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To scientists’ relief, key research reactor to restart 2 years after accident

More than 2 years after an accident that caused a small and fleeting release of radiation, a research reactor that serves as a key source of neutrons for studying materials should soon be back online. On 9 March, the U.S. Nuclear Regulatory Commission (NRC) authorized officials at the National Institute of Standards and Technology (NIST) to restart the 54-year-old reactor in Gaithersburg, Maryland, which prior to the accident supported nearly half of all neutron-scattering research in the United States. The tiny reactor will come on slowly over the next few months, NIST officials say.

“It’s fantastic news,” says Claire White, a materials scientist at Princeton University and chair of the executive committee of the users group for the NIST Center for Neutron Research (NCNR). “It was unfortunate what happened, but it’s really good to see that they’ve been able to work through the situation.” Robert Dimeo, director of NCNR, says, “We faced many challenges during this time, so all the NIST staff are excited to restart operations.”

The accident occurred on 3 February 2021, as operators were restarting the reactor after a routine shutdown for refueling. One of the reactor’s 30 rodlike, uranium-filled fuel elements was not properly latched into place, according to an investigation. So, as the reactor’s cooling water began to circulate, the rod popped out of place, overheated, and partially melted. The mishap exposed 10 workers to a radiation dose about equal to that from a CT scan, according to the NIST website. Radiation at the perimeter of the 2.34-square-kilometer NIST campus remained at safe levels the whole time.

As designed, an automated system abruptly shut down the reactor, which feeds 30 experimental stations and serves about 2000 researchers annually. The reactor could not be restarted without NRC approval. In March 2021, an NRC inspection found seven “apparent violations” of the reactor’s operating license, including allowing the cladding on a fuel element to exceed a temperature of 450°C. NRC and NIST then negotiated a plan, finalized in August 2022, to improve procedures for ensuring the latching of the fuel elements and remedying other problems. Last week, NRC released a report that found NIST’s corrective actions sufficient and authorized the restart of the reactor.

Operators are now testing the reactor in “subcritical” conditions that don’t support a nuclear chain reaction and will ramp up from there, Dimeo says. If all goes well, he says, the facility should start to serve users in May or June. That will come as a relief to thousands of researchers studying materials ranging from crystalline solids and magnets to polymers and biological samples. During the shutdown they have been scrambling to find beam time elsewhere, including at the High Flux Isotope Reactor and the accelerator-driven Spallation Neutron Source, both at Oak Ridge National Laboratory. “I know people who went overseas,” says Michael Hore, a polymer physicist at Case Western Reserve University. “Some people went to Australia.”

In addition to slashing available beam time, the shutdown also took unique instruments offline, Hore notes. “We are waiting on a very particular instrument at NCNR, so we just haven’t done any neutron measurements.” More important, the shutdown cut off access to the expertise of the NCNR staff scientists, says Nairiti Sinha, a soft-matter scientist at the University of California, Santa Barbara. “They are the best neutron scientists in the world,” she says. “The NCNR is more willing to try new sample environments or types of experiments that would require a lot more convincing at Oak Ridge,” she says.

Once the NIST reactor is up and running, NCNR officials will strive first to work through the backlog of experiments waiting for beam time, Dimeo says. Unfortunately for users, before long, the NIST reactor will shut down again. NIST plans to replace a key system for cooling and slowing some of the neutrons, an upgrade that will take a year. The reactor will likely run through four or five fueling cycles, each of which lasts about 7 weeks, before commencing with the upgrade, Dimeo says. However, he says, NIST has not yet determined precisely when the reactor will go idle again.

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