Study of radon exhalation rates and its influence on ambient radon concentrations in the city of Huelva

Abstract

According to United Nations Committee on Ionizing Radiation (UNSCEAR), radon gas is responsible for 42 % of the dose from natural radiation sources. The most common radon isotope, 222Rn, is emitted from materials with a high radium content such as phosphogypsum (PG), a residue generated in the production of phosphoric acid. This material contains high concentrations of radium (about 20 times that of uncontaminated soil) and is a potential radon source. In the city of Huelva, a phosphogypsum repository with 1000 ha, similar to the city area, is located less than 1 km from the city. The objective of this doctoral thesis was to improve radon exhalation methods and analyze radon atmospheric concentrations in Huelva. Firstly, to study radon exhalation from phosphogypsum, two reference exhalation chambers were made with a layer of this material. Its design allowed to close the box and measure its exhalation without perturbing the emitting layer. The performance of the closed-circuit accumulation chamber technique and the exponential and linear radon exhalation fitting methods was studied using different measurement devices and accumulation chambers. Although the linear approximation is routinely employed in the literature, the effective time constant of the measurement system is often not adequate to apply this approximation, leading to significative underestimations of the exhalation. To extend the applicability of the linear fit, the influence of lateral transport was studied using six cylindrical accumulation chambers that allowed to increase the insertion depth in the soil, showing that increasing the insertion depth effectively reduced the effective decay constant of the measurement system. This result increases the range of applicability of the linear fit, obtaining reliable exhalation measurements once a minimum insertion depth of 6 cm was used. These results were verified on measurements carried out on the phosphogypsum repository. Secondly, radon atmospheric activity concentration was measured in Huelva at two stations located at both sides of the repository. The first station was installed on El Carmen university campus, to the north of the phosphogypsum piles the second station, La Rabida, is located to the south of the repository. Measurements taken on the first station between March 2015 and March 2016 were used to study the general radon daily behavior using clustering algorithms. Typical daily curves with seasonal variations were observed, with maximums between 06:00 and 08:00 UTC (Coordinated Universal Time) and minimums around noon. The mean seasonal values oscillated between 5.6 and 10.9 Bq m-3 and maximum ranged between 36.4 and 53.4 Bq m-3 . Four events were analyzed using local meteorology measurements and atmospheric modelling obtained with HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory) and WRF (Weather Research and Forecasting) models. These results suggested that contribution from long-distance sources may have a significant impact on high radon events in the area. The combined measurements of La Rabida and El Carmen stations during the year 2018 were used to study local transport from the repository. Hierarchical clustering identified daily radon patterns, finding differences between stations and a higher occurrence of high radon clusters in El Carmen station. Atmospheric WRF simulations were used as an input for lagrangian transport model FLEXPART (FLEXible TRAjectory Model). This study suggested that El Carmen station was influenced by radon diffusion from the repository while La Rabida station was only affected by direct radon transport.