgrade, a new instrument being built in an NSF-funded partnership with the University of Delaware will be commissioned. Known as a neutron spin-echo instrument, it will probe motion in soft materials such as polymers and biomolecules.

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