Introduction

The Czech Republic has two nuclear power plants (NPPs) equipped by light water pressurized reactors (LWPR), Temelín and Dukovany, with the installed power output 2 × 1,000 MW and 4 × 440 MW, respectively. The monitoring in the surrounding environment of Czech nuclear power plants Dukovany and Temelín consists of routine determinations, performed by NPP’s staff, and also extended sampling which is performed by research institutions [1–3].

Environmental compartments contain a mixture of two stable carbon isotopes (12C and 13C) and one radioactive isotope 14C (radiocarbon). This radionuclide of global occurrence and a half-life of 5,730 year is partly of anthropogenic origin. In the nature, 14C is produced by nuclear reactions generated by cosmic rays in the atmosphere [4, 5].

At present, the most significant artificial sources of radiocarbon in the environment are effluents from nuclear power facilities, even though it is a minor contribution in comparison with its natural production. Nevertheless, radiocarbon is responsible for dominant contribution to the collective effective dose from all radionuclides released by nuclear power plants (NPP) with light-water pressurized reactors (LWPR) during normal operation [6]. Part of 14C is discharged by NPPs into the surrounding environment during normal operation as gas effluents, in the case of LWPR it reaches about 95% of released 14C [7, 8]. Radiocarbon from the gas releases of the NPP can be captured in the surrounding or dissipated in the atmosphere depending on its chemical form. The stable chemical forms are hydrocarbons with prevailing 14CH4 [7], which is not significantly captured in the vicinity of the NPP and contributes to the increase of the 14C activity level more on a regional or global scale [9]. Abundance of 14CO2 in airborne effluents from LWPR varies between 5 and 43% [7, 8, 10–13]. The biota in the surrounding of NPPs intakes 14CO2, especially during: calm, rainfall, haze, or atmospheric inversion. Radiocarbon in the form of CO2 is assimilated by plant photosynthesis and afterwards transferred also into the food chain [6].

In the last century, nuclear weapons tests were important sources of anthropogenic 14C. Consequently, 14C activity in the atmosphere of the Northern Hemisphere was double the natural level in 1963 [14, 15]. Since the nuclear moratorium on atmospheric nuclear bomb tests was signed in 1963, the 14C concentration in the atmosphere has been decreasing due to its intensive transfer to oceanic and terrestrial carbon reservoirs [16–23]. Currently, 14C activity in the atmosphere is gradually approaching the level that was seen before the nuclear age.

There is another significant anthropogenic influence on 14C levels in the atmosphere and biosphere - the Suess effect [24]. This effect causes a decrease of the 14C activity on global, regional, and local scales as a result of the dilution of the carbon isotopic mixture by fossil carbon [5, 17, 18, 25–33].

Time behavior of atmospheric 14CO2 activity can be characterized by linear interannual decrease since the beginning of 1990s. This long-term trend is caused by global Suess effect. Seasonal fluctuations with minima during cold parts of year are amplified by more intensive local and regional Suess effect, what is evident from the time series of atmospheric 14CO2, see Fig. 1 [22, 29, 34].

Fig. 1
figure 1

Time series of atmospheric 14CO2: Prague-Bulovka (local and regional Suess effect), Košetice (regional Suess effect), and Jungfraujoch (clean-air Alpine monitoring station, simulation). 14C activities are reported (y-axis) in ‰ of Δ14C [54]. Interannual decrease of 14C activities is visible in all time series [22, 29, 34]

Full size image

Materials and methods

Monitoring of 14C surrounding the NPPs and in reference localities can be performed by two possible ways:

  1. 1.

    Monitoring of atmospheric air. This monitoring can be performed during whole year, without limitations given by vegetation period. Drawbacks of such monitoring are greater time and economy requirements and also limited number of monitoring facilities in fixed localities. Such monitoring can provide the samples with exactly known duration of sampling period [28, 34–39].

  2. 2.

    Monitoring of 14C activity in biota [39–47]. Time interval of 14C activity record depends on period of biomass accumulation in a given plant. Samples (parts of plants) should be selected with care to avoid contamination by biomass originated in previous years. During sampling, a great number of biota samples can be collected and the NPP surrounding area can be covered by dense network of sites, if necessary. Positions of sampling localities can be changed easily according to atmospheric dissipation conditions in the given year. Obviously, 14C monitoring in biota is restricted on the part of vegetation period.

Annual sampling of biota and agricultural products for 14C activity monitoring in the surrounding of both Czech NPPs and in reference areas was launched in 2002 by Nuclear Physics Institute AS CR in the cooperation with the National Institute of Radiation Protection.

Sampling of biota was performed in the sites of Temelín and Dukovany NPPs at the distance from 0.5 to 9 km during the period of 2002–2005. Prevailing part of biota samples were leaves of deciduous trees, analogically to published studies performed in the vicinity of other NPPs [41–47]. Leaves of Sambucus nigra (pipe tree) were preferred, because this tree is widespread in the Czech Republic and it can be easily identified. Agricultural plants (spikes of wheat and barley) made a smaller part of collected samples [1]. In the vicinity of NPPs Temelín and Dukovany several roads are situated and there are also some smaller cities and villages [1]. The influence of local Suess effect could be estimated/quantified with difficulties, due to the lack of data on local fuel combustion and density of surrounding traffic. To compare 14C activities in the NPPs surroundings with relevant 14C activity level in the environment two types of areas with different load of Suess effect were selected. It can be supposed that actual size of Suess effect influencing NPPs surrounding will be in the interval demarked by these two types of reference areas [1, 29]. (A) Localities, in greater distances from fossil carbon sources, where only small local influence of Suess effect was supposed (Košetice, Kleť, Sudoměřice u Bechyně, Krokočín). (B) Localities where extended local Suess effect influence can be expected (bordering parts of Prague).

Since 2007 the type of sampling material has been changed to above ground parts of Urtica dioica (stinging nettle) collected in November. Stinging nettle or common nettle is a herbaceous perennial flowering plant, native to Europe, Asia, northern Africa, and North America, and is the best-known member of the nettle genus Urtica [48]. This perennial plant has only exiguous annual biomass supply and thin root system. Vegetation period of this plant is between the end of March and beginning of November in the Central Europe. Besides, this herb forms internodia with relatively uniform rate. On each internodium biomass cumulates from its origin till the end of vegetation period [49]. Hence, more detailed time resolution of 14C activity advancement could be achieved during vegetation period using this plant, if necessary.

Dusty biota samples were washed with 10% HCl and distilled water, dried (105 °C) and homogenized. Washing with diluted HCl was suspended if presence of dust on a sample surface was not evident. Dried samples were combusted and the produced CO2 was purified. In the NPI AS CR a routine of sample processing based on benzene synthesis was followed [1, 29, 30, 50–52]. 14C activity was measured by liquid scintillation spectrometer Quantulus 1,220 in 3 mL low-background Teflon vials. Total counting time was about 3,000 min per sample. Benzene distributed by Sigma-Aldrich (spectrophotometric grade) was used as a blank sample. Calibration was performed using oxalic acid NIST (NBS) HOX II, SRM 4990-C [53]. Resulting activities were reported in ‰ of Δ14C following Stuiver-Polach convention [54]. Combined uncertainties of observed values (in the interval 6.1–8.1 ‰ of Δ14C) include the individual uncertainties of measured sample, blank sample, calibration, quenching corrections, and uncertainty of the δ13C value [55].

Results

During the period of 2002–2005, 77 biota samples for 14C analyses were collected in the vicinity of NPPs Dukovany (EDU) and Temelín (ETE). Likewise, 30 samples were collected in reference areas influenced with slight (A) and extended (B) local Suess effect. Basic statistical parameters of results (EDU, ETE, A, B) are reported in Table 1. Standard deviations of couples EDU-B, ETE-A, and ETE-B are equal on the base of F tests performed (Fischer-Snedecor test). In the next step results from each type of area were compared utilizing t test (student test, unpaired, probability of first kind of observation error 5%), see Table 2.

Table 1 Basic statistical parameters of biota samples collected in the vicinity of NPPs Dukovany (EDU) and Temelín (ETE) and in reference localities with slight (A) and extended (B) local Suess effect
Full size table
Table 2 Comparisons of activities of observed results from each type area (group of the data), values of T reported in table: T o (observed) and T c (critical); probability of first kind of observation error 5%
Full size table

In the period of 2007–2008 samples of Urtica dioica were collected in the surrounding of EDU7,8 and ETE7,8 (18 samples, adjacent – in distances from 0.7 to 1.2 km, without preferred direction). In the comparison with previous sampling, distances were reduced to achieve areas with expected maximal possible 14C activity surplus [41, 43, 44]. Reference samples (11 samples, 10–20 km from a given NPP, without preferred direction) from more distant surrounding were collected in localities with estimated densities of roads and traffic loads similar to those in the direct vicinity of NPPs [41]. Basic statistical parameters of results (EDU7,8, ETE7,8, refEDU, refETE) are reported in Table 3. Variances for all couples EDU7,8-ETE7,8, EDU7,8-refEDU, ETE7,8-refETE, and refEDU-refETE do not differ significantly on the base of F-tests performed. In the next step results from each type of area were compared utilizing t-test (unpaired, probability of first kind of observation error 5%), see Table 4.

Table 3 Basic statistical parameters of biota samples collected in the direct neighborhood of NPPs Dukovany (EDU7,8), Temelín (ETE7,8) and in corresponding reference localities (refEDU, refETE) with similar estimated local Suess effect, sampling period 2007–2008
Full size table
Table 4 Comparisons of activities of observed results from each type area (group of the data), values of T reported in table: T o (observed) and T c (critical); probability of first kind of observation error 5%, sampling period 2007–2008
Full size table

Discussion

For the period of 2002–2005, statistical evaluation of the results confirmed significantly greater 14C activity level in biota from both NPPs surroundings in comparison with reference area B (greater load from fossil fuel combustion – bordering parts of Prague), see Table 2. Significant differences were found also between reference localities A and B. Likewise, the difference of 14C activity level between biota from NPP Temelín vicinity and biota from reference area A (minor load from local fossil fuel combustion) was found to be statistically significant also. In the point of view of local Suess effect, it can be supposed that relevant reference 14C activity level for NPPs surroundings with relatively traffic-loaded roads is situated in the interval between reference areas A and B.

Observed values of 14C activity for each type of locality are charged with relatively great variations, probably caused by local Suess effect from surrounding roads, for samples from NPPs surroundings namely. Another reason of fluctuation can be caused also by relatively short time interval of biomass accumulation in leaves of deciduous trees (about four or 5 weeks in April and May). At this part of year, the activity of atmospheric 14CO2 changes rather quickly, what is visible also from Fig. 1 [17, 22, 25, 28, 29, 56]. The exact duration of the period of plant biomass accumulation in tree leaves is depending on the local microclimatic conditions (atmospheric precipitations, soil moisture, and sunlight exposure). Hence, the local microclimatic differences can cause small time shift of the period for dominant atmospheric 14CO2 intake by tree leaves and thus also differences in resulted 14C activities. Additional reason of 14C activity variations in NPPs surroundings is given by relatively greater variation of distances from NPPs stacks (below 9 km) in certain sample collection sites. It can be supposed, that 14C activity surplus in biota at distances exceeding 10 km is minimal [41]. Potentially most influenced zones around NPPs can be probably found in the distance up to 2 km, on the basis of 14C dissipation model [41, 57]. Direct results of 14C of biota monitoring performed in the vicinity of NPPs with boiling water reactors (BWR)Footnote 1 also confirm similar distances from stacks for maximal surplus of 14C activity in biota [43, 44, 58].

In the case of nettle samples (2007–2008), observed activities of 14C seem to be more uniform, namely due to longer biomass accumulation period, probably reducing influence of microclimatic variations. In comparison with previous types of samples, reduced activities of nettle samples are evident for each group. Such difference is given particularly by extended period of biomass accumulation (end of March till beginning of November). During colder seasons, the activity of atmospheric 14CO2 decreases as a result of regional and local Suess effect, see Fig. 1 [5, 17, 18, 25–29]. Likewise, the difference between mean values of 14C activities observed in periods of 2002–2005 and 2007–2008 is partly given also by the interannual decrease of atmospheric 14CO2 activity. This interannual decrease of atmospheric 14CO2 activity is estimated to be about 5 ‰ of Δ14C [34]. Due to large variations of activities, the interannual decrease was not observed (insignificant statistically) when leaves of deciduous trees were used in the period of 2002–2005.

In the case of nettle sampling, difference of several per mille in each group of samples seems to occur between sampling years 2007 and 2008, including reference samples. The corresponding reference areas were selected in the distances 10–20 km from a given NPP, supposing minimal 14C activity surplus at such distances [41, 43, 44]. Utilizing Student test (unpaired, probability of first kind of observation error 5%), a significant difference between vicinity of ETE (ETE7,8) and corresponding reference localities (ref.ETE) was found. Difference of 5.4 ‰ of Δ14C (ETE7,8 - ref.ETE) for nettle sampling is in a good agreement with observed difference 4.8 ‰ of Δ14C (ETE - A, significant for 5% probability of the first kind of observation error) obtained on the base of previous sampling campaign.

Application of nettle plants as a sampling material seems to reduce variations of 14C activities caused by microclimatic differences. Likewise, this sampling material is widespread and can assure relatively long period of 14C activity record.

Due to formation of nettle internodia with relatively uniform rate, time resolution of 14C activity changes (in the surrounding air) in a given year with precision of several weeks can be achieved, if necessary. To validate such possibility, small outdoor experiments are intended in 2011. The group of nettle plants will be partly sealed by a polyethylene bag and exposed to 14CO2 (about several kBq) for several hours in June or July. During November the plants will be collected and the material will be cut node to node. The response curve of 14C activity vertical distribution (in dependence on internodium number) will be compared with the data of a model of accidental release of 14CO2.

Conclusion

Results of 14C monitoring in the biota of the surroundings of NPPs Dukovany and Temelín were briefly reported. On the base of biota monitoring around Czech NPPs a small surplus of 14C activity level in the close surrounding of NPP Temelín was observed for both sampling campaigns (2002–2005, utilizing mainly leaves of deciduous trees, and 2007–2008, sampling of nettle plants). In comparison with reference areas minimally locally loaded by fossil fuel combustion (A), the mean numeric value of the excess is 4.8 ‰ of Δ14C, (t test, unequal variations, probability of first kind of observation error 5%). Applying a new sampling routine in the period of 2007–2008, the surplus of 5.4 ‰ of Δ14C was observed in the neighborhood of ETE compared with reference sites in distance 10–20 km from the NPP (t test, equal variations, probability of first kind of observation error 5%). Nevertheless, this excess of 14C activity was not significant for 1% probability of first kind of observation error. Low/insignificant 14C activity surplus in biota around these NPPs with LWPRs is given by small abundance of 14CO2 form in gas releases of 14C from both NPPs (about 5%). Other 14C chemical forms, with prevailing 14CH4, are not responsible for radiocarbon intake by surrounding biota.

During campaign in 2007–2008, samples of nettle plants were collected. With regard to the extended period of biomass accumulation by this plant, resulting activities in each group of samples seems to be charged by smaller variations compared to classical types of biota samples. Several week period of biomass accumulation in tree leaves can be also in the partial coincidence with the several weeks of service period of reactor outage in a given NPP (14C releases are minimal at such case).

Campaign performed with a new type of sampling material covers only 2 years and two localities in Czech Republic. Due to a widespread occurrence of the nettle plants, a validation of this unconventional sample material suitability also in other geographical positions and slightly different climatic conditions would be a benefit.