Representative Sampling of High-Level Radioactive Waste Tank Headspaces

Headspaces of the underground high-level radioactive waste-storage tanks at the U.S. Department of Energy's Hanford Site have been sampled to resolve tank safety and industrial hygiene issues and to estimate regulated air pollutant emissions. Because sampling these tanks is difficult and expensive, samples have been collected from a single location of the headspaces, based on the supposition that this would provide representative samples. In most tanks, mixing of vapors occurs because of thermally driven convection from heat generated by radioactive decay of the waste. However, in some low-temperature tanks, the ground temperature above the tank may be warmer than the waste, minimizing thermally induced convection, and raising the concern that samples from a single location may not be representative. To resolve this issue, six samples at different vertical and horizontal locations were taken from each of three low-temperature tanks and analyzed for ammonia, water, permanent gases, total non-methane organic compound concentration, and selected organic vapors. Statistical analysis of the data indicated that the tanks did not exhibit significant horizontal or vertical concentration gradients.