Radon in indoor air and water: design and development of experimental apparatuses and measurement protocols

Exposure to the decay products of radon (used in brief to refer to the isotope 222 of Rn) and thoron (used to refer to the isotope 220 of Rn) represents, on average, approximately half of the overall effective dose from natural sources suffered per year by the global population, i.e. 2.4 mSv per year. In particular, the UNSCEAR Report from 2008 reports effective doses due to radon and thoron inhalation of 1.15 mSv per year and 0.1 mSv per year, respectively. Due to the relatively low outdoor concentration, most of the exposure to radon occurs indoor where a wide variability exists in radon daughters concentration. Both temporal and spatial variability of the equilibrium factors surely reflects on radon progeny variability, but the latter is mainly caused by the difference in indoor radon concentration over time and from an indoor place to another. According to the cite{WHO2009}, indoor radon concentration depends on two main factors: typology of building construction and ventilation specifications and habits. Both of them affect the indoor radon concentration by influencing the relative contributions of the various radon sources. Several authors suggest that radon can enter building interiors: directly from soil due to radium-containing rocks still in the crust; via radon-carriers utilities such as water and, in principle, natural gas; indirectly from crustal materials no longer incorporated in crust but contained into building structure in the form of concrete, brick and the like. end{itemize} The relative contribution of each of these entry patterns obviously depends on specific circumstances, i.e. building characteristics (including building materials, construction typology and floor level), morphology and composition of the underlying soil, ventilation features, occupancy patterns and living habits of occupants. Council Directive 2013/59/EURATOM requires Member States to emph{consider} any source of radon ingress, whether from soil, building materials or water. This is required when preparing the national action plan to address long-term risks from radon exposure. In particular, regarding building materials, Member States are explicitly required to identify (and/or develop) strategy, including methods and tools, to identify building materials with significant radon exhalation rate. Pertaining to building materials, several measurements of radon exhalation rate have been reported on literature through the years (more references are available in the text). However, as firstly highlighted by Sahoo (in 2011), the difference between radon exhalation from samples and that from walls has not been investigated enough. As a result of this lack of knowledge, measurements of radon exhalation from building materials samples have been commonly used to assess the effective dose attributable to radon exhaled from walls in indoor environment even though different geometries and boundary conditions characterize the two scenarios. In fact, if the one dimensional (1-D) geometry better describes the radon flux from masonry surfaces, mainly walls, it is not suitable to the radon exhalation from a sample of building material, the latter better modelled as a three dimensional (3-D) phenomenon. At the present state of knowledge and technology, two main possibilities exist to provide reliable values of radon exhalation rate from a wall. The first is estimating the radon surface exhalation rate from a wall as related to that from a building material sample. This approach has been proposed by and recalled by Orabi (n 2018). It relies on the comparison between the 3-D model of the radon flux from the building material sample and the 1-D model used to assess flux from a wall made up by the same material. The 1-D model solution proposed by the authors -- firstly reported by and later recalled by -- is obtained under three main conditions: i) the diffusion is the only mechanism governing the radon transport, ii) the radon concentration inside the wall is an even function, symmetric with respect to the wall half thickness and iii) the room inner volume is much higher than the void wall volume. In other words, the solution is valid only when advective contribution is negligible and both wall surfaces are free to exhale into a radon-free space. As a matter of fact, the boundary conditions adopted by Sahoo and Orabi, despite being far from commonly verified, are not even declared at all. The resulting formulations can so lead to misleading predictions of radon surface flux from an existing wall. The issue just described is an evidence of the need to provide a systematic review of the differential equations describing the radon migration, as well as the corresponding solutions for any reasonable boundary conditions sets. This task has been accomplished by the Chapter 2 of this work. The aim is to provide the readers with a comprehensive description of how typical scenarios for radon transport are mathematically modelled as well as to clarify the assumptions underlying the solutions. In particular, the review has addressed either diffusive, advective and diffusive plus advective transport of radon through a slab, the latter containing radium sources or not, in different scenarios. The second possibility to provide reliable values of radon exhalation rate from a wall is in-situ measuring radon exhalation rate directly from the wall surface, i.e. through the so-called accumulation method described by ISO 11665-7. To the author knowledge, no published study exists of in-situ applications of the ISO method to vertical surfaces. This is mainly due to the several issues affecting the measuring apparatuses available on the market. Firstly, they are not specifically designed for vertical surfaces so they are not self-standing and equipped with a frame supporting the accumulation can. Secondly, they are not provided with sealing systems of any kind and the air exchanges between inside and outside the accumulation container, other than not being prevented, are not traceable at all. Besides, they are sold by the continuous radon monitor manufacturers, so the compatibility is assured only with a specific model of a specific detector. Furthermore, the analysis of the radon concentration registered to obtain exhalation rate value is a quite slow, multi-step and not automatized process completely up to the operator. Chapter 3 of this work deals with design, commissioning and realization of the first custom apparatus specifically conceived to in-situ measure the radon exhalation rate directly from walls vertical surfaces. The prototype, fully developed at the Laboratory of Radioactivity of the Italian National Institute of Health, is intended to solve such critical issues that have prevented similar apparatuses from being adopted by the radon experts: mechanically sustaining the accumulation can during the measurement without interfering with the measurements itself (i) and assuring the sealing of the chamber relative to the radon detector (ii) and the wall under investigation (iii). The prototype also aims to avoid the interfering effect of the chamber pressurization during the measurement and to reduce the effect of the back-diffusion on the accumulation process. The apparatus presented has been already successfully used in some surveys in large buildings to reconstruct the likely radon migration path through some surface flux measurements at different locations in different rooms. The apparatus has been designed for a specific continuous radon monitor model but the configuration can be adapted, with very few modifications, to other radon detectors. The choice comprehends the large number of low-cost detectors that entered, and are still entering, the market in the last few years. This quite recent and sudden entry into the market of a large number of different detectors for both professional and "domestic" purpose has turn a spotlight on the need of increasing the number of testing facilities and calibration apparatuses. These facilities should always rely on radon chambers that are designed to produce reference atmospheres whose radon activity concentration depends on the radium source employed and on the chamber volume. According to the current state of the art, radon chambers are characterized by significant costs as design, construction, commissioning, and maintenance are concerned. In particular, critical issues are i) materials used for the structure and the sealing, ii) fan system for concentration homogenization, iii) source-chamber interface circuit and iv) control instrumentation. Furthermore, industries, agencies or institutions managing a radon chamber need as many radium sources as the radon concentrations required by the different calibration protocols. Holding more than one source complicates the licensing requirements concerned with radioactive materials possession established by the national transpositions of the Council Directive 2013/59/Euratom. Chapter 4 of this work describes an innovative 0.1 m^3 radon chamber fully designed, built and tested at the laboratory of Radiation Protection of Sapienza -- University of Rome. It has been conceived as and easy-to-assemble, cheap and small facility dedicated to research on radon and calibrations services. The main innovation stands in the way radon activity concentration is varied and controlled within the chamber atmosphere: the system, in fact, may allow to establish a wide range of Rn concentrations through a single Ra source placed outside the control volume and by means of two air circulation circuits controlled by specific electric pumps remotely controlled and actuated. On view of this, the apparatus is intended to be suitable for several applications, such as: i) calibrating both passive and active radon detectors at different radon concentrations, ii) checking the response linearity of both passive and active radon detectors and iii) studying the dynamic response of the continuous radon monitors to sudden changes in radon concentration. Pertaining to the water as an indoor exposure source to radon, the Council Directive 2013/51 introduced several requirements to Member States about radon concentration in water, including: i) to adopt a parametric value above which the risk has to be evaluated and remedial actions have to be considered, and ii) to carry out representative surveys in order to identify water sources whose radon content might exceed such a parametric value. The implementation of the Council Directive has led to a considerable increase of radon concentration measurements in drinking waters. The Directive indicates for the method of analysis a minimum limit of detection (or detection limit, DL) of 10 Bq per L, i.e. 10% of the parametric value. Test methods satisfying such a limit are, mainly, gamma-ray spectrometry, liquid scintillation counting, and emanometry, whose achievable lowest detection limit are 10, 0.05 and 0.04 Bq per L, respectively. Findings from previous studies showed no statistically significant differences between results from the three different measuring techniques. The test method using emanometry, regulated by the international standard ISO 13164-3, has been used in several surveys thanks to its advantages: mainly, the possibility to use different detectors with low-to-moderate costs (i.e. 1-20 k€), the low achievable uncertainty (i.e. up to 5%), the suitability for in-situ measurements and the very short turnaround time. Chapter 5 of this work deals with the development of a specific quality assurance (QA) protocol for measurements of radon in water contemporary performed with different measuring chains by emanometry technique. This protocol is intended to allow increasing the number of measurements performed, i.e. samples analysed per day, considering that, for the emanometry test method, the water samples have to be analysed one at a time. The effectiveness of such a protocol has been evaluated by studying the results reproducibility and participating to an international proficiency test organized by the European Commission Joint Research Centre (JRC). The quality assurance protocol has been so adopted, with excellent results, during the first survey addressing the radon concentration in self-bottled mineral spring waters.