Root causes of NIST reactor alert point to operator training

October 6, 2021, 3:02PMNuclear News
A rendering of the core of the NBSR, which consists of 30 aluminum-cladded plate-type U3O8 fuel elements with a 17.8-cm gap between elements. (Image: NCNR Technical Working Group, Root Cause Investigation of February 2021 Fuel Failure) (CLICK IMAGE TO ENLARGE)

The National Institute of Standards and Technology (NIST) has submitted two reports and supplemental information to the Nuclear Regulatory Commission after conducting a root cause analysis of the February 2021 fuel failure and resultant alert at the NIST Center for Neutron Research (NCNR) in Gaithersburg, Md. While the 20-MWt NCNR research reactor remains shut down, scuttling the plans of researchers who rely on it as a source of both cold and thermal neutrons, NIST states in an October 4 update that it has requested permission to restart the reactor, contingent upon meeting all 18 corrective actions identified.

What happened: On February 3, the reactor was being started up after a January 4 refueling. As the reactor approached full power, several radiation monitors, including the stack radiation monitor, showed a sudden increase, indicating a release of fission products and a probable fuel cladding failure. The reactor automatically scrammed, and an alert was declared. According to NIST, a post-incident in-core video inspection revealed that one fuel element, which had not been properly latched, had lifted out of position in the lower core grid plate and become skewed. In that position, the fuel element did not have adequate cooling, and a fuel cladding failure occurred in a matter of minutes. All doses were confirmed to be below the limits established for radiation workers, and no reactor structures, apart from the single fuel element, were damaged.

Root causes: According to NCNR’s initial root cause investigation, as described in Root Cause Investigation of February 2021 Fuel Failure, Revision 2, because the fuel element was unlatched, it was displaced by the primary coolant flow once that flow was started. The NCNR Technical Working Group that developed the report determined that “element 1175 was unlatched as a result of inadequate training of reactor operators, inadequate refueling procedures, a lack of procedural adherence and enforcement, and inadequate methods for verifying elements were latched.”

The consequences of inadequate training were amplified by the relative inexperience and lack of plant knowledge in the operations workforce, according to the report. Since 2015, a total of 15 licensed operators either retired or left the organization, and the hiring of new and relatively inexperienced operators was found to be a contributing factor to the incident, lowering the average years of experience of operators and supervisors by more than half.

The COVID-19 pandemic contributed to a lack of hands-on experience and altered or delayed operator licensing exams and fuel handling qualification. According to the report, “The COVID shutdown from March to July of 2020 had a fairly substantial impact on training in that candidates did not have the opportunity to participate in refuelings, among other reactor operations duties. . . . Fuel handling qualification of some candidates . . . were made by discussion and observation on the practice stand by the shift supervisor. As a result, there were four individuals that received an operating license without ever having performed actual fuel movements on the reactor top.”

Investigation and response: The initial root cause investigation by the NCNR Technical Working Group identified five root causes. A subcommittee of NIST’s Safety Evaluation Committee, which included NIST staff members and an outside expert, performed a further review of the technical working group’s analysis and identified two additional root causes. Both groups identified a total of 18 corrective actions that NIST will take to address the root causes. According to NIST, four external experts will review the analyses, corrective actions, and organizational response to the event, and the NRC will complete a special inspection and issue its own report before fully reviewing the restart request.

The seven root causes identified in the reports are: (1) the training and qualification program for operators was not on par with programmatic needs; (2) written procedures did not capture necessary steps in ensuring that fuel elements were latched in place; (3) procedural compliance was not enforced; (4) the equipment and tools used to determine whether fuel elements were securely latched in place were inadequate; (5) management oversight of refueling staffing was inadequate; (6) the NCNR’s change management program was insufficient; and (7) the reactor operations group had a culture of complacency.

“We take these findings very seriously, as they absolutely are not consistent with NIST’s dedication to safety and excellence,” said James Olthoff, who is performing the non-exclusive functions and duties of the undersecretary of commerce for standards and technology and NIST director. “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.”

While corrective actions are focused on operations, a review of NIST’s response to the alert by the safety subcommittee determined that “safety systems functioned as intended, defined roles were fulfilled quickly and correctly, and defined processes and procedures were implemented.”

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