He and SF6 as tracers for determining ventilation rates of high-level radioactive waste tanks

Helium (He) and sulphur hexafluoride (SF6) were used as tracers to determin e passive ventilation rates of eight high-level radioactive waste storage t anks at the US Department of Energy's Hanford, Site. Measurements of passiv e ventilation rates are needed for resolution of several safety issues, inc luding rates of flammable gas ventilation, drying rates of organic salt-nit rate salt mixtures, and estimation of organic liquid waste surface area. Bo th He and SF6 met the criteria of having low background levels in the tank headspaces, being rapidly dispersed in the headspace, and remaining well-mi xed for the duration of the experiments. However, SF6 was lost at a faster rate than He from higher-temperature tanks. This accelerated loss was attri buted to beta radiolysis of the SF6 associated with its large electron-capt ure cross-section. Thus, SF6 is not a reliable tracer in these high-level r adioactive waste storage tanks. Passive ventilation rates were significantl y different among the tanks studied and also exhibited temporal variation w ithin given tanks. Ventilation rates were higher than expected based on mod eling of barometric pressure fluctuations. Rates may be influenced by a com bination of factors, including wind scavenging, temperature differentials, and available airflow pathways.