The author removing one of the impaction rods out of the AEA-designed large particle impaction rod sampler at Sellafield 1993

Conclusions:

The experimental work undertaken in this project has determined high rates of dry and total (wet & dry) deposition velocities for plutonium and Cs at the Met. Station and to a lesser extent the off-site Petersburgh Farm. Values of dry particle deposition velocity for plutonium as defined by Pm10 air concentrations ranged from 3.1E-02 m s-1 to 2.4E-01 m s-1.

The Pm10 air concentrations, however, include an under-estimation of Pu activities due to to the presence of large particle-associated radioactivity. It is more appropriate to combine the air concentrations of the Pm10 and rod 3 samplers which is representative of the size distribution for the total aerosol. This reduces Pu Vg at the Met. Station for experimental run 1 from a Pm10-referenced value of 2.4E-01 m s-1 to 3.2E-02 m s-1.

On the other hand, even where air concentration and deposition flux were better size-matched, Vg for Pu and Cs were still higher than the commonly assumed value of 1E-03 m s-1. Atmospheric dispersion modellers calculate the rate of deposition (Bq m-2 s-1) by multiplying the air concentration by an assumed value of Vg. If the value of Vg is not representative of small particles typically 1 um aerodynamic diameter (AMAD), consequent deposition-relating dose modelling will be under-estimated.

This work also verified the presence of hot particles containing plutonium, americium and curium within the deposition flux at 1 m above ground level. The presence of these particles lead to a higher derived value of Vg which may or may not be a typical value of Vg for this area of Sellafield. It is possible that hot particle deposition may be due to the resuspension of historically deposited material or it may indicate this material may arise from fugitive emissions.

KEY CONCLUSIONS:

Air samplers with different particle size collection ranges were used to identify errors in field measurements of dry particle deposition velocity which result from differences in particle sizes between the collected deposition flux and air sample concentrations. Differences between field sample values and a value typical of small particles 1 um (AMAD) may be due to resuspension processes or fugitive (undocumented) site emissions.

An important use of Vg in modelling studies is to predict the deposition and dose consequences from stack discharges. For this purpose it is important that the value of Vg used is appropriate for the particle sizes present in the plume. In the case of modern, well-filtered discharge systems, particles present in the plume will be mainly small, less than a few um diameter. In such cases, a field measurement of Vg which may include the effects of large particles of resuspended material will be less appropriate. It is not possible to argue from  the work undertaken in this study to provenance the origin of hot particles as being historic material resuspended from the ground or whether this material arises from stacks and old disused reprocessing facilities as fugitive emissions.

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