The atmospheric diffusion of gases discharged from the chimney of the Harwell reactor BEPO.

Abstract

The 61 m chimney of the BEPO reactor discharges warm air containing a small amount of radioactive argon. This gas has been used as a tracer to study the diffusion of the plume in the atmosphere downwind of the chimney. Surveys have been carried out at ground level up to a distance of 10,000 m and, with instruments suspended from a balloon, up to a height of 300 m at a maximum range of 1000 m. The results have been correlated with the gradient of potential temperature in the atmosphere near ground level. The effective height of the plume is found to depend on the wind velocity u and on the temperature gradient, having a mean value of 125 m under adiabatic conditions when u = 7 m/sec. The diffusion of the plaume over the range studied occurs in three distinct phases. Near the chimney, the rate of vertical diffusion is about twice that in the horizontal direction as a result of the discharge conditions. As the plume diffuses towards ground level it enters a region dominated by building turbulence and the rate of diffusion in both directions is greatly increased. Finally, as it travels beyond the built-up area, themore » rate of diffusion reverts to that characteristic of diffusion over open country. Sutton's diffusion equations cannot be simply applied to this complex type of problem, but they provide a useful model for classifying the data, and effective values of the virtual diffusion constants C and C/sub z/ are derived for each stage of the diffusion process. Under adiabatic and stable conditions, peak groundlevel concentrations between 1.3 x 10/sup -6/ and 3 5 x 10/sup -6/ wind with wind velocities in the range 6 to 12 m/sec and a chimney emission rate of 1 unit/sec. Under statically stable conditions no detectable concentrations were found on the ground within 10,000 m of the chimney. (auth)« less