A theoretical approach to the re-suspension factor

ARPA Piemonte, Dipartimento Tematico Radiazioni Via Jervis 30, 10015 Ivrea (TO), Italy

Abstract

The atmospheric re-suspension of radionuclides is a well-known phenomenon that consists in the re-injection into the atmosphere of previously deposited radioactivity. The process is driven by the action of wind on surfaces and can act as an additional source of radiation exposure by inhalation, after the deposition has finished. It is thus defined as the re-suspension factor, a parameter K generally considered as a time depending function and defined as the ratio of C_a, the volumetric air activity concentration (Bq m⁻³) and I₀ (Bq m⁻²), the radioactivity deposition at time zero.

The re-suspension factor concept is very useful in radioprotection in order to estimate the inhalation of radionuclides re-suspended from contaminated surfaces when direct atmospheric measurements are lacking or difficult to perform. However, the choice of the proper values of K is usually not a simple task, being quite site-specific and related to the meteorological, géomorphologie and environmental characteristics of the area to be studied. Moreover, several investigations showed clearly that the values of K are a decreasing function of time. For that reason, K values span several orders of magnitude: typical values in the range 10⁻⁵–10⁻¹⁰ m⁻¹ are reported in literature for different environmental conditions and time elapsed since the deposition event. The current available models for the re-suspension factor are based on empirical formulas whose parameters are highly site dependent and cannot easily be related to some physical quantity.

In this paper a simple physical model for the re-suspension factor is proposed and tested with available environmental radioactivity data (¹³⁷Cs), collected since 1986 (Chernobyl fallout). The new model not only allows a satisfactory description of the experimental data like even the current empirical models do, but it is also able to connect the K values to quantities with a physical meaning (such as, for example a diffusion-dispersion coefficient) and that are related to processes underlying resuspension.