Measurements on the release of gaseous activity from a short stack

Abstract

The behaviour of gaseous effluent released from a short stack at Hinkley Point “A” Nuclear Power Station has been examined using argon-41, produced in the shield-cooling air, as a tracer. The rate of release of activity from the stack was determined and the resulting ground-level gamma fluxes were measured and correlated with wind speed and direction. The results are identified with two types of plume behaviour characterized by the ability of the effluent to penetrate the shear and wake region over the building. If the effluent does penetrate this region an elevated plume is formed apparently unaffected by surrounding buildings; otherwise the effluent is entrained into the low-pressure field above and in the lee of the building where rapid mixing occurs and a large area of the building appears to emit the plume. At low wind speeds the observed gamma fluxes were found to be roughly independent of wind speed whilst at high wind speeds the fluxes exhibited an inverse dependence on wind speed and could be associated with a uniform distribution of effluent within a 70 m radius semi-cylinder with its axis at ground level and extending from the building to infinity downwind. The results were compared with a simple model which assumes a gaussian distribution of wind speeds and a “switch” from the elevated plume to entrainment at a critical wind speed. Good agreement was obtained. Values of the ratio of efflux velocity to critical wind speed were dependent on direction and were in the range 1·5–3·0.