In the 13 months since a fuel element failure triggered a scram of the research reactor at the National Institute of Standards and Technology’s NIST Center for Neutron Research (NCNR), the event and its causes have been scrutinized by both NIST and the Nuclear Regulatory Commission.
Initial conclusions from an NRC special inspection released on March 16 confirm that while public health and safety was maintained during and after the event, and doses to reactor facility staff were well below regulatory limits, a safety limit was violated when the temperature of the fuel cladding of a single fuel element in the 20-MWt research reactor reached a temperature high enough to partially melt the element.
The incident: On February 3, 2021, the NCNR reactor was being restarted after a refueling outage. As the reactor approached full power, several radiation monitors, including the confinement exhaust stack radiation monitor, showed a sudden increase, indicating a release of fission products and a probable fuel cladding failure. The reactor automatically scrammed, and staff declared an alert. Investigations showed that one fuel element had not been properly latched in place and was not in normal grid position in the reactor core. This prevented adequate coolant flow to the fuel element, and a fuel cladding failure occurred in a matter of minutes. No reactor structures, apart from the single fuel element, were damaged, and the reactor has remained in a stable shutdown condition since the incident.
The inspection: The NRC launched an inspection on February 9, 2021, and issued an interim inspection report in April 2021. The NRC inspectors reviewed NIST’s records of the event, the facility staff’s response, and NIST’s root cause analysis, proposed corrective actions, and related documentation. They also interviewed NIST staff and management.
Mohamed K. Shams, director of the Division of Advanced Reactors and Non-Power Production and Utilization Facilities in the NRC’s Office of Nuclear Reactor Regulation, in a March 16 letter to Robert Dimeo, director of the NCNR, stated, “The NRC inspection activities related to the violation of the [technical specifications] safety limit determined that the fuel cladding reached a temperature that resulted in the partial melt of a single fuel element.
“Based on the results of this inspection, the NRC staff identified seven apparent violations, which are being considered for escalated enforcement action, including a civil penalty, in accordance with the NRC Enforcement Policy,” Shams wrote.
The violations include five related to exceeding the fuel temperature safety limit and damaging a fuel element, according to the NRC. The other apparent violations are related to emergency planning and equipment modification.
NIST response: “NIST takes very seriously the findings of the NRC special inspection team and is committed to taking all corrective actions that will ensure the safe operation of this vital national resource,” said James Olthoff, acting director of NIST, in a March 16 press release. “We have already begun making changes to our organization and procedures and will work closely with the NRC to make sure an incident like this does not happen again.”
According to NIST, “The [NRC] report also confirms and expands on many aspects of NIST’s analysis of the incident, pointing to deficiencies in policies, procedures, training, and safety culture as contributing to the incident.”
The seven root causes that NIST identified are (1) the training and qualification program for operators was not on par with programmatic needs; (2) written procedures did not capture the necessary steps to ensure 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.
Restart potential: After conducting a root cause analysis and submitting two reports and supplemental information to the NRC in October 2021, NIST requested permission to restart the reactor, contingent on meeting the 18 corrective actions identified.
Because the event violated a fuel temperature safety limit, the NRC must formally approve the restart of the reactor. After restart, the facility will be subject to increased NRC oversight.
The NIST event was discussed at a technical session held on March 9 during the NRC’s 2022 Regulatory Information Conference. According to a presentation prepared by Josh Borromeo, NRC branch chief for non-power production and utilization facility licensing, “The restart decision will be informed by several NRC activities, including a technical review, inspections, and enforcement actions. While the NRC is prioritizing the review, the agency will not authorize restart until it has determined that restart will be protective of public health and safety.”
NIST can choose to accept the NRC’s findings, provide additional information in writing or during a regulatory conference, or request alternative dispute resolution.