ESTABLISHING NO CREDIBLE RISK OF INADVERTENT CRITICALITY FOR THE K-25 DEMOLITION PROJECT

摘要

Demolition of the K-25 Gaseous Diffusion Plant required establishing a non-credible risk of inadvertent criticality because a Criticality Accident Alarm System (CAAS) could not be reliably maintained operable as the building was being torn down. Preparation for, and demolition of, the facility was a multi-year long effort with significant changes in approach. The most significant change was to demolish the facility with as much of the process gas equipment and piping left in the facility as possible, rather than removing all process gas equipment and piping from the facility prior to demolishing the building structure only. The contractor had to adapt their approach to accommodate these changes. The normal approach to demolition was to remove all process gas equipment and piping prior to demolition, using Nuclear Criticality Safety (NCS) controls based on the double contingency principle, while CAAS coverage was intact. Then, the building structure was demolished with no requirement for a CAAS. This was the method used to demolish the K-33 Building, for example. In the case of K-25 demolition, all equipment and piping that met the Environmental Management Waste Management Facility (EMWMF) disposal cell Waste Acceptance Criteria (WAC) was left in the building and removed as part of the building demolition process. This resulted in significant quantities of fissionable material remaining in the building and being handled during demolition. The NCS Organization worked closely with several groups during preparation activities. A Data Management organization was established to specifically identify every piece of process gas equipment and piping in the facility so that it could be accounted for and characterized. Deactivation & Decommissioning (D&D) personnel conducted internal visual inspections, by remote camera, of all process gas equipment and piping larger than 3" diameter, to reveal any large deposits that would require removal. Non-Destructive Assay (NDA) of the vast majority of equipment and piping was required to quantify the amount of fissionable material remaining. Finally, all process gas equipment and piping larger than 3" diameter was internally filled with foam, which was credited by NCS with immobilizing the remaining fissionable material such that large migration and accumulation could not occur during demolition activities. NCS personnel worked closely with all of these groups in order to acquire the data and information needed to establish a non-credible risk of inadvertent criticality during building demolition. Good communication and a close working relationship between NCS, the various D&D/characterization personnel, and the Department of Energy (DOE) ensured the success of the modified approach to building demolition. The same successful approach is currently underway in the K-27 Building as well.