Event at the National Institute of Standards and Technology (NIST) Center for Neutron Research

Summary

On February 3, 2021, the NIST test reactor shut down automatically in response to a higher than normal radiation level in the air leaving the reactor building. All safety systems functioned properly to shut down the reactor and the reactor remained safely shut down while NIST took appropriate actions to repair the reactor and to prevent future incidents. The Nuclear Regulatory Commission (NRC) staff are confident that public health and safety were protected during and after the incident. Radiation doses inside NIST property were a small fraction of the regulatory limits for members of the public. Radiation doses beyond the NIST property would have been even lower.

On March 9, 2023, the NRC authorized NIST to restart the reactor. NIST is gradually returning to full power operations in a controlled and methodical manner, monitoring for any unexpected indications. The NRC inspection staff continues its increased oversight consistent with the confirmatory order.

Description of the Event and NIST and NRC Responses

On February 3, 2021, NIST restarted its test reactor after a shutdown for refueling and maintenance. During the startup, shortly after 9 a.m., the safety system automatically shut down the reactor because detectors showed much higher than normal radiation levels in the air leaving the reactor building through the ventilation system. The reactor operators declared an “Alert” in accordance with the NIST emergency instructions and reported it to the NRC shortly before 10 a.m. An “Alert” designates an event that could be hazardous to personnel in the facility or indicate a problem with the reactor, but shouldn’t have any significant consequences for the public or environment. After the reactor was shut down, the workers left the building and monitored the reactor using a special station designed for that purpose. NIST ended the event later that day at 7:35 p.m. Eastern Standard Time because radiation levels were below the criteria in the emergency instructions.

The event had no significant radiological consequences for NIST workers, the public, or the environment. No one was injured. Several NIST workers who were inside the building during the event were contaminated with radioactive material and exposed to higher than normal radiation levels. All workers were decontaminated, and radiation exposures were well below regulatory limits for radiation workers. Measurements made near the boundary of the NIST campus, about 400 meters from the reactor, showed that radiation levels were near naturally occurring levels. During the event, potential radiation doses beyond NIST property would have been less than one millirem, a very small fraction of the regulatory annual public dose limit of 100 millirem (the limit is roughly equivalent to several chest X-rays in one year). The NRC website provides more information about radiation doses in our daily lives.

On February 9, 2021, the NRC began a Special Inspection to examine NIST’s response to the event. Since then, the NRC inspectors and additional NRC technical experts have been monitoring NIST’s response to the event using onsite inspections at the reactor, virtual attendance of meetings, and remote inspection of documents. The NRC issued an interim inspection report on April 14, 2021. The final special inspection team report was issued on March 16, 2022, and is discussed in detail below.

On February 10, 2021, NIST conducted a public meeting to inform the local community about the event. Several NRC staff attended the meeting to help address public questions and concerns. Questions and answers from the meeting are available on NIST's website.

As required by the NRC license for the reactor, NIST submitted a report on February 16, 2021, that described the circumstances of the event. NIST updated the report on March 4, 2021, revising the concentration of airborne radioactive material released during the event slightly upward, but releases were still a small fraction of the regulatory limits.

On March 2, 2021, NIST notified NRC that the temperature of one fuel element exceeded the safety limit during the event and submitted an additional report to the NRC on March 5, 2021. The report explained that a complete evaluation of the conditions in the reactor and cause of the event will likely take several months. NIST provided an update to Event Report #55120 on May 7, 2021, and submitted a follow-up written report on May 13, 2021, with a preliminary analysis of the cause of the event. The report also notified NRC that NIST did not meet several operational requirements in the facility license during the event, in addition to exceeding the fuel temperature safety limit. NRC regulations require NIST to receive approval from the NRC before restarting the reactor.

NIST submitted a report to the NRC on October 1, 2021, as supplemented on October 15, and 21, 2021, containing the results of its root cause investigations, an analysis of proposed improvements to the fuel element latching mechanism, and a list of recovery items related to the event. The report also included proposed corrective actions to be completed prior to and following restart of the NIST reactor, and a request to restart, upon completion of the necessary corrective actions.

The NIST root cause investigations revealed one fuel element was not properly latched during the routine refueling outage performed in January 2021. The improperly latched fuel element became displaced/tilted which blocked the coolant flow into the element and caused the element to overheat while the reactor power level was increased.

The corrective actions that NIST proposed include improvements to the latching procedures and enhancements to operator training. The list of recovery items includes cleanup of the primary system, evaluation of the fuel elements in the core at the time of the event for reuse, removal of the damaged element, and an operational review of the balance of plant systems.

In response to NIST’s report and request, as supplemented, the NRC staff identified additional information necessary to inform the restart decision. The NRC sent the information request to NIST in a letter dated November 18, 2021. NIST submitted its response to this request for supplemental information on December 3, 2021.

The NRC staff’s review of the event and the licensee’s subsequent corrective actions were documented in a technical evaluation report (TER) issued on March 9, 2023. The TER outlined the basis to authorize restart. The NRC staff conducted a detailed technical review of the impacts of the event on the structures, systems, and components to ensure there was no functional damage to preclude safe operation of the facility. The NRC staff also evaluated NIST's procedures and practices to ensure they provide reasonable assurance that the reactor will be operated consistent with its license and the NRC's regulations. Furthermore, the NRC staff and NIST agreed to a series of additional corrective actions in response to the violations identified related to the event. These actions were memorialized in a Confirmatory Order on August 1, 2021, and the NRC continues to provide enhanced oversight of the facility. The TER documents the NRC staff's review in each of these areas, along with other factors considered in the restart decision. The following actions informed the NRC's restart review of the NIST test reactor.

On March 16, 2022, the NRC issued the special inspection team report, which identified seven apparent violations of NRC requirements. The apparent violations include:

1. Technical Specification (TS) 2.1, "Safety Limit," which states the reactor fuel cladding shall not exceed 842°F (420°C) for any operating conditions of power and flow
2. TS 3.1.3, which states that, "the reactor shall not operate unless all grid positions are filled with full length fuel elements or thimbles, except during subcritical and critical startup testing with natural convection flow"
3. inadequate refueling procedure
4. inadequate startup procedure
5. inadequate emergency response procedure
6. TS 3.9.2.1, "Fuel Handling, Within the Reactor Vessel," which states, that following handling of fuel within the reactor vessel, the reactor shall not be operated until all fuel elements that have been handled are inspected to determine that they are locked in their proper positions in the core grid structure; and
7. 10 CFR 50.59(c)(1)(i) which states in part that a licensee may make changes to a facility (refueling tooling) without obtaining a license amendment only if a change to the technical specifications is not required.

In accordance with the NRC’s Enforcement Policy, the NRC and NIST participated in alternative dispute resolution (ADR) to resolve the issues identified by the special inspection. As a result of ADR, agreement was achieved on corrective actions and timelines for implementation. On August 1, 2022, the NRC issued a Confirmatory Order to NIST outlining robust and comprehensive corrective actions required to be implemented at NIST.

On August 1, 2022, the NRC issued a supplemental inspection plan to NIST. The supplemental inspections of corrective actions and improvements made by NIST informed the decision to authorize restart of the reactor.

As part of its corrective actions to restart, NIST submitted three license amendment requests (LARs). The first LAR, dated December 23, 2021, modified the technical specifications associated with latch verification increasing the confidence that a fuel element will be in its properly latched position during reactor operations. The second LAR, dated October 19, 2022, authorized revisions to the Final Safety Analysis Report (FSAR) related to a small quantity of debris remaining in the primary coolant system following cleanup efforts. The revised FSAR chapters address the potential mechanical and radiological impacts of operating the reactor with some debris remaining in the primary coolant system. The third LAR, dated February 1, 2023, authorized revisions to the FSAR related to fuel management schemes and specified an analysis method to allow alternative fuel management scheme development. The LARs were approved on July 22, 2022, February 1, 2023, and March 2, 2023, respectively.

To support the inspection and licensing activities, NIST provided supplements to support the NRC review. These include:

On June 29, 2022, and August 15, 2022, NIST provided supplements, including an analysis of the event on the reactor vessel, reactor vessel internals and undamaged fuel assemblies and its restart plan.

On November 17, 2022, NIST provided a supplement, including its response to NRC questions regarding the primary and effluent monitoring systems related to the October 19, 2022, NIST license amendment request.

On March 9, 2023, the NRC authorized restart of the reactor. The restart decision was informed by numerous activities as described above. The NRC’s decision was based on a finding of reasonable assurance that the facility will be operated in a safe manner and within its licensing basis. NIST is gradually returning to full power and the NRC is continuing its enhanced oversight of the facility in accordance with the supplemental inspection plan.