Atmospheric stability effects on potential radiological releases at a nuclear research facility in Romania: Characterising the atmospheric mixing state Scott D. Chambers a, * , Dan Galeriu b , Alastair G. Williams a , Anca Melintescu b , Alan D. Griffiths a , Jagoda Crawford a , Leisa Dyer a , Marin Duma b , Bogdan Zorila b, c a Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia b “Horia Hulubei” National Institute for Physics and Nuclear Engineering, 30 Reactorului St., POB MG-6, 077125 Bucharest, Magurele, Romania c Department of Electricity, Solid Physics and Biophysics, Faculty of Physics, University of Bucharest, Magurele, Romania article info Article history: Received 10 November 2015 Received in revised form 13 January 2016 Accepted 17 January 2016 Available online xxx Keywords: 222 Rn Atmospheric stability Mixing depth Radon flux Radioactive releases Tritium abstract A radon-based nocturnal stability classification scheme is developed for a flat inland site near Bucharest, Romania, characterised by significant local surface roughness heterogeneity, and compared with tradi- tional meteorologically-based techniques. Eight months of hourly meteorological and atmospheric radon observations from a 60 m tower at the IFIN-HH nuclear research facility are analysed. Heterogeneous surface roughness conditions in the 1 km radius exclusion zone around the site hinder accurate char- acterisation of nocturnal atmospheric mixing conditions using conventional meteorological techniques, so a radon-based scheme is trialled. When the nocturnal boundary layer is very stable, the Pasquill eGifford “radiation” scheme overestimates the atmosphere's capacity to dilute pollutants with near- surface sources (such as tritiated water vapour) by 20% compared to the radon-based scheme. Under these conditions, near-surface wind speeds drop well below 1 m s 1 and nocturnal mixing depths vary from ~25 m to less than 10 m above ground level (a.g.l.). Combining nocturnal radon with daytime ceilometer data, we were able to reconstruct the full diurnal cycle of mixing depths. Average daytime mixing depths at this flat inland site range from 1200 to 1800 m a.g.l. in summer, and 500e900 m a.g.l. in winter. Using tower observations to constrain the nocturnal radon-derived effective mixing depth, we were able to estimate the seasonal range in the Bucharest regional radon flux as: 12 mBq m 2 s 1 in winter to 14 mBq m 2 s 1 in summer. Crown Copyright © 2016 Published by Elsevier Ltd. All rights reserved. 1. Introduction Nuclear facilities are commonly required to monitor their emissions of radioactive gases and aerosols to the environment, in order to gauge the integrated environmental impacts of routine releases of pollutants, and to help in the forecasting of potential health risks associated with accidental releases (Galeriu et al., 2014; IAEA, 2011a,b; EURATOM TREATY, http://www.euratom.org/). An important component of regulatory monitoring programs is the routine measurement of meteorological quantities that can be used to characterise the state of the atmosphere and its ability to dilute the emitted pollutants and transport them away from the area. Near-surface concentrations of hazardous air-borne pollutants to which workers, residents, wildlife or crops may be exposed, are primarily a function of the source strength, the volume of the at- mosphere into which they mix, and entrainment processes (e.g. Pal, 2014). For pollutants with sources typically below the height of the nocturnal inversion layer, concentrations will usually peak in calm pre-dawn conditions, when the lower atmosphere is most poorly mixed (e.g. Avino et al., 2003; Baciu, 2005; Galmarini, 2006; Pearce et al., 2011; Crawford et al., 2016; Grundstr€ om, 2015; Chambers et al., 2015a,b). On the other hand, emissions from elevated sour- ces (e.g. tall stacks) frequently result in peak concentrations shortly after dawn (so-called “fumigation events”; Oke, 1987), when tur- bulence erodes the nocturnal inversion and incorporates the overlying air into the developing convective boundary layer. Since establishing and maintaining dense regional monitoring networks is logistically and economically prohibitive, efforts to understand the impact of radioactive releases into the atmosphere * Corresponding author. E-mail address: szc@ansto.gov.au (S.D. Chambers). Contents lists available at ScienceDirect Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad http://dx.doi.org/10.1016/j.jenvrad.2016.01.010 0265-931X/Crown Copyright © 2016 Published by Elsevier Ltd. All rights reserved. Journal of Environmental Radioactivity 154 (2016) 68e82