Journal of Environmental Radioactivity 145 (2015) 48e57
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Anthropogenic radionuclide fluxes and distribution in bottom sediments of the cooling basin of the Ignalina Nuclear Power Plant D. Mar ciulioniene_ a, *, J. Ma zeika a, B. Luksiene_ b, O. Jefanova a, R. Mikalauskiene_ a, R. Paskauskas a a b
State Research Institute Nature Research Centre, Akademijos str. 2, LT-08412 Vilnius, Lithuania State Research Institute Center for Physical and Technological Sciences, Savanoriu av. 231, LT-02300 Vilnius, Lithuania
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 June 2014 Received in revised form 5 March 2015 Accepted 6 March 2015 Available online
Based on g-ray emitting artificial radionuclide spectrometric measurements, an assessment of areal and ksiai vertical distribution of 137Cs, 60Co and 54Mn activity concentrations in bottom sediments of Lake Dru was performed. Samples of bottom sediments from seven monitoring stations within the cooling basin were collected in 1988e1996 and 2007e2010 (in JulyeAugust). For radionuclide areal distribution analysis, samples from the surface 0e5 cm layer were used. Multi sample cores sliced 2 cm, 3 cm or 5 cm thick were used to study the vertical distribution of radionuclides. The lowest 137Cs activity concentrations were obtained for two stations that were situated close to channels with radionuclide discharges, but with sediments that had a significantly smaller fraction of organic matter related to finest particles and consequently smaller radionuclide retention potential. The 137Cs activity concentration was distributed quite evenly in the bottom sediments from other investigated monitoring stations. The ksiai were measured in the highest 137Cs activity concentrations in the bottom sediments of Lake Dru period of 1988e1989; in 1990, the 137Cs activity concentrations slightly decreased and they varied insignificantly over the investigation period. The obtained 238Pu/239,240Pu activity ratio values in the ksiai represented radioactive pollution with plutonium from nuclear bottom sediments of Lake Dru weapon tests. Higher 60Co and 54Mn activity concentrations were observed in the monitoring stations that were close to the impact zones of the technical water outlet channel and industrial rain drainage ksiai system channel. 60Co and 54Mn activity concentrations in the bottom sediments of Lake Dru significantly decreased when operations at both INPP reactor units were stopped. The vertical distribution of radionuclides in bottom sediments revealed complicated sedimentation features, which may have been affected by a number of natural and anthropogenic factors resulting in mixing, resuspension and remobilization of sediments and radionuclides. The associated with particles 137Cs flux was 129 Bq/(m2 year). The 137Cs transfer rate from water into bottom sediments was 14.3 year1 (or, the removal time was 25 days). The Kd value for 137Cs in situ estimated from trap material was 80 m3/kg. The associated with particles 60Co flux was 21 Bq/(m2 year), when 60Co activity concentration in sediment trap particles was 15.7 ± 5 Bq/kg. 60Co activity concentration in soluble form was less than the minimum detectable activity (MDA ¼ 1.3 Bq/m3). Then, the conservatively derived Kd value for 60Co was >90 m3/kg. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Bottom sediment 60 Co 54 Mn 137 Cs 239,240 Pu Areal and vertical distribution
1. Introduction The Ignalina Nuclear Power Plant (INPP) is located in the northeastern part of Lithuania near the borders with Belarus and Latvia. The two reactor units, Unit 1 and Unit 2, were put into operation in
* Corresponding author. _ E-mail address: radeko@ar.fi.lt (D. Mar ciulioniene). http://dx.doi.org/10.1016/j.jenvrad.2015.03.007 0265-931X/© 2015 Elsevier Ltd. All rights reserved.
December 1983 and August 1987, respectively. As a part of the obligations of the European Union Accession Treaty, Lithuania was required to shut down Units 1 and 2 of the INPP and to decommission them as soon as possible. Unit 1 was shut down on December 31, 2004 and Unit 2 on December 31, 2009. ksiai as a natural During operation, the INPP used Lake Dru reservoir for cooling water and produced 307.9 billion kWh of electricity: Unit 1e136.9 billion kWh and Unit 2e170.2 billion kWh. The total amount of electricity sold was 279.8 billion kWh.
D. Marciulioniene_ et al. / Journal of Environmental Radioactivity 145 (2015) 48e57
The INPP operational history and radiation in the environment monitoring data showed that INPP was operated safely with a very low radiation impact on terrestrial ecosystems. This is evident from the results of the extensive investigations of the distribution of artificial gamma-ray emitting radionuclides in the top soil layer in the vicinity of the INPP and distant regions in Lithuania in 1996e2008. The mean 137Cs activity concentrations in the top soil layer in the vicinity of the INPP were found to be significantly lower than those in other studied regions severely impacted by global fallouts. 60Co, the INPP origin radionuclide, was detected in some samples only in 1996 and 2000. The activity concentration of 60Co was found to be in the range of 0.4e7.0 Bq/kg at the sampling locations nearest to the INPP (Luksiene_ et al., 2012b). However, the INPP itself as a big industrial complex with complementary objects in the whole region of urban-industrial development had an influence on the environment and on the Lake ksiai ecosystem in particular. Structural as well as functional Dru ksiai occurred as a result of changes in the ecosystem of Lake Dru growing anthropogenic pressure expressed as thermal and chemical pollution, especially after the commissioning of the INPP Unit 2 te_ and Jankevi (Mar ciulioniene_ et al., 1992; Jankevi ciu cius, 1992; _ 1992). Changes occurring in a lake may influence Trainauskaite, the particulate material and bottom sediments composition which commonly can be comprised of three basic components (Garunkstis, 1975; Kelts, 1988; Schnurrenberger et al., 2003): (1) Clastic sediments (could be referred to as mineral/terrestrial), (2) Chemical sediments (could be referred to as carbonates), and (3) Biogenic sediments (could be referred to as organic carbon). Many artificial radionuclides are very quickly associated to settling particles when they reach water bodies and are distributed consequently in a certain interval of bottom sediments because of diffusion, mechanical mixing and the effects of bioturbation. The association of radionuclides to sediments is governed by several key processes (Kansanen et al., 1991): sedimentation of insoluble particles of aerosols containing radionuclides; adsorption or precipitation of radionuclides on inorganic compounds (carbonates, clay particles); sedimentation of radionuclides with humic matter; sedimentation of radionuclides with autochthonous matter after their assimilation or absorption on cell surface; direct uptake of radionuclides by biota occurring on the surface of bottom sediments; direct adsorption of radionuclide on surface of sediments. The activities of annually discharged radionuclides (137Cs, 134Cs, 54 Mn, 58Co, 60Co, 59Fe, 51Cr, 95Zr, 95Nb, 89Sr, 90Sr, 131I mainly, and 3H and 14 C for only the last period of INPP operation) from the INPP to Lake ksiai in 1984e2010, as summarized from Reports of the EnviDru ronment Protection Laboratory of the INPP, are presented in Fig. 1.
ksiai in Fig. 1. Activities of radionuclides discharged from the Ignalina NPP to Lake Dru 1984e2010 (Van der Stricht and Janssens, 2001, 2005, 2010).
49
The annual discharge rates of most radionuclides were variable over the course of time. Additionally, the radioactive discharge monitoring system also changed: in the period 1983e1991, only the total activity discharge rate was reported with no indication of specific radionuclides, whilst since 1992, the activity discharge rates of specific radionuclides have been estimated. The key radionuclides in liquid discharge were 137Cs and 60Co. Based on annual discharge rates, the following years of INPP operation have to be mentioned: 1989 with total activity discharged 1.5Eþ10 Bq, 1990 with total activity discharged 2.6Eþ10 Bq, 1992 with maximum estimated 60Co discharge rate of 8.1Eþ09 Bq/year and 1995 with maximum estimated 137Cs discharge rate of 6.5Eþ09 Bq/year. The time span from 1989 to 1997 can be distinguished by highest ksiai. discharge rates of radionuclides from the INPP to Lake Dru Later, the annual discharge rates of radionuclides were significantly reduced due to technical development, operational and safety culture upgrading in the frame of many international projects. The aim of this study was to determine the areal distribution of 137 Cs, 60Co, 54Mn and 239,240Pu in 1989e1996 and 2007e2009, as well as the vertical distribution of 137Cs and 60Co in 1989, 1996 and ksiai e the cooling basin of 2010 in bottom sediments of Lake Dru the Ignalina NPP. On the basis of analysis and comparison of data, the study aimed to assess the distribution of radionuclides in Lake ksiai bottom sediments in the periods when two reactor units Dru and one reactor unit of the INPP were in operation. 2. Material sampling and methods The samples for the evaluation of the areal distribution of 137Cs, Co, 54Mn and 239,240Pu in bottom sediments were taken only from the top 0e5 cm layer using the Ekman bottom grab sampler (the area of sampler 20 20 cm, the total length up to 30 cm) in the lake ksiai monitoring stations 1e7 in JulyeAugust in 1988e1996 and Dru only 1, 4, 6 ant 7 monitoring stations in 2007e2009 (Fig. 2). The samples for the evaluation of the vertical distribution of 137 Cs and 60Co in bottom sediments were taken by the Ekman bottom grab sampler (up to 20e30 cm in depth) in 1989 and 2010, in JulyeAugust. The cores were separated into 5 cm thick slices in € 1989 and into 3 cm thick slices in 2010. In 1996, the Niemisto gravity corer (inner diameter 54 mm, up to 80 cm in length) was used for sediment sampling. The cores to 60e80 cm were taken and separated into 2 cm thick slices. The samples were dried in the laboratory at 20e22 C, were homogenized using a ceramic pestle, and were passed through sieves with holes 0.8 mm in diameter (Marciulioniene_ et al., 1992; Trapeznikov et al., 2007). Measurements of 137Cs and 60Co in the samples of bottom sediments were accomplished on gamma-ray spectrometers with Ge(Li) and HPGe detectors at the Centre for Physical Sciences and Technology, Vilnius, and at the Nature Research Centre, Vilnius, as described in Gudelis et al. (2000). However the majority of sediment samples were measured by gamma ray spectrometers (two) with HPGe GWL-series detectors as the dried and sliced sediment samples were small; actually maximum filled measuring vials volume was 3 cm3, and apart of anthropogenic radionuclides the Pb-210 was radionuclide of concern. For calibration the working solution in the water was prepared from Amersham reference solution (radionuclides presented, Mn54, Co-57, Zn-65, Sr-85, Y-88, Ba-133, Cs-137, Ce-139) through the dilution of initial material. Using this solution there were three standards prepared in non-liquid matrices (perlite, sediment and quartz sand) of densities of 0.6, 1, and 1.4 g/cm3 for density correction and three standards of density 1 g/cm3 of three filling heights of 10, 20 and 30 mm for geometry correction. From the all recorded full-energy peaks only ones attributed to single-photon 60
50
D. Marciulioniene_ et al. / Journal of Environmental Radioactivity 145 (2015) 48e57
ksiai bottom sediments distribution with sites for bottom sediment cores collection (IRD-1, 2 e industrial rain drainage channel, TWO e technical water Fig. 2. Scheme of Lake Dru outlet channel, 1, 2, 3, 4, 4a, 6, 7 e monitoring stations): 1 e till (morainic clay, loam and sandy loam), 2 e carbonate sediments, 3 e gravel, 4 e coarse-grained sand, 5 e variousgrained sand, 6 e fine-grained sand, 7 e silty sand, 8 e coarse silt mud, 9 e fine silt mud, 10 e silt-pelitic mud. Modified after Joksas et al., 1995; Galkus, 1997.
emitting radionuclides Cs-137, Mn-54, Zn-65, and Co-57 which had no significant coincidence-summing were considered. In order to define counting efficiency for Pb-210 and to resolve the coincidence-summing effect for Co-60, other single radionuclide solution standards (namely Pb-210 and Co-60) were used for calibration experiments with the same matrices. By measuring actual dry sediment samples the quantity of material was enough for full filling of vial; and the dry material density was not significantly varying between 0.95 and 1.1 g/cm3. In this case geometry and density corrections were not essential, and radionuclide-specific (Mn-54, Co-60, Cs-137 and Pb-210) counting efficiency values for 30 mm high and for density of 1 g/cm3 were used. For the comparison radionuclides activity of sediment samples were recalculated using calibration curves and counting efficiency values based on commercially available multi radionuclides (Mn-54, Co-57, Co-60, Y-88, Ba-133, Cs-137, Pb-210, and Am-24) and single radionuclide (Co-60, Cs-137 and Pb-210) standard sources of CBSS 2 type (Czech Metrology Institute), where radioactive material was by producer homogeneously dispersed in silicone resin (mass e 2.94, density 0.98 g/cm3, volume e 3 cm3, matrix composition, mass ratio: C e 0.324, H e 0.0816, O e 0.216, Si
e 0.379). The deviation of radionuclide activity values using two calibration approaches was in the range of expanded uncertainty evidencing similar absorption properties of sediment and silicon resin (actually density and atomic composition of both materials is also similar). The detection limit for the counting time of 200,000 s was about 0.014 Bq for 137Cs and 0.065 Bq for 210Pb, while measurement errors did not exceed 8% and 15% for 137Cs and 210Pb respectively. The normal precision of gamma spectrometric measurements of our laboratories was recognized during the comparison exercise of 2013 organized on national level by Lithuanian Metrology Inspectorate and Centre for Physical Sciences and Technology, and intercomparison of 2014 organized by the STUK e Finish Radiation and Nuclear Safety Authority (Klemola et al., 2014). To determine plutonium analytes, sample destruction was inevitable. Organic matter was decomposed by heating at 550 C overnight and later to 700 C for 2 h and organic matter content on loss of ignition was determined. Samples were spiked with 0.0125 Bq 242Pu as yield monitor. Acid leaching with aqua regia and concentrated HNO3 was employed. The chemical purification was based on two separation column methods and electrodeposition of
D. Marciulioniene_ et al. / Journal of Environmental Radioactivity 145 (2015) 48e57
Pu analytes onto one-side polished stainless steel discs (Luksiene_ et al., 2006, 2012a; Druteikiene_ et al., 2011). The initial separation of Pu from uranium/fission products and other matrix elements was accomplished on a Bio RaD AG-1X8 anion exchange column (Talvitie, 1971; Holm, 1984; Xu et al., 2014). The eluted from the anion exchange column Pu solution was evaporated and prepared for further purification by the extraction chromatography method (Horwitz et al., 1995). The plutonium isotopic composition (238Pu and 239,240Pu) and their amount in the samples were determined by using the semiconductor Alpha spectrometer OctetePlus with an ORTEC large square (450 mm2) Si detector (BU-020-450-AS) with a resolution capability of 20 keV. The spectra were analyzed applying AlphaVision and Maestro programs. The detection limit for the counting time of 86,400 s was about 103 Bq for 239,240Pu. Some of the samples (usually from double cores) were examined for the main components of the sedimentary matrix (Bengtsson and Enell, 1986). Firstly, the samples were slowly dried at 50e80 C and later at 105 C till constant weight, disaggregated and sieved to pass through a 0.25 mm mesh. Organic matter was estimated using loss of weight on ignition at 850 C, carbonates by titration with HCl and clastic component of sediment matrix were calculated. The water content and dry bulk density of sediments were estimated by weighing standard volume samples dried at 105 C. The main sources and fluxes (F) of anthropogenic radionuclides in the lake system are the following (Wieland et al., 1993): total input of radionuclides into the lake with transfer from the catchment and from direct fallout (I), radionuclide outflow from the lake with surface water outflows (o), exchange of radionuclides in soluble form by vertical mixing of water between epilimnion and hypolimnion (e), radionuclide sorption to suspended particulates (s), radionuclide uptake by bottom sediments in “wateresediment” interface (u), particular processes of radionuclides transfer from bottom sediments to water, i.e., resuspension and remobilization (r). For a stratified water body, the radionuclides balance can be expressed as follows:
dI ¼ ðFi Fe Fu þ Fr Þ FsðepiÞ dt . Bq m2 day for epilimnion ; Fo
(1)
dI ¼ FsðepiÞ FsðhypoÞ þ ðFe Fu dt . þ Fr Þ Bq m2 day for hypolimnion :
(2)
Total input of radionuclides in a lake can be evaluated as total P activity ð aÞ in the lake water column h:
I¼
X
Z a¼
Cw þ Cp S dh
h 0
. Bq m2 ;
(3)
where S e particulates concentration in water column. Although the fraction of 137Cs associated with particles is up to 5e10% of total 137Cs activity presented in the water column, the particle sedimentation is the main process of 137Cs removal from the lake water. Therefore, the radionuclide flux (Fs) can be associated with the particle flux (Fp):
Fs ðtÞ ¼ Fp ðtÞ Cp ðtÞ
. Bq m2 day :
(4)
The radionuclides transfer from water into bottom sediments rate (lR) or removal time (tR) can be evaluated:
lR ¼ 1=tR ¼ Fs ðtÞ=IðtÞ
day1 :
51
(5)
Because inequality Cw [ CpS (when S ¼ 1e5 mg/l) is valid, radionuclides transfer from water into bottom sediments rate is expressed:
lR ¼ Fp ðKd =hÞ:
(6)
3. Results and discussion 3.1. The areal distribution of anthropogenic radionuclide activity ksiai concentrations in the bottom sediments of Lake Dru The data on the areal distribution of anthropogenic radionuclides activity concentrations in the top layer of bottom sediments ksiai are presented in Figs. 3e7. from Lake Dru Component-based analysis of bottom sediments started already at the construction stage of INPP (Tamosaitis et al., 1986; Tamosaitis nas, 1992). Later, a more detailed lacustrine sediment and Bajoru typology in the form of a lithological map (Fig. 2), based mainly on grain size analysis with ten types of sediments, was presented (Galkus, 1997). These sediment types were distinguished on the basis of following particle size intervals: median diameter of particles D50 > 1 mm e gravel; 1e0.3 mm e coarse sand; 0.3e0.063 mm e fine sand; 0.063e0.03 mm e coarse silt sediments; 0.03e0.01 mm e fine silt sediments;
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Anthropogenic radionuclide fluxes and distribution in bottom sediments of the cooling basin of the Ignalina Nuclear Power Plant D. Mar ciulioniene_ a, *, J. Ma zeika a, B. Luksiene_ b, O. Jefanova a, R. Mikalauskiene_ a, R. Paskauskas a a b
State Research Institute Nature Research Centre, Akademijos str. 2, LT-08412 Vilnius, Lithuania State Research Institute Center for Physical and Technological Sciences, Savanoriu av. 231, LT-02300 Vilnius, Lithuania
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 June 2014 Received in revised form 5 March 2015 Accepted 6 March 2015 Available online
Based on g-ray emitting artificial radionuclide spectrometric measurements, an assessment of areal and ksiai vertical distribution of 137Cs, 60Co and 54Mn activity concentrations in bottom sediments of Lake Dru was performed. Samples of bottom sediments from seven monitoring stations within the cooling basin were collected in 1988e1996 and 2007e2010 (in JulyeAugust). For radionuclide areal distribution analysis, samples from the surface 0e5 cm layer were used. Multi sample cores sliced 2 cm, 3 cm or 5 cm thick were used to study the vertical distribution of radionuclides. The lowest 137Cs activity concentrations were obtained for two stations that were situated close to channels with radionuclide discharges, but with sediments that had a significantly smaller fraction of organic matter related to finest particles and consequently smaller radionuclide retention potential. The 137Cs activity concentration was distributed quite evenly in the bottom sediments from other investigated monitoring stations. The ksiai were measured in the highest 137Cs activity concentrations in the bottom sediments of Lake Dru period of 1988e1989; in 1990, the 137Cs activity concentrations slightly decreased and they varied insignificantly over the investigation period. The obtained 238Pu/239,240Pu activity ratio values in the ksiai represented radioactive pollution with plutonium from nuclear bottom sediments of Lake Dru weapon tests. Higher 60Co and 54Mn activity concentrations were observed in the monitoring stations that were close to the impact zones of the technical water outlet channel and industrial rain drainage ksiai system channel. 60Co and 54Mn activity concentrations in the bottom sediments of Lake Dru significantly decreased when operations at both INPP reactor units were stopped. The vertical distribution of radionuclides in bottom sediments revealed complicated sedimentation features, which may have been affected by a number of natural and anthropogenic factors resulting in mixing, resuspension and remobilization of sediments and radionuclides. The associated with particles 137Cs flux was 129 Bq/(m2 year). The 137Cs transfer rate from water into bottom sediments was 14.3 year1 (or, the removal time was 25 days). The Kd value for 137Cs in situ estimated from trap material was 80 m3/kg. The associated with particles 60Co flux was 21 Bq/(m2 year), when 60Co activity concentration in sediment trap particles was 15.7 ± 5 Bq/kg. 60Co activity concentration in soluble form was less than the minimum detectable activity (MDA ¼ 1.3 Bq/m3). Then, the conservatively derived Kd value for 60Co was >90 m3/kg. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Bottom sediment 60 Co 54 Mn 137 Cs 239,240 Pu Areal and vertical distribution
1. Introduction The Ignalina Nuclear Power Plant (INPP) is located in the northeastern part of Lithuania near the borders with Belarus and Latvia. The two reactor units, Unit 1 and Unit 2, were put into operation in
* Corresponding author. _ E-mail address: radeko@ar.fi.lt (D. Mar ciulioniene). http://dx.doi.org/10.1016/j.jenvrad.2015.03.007 0265-931X/© 2015 Elsevier Ltd. All rights reserved.
December 1983 and August 1987, respectively. As a part of the obligations of the European Union Accession Treaty, Lithuania was required to shut down Units 1 and 2 of the INPP and to decommission them as soon as possible. Unit 1 was shut down on December 31, 2004 and Unit 2 on December 31, 2009. ksiai as a natural During operation, the INPP used Lake Dru reservoir for cooling water and produced 307.9 billion kWh of electricity: Unit 1e136.9 billion kWh and Unit 2e170.2 billion kWh. The total amount of electricity sold was 279.8 billion kWh.
D. Marciulioniene_ et al. / Journal of Environmental Radioactivity 145 (2015) 48e57
The INPP operational history and radiation in the environment monitoring data showed that INPP was operated safely with a very low radiation impact on terrestrial ecosystems. This is evident from the results of the extensive investigations of the distribution of artificial gamma-ray emitting radionuclides in the top soil layer in the vicinity of the INPP and distant regions in Lithuania in 1996e2008. The mean 137Cs activity concentrations in the top soil layer in the vicinity of the INPP were found to be significantly lower than those in other studied regions severely impacted by global fallouts. 60Co, the INPP origin radionuclide, was detected in some samples only in 1996 and 2000. The activity concentration of 60Co was found to be in the range of 0.4e7.0 Bq/kg at the sampling locations nearest to the INPP (Luksiene_ et al., 2012b). However, the INPP itself as a big industrial complex with complementary objects in the whole region of urban-industrial development had an influence on the environment and on the Lake ksiai ecosystem in particular. Structural as well as functional Dru ksiai occurred as a result of changes in the ecosystem of Lake Dru growing anthropogenic pressure expressed as thermal and chemical pollution, especially after the commissioning of the INPP Unit 2 te_ and Jankevi (Mar ciulioniene_ et al., 1992; Jankevi ciu cius, 1992; _ 1992). Changes occurring in a lake may influence Trainauskaite, the particulate material and bottom sediments composition which commonly can be comprised of three basic components (Garunkstis, 1975; Kelts, 1988; Schnurrenberger et al., 2003): (1) Clastic sediments (could be referred to as mineral/terrestrial), (2) Chemical sediments (could be referred to as carbonates), and (3) Biogenic sediments (could be referred to as organic carbon). Many artificial radionuclides are very quickly associated to settling particles when they reach water bodies and are distributed consequently in a certain interval of bottom sediments because of diffusion, mechanical mixing and the effects of bioturbation. The association of radionuclides to sediments is governed by several key processes (Kansanen et al., 1991): sedimentation of insoluble particles of aerosols containing radionuclides; adsorption or precipitation of radionuclides on inorganic compounds (carbonates, clay particles); sedimentation of radionuclides with humic matter; sedimentation of radionuclides with autochthonous matter after their assimilation or absorption on cell surface; direct uptake of radionuclides by biota occurring on the surface of bottom sediments; direct adsorption of radionuclide on surface of sediments. The activities of annually discharged radionuclides (137Cs, 134Cs, 54 Mn, 58Co, 60Co, 59Fe, 51Cr, 95Zr, 95Nb, 89Sr, 90Sr, 131I mainly, and 3H and 14 C for only the last period of INPP operation) from the INPP to Lake ksiai in 1984e2010, as summarized from Reports of the EnviDru ronment Protection Laboratory of the INPP, are presented in Fig. 1.
ksiai in Fig. 1. Activities of radionuclides discharged from the Ignalina NPP to Lake Dru 1984e2010 (Van der Stricht and Janssens, 2001, 2005, 2010).
49
The annual discharge rates of most radionuclides were variable over the course of time. Additionally, the radioactive discharge monitoring system also changed: in the period 1983e1991, only the total activity discharge rate was reported with no indication of specific radionuclides, whilst since 1992, the activity discharge rates of specific radionuclides have been estimated. The key radionuclides in liquid discharge were 137Cs and 60Co. Based on annual discharge rates, the following years of INPP operation have to be mentioned: 1989 with total activity discharged 1.5Eþ10 Bq, 1990 with total activity discharged 2.6Eþ10 Bq, 1992 with maximum estimated 60Co discharge rate of 8.1Eþ09 Bq/year and 1995 with maximum estimated 137Cs discharge rate of 6.5Eþ09 Bq/year. The time span from 1989 to 1997 can be distinguished by highest ksiai. discharge rates of radionuclides from the INPP to Lake Dru Later, the annual discharge rates of radionuclides were significantly reduced due to technical development, operational and safety culture upgrading in the frame of many international projects. The aim of this study was to determine the areal distribution of 137 Cs, 60Co, 54Mn and 239,240Pu in 1989e1996 and 2007e2009, as well as the vertical distribution of 137Cs and 60Co in 1989, 1996 and ksiai e the cooling basin of 2010 in bottom sediments of Lake Dru the Ignalina NPP. On the basis of analysis and comparison of data, the study aimed to assess the distribution of radionuclides in Lake ksiai bottom sediments in the periods when two reactor units Dru and one reactor unit of the INPP were in operation. 2. Material sampling and methods The samples for the evaluation of the areal distribution of 137Cs, Co, 54Mn and 239,240Pu in bottom sediments were taken only from the top 0e5 cm layer using the Ekman bottom grab sampler (the area of sampler 20 20 cm, the total length up to 30 cm) in the lake ksiai monitoring stations 1e7 in JulyeAugust in 1988e1996 and Dru only 1, 4, 6 ant 7 monitoring stations in 2007e2009 (Fig. 2). The samples for the evaluation of the vertical distribution of 137 Cs and 60Co in bottom sediments were taken by the Ekman bottom grab sampler (up to 20e30 cm in depth) in 1989 and 2010, in JulyeAugust. The cores were separated into 5 cm thick slices in € 1989 and into 3 cm thick slices in 2010. In 1996, the Niemisto gravity corer (inner diameter 54 mm, up to 80 cm in length) was used for sediment sampling. The cores to 60e80 cm were taken and separated into 2 cm thick slices. The samples were dried in the laboratory at 20e22 C, were homogenized using a ceramic pestle, and were passed through sieves with holes 0.8 mm in diameter (Marciulioniene_ et al., 1992; Trapeznikov et al., 2007). Measurements of 137Cs and 60Co in the samples of bottom sediments were accomplished on gamma-ray spectrometers with Ge(Li) and HPGe detectors at the Centre for Physical Sciences and Technology, Vilnius, and at the Nature Research Centre, Vilnius, as described in Gudelis et al. (2000). However the majority of sediment samples were measured by gamma ray spectrometers (two) with HPGe GWL-series detectors as the dried and sliced sediment samples were small; actually maximum filled measuring vials volume was 3 cm3, and apart of anthropogenic radionuclides the Pb-210 was radionuclide of concern. For calibration the working solution in the water was prepared from Amersham reference solution (radionuclides presented, Mn54, Co-57, Zn-65, Sr-85, Y-88, Ba-133, Cs-137, Ce-139) through the dilution of initial material. Using this solution there were three standards prepared in non-liquid matrices (perlite, sediment and quartz sand) of densities of 0.6, 1, and 1.4 g/cm3 for density correction and three standards of density 1 g/cm3 of three filling heights of 10, 20 and 30 mm for geometry correction. From the all recorded full-energy peaks only ones attributed to single-photon 60
50
D. Marciulioniene_ et al. / Journal of Environmental Radioactivity 145 (2015) 48e57
ksiai bottom sediments distribution with sites for bottom sediment cores collection (IRD-1, 2 e industrial rain drainage channel, TWO e technical water Fig. 2. Scheme of Lake Dru outlet channel, 1, 2, 3, 4, 4a, 6, 7 e monitoring stations): 1 e till (morainic clay, loam and sandy loam), 2 e carbonate sediments, 3 e gravel, 4 e coarse-grained sand, 5 e variousgrained sand, 6 e fine-grained sand, 7 e silty sand, 8 e coarse silt mud, 9 e fine silt mud, 10 e silt-pelitic mud. Modified after Joksas et al., 1995; Galkus, 1997.
emitting radionuclides Cs-137, Mn-54, Zn-65, and Co-57 which had no significant coincidence-summing were considered. In order to define counting efficiency for Pb-210 and to resolve the coincidence-summing effect for Co-60, other single radionuclide solution standards (namely Pb-210 and Co-60) were used for calibration experiments with the same matrices. By measuring actual dry sediment samples the quantity of material was enough for full filling of vial; and the dry material density was not significantly varying between 0.95 and 1.1 g/cm3. In this case geometry and density corrections were not essential, and radionuclide-specific (Mn-54, Co-60, Cs-137 and Pb-210) counting efficiency values for 30 mm high and for density of 1 g/cm3 were used. For the comparison radionuclides activity of sediment samples were recalculated using calibration curves and counting efficiency values based on commercially available multi radionuclides (Mn-54, Co-57, Co-60, Y-88, Ba-133, Cs-137, Pb-210, and Am-24) and single radionuclide (Co-60, Cs-137 and Pb-210) standard sources of CBSS 2 type (Czech Metrology Institute), where radioactive material was by producer homogeneously dispersed in silicone resin (mass e 2.94, density 0.98 g/cm3, volume e 3 cm3, matrix composition, mass ratio: C e 0.324, H e 0.0816, O e 0.216, Si
e 0.379). The deviation of radionuclide activity values using two calibration approaches was in the range of expanded uncertainty evidencing similar absorption properties of sediment and silicon resin (actually density and atomic composition of both materials is also similar). The detection limit for the counting time of 200,000 s was about 0.014 Bq for 137Cs and 0.065 Bq for 210Pb, while measurement errors did not exceed 8% and 15% for 137Cs and 210Pb respectively. The normal precision of gamma spectrometric measurements of our laboratories was recognized during the comparison exercise of 2013 organized on national level by Lithuanian Metrology Inspectorate and Centre for Physical Sciences and Technology, and intercomparison of 2014 organized by the STUK e Finish Radiation and Nuclear Safety Authority (Klemola et al., 2014). To determine plutonium analytes, sample destruction was inevitable. Organic matter was decomposed by heating at 550 C overnight and later to 700 C for 2 h and organic matter content on loss of ignition was determined. Samples were spiked with 0.0125 Bq 242Pu as yield monitor. Acid leaching with aqua regia and concentrated HNO3 was employed. The chemical purification was based on two separation column methods and electrodeposition of
D. Marciulioniene_ et al. / Journal of Environmental Radioactivity 145 (2015) 48e57
Pu analytes onto one-side polished stainless steel discs (Luksiene_ et al., 2006, 2012a; Druteikiene_ et al., 2011). The initial separation of Pu from uranium/fission products and other matrix elements was accomplished on a Bio RaD AG-1X8 anion exchange column (Talvitie, 1971; Holm, 1984; Xu et al., 2014). The eluted from the anion exchange column Pu solution was evaporated and prepared for further purification by the extraction chromatography method (Horwitz et al., 1995). The plutonium isotopic composition (238Pu and 239,240Pu) and their amount in the samples were determined by using the semiconductor Alpha spectrometer OctetePlus with an ORTEC large square (450 mm2) Si detector (BU-020-450-AS) with a resolution capability of 20 keV. The spectra were analyzed applying AlphaVision and Maestro programs. The detection limit for the counting time of 86,400 s was about 103 Bq for 239,240Pu. Some of the samples (usually from double cores) were examined for the main components of the sedimentary matrix (Bengtsson and Enell, 1986). Firstly, the samples were slowly dried at 50e80 C and later at 105 C till constant weight, disaggregated and sieved to pass through a 0.25 mm mesh. Organic matter was estimated using loss of weight on ignition at 850 C, carbonates by titration with HCl and clastic component of sediment matrix were calculated. The water content and dry bulk density of sediments were estimated by weighing standard volume samples dried at 105 C. The main sources and fluxes (F) of anthropogenic radionuclides in the lake system are the following (Wieland et al., 1993): total input of radionuclides into the lake with transfer from the catchment and from direct fallout (I), radionuclide outflow from the lake with surface water outflows (o), exchange of radionuclides in soluble form by vertical mixing of water between epilimnion and hypolimnion (e), radionuclide sorption to suspended particulates (s), radionuclide uptake by bottom sediments in “wateresediment” interface (u), particular processes of radionuclides transfer from bottom sediments to water, i.e., resuspension and remobilization (r). For a stratified water body, the radionuclides balance can be expressed as follows:
dI ¼ ðFi Fe Fu þ Fr Þ FsðepiÞ dt . Bq m2 day for epilimnion ; Fo
(1)
dI ¼ FsðepiÞ FsðhypoÞ þ ðFe Fu dt . þ Fr Þ Bq m2 day for hypolimnion :
(2)
Total input of radionuclides in a lake can be evaluated as total P activity ð aÞ in the lake water column h:
I¼
X
Z a¼
Cw þ Cp S dh
h 0
. Bq m2 ;
(3)
where S e particulates concentration in water column. Although the fraction of 137Cs associated with particles is up to 5e10% of total 137Cs activity presented in the water column, the particle sedimentation is the main process of 137Cs removal from the lake water. Therefore, the radionuclide flux (Fs) can be associated with the particle flux (Fp):
Fs ðtÞ ¼ Fp ðtÞ Cp ðtÞ
. Bq m2 day :
(4)
The radionuclides transfer from water into bottom sediments rate (lR) or removal time (tR) can be evaluated:
lR ¼ 1=tR ¼ Fs ðtÞ=IðtÞ
day1 :
51
(5)
Because inequality Cw [ CpS (when S ¼ 1e5 mg/l) is valid, radionuclides transfer from water into bottom sediments rate is expressed:
lR ¼ Fp ðKd =hÞ:
(6)
3. Results and discussion 3.1. The areal distribution of anthropogenic radionuclide activity ksiai concentrations in the bottom sediments of Lake Dru The data on the areal distribution of anthropogenic radionuclides activity concentrations in the top layer of bottom sediments ksiai are presented in Figs. 3e7. from Lake Dru Component-based analysis of bottom sediments started already at the construction stage of INPP (Tamosaitis et al., 1986; Tamosaitis nas, 1992). Later, a more detailed lacustrine sediment and Bajoru typology in the form of a lithological map (Fig. 2), based mainly on grain size analysis with ten types of sediments, was presented (Galkus, 1997). These sediment types were distinguished on the basis of following particle size intervals: median diameter of particles D50 > 1 mm e gravel; 1e0.3 mm e coarse sand; 0.3e0.063 mm e fine sand; 0.063e0.03 mm e coarse silt sediments; 0.03e0.01 mm e fine silt sediments;
