Uranium Holdup Measurements at Building 9206

A holdup measurement campaign is in progress at the Oak Ridge Y-12 Plant as part of facility deactivation. When holdup measurements confirm that all significant deposits have been removed, the building’s physical security and nuclear criticality controls can be downgraded. In this campaign, holdup measurements are “endpoints-driven”. Process equipment in the facility has been divided into several subsystems, each subsystem corresponding to an endpoint in the facility’s deactivation plan. Holdup in each subsystem is quantified individually; results are reported to the program manager and reviewed for nuclear criticality safety impact. Before making measurements, analysts and process engineers inspect each equipment unit and identify its boundaries. Photographs and drawings are made. A gamma ray detector/ratemeter is used to identify “hot spots” in the equipment. Ratemeter readings are recorded on the drawings. The equipment is mathematically modeled as a combination of points, lines, and areas using the Generalized Geometry Holdup model. Bar code labels are attached at appropriate intervals, and quantitative measurements are made. Data are analyzed and preliminary results are calculated. The results are assessed and corrections are applied if warranted. Corrections may include finite source and self-attenuation corrections and corrections for spectral interferences. Spectral interferences may occur if the uranium has been irradiated in a reactor. Pb-212, a commonly occurring contaminant, emits a gamma ray at 238 keV, which interferes with the 186- keV signal from U-235 in low-resolution spectra. The two gamma rays may be resolved using higher-resolution cadmium-zinc-telluride detectors. A typical piece of equipment may require 100 measurement points for proper characterization. Each measurement requires about 50 parameters including calibration, quality control, and geometric correction factors. Due to the large amount of data, measurement parameters are tracked using the HMS3 database manager developed at Y-12. HMS3 also computes results, prints reports, and tracks completion status. This paper will discuss the measurement process in detail, and relate field experiences. Spectra will be displayed illustrating the Pb-212 problem.