Spectral shape analysis for contaminant logging at the Hanford site

A spectral shape factor log has been developed to provide information on the spatial distribution of gamma-ray-emitting contaminants detected by passive logging of boreholes that surround the buried high-level nuclear waste tanks at the U.S. Department of Energy´s Hanford Site near Richland, Washington. Many boreholes intersect plumes of contaminants that leaked from the tanks. Spectral shape analysis has been able to distinguish ¹³⁷Cs and ⁶⁰Co uniformly distributed in the formation from borehole-localized and radially remote distributions of these contaminants. A uniform distribution is evidence for a formation-migration plume while a borehole-localized contaminant distribution indicates that the borehole was a preferential pathway for contaminant migration. Knowledge of the contaminant distribution assists the process of formation contamination assessment and of estimation of remediation costs. In a few cases, the shape factor log has identified the bremsstrahlung-producing contaminant ⁹⁰Sr/⁹⁰Y. Shape factor data processing and interpretation methods were optimized using results of Monte Carlo simulations of the logging measurements. The accuracy of the Monte Carlo simulations was confirmed through experiments with known source distributions