Plutonium Hold-up Measurements at Pacific Northwest National Laboratory

Estimation of plutonium quantities contained in various process equipment is necessary to comply with criticality, safeguards and safety basis requirements during operation and decommissioning of nuclear facilities. Pacific Northwest National Laboratory (PNNL) gained extensive experience performing plutonium holdup measurements at various nuclear facilities over the past two decades. Both neutron and gamma techniques are used for the determination of plutonium quantities in the presence of other radioactive materials such as fission and activation products. An additional challenge is to perform such measurements at locations surrounded by materials in process and sealed sources when a facility is in operations mode. The neutron slab counter is calibrated for measurements of various sized objects at different distances. The measurement technique that is utilized accounts for the interferences from these background sources. Using a high purity germanium detector provides the capability to distinguish plutonium gamma activities from other radioactive materials and obtain additional information useful for analysis. The infinite energy extrapolation technique is applied (when necessary) to correct for attenuation not otherwise accounted for. Since both neutron counting and high resolution gamma spectrometry require relatively long count times for a single measurement, the preliminary planning and simple dose rate survey data are extremely helpfulmore » to optimize the number of quantitative measurements. The paper describes the specific measurement approaches for both neutron and gamma assay techniques and also presents a few application examples based on measurements taken at several facilities.« less