The conditions under which the gradient method can be used to determine the wind lift coefficient of radioactive dust from the earth's surface

Abstract

Tile wind lift coefficient ce of dust from the ground is defined as the ascending current of dust, lifted by the wind, per unit density of surface contamination A [1]. There are three main methods of determining ce: the diffusion method, the gradient method, and using radionuclide ratios [2]. The gradient method is based on a calculation of the ascending current of radioactive dust from the measured vertical dust concentration gradient in the air. The simplicity and small number of input quantities are its advantages, but its drawback is its low accuracy. This method has been repeatedly used to determine oe both in the contaminated zone and on its periphery. Since measurements of the concentration of radioactive dust in air must be accompanied by a gradient determination of meteorological quantities, its use in a zone of high contamination without using automation becomes dangerous for the health of the servicing personnel. Hence, at the present time measurements are usually carried out on the periphery of the contaminated zone. Air-filtering equipment [3] or gauze cones through which the wind blows [4], are used to collect samples of radioactive dust from the air.Gradient measurements using gauze cones were carried out for the first time in Russia in the Southern Urals in the summer of 1959 [5] after an accident by the "Mayak" Industrial Association (1957). Simultaneously, oe was also determined with greater accuracy by the diffusion method. The results of the measurements were summarized in [2]. After the accident at the Chernobyl Nuclear Power Plant, gradient measurements of oe were made on the southern periphery of the contaminated region [6] and in the Novozybkov Bryansk region [7].The wind uplift coefficient of radioactive dust from the underlying surface is usually calculated in the gradient method from the formula (1) where K z is the mean vertical turbulent mixing coefficient in the terrestrial layer of the atmosphere between levels z I and z 2, qt.2 is the concentration of radioactive dust at these levels, and A is the density of radioactive contamination of the locality. Formula (l), strictly speaking, only holds for estimating the wind uplift factor of an unsettling impurity over an inf'mite horizontally uniform plane under fixed meterological conditions. However, in practice, these conditions can never be exactly realized. Hence, the difficulty involved in organizing natural spatial measurements to estimate advection, in calculating the washing-out of impurity by atmospheric precipitation, in determining the rate of deposition of polydisperse radioactive dust under varying meterological conditions, and the complex field of the density of radioactive contamination at the actual locality, usually necessitate using (1), while acknowledging that the values of oe obtained are approximate.A serious drawback of the gradient method when making measurements on the periphery of the contamination zone is its low sensitivity. This is due to the difficulty of choosing a prolonged period of steady meterological...