Radiocarbon in the Atmosphere of the Žlkovce
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Radiocarbon, Vol 57, Nr 3, 2015, p 355–362 DOI: 10.2458/azu_rc.57.18364 © 2015 by the Arizona Board of Regents on behalf of the University of Arizona RADIOCARBON IN THE ATMOSPHERE OF THE ŽLKOVCE MONITORING STATION OF THE BOHUNICE NPP: 25 YEARS OF CONTINUOUS MONTHLY MEASUREMENTS P P Povinec1,2 • A Šivo1 • M Ješkovský1 • I Svetlik3 • M Richtáriková1 • J Kaizer1 ABSTRACT. Radiocarbon variations in the atmosphere have been observed at the Žlkovce monitoring station of the Bohu- nice nuclear power plant (NPP), situated only 5 km ESE from the NPP. The observed 14C levels provide unique evidence of a decreasing long-term impact of the Bohunice NPP on the region. Simultaneously, decreasing emissions of fossil fuel carbon dioxide in the atmosphere of the monitoring site have been found. The observed ∆14C variations with time have attenuating amplitudes and decreasing mean values, showing maxima in summer and minima in winter, the latter primarily caused by 14 increased emission of fossil CO2 in winter months. Sporadic short-term releases of C from the Bohunice NPP were observed at the Žlkovce station. The annual atmospheric ∆14C variations compared with tree-ring data collected at the Žlkovce village show reasonable agreement. The observed ∆14C levels after 2005 are close to the European clean-air levels as measured at the Jungfraujoch (3450 m asl) monitoring station. INTRODUCTION Investigations of radiocarbon in the atmosphere have been carried out by the Bratislava group since 1967 as a response to construction of the first nuclear power plant (NPP) at Jaslovské Bohunice (called A-1) in the former Czechoslovakia. The nuclear reactor KS-150 (150 MWel power) used natural uranium as a fuel, heavy water as a moderator, and carbon dioxide as a coolant. As the carbon dioxide contained traces of nitrogen, the nuclear reaction 14N (n, p) 14C (the same one that produces cosmogenic 14C in the atmosphere by interactions of cosmic-ray neutrons with nitrogen) was responsible for 14C production in the coolant. The first 14C measurements started in 1967 in Bratislava (Povinec et al. 1968), and from 1968 also at the Bohunice NPP, with the idea to obtain pre-operational data on 14C concentrations in the atmosphere. A short-term sampling during a few hours (undertaken weekly) was carried out up to 1972 at the Bohunice NPP, when the sampling site was shifted to a new monitoring station at Žlkovce. Starting from 1987, monthly sampling was introduced at the Žlkovce monitoring station. The KS-150 reactor operated from 1972 until 1979 when an accident forced reactor operations to be shut down. During its operation, the ∆14C levels at the Bohunice NPP reached values around 1000‰, confirming the expectation that14 C would deliver an important component of radiation dose to the public (Cimbák et al. 1986; Povinec et al. 2009). The second NPP (called V-1) constructed at the Jaslovské Bohunice site used two 440-MWel pres- surized light-water reactors of Russian origin (VVER-440, type V 230). The first reactor started to operate in December 1978, and the second one of the same design in March 1980. Two reactors of the third NPP (called V-2), of a similar construction as V-1 (the reactor type V 213), started oper- ation at the Bohunice NPP in August 1984 and August 1985, respectively. The two reactors of the V-1 NPP were shut down at the end of 2006 and 2008, and are presently undergoing decommission- ing. The other two reactors of the V-2 NPP are still in operation. In the pressurized light-water nuclear reactors of VVER- 440 type, several nuclear reactions are pos- sible to produce 14C (Chudý and Povinec 1982): 14N (n, p) 14C; 17O (n, α) 14C; and 13C (n, γ) 14C. The 14C is released from nuclear reactors in different chemical forms. While in the A-1 nuclear reactor 14 the main form was CO2 (Povinec et al. 1986a), in the VVER-440 reactors C was predominantly released as hydrocarbons (70–95%), and the remainder as CO2 (Uchrin et al. 1998). 1. Comenius University, Faculty of Mathematics, Physics and Informatics, Department of Nuclear Physics and Biophysics, 84248 Bratislava, Slovakia. 2. Corresponding author. Email: [email protected]. 3. Nuclear Physics Institute CAS, 180 86 Prague, Czech Republic. Proceedings of the 1st International Radiocarbon in the Environment Conference 18–22 August 2014, Queen’s University Belfast, Belfast, Northern Ireland, UK Edited by Evelyn Keaveney and Paula Reimer 356 P P Povinec et al. Several papers have been published studying 14C releases from nuclear reactors, and their impact on the local and regional environments (Kunz 1985; Cimbák et al. 1986; Obelić et al. 1986; Povinec et al. 1986a,b, 2008, 2009, 2012, 2015; Levin et al. 1988; Hertelendi et al. 1989; Loosli and Oeschger 1989; Milton et al. 1995; Stenström et al. 1996, 1998; Uchrin et al. 1998; Roussel-Debet et al. 2006; Svetlik et al. 2007, 2010, 2012). In all these investigations, small but noticeable evidence has been found of the impact of NPPs on local/regional environments. The present paper summarizes the results from 25 yr of continuous monthly monitoring of atmo- 14 spheric C in CO2 at the Žlkovce monitoring station (situated ~5 km ESE of the Bohunice NPP). In addition, recently tree-ring samples were collected at Žlkovce village by coring living trees and measuring 14C in single annual tree rings by accelerator mass spectrometry (AMS). A comparison of the atmospheric 14C data with annual tree-ring data has been made, and differences in both 14C records are discussed. Furthermore, we compare the Žlkovce 14C data with the Czech data, which were obtained for the Dukovany NPP (Czech Republic), operating four nuclear reactors of similar design as the Bohunice NPP. SAMPLING AND MEASURING TECHNIQUES Monthly sampling of atmospheric CO2 was carried out from 1987 at the Žlkovce monitoring station. Žlkovce (48°29′N, 17°40′E) is a small village, situated approximately 60 km NE from Bratislava (Figure 1) in a flat agricultural area (162 m asl). The nearest pollution source that can influence the 14C concentration in the atmosphere is the Bohunice NPP, situated 5 km WNW from Žlkovce (Figure 1). Monthly CO2 samples were collected by bubbling air (1 L/min) through a 0.7M NaOH solution (contained in 2-L flasks). The total volume of collected air was around 43 m3, so the final amount of carbonates did not exceed 30% of the saturation capacity of the solution. Samples were then precipitated in the laboratory as BaCO3, and CO2 was liberated in a vacuum line by adding H3PO4. The collected CO2 gas (~8 L) was measured volumetrically in a calibrated volume. A de- tailed description of the sampling has already been described (Usačev et al. 1973; Povinec et al. Figure 1 Location of the Bohunice (Slovakia) and Dukovany (Czech Republic) NPPs and the Žlkovce monitoring station. 14C in the Atmosphere of the Bohunice NPP 357 1986a). Almost all produced CO2 was used for preparation of CH4 in a reactor over a heated ruthe- nium catalyst (Povinec 1972). A low-level gas proportional counter was used for counting of 14C decays (Povinec 1978). A few mL of CO2, collected from the samples, were analyzed using a stable isotope mass spectrometer for determination of the 13C/12C isotopic ratio. The uncertainty of δ13C -at the 1σ level. 14C results are presented as ∆14C values, which were calcu ‰0.1 ﰈ± measurements was lated relative to the NIST (National Institute of Standards and Technology, Gaithersburg, USA) 14C oxalic acid standard, and corrected for isotopic fractionation following Stuiver and Polach (1977). 13 When correcting for fractionation, the measured δ C values represent mainly fossil CO2 as the 13 14 reactor-derived CO2 is negligible. Typical uncertainties in ∆ C values at the 1σ level were ±0.3‰. Core sampling of tree rings from a living lime tree (Tillia cordata) was also carried out at Žlkovce village. The tree rings were separated under microscope (as it was difficult to separate the light and dark parts of the rings, they were sampled together as a single ring sample/year), oven-dried, chem- ically cleaned (acid-base-acid method), then combusted to evolve CO2, and finally graphite targets were prepared. The AMS measurements were carried out in the VERA laboratory of the University of Vienna (Ješkovský et al. 2015). RESULTS AND DISCUSSION Atmospheric Radiocarbon Data The impact of the Bohunice NPP on atmospheric 14C can be clearly recognized in the Žlkovce monthly 14C data (Figure 2). Operation of the V-1 and V-2 NPPs did not produce high 14C concen- trations in the atmosphere, which would be comparable with emissions from the A-1 NPP (Povinec et al. 2009). However, sharp ∆14C peaks were observed during summer months, which were associ- ated with works carried out at the Bohunice NPP. In some years, e.g. in 1989 and 1991, ∆14C values around 300‰ were measured in the Žlkovce air due to 14C emissions from the Bohunice NPP. Later, the ∆14C values decreased below 180‰, and during the 2000s they were only sporadically above 100‰, e.g. 160‰ in July 2004. This was due to the fact that the 14C releases from the Bohunice NPP decreased considerably (from about 1100 GBq/yr in 1994 to about 300 GBq/yr after 1997; Povinec et al. 2008) thanks to improvements in the operation of the NPP. The observed variations in 14C activities are also due to fluctuations in the local Suess effect, which can be observed mainly during winter months [e.g. due to heating of houses and increased occur- rence of atmospheric (temperature) inversions].