At a fuel fabrication facility (BWX Technologies, Inc.), unanticipated deposits of high-enriched uranium were discovered in two unfavorable geometry containers. Prior to discovery, the containers were assumed to be non-uranium bearing and were not included in the licensee’s safety analysis.
BWX Technologies is authorized to use high-enriched uranium to fabricate research and test reactor nuclear fuel assemblies. Unexpected deposits of uranium-bearing material were found in two desiccant containers serving a dry air purification system on a uranium processing glove box line in the research and test reactors area. The two containers were located within close proximity to one another and initial surveys indicated that they may have contained a combined total of approximately 1 kilogram U-235 in the form of a powder aluminum uranium compound. The function of the desiccant is to remove oxygen and moisture from the process. The air purification system for the glove box was considered to be non-uranium bearing, and therefore had no documented controls in the Integrated Safety Analysis to prevent accumulations and was not routinely surveyed for uranium accumulations. There were no documented controls implemented to prevent criticality. (NRC EN52840)
The containers were unfavorable geometries. Uranium mass and moderation were the only parameters required for criticality to occur. A glove box pre-filter served to limit the accumulation rate; however, it was not effective at preventing all accumulation mechanisms as demonstrated by the event. The containers were not included in routine surveys to identify any accumulation. Therefore, mass was not effectively controlled, and the potential for an unsafe mass to accumulate undetected still existed. The containers were located in a moderator controlled area and the desiccant material did not possess any appreciable moderating characteristics; however, there were credible sources of moderation present, including a potential leak from two nearby heat exchangers. The water servicing the heat exchangers contained boron (a neutron absorber). However, there was no requirement that the heat exchangers be serviced by borated water or this particular water line. The licensee identified that a barrier did exist which limited the likelihood of an introduction of moderation. The design of the heat exchangers utilized an indirect heat transfer such that in order to introduce moderator into the desiccant containers, a breach of either heat exchanger’s double wall would be required. Although not specifically established for NCS purposes, the design and physical integrity of the heat exchangers served to keep moderation within safe limits. Therefore, a barrier was in place which served to limit the likelihood of an introduction of moderation to the desiccant containers.
Given the unfavorable geometry of the containers and the potential for mass and moderation to be present, only slightly different conditions would have been required for criticality to occur, such as an upset involving a leak from either of two nearby heat exchangers. The likelihood of a moderation introduction, however, was limited by the design and physical integrity of the heat exchangers.
Location: Lynchburg, Virginia / BWX Technologies, INC. Event date: Tue, 04-07-2017