On-site characterization and verification for removal of cesium contamination along an active rail spur

A rapid, cost-effective sampling and analysis approach was used to conduct on-site analysis of soil and ballast for 137Cs contamination along an active rail spur in Oak Ridge, Tennessee. Savings in time and analytical costs were realized for both the characterization and cleanup phases of this project. For characterization, a procedure was developed to conduct gamma-logging of shallow boreholes. Portable gamma radiation survey instruments consisting of a 5- by 5-cm NaI detector coupled to a digital ratemeter/scaler with a single-channel analyzer were calibrated to a 137Cs source, then used for both surface and subsurface gamma radiation surveys. A correlation was made between the direct radiation measurements and 137Cs concentrations in surface and subsurface soil and ballast. Borehole logs of gamma-ray activity as a function of depth were used to estimate vertical profiles of the subsurface distribution of 137Cs. Information from these surveys was used to develop accurate 3-dimensional isopleths of contaminated areas, including estimates of waste volumes. After a remedial alternative was selected, on-site analytical methods were again used to verify that the cleanup level was achieved. Samples from excavated areas were analyzed for 137Cs by comparative gamma-ray counting using a multi-channel pulse height analyzer and a 3-in. diameter by 2-in. thick NaI crystal. The counting system was housed in a mobile field laboratory. To better direct ongoing excavation, a correlation factor between samples analyzed by gamma spectroscopy and measurements made by field instruments was determined. In both the characterization and the cleanup phases, sample results were verified by independent off-site laboratory analyses