Print View Posted on: 31 July 2017
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 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 established 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 a nearby heat exchanger. The water servicing the heat exchanger contained boron (a neutron absorber). However, there was no requirement that the heat exchanger be serviced by borated water or this particular water line. Therefore, moderation was not effectively controlled, and the potential for sufficient moderation still existed. Given the unfavorable geometry of the containers and the potential for mass and moderation (both of which were not effectively controlled) to be present, only slightly different conditions would have been required for criticality to occur, such as an upset involving a leak from a nearby heat exchanger.