Remote Gamma Ray Survey of Underground Ductwork at Y-12

Building 9206 opened at Y-12 in about 1944 for special chemical processing. For the first decade of its existence, a system of underground ducts provided the building's room ventilation and chemical process exhaust. The underground ductworks were removed from service and replaced with rooftop units when air conditioning became commercially available. The underground ducts were sealed and remained so until this year. No quantitative assessment of uranium holdup had been made. In the past few years, questions had been asked about the potential for a nuclear criticality accident that might occur because of the uranium holdup in the large underground ducts. No definitive statement could be made about nuclear criticality safety until a holdup survey had been performed. Such measurements are very difficult because access to the ductwork is limited, and because many measurement conditions are unknown and uncontrollable. A gamma ray detector and a miniature multi-channel analyzer were attached to a remote-controlled crawler, then driven up into the ductwork as far as possible. The crawler was equipped with lights and a video camera as well. Although a complete survey was not performed, six ducts were evaluated and about 800 feet of ductwork were scanned. Approximately 330 grams of uranium holdup was found throughout the ducts. This paper will outline the measurement conditions and assumptions required to calculate the holdup. Experimental difficulties are discussed, especially the factors that contribute to measurement uncertainty.