Dottorato in ENERGIA E AMBIENTE

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

Exposure to the decay products of radon and thoron represents approximately half of the worldwide average overall effective dose, i.e. 2.4 mSv year-1 (United Nations Scientific Committee on the Effects of Atomic Radiation, 2008), suffered per year by the global population from naturally occurring radionuclides. In particular, UNSCEAR 2008 Report refers values for effective dose due to inhalation of radon and thoron of 1.15 mSv year-1 and 0.1 mSv year-1, respectively. Due to the relatively low outdoor radon concentration, most of the exposure to radon progeny occurs indoor, the latter environment being characterized by a wide variability in daughters concentration (Nero and Nazaroff, 1984). This variability arises partly from different equilibrium factors (several studies dealt with equilibrium factor temporal (e.g. Tokonami et al., 2003; Iimoto, 2000; Hopke et al., 1995) and spatial (e.g. Prasad et al., 2016; Tokonami et al., 2003; Ramachandran and Ramu, 1994) variability), but is caused mainly by the difference in indoor radon concentration from one building to another. According to the WHO (World Health Organization, 2010), indoor radon concentration varies with two main factors: typology of building construction and ventilation characteristics/habits. Although the indoor radon concentration is affected by ventilation rate (in the recent past, several works addressed such a correlation with a systematic approach, e.g. Chao et al., 1997), the major source of the variability in radon concentration from one building to another has been observed to be the different entry rates of its various sources. Several authors (e.g. Nazaroff and Nero, 1988) suggest that radon can enter building interiors: i) directly from soil due to radium-containing rocks still in the crust; ii) via radon-carriers utilities such as water and, in principle, natural gas; iii) indirectly from crustal materials, no longer incorporated in crust but contained into building structure in the form of concrete, brick and the like. The relative contribution of each of these entry patterns obviously depends on specific circumstances, i.e., not listed in order of importance, building characteristics (including building materials, construction typology, floor levels, etcetera), 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 consider any source of radon ingress, whether from soil, building materials or water (European Commission, 2014). This is required when preparing the national action plan to address long-term risks from radon exposure. In particular, regarding building material, Member States are explicitly required to identify (and/or develop) strategy, including methods and tools to identify building materials with significant radon exhalation rate. Knowing the radon exhalation rate from building boundaries is also crucial when choosing the best remedial strategies to reduce indoor radon concentration. Several measurements of radon exhalation rate from building materials samples have been reported on literature through the years (e.g. Avramovic et al., 2019; Leonardi et al., 2018) (more references are available in the text). However, as firstly highlighted by Sahoo et al. (2011), the difference, or the existing relationship, between radon exhalation from samples and that from walls has not been considered enough. Results from measurements of radon exhalation from building materials samples have been currently used to assess the effective dose attributable to radon exposure in indoor environment (Feng and Lu, 2016; Gupta et al., 2013; Mahur et al., 2008; Ujic et al., 2010). The one dimensional (1-D) geometry, considered when modelling the radon flux from masonry surfaces (mainly walls) is not applicable at all to the radon exhalation from a block of building materials, the latter much better described by a three dimensional (3-D) model. This leads to different radon exhalation per unit surface area of the matrix and per unit time, in the two scenarios. Given the above, two main possibilities are capable of providing reasonable 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 its building material sample. This approach has been proposed by Sahoo et al. (2011) and recalled by Orabi (2018). It relies on the comparison between the 3-D modelling of the radon flux from building material sample with that 1-D used to assess flux from wall made up by the same material. The 1-D model solution proposed by the authors – firstly reported by Jonassen and McLaughlin (1980) and later recalled by Nazaroff and Nero (1988) – 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. Furthermore, the boundary conditions implicitly adopted by Sahoo et al. (2011) and Orabi (2018), despite being far from commonly verified, are not declared at all by the authors. 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. 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 only diffusive and diffusive plus advective transport of radon through slabs with and without inner radium sources by assuming different boundary conditions, i.e. fixed radon concentration on both finite slab sides, fixed radon concentration on a side of an infinite slab and finite slab inside a vessel with unknown radon concentration. 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 (International Organization for Standardization, 2012a). In-situ applications of this method to vertical surfaces, i.e. walls, are rare. To the author knowledge, no published study exists documenting measurements performed by sealing an accumulation can directly on a vertical surface of a wall. The solution available on the market for measuring radon exhalation rate from vertical surfaces are generally affected by several problems. Firstly, they are not specifically designed for vertical surfaces so they are not self-standing apparatuses equipped with a frame supporting the accumulation can. Secondly, they are not provided with sealing system 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 manufacturer, 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 Italian National Institute of Health, is intended to solve the main critical issues associated to this kind of measurements that, until now, have prevented a similar apparatus from being commercially available on the market of radon industries: 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 in order to reconstruct the likely radon migration path by measuring the surface flux 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 few modifications to other commercial radon detectors, including the large number of low-cost detectors that entered, and are still entering, the market in the last few years. The availability of a large number of different detectors for both professional and "domestic" use has recently led to an increase of the request 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 (European Commission, 2014). Chapter 4 of this work describes an innovative 0.1 m3 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 radon concentrations through a single radium 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 employed in several applications, such as: i) calibrating at different radon concentrations both passive and active radon detectors, ii) checking the response linearity of both passive and active radon detectors and iii) studying the dynamic response of the continuous radon to sudden changes in radon concentration. Pertaining to the water as an exposure source to radon, the Council Directive 2013/51 (European Commission, 2013) for the protection of the health of the general public with regard to radioactive substances in water contains several requirements to Member States about radon concentration in water, including: i) to adopt a parametric value (equal to 100 Bq L-1) 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 such 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 L-1, i.e. 10% of the parametric value. Test methods capable of measuring radon activity concentration satisfying this limit are, mainly, gamma-ray spectrometry, liquid scintillation counting, and emanometry, whose achievable lowest detection limit are 10, 0.05 and 0.04 Bq L-1, respectively (Jobbágy et al., 2017). Findings from previous studies showed no statistically significant differences between results from the three different measuring technique (Pujol and Pérez-Zabaleta, 2017). The test method using emanometry, regulated by the international standard ISO 13164-3 (International Standard Organization, 2013), 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%) (Jobbágy et al., 2017), the suitability for in-situ measurements and the very short turnaround time (International Organization for Standardization, 2014). 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 during the first survey addressing the radon concentration in self-bottled mineral spring waters.