A Sulphur Hexafluoride Tracer Dispersion Study at the Sellafield Reprocessing Plant

Abstract

Sulphur hexafluoride tracer was used in a series of four experiments to study the dispersion from a 60m emission point and conditions varying from unstable to neutral. The tracer concentrations downwind and across the axis of the plume were compared with those predicted by a simple Gaussian dispersion model. The Gaussian dispersion model published by the Atmospheric Dispersion Modelling Working Group (Clarke, 1979) can be used to predict air concentrations per unit release rate under idealistic conditions. More complex models have been proposed by the Working Group to take into account the effects of deposition, coastal sites, plume rise, and building effects but the main interest in the present study was to examine the extent of the differences between the simple idealistic Gaussian model and actual dispersion experienced under real conditions on particular occasions. The tracer experiments used release rates of 25 to 30 kgh−1 of SF, for a few hours from an outlet 60m above ground level. The release point used was a high flowrate ventilation system (1.5 E5 m3 h−1) having two outlet grills in a vertical plane either side of a large stack whose total height was 120m. It was expected that the turbulent wake of the stack, as well as that of nearby uildings, would affect the dispersion of the plume.