Inadequate training and licensing of operators was one of several factors that contributed to the accident earlier this year at NIST’s research reactor, according to a NIST report submitted to the Nuclear Regulatory Commission (NRC) on 1 October.

The report analyzes the root causes of the incident on 3 February, when the reactor at the NIST Center for Neutron Research (NCNR) in Gaithersburg, Maryland, released radiation into the surrounding facility. According to the report, during a routine refueling operation, inexperienced staff members failed to identify an improperly secured fuel element that, when the reactor restarted, led to a disruption in coolant flow, which caused the element to overheat and deform.

NIST explains that the reactor is refueled manually and that hands-on experience is needed to ensure that certain steps of the process are performed properly, including latching. NIST also points out in the report that turnover has led to a decline in the aggregate experience of the reactor’s staff and that recently trainees have had less opportunity to participate in refueling operations due to the pandemic.

The root-cause analysis is a key step toward restarting the reactor, which has been offline since the incident. The NCNR reactor typically serves more than 2500 researchers annually and accounts for nearly half of all neutron-scattering research in the US. It is the nation’s preeminent source for cold neutrons, which travel at slow speeds and are optimal for certain kinds of experiments.

Deficiencies in training and personnel experience flagged

The report details NCNR’s loss of experienced reactor operators and recent shortcomings in training new operators. Fifteen licensed reactor operators have either retired or left the center since 2015. In most years the agency has hired several operator candidates as replacements. The report also notes that the new chief of reactor operations whom NIST hired in 2019 left a year later; since then, the reactor has had three interim chiefs.

With that turnover, the number of highly experienced operators has dropped off markedly over the last decade, whereas the number of reactor operators with less than three years of experience has risen sharply in just the last two years. By earlier this year, it was typical to have two licensed senior reactor operators and two unlicensed trainees working on a shift, the report states.

To refuel the reactor, operators must stand atop it and use specialized tools to maneuver fuel elements into their latched position without being able to visually observe their placement inside the reactor. According to the report, all trainees practice fuel-handling using a mockup test stand, but the latching process cannot be simulated on the stand and must be learned during actual refueling.

When NCNR was closed during the early phases of the pandemic, trainees had no opportunities to participate in refueling operations. NIST postponed operator licensing exams twice at the NRC’s request before finally giving one in June 2020, relying on ‘discussion and observation on the practice stand by the shift supervisor’ to assess qualifications for fuel handling. ‘As a result,’ the report states, ‘there were four individuals that received an operating license without ever having performed actual fuel movements on the reactor top.’

Because of the declining experience of reactor operators, ‘NCNR started a transition from knowledge-based performance to a greater reliance on written procedures.’ The report documents that certain aspects of fuel latching and latch checking were not captured in written procedures. It also observes that the latch-check process was ‘not stressed’ during fuel-handling training.

Series of mistakes made in run-up to incident

NIST’s report documents that the critical errors leading to the radiation incident occurred during a refueling operation on 4 January, a month before the reactor was started up. A trainee was atop the reactor learning how to move fuel and had difficulty undertaking a ‘height check’ to ensure it was properly latched.

Quoting notes taken during post-incident interviews, the report states, ‘A licensed operator resumed control and was able to ‘wiggle’ the tool into proper position. The height check passed after moving the tool into position. Both operators stated they believe the element was latched or at least partially latched upon completion of the procedure. One operator stated that it is difficult to verify the steps of the procedure without the ‘feel’ while visually watching.’

After completing the refueling, the operators were replaced by the next shift, which comprised two trainees and one of the operators who had not performed a refueling operation before being licensed. The supervisor for that shift had been recently promoted and had performed only four latch checks previously.

Video surveillance footage and interviews revealed that the crew performed follow-up latch checks improperly, the report states. The operators used their tools incorrectly, failed to consult written procedures, and attributed an aberrant height check to new tools that were known to handle differently from the old ones. The newly licensed operator subsequently stated they were unaware that an improperly latched fuel element could result in a radiation incident.

After the 4 January refueling, reactor startup was postponed to 3 February due to concerns that NCNR personnel had been exposed to COVID-19. During that interval, the reactor’s coolant pumps were started and stopped 44 times to maintain appropriate temperatures. According to the report, it is believed that those actions moved the fuel element further out of place.

NIST’s procedures call for checks to be performed after the pump is started for the last time prior to restarting, but the report states that those checks were not carried out. The requirement is not captured in training, the report notes, and operators were therefore apt to miss it, especially in a situation in which the pumps were run so many times.

When the reactor was started up, it did not exhibit the same magnitude of power oscillations that had indicated previous cases of unlatched elements, in 1981 and 1993. As it ramped up to full power, rising radiation readings prompted an immediate emergency shutdown and the sealing of the reactor facility.

NIST has reported that the radiation exposure of NCNR personnel remained within regulatory limits and that there was only a very small and fleeting increase in radiation in the environment outside the facility. Aside from the one deformed fuel element, the reactor was undamaged.

Other NIST facilities also under strain

NIST’s report enumerates 15 corrective actions that NCNR is taking to prevent a repeat incident, which mainly involve changes in training and procedure. Approval from the NRC to restart the reactor will follow external reviews of NIST’s analysis and actions, the full implementation of those actions, the filtering of fission products from the reactor’s coolant, and the completion of an NRC inspection. A specific timetable for restarting the reactor has not been determined.

In the meantime, researchers will continue to work around the facility’s absence. Researchers have reported that the shutdown has led them to further analyze past data, seek out time at comparable facilities in and outside the US, and use alternative experimental methods that may be less well suited for their work, such as approaches that employ x rays rather than neutrons.

In a statement, interim NIST head James Olthoff remarked on the report- ‘We take these findings very seriously, as they absolutely are not consistent with NIST’s dedication to safety and excellence. We have already begun implementing many corrective actions and I’m confident that these changes will strengthen our program and ensure the safe operation of this important national resource for years to come.’

Notwithstanding NIST’s response to the current incident, the agency remains under considerable strain due to not only staff turnover but also aging facilities across its campuses in Maryland and Colorado. In its latest budget request to Congress, NIST notes that its backlog of deferred maintenance now requires expenditures of more than $800 million, which is comparable to its entire annual budget. This has left some of its facilities in a dangerous situation. ‘Numerous major utility infrastructure systems are currently in critical condition, creating risks of catastrophic failure of entire laboratory buildings,’ the agency warns.

Editor’s note- This article is adapted from an 8 October post on FYI, which reports on federal science policy. Both FYI and Physics Today are published by the American Institute of Physics.