Radiocarbon in the Atmosphere of the Žlkovce

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 pressurized 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 operation 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 concentrations 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 associated 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]. The Suess effect is much stronger in the Bratislava 14C record (Figure 2), reflecting contributions to air pollution from local industries, including oil- fired energy production, domestic heating, and vehicle emissions. The quality of the Bratislava air has improved, however, during recent years when compared with the situation before 1990.

The average monthly ∆14C values at the Žlkovce station have been showing maxima in summer months (June–September) and minima in winter (December–January). These variations are no lon- ger associated with the stratospheric input of bomb-produced 14C to the troposphere, but by the regional Suess effect, which decreases winter 14C levels compared to summer values (Povinec et 14 al. 2012). The fossil CO2 impact on the ∆ C values is clearly seen in Figure 3, when several deep minima were observed, e.g. in winter 1994. This was due to specific meteorological conditions in the Jaslovské Bohunice–Žlkovce region when local dispersion conditions were under an influence of low pressure, which kept the contaminated air in the region for over 2 weeks (as was observed at the Jaslovské Bohunice meteorological station).

A long-term trend in the ∆14C of the clean air over the Europe has been measured at the Jungfraujoch (3450 m asl, Swiss Alps) monitoring station (Levin and Kromer 2004; Levin et al. 2008, 2010). We 358 P P Povinec et al.

14 Figure 2 Radiocarbon monthly variations in the atmospheric CO2 of Bratislava and Žlkovce. Clean-air C data for the high-altitude Jungfrajoch station (Swiss Alps) are also shown (Levin et al. 2010).

Figure 3 Radiocarbon trend in the atmospheric CO2 and tree rings collected at the Žlkovce monitoring station situated 5 km ESE from the Bohunice NPP. Clean-air 14C data for the high-altitude Jungfrajoch station (Swiss Alps) are also shown (Levin et al. 2010). see from Figure 2 that up to 1994 the ∆14C values at the Žlkovce station were significantly below the clean-air values represented by the Jungfrajoch station (except for 1989 and 1991). In recent years, however, the European clean-air trend line is closer to the Žlkovce ∆14C line. Since 1995, the contamination of the air by fossil CO2 was lower in Slovakia due to a decrease in the CO2 emission rates from about 60 Tg CO2/yr in 1990 to about 40 Tg CO2/yr in 1994 and afterwards (MESR 2005). 14C in the Atmosphere of the Bohunice NPP 359

Tree-Ring Radiocarbon Data One-year time resolution of tree-ring data, which are based on the AMS 14C measurements with only submilligram samples, opened a new possibility to investigate the integrated 14C impact of NPPs on the environment. In the past, we used different 14C biota monitors, even exotic ones such as bottled wines (Burchuladze et al. 1980, 1989). The advantage of tree-ring samples is that they enable the re- construction of a 14C chronology in a given locality over several decades. The results of 14C activity measurements in a tree-ring sequence from a lime tree (Tillia cordata), collected at the Žlkovce village, are presented in Figure 3. Annual 14C averages calculated from the atmospheric data compared with the tree-ring 14C data show good agreement, although some differences in recent years can be seen between the atmospheric and tree-ring data. Biota samples, like tree rings, which are growing during the spring-summer-autumn seasons, could not record atmospheric 14C levels observed during the winter. As tree rings integrate the 14C concentrations during a long period, they are not sensitive enough to record short-term increases caused by single releases of 14C from the NPP (or decreases due to the Suess effect).

The annual mean atmospheric ∆14C values measured in Žlkovce are compared with the long-term trend annual mean ∆14C line representing clean air over Europe, obtained from the high-altitude clean-air station at Jungfraujoch (3450 m asl) data set (Levin and Kromer 2004; Levin et al. 2008, 2010). Up to 1994, the annual mean ∆14C values were about 48‰ below the ∆14C values in clean air. In recent years, however, the observed 14C values are comparable with the levels observed at the Jungfraujoch station, as well as with the surface air measured at the Košetice station, Czech Repub- lic (Svetlik et al. 2010; Povinec et al. 2015).

Due to wood biomass accumulation from May to September, a small decrease in the 14C levels in 14 tree-ring samples when compared with atmospheric CO2 levels can be expected (Figure 3). The tree-ring method is limited by the age of the tree, but 14C data over several decades can be recon- structed from old trees. The 14C tree-ring data can provide information on average 14C concentrations during spring and early summer (when new wood, light in color and usually softer, is formed, so- called softwoods) and during the late summer and autumn growing season (when new cells formed are smaller, more dense, and have darker thicker walls, so-called hardwoods). Therefore, short-term effects like the injection of stratospheric masses, a local Suess effect, or short-term releases from NPPs are usually not clearly visible in tree rings. The tree rings during the growing season can integrate impacts from both nuclear and fossil CO2 sources on the local environment, indicating long- term trends in 14C biospheric concentrations, and can be used to assess radiation doses to the public.

Comparison of Slovak and Czech Radiocarbon Data around NPPs

The Dukovany NPP (Czech Republic) has four nuclear reactors VVER-440 MWel of the same type as the Bohunice NPP. They were put into operation between 1985 and 1987. The Dukovany NPP is located close to the Dukovany village, in the southeastern part of the Czech Republic (49°05′06″N, 16°08′56″E), about 120 km northwest of the Bohunice NPP (Figure 1).

In the period of 2007–2008, 10 samples of stinging nettle (Urtica dioica) were collected and analyzed in the surroundings of the Dukovany NPP (Table 1). The samples were collected at locations east of the Dukovany NPP (in the direction of prevailing winds from the NPP). Distances of sampling points from the NPP were in the range of 0.7 to 1.2 km, where the maximum 14C activity surplus could be expected. The Dukovany data could be compared with data from reference lo- calities with similar estimated local Suess effect, but no Dukovany NPP influence. Stinging nettle (or common nettle) is an herbaceous perennial flowering plant having an exiguous annual biomass 360 P P Povinec et al. supply and a thin root system. The vegetation period of this plant in central Europe is between the end of March and the beginning of November. Table 1 Radiocarbon in stinging nettle (Urtica dioica), collected during 2007–2008 at the locations east of the Dukovany NPP and at the reference sites. Activities are reported in ‰ of Δ14C. Dukovany NPP Reference sites Average 37.9 33.2 Median 35.5 33.4 Standard deviation 7.2 4.7 Variation 52.2 22.1 Number of observations 10 6 Observed maximum 52.7 39.7 Observed minimum 30.0 26.6

The observed Δ14C levels in the nettle samples ranged from 30.0 to 52.7‰, with an average value of 37.9‰ (Table 1). Corresponding values for a reference site with no reactor impact were from 26.6‰ to 39.7‰, with an average value of 33.2‰. The Dukovany NPP contribution for the years 2007–2008 is thus estimated to be 4.7‰, and is in agreement with Δ14C data collected from the pre- vious sampling campaign carried out in 2002–2005 when leaves of deciduous trees were collected and analyzed. Utilization of nettle plants reduced variations in 14C activities by half in comparison to that of tree-leaf samples (Svetlik et al. 2007). This could be due to the relatively short time interval of biomass accumulation in tree leaves (about 4–5 weeks in April and May), when the activity of 14 atmospheric CO2 changes relatively quickly (Figure 3). The biomass accumulation in tree leaves also depends on local microclimatic conditions (atmospheric precipitation, soil moisture, and sun- 14 light exposure), which can cause shifts in the period of atmospheric CO2 uptake by tree leaves (Magnusson et al. 2004; Roussel-Debet et al. 2006).

The impact of the Dukovany and Bohunice NPPs on the local environment, as obtained from 14C analyses of nettle and tree-ring samples, respectively, is around 5‰, and is very close to the statistical limit. Such a low increase in 14C concentration around both NPPs has a negligible contribution to the radiation dose delivered to local inhabitants (by about three orders of magnitude below the dose limit for the public; Povinec et al. 2009).

CONCLUSIONS 14C variations in the atmosphere observed at the Žlkovce monitoring station have been influenced by 14 releases of C from the Bohunice NPP, as well as by non-radioactive CO2 from the combustion of fossil fuels. The observed Δ14C variations have attenuating amplitudes and decreasing mean values with time. The deep winter minima are due to the local Suess effect, while several 14C emissions from the Bohunice NPP can be identified. However, the total impact of the Bohunice NPP on 14C 14 levels in Žlkovce, as represented both by the atmospheric CO2 and the tree-ring record, is negligible. In recent years, the observed 14C values are comparable with the levels observed at the high- altitude clean-air station at Jungfraujoch.

The impact of the Dukovany NPP (Czech Republic) and the Bohunice NPP (Slovakia) on the local environment, obtained from 14C analyses of nettle and tree-ring samples, respectively, is around 5‰, and very close to the statistical limit. The low increase in 14C concentration around both NPPs is negligible in terms of contribution to radiation dose to local inhabitants (approximately three orders 14C in the Atmosphere of the Bohunice NPP 361 of magnitude below the dose limit for the public; Povinec et al. 2009).

ACKNOWLEDGMENTS The authors are indebted to Prof P Steier of the Vienna University for support during AMS analysis of the tree-ring samples. Part of this research was supported by the International Atomic Energy Agency (project No. TCP-SLR/008), and the EU Research and Development Operational Program funded by the ERDF (projects No. 26240120012, 26240120026 and 26240220004). IS acknowl- edges support from the institutional funding of the Nuclear Physics Institute CAS (RVO61389005).

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