A study of the permeation of radon through geomembranes

Xiaotian Mao, Purdue University

Abstract

Geomembranes are thin sheets of polymeric materials which have been widely used as linings and covers of containers for liquid or solid waste. In this research various types and thicknesses of geomembranes were evaluated in order to determine which geomembranes could be used as suitable barriers for radon. The permeation of radioactive gases through geomembranes cannot be measured using the method described by the American Society for Testing and Materials (ASTM). The ASTM method estimates the permeation of a single pure gas through a polymeric membrane. The radioactivity of radon would be very high if this method was used. The method developed in this study utilized radon permeation scintillation cell which consisted of a source chamber, a measuring chamber, a radium source, and a geomembrane specimen. The concentration of radon within the source chamber and the measuring chamber was quantified by counting the alpha particles emitted by the decay of radon and its daughters. Based on the percentage of radon permeation through the membranes (defined as the reduction factor), the permeance was calculated. The data indicated that 0.8-mm thick geomembranes such as chlorinated polyethylene, polyurethane and ethylene interpolymer alloy provided a permeance that was less than 500 fmol m$\sp{-2}$s$\sp{-1}$ Pa$\sp{-1}$. These geomembranes are suitable barriers for radon. The permeance of 0.15-mm polyethylene sheet, which has been used as a radon barrier, is 1.47 $\times$ 10$\sp{4}$ fmol m$\sp{-2}$s$\sp{-1}$ Pa$\sp{-1}$. The effect of certain parameters on radon permeation was quantitatively studied. Such parameters included the difference in air pressure between the two sides of a geomembrane, the thickness of the geomembrane, and the temperature. It was found that a difference of 5 cm Hg in air pressure between the two sides of a geomembrane did not significantly influence the radon permeation. Radon permeation decreased exponentially with increasing thickness of the geomembranes. Radon permeation increased according to an exponential function as the temperature increased. The values measured for the activation energy for the permeation of radon through different geomembranes were between 26 and 110 kJ mol$\sp{-1}$. In studies of the effect of temperature on radon permeation our results showed that the values of the activation energies and the pre-exponential factors measured for radon through a wide range of geomembranes gave very good linear plots of the activation energies and the logarithm of the pre-exponential factors. This relationship implies that a general mechanism of permeation is utilized in all geomembranes studied, differing only in magnitude.

Degree

Ph.D.

Advisors

Kessler, Purdue University.

Subject Area

Environmental science|Radiation|Radiology

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