Study of Radon-222 exhalation of phosphogypsum blocks used as building materials. Comparison with modeling

摘要

Phosphogypsum is a by-product of the phosphate fertilizer industry, which is stockpiled in large quantities worldwide. It consists mainly of dihydrate gypsum but contains elevated concentrations of ~(226)Ra and other inorganic species, which originate from the processing of phosphate rocks. ~(222)Rn is the first decay product of ~(226)Ra and ~(222)Rn gas evolution has been identified as one of the major environmental concerns associated with phosphogypsum. We present here a laboratory method for the determination of radon exhalation rate from gypsum used in the fabrication of building materials. In this study, the phosphogypsum slabs are placed under a hemispherical chamber in order to determine the radon concentration in samples by alpha spectroscopy and the uni-directional radon exhalation. The radon exhalation rate from phosphogypsum at various moisture contents is also studied: with increasing moisture content, a significant increase of ~(222)Rn exhalation can be shown. On the other hand, we performed a 3D simulation of the transport and exhalation of radon in phosphogypsum samples using a finite-element method. The theoretical study characterized the equation of radon transport in a building material, which combines the diffusive migration of radon and the effects of radon decay. This simulation follows the evolution of ~(222)Rn concentration in the phosphogypsum and we obtain an evaluation of the theoretical ~(222)Rn exhalation rate in the chamber. Finally, the experimental and theoretical results are compared.