Methodology and Levels of 137 Cs, 90 Sr and Pu in Marine Sediments from the Brazilian Southern Coast

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1 Congreso Internacional Conjunto Cancún 2004 LAS/ANS-SNM-SMSR/International Joint Meeting Cancun 2004 LAS/ANS-SNM-SMSR XV Congreso Anual de la SNM y XXII Reunión Anual de la SMSR/XV SNM Annual Meeting and XXII SMSR Annual Meeting Cancún, Q.R., México, de Julio, 2004/Cancún, Q.R., Mexico, July 11-14, 2004 Methodology and Levels of 137 Cs, 90 Sr and Pu in Marine Sediments from the Brazilian Southern Coast R. C. L. Figueira 1, M. G. Tessler 2 1 Universidade Cruzeiro do Sul CETEC/UNICSUL São Paulo/SP Brazil 2 Instituto Oceanográfico da Univesidade de São Paulo IOUSP São Paulo/SP - Brazil figueira@curiango.ipen.br; mgtessle@usp.br M. M. de Mahiques 2 and I. I. L. Cunha 3 3 Instituto de Pesquisas Energéticas e Nucleares IPEN/CNEN-SP São Paulo/SP Brazil mahiques@usp.br; iedacunha@ig.com.br Abstract Methodologies for analysis of 137 Cs, 90 Sr and Pu were developed and applied to sediment samples collected along the Brazilian coast. This paper presents the instrumental steps for 137 Cs analysis and the radiochemical steps necessary to analyse 90 Sr and Pu. The analytical quality control for radionuclide analysis was verified by participation in international intercomparison exercise runned by IAEA as well as in reference material analysis. Further, methodologies were applied in sediments collected from 1976 to 1986 at the Brazilian southern coastal region. 137 Cs levels ranged from 0.37 to 2.03 Bq kg -1, for 90 Sr from MDC to 8.2 Bq kg -1 and for Pu from MDC to 227 m Bq kg -1. The minimum detectable concentration values (MDC) for 90 Sr and Pu were of 0.3 Bq kg -1 and 4.2 mbq kg -1, respectively. The artificial radioactivity levels in Brazilian sediments are typical values due to fallout deposition and represent reference values for our country. 1. INTRODUCTION The environmental contamination from radionuclides originating from nuclear weapons tests, dumping operation and nuclear accidents have introduced artificial radionuclides to the environment, in particular, the marine environment. The radionuclides can be transported across long distances from their emission source [1,2]. Considering this problem and the lack of information in our country, methodologies for analysis of 137 Cs, 90 Sr and Pu were developed and applied to marine sediment samples. Radioactive tracers of 242 Pu and 85 Sr were used to determine radionuclide recovery yields. 137 Cs was determined by instrumental gamma spectrometry. 90 Sr, after acid leaching of the sample, it was purified by precipitation with iron hydroxide. After the radioactive equilibrium be reached, yttrium was precipitated as hydroxide, further as oxalate and assayed by beta counting Pu was coprecipitated with iron hydroxide, purified by using an anionic exchange column (BIORAD Memorias CIC Cancún 2004 en CDROM 1/11 Proceedings IJM Cancun 2004 on CDROM

2 Figueira, R. C. L. et al, Methodologies and Levels of Cs-137,Sr-90 and Pu-239 in Marine Sediments... AG1X8 resin), electrodeposited and counted by alpha spectrometry. In order to verify if the methods here developed are appropriated to the radionuclide analysis, they were applied to reference materials. Further, methodologies were employed to analyse radionuclides of interest in marine sediments collected in the southern Brazilian coast. 2.1 Sediment Sampling 2. EXPERIMENTAL Sediment cores were collected with a standard box core of 20 x 30 cm cross-sectional area by the Oceanographic Institute from the University of Sao Paulo, Brazil, at the continental shelf. Cores were dried, homogenized and transferred to plastic containers and stored in the freezer. The contents of organic matter and humidity were determined in every core. 2.2 Methodology for 137 Cs Analysis 137 Cs was assayed by gamma counting by means of its photopeak of 661 kev as described below. The method consisted of detector calibration, determination of detector counting efficiency, cumulative countings of both background and samples in regular intervals of counting time, photopeak smoothing and linear regression. Detector calibration was performed by means of several gamma ray emitting nuclides. IAEA reference materials were employed to determine the detector counting efficiency in the radionuclide photopeak region Background radiation The environmental samples present a low activity, so any background radiation influences the detection of the radionuclide. A study of the background radiation variation in the region of 137 Cs photopeak as a function of time was performed. Cumulative background counting was carried out for 150,000 seconds, in intervals of 10,000 seconds. The counting time to be employed depended on the 137 Cs level in the sample in order to improve counting statistics. A linear regression was fitted through data ranging from 70,000 to 150,000 seconds. The gamma spectrum was smoothed according to a binomial algorithm, which replaces the original data, channel by channel, with the smoothed data. It was noticed that the ideal number of channels for the analysis was eleven and any variation in the channel versus energy function could lead to error in the analysis (change in the radionuclide photopeak). Therefore, the spectrum must be often re-calibrated to the channel position previously specified [3]. The same process carried out to obtain background accumulative counting was repeated for the sediment sample. From 50 to 100 g of sediment (dried and homogenized) was used for analysis depending on the radionuclide level. Comparison between the linear regression curves of Memorias CIC Cancún 2004 en CDROM 2/11 Proceedings IJM Cancun 2004 on CDROM

3 Congreso Internacional Conjunto Cancún 2004 LAS/ANS-SNM-SMSR /International Joint Meeting Cancun 2004 LAS/ANS-SNM-SMSR background and sediment sample allowed the determination of sediment radionuclide activity, discounting background activity for each registered time. 137 Cs activity was calculated according to the equation (1): ( A 37 ± σ Cs Cs ) = ( C S ± σ ) ( C S m t ( ε ± ± σ Bg σ ε ) Bg ) (1) where: A 137Cs is the 137 Cs activity in the sample (Bq kg -1 ); C s is the 137 Cs gross counting in the sample; C Bg is the background counting; m is the sample mass (kg); t is the counting time (s); ε is the counting efficiency for the 137 Cs photopeak (661 kev) σ 137Cs is the standard deviation of the variables 2.3 Methodology for 90 Sr Analysis Sediment samples (about 50g) were lixiviated with nitric acid solution (8 moles L -1 ) and a few drops of hydrogen peroxide (30%v/v),in the presence of 500mg of strontium carrier and 85 Sr tracer. The solution was filtered, evaporated until 50 ml and 10 mg of iron carrier was added. In order to precipitate iron hydroxide, 20 mg of iron carrier was added and the ph was increased up to 7-8 ( by adding ammonium hydroxide). 85 Sr was counted by gamma spectrometry and it was used to calculate the yield of the process. The precipitate was stored by 14 days in order to obtain the radioactive equilibrium. The strontium precipitate was dissolved with concentrated nitric acid and yttrium carrier (10 mg) was added. Yttrium was precipitated as hydroxide (ph=8.0 by adding ammonium hydroxide), dissolved with concentrated hydrochloric acid, and yttrium was again precipitated as hydroxide. This purification process was repeated for 4 times. Finally, the yttrium hydroxide was dissolved with concentrated hydrochloric acid and precipitated as oxalate (1 M oxalic acid solution). The yttrium oxalate was assayed by beta counting and yttrium was determined gravimetrically. Corrections for building up and 90 Y decay, background counting, detector efficiency, strontium and yttrium yields and blank value were employed in order to obtain the 90 Sr activity. Blank analysis was performed simultaneously to the sediment analysis. 90 Sr activity was determined by means of the equation 2: A 90 Sr cps BG = t1 λt2 RSr RY EY (1 e ) e m λ (2) where: A 90Sr is the 90 Sr activity in the sample (Bq kg -1 ); cps is the 90 Y counting in the sample; BG is the background counting; Memorias CIC Cancún 2004 en CDROM 3/11 Proceedings IJM Cancun 2004 on CDROM

4 Figueira, R. C. L. et al, Methodologies and Levels of Cs-137,Sr-90 and Pu-239 in Marine Sediments... m is the sample mass (kg); R Sr is the strontium yield obtained by means of 85 Sr tracer; R Y is the yttrium yield obtained by gravimetric analysis; E Y is the counting efficiency of 90 Y; t 1 is the time of growth of 90 Y, after precipitation of 90 Sr with hydroxide; t 2 is the decay time of 90 Y, after precipitation of 90 Y with hydroxide. 2.4 Methodology for Pu Analysis About 15 grams of the sediment was lixiviated with nitric acid solution ( 8 moles L -1 ) and a few drops of hydrogen peroxide (30%v/v),in the presence of 242 Pu tracer. Plutonium was coprecipitated with iron hydroxide. The precipitate was dissolved by using 8 M nitric acid solution, after addition of sodium nitrite, under heating. The solution was loaded onto the conditioned anion exchange column (AG1-X8). Americium was eluted by 8 M nitric acid solution, while thorium, uranium and iron were desorbed from the column by passing through 10 M hydrochloric acid. Plutonium was eluted by using 10 M hydrochloric acid in the presence of 0.6 g of iodine ammonium. Iodine was eliminated by heating of the solution and addition of concentrated nitric acid. The solution was evaporated almost dryness and 0.3M sodium sulphate solution (1mL) was added. After the solution evaporation, 300 µl of conc. sulphuric acid was added and plutonium was electrodeposited (ph=1.5 to 2.2, constant current of 1.8 A for 75 minutes) and assayed by alpha spectrometry. Blank analysis was performed simultaneously to the sediment analysis Pu activity was determined by means of equation 3: A = C where, A Pu is the activity of Pu, in Bq kg -1 ; C Pu is the counting of Pu, in cps ; t is the counting time in seconds; R Pu is the plutonium yield obtained by means of 242 Pu tracer; ε is the counting efficiency of the detector; m is the mass of sediment, in kg. Pu Pu (3) t RPu ε m 2.5 Equipments The following equipments were used to analyze the studied radionuclides in the marine sediments: hyperpure Ge detector with resolution of 1.9 kev for the peak of kev of 60 Co, GEM model, by EG&ORTEC, and associated electronics. Alpha spectrometer acopled to a surface barrier detector, 576 A model, by EG&ORTEC, and associated electronic. Geiger-Müller counter, GM-25-5 model, by RISO NATIONAL LABORATORY (Denmark). This counter is gas-flow type, showing a low background radiation (< 1.2 cpm) and a counting efficiency of 28% for 90 Y. Memorias CIC Cancún 2004 en CDROM 4/11 Proceedings IJM Cancun 2004 on CDROM

5 Congreso Internacional Conjunto Cancún 2004 LAS/ANS-SNM-SMSR /International Joint Meeting Cancun 2004 LAS/ANS-SNM-SMSR 3.1 Methodology 3. RESULTS AND DISCUSSION Tables I and II present the results obtained in the radionuclide analysis in reference samples and in the intercomparison exercices, respectively. Results for cesium presented accuracy between 0.7 and 23.8% and precision between 1.8 and 15.1%, showing that the methodology is appropriated to detect low 137 Cs concentrations. For samples with 137 Cs levels upper 5 Bq kg -1, the error of the analysis was less than 4%. The methodology developed for strontium showed accuracy of 15.5% and precision of 21.1%. For plutonium, the precision ranged from 2.5 to 25 % and the accuracy varied from 1.4% to 15.4%. Plutonium recovery varied from 30% to 75% that is in good agreement with the literature. The variation observed is related to the lost of plutonium by volatilization as well as to the interference in the plutonium deposition. It can be pointed out that even samples with low levels of plutonium, such as 0.3 Bq kg -1 can be analysed by this method, presenting errors of about 14%. We have also participated in a comparison analysis by the International Atomic Energy Agency (IAEA). The results obtained for the analysis of the IAEA-384 sample (Fangataufa sediment sample) and a mineral matrix (IAEA) are presented in Table II. It can be verified that the values obtained by our laboratory are in good agreement with the certified values, indicating that the methods developed here can be applied to the analysis of 137 Cs, 90 Sr and Pu. Table I. Radionuclide levels in reference materials. Reference Material (IAEA) Certified value (Bg kg -1 ) Our result (Bg kg -1 ) 137 Cs Soil ( ) (1) 54 ± 1 IAEA ( ) (1) 4.7 ± 0.5 IAEA ( ) (1) 3.3 ± 0.5 IAEA/SD-N ( ) (1) 0.61 ± 0.09 t 90 Sr IAEA (62 129) (1) 118 ± Pu IAEA ( ) (1) 3.6 ± 0.6 IAEA ± 1 ( ) (1) IAEA ± 2 (29 34) (1) IAEA ( ) (1) 0.34 ± 0.04 t Soil ± 0.3 ( ) (1) (1) confidence interval Memorias CIC Cancún 2004 en CDROM 5/11 Proceedings IJM Cancun 2004 on CDROM

6 Figueira, R. C. L. et al, Methodologies and Levels of Cs-137,Sr-90 and Pu-239 in Marine Sediments... The methodology developed for strontium showed accuracy of 15.5% and precision of 21.1%. For plutonium, the precision ranged from 2.5 to 25 % and the accuracy varied from 1.4% to 15.4%. Plutonium recovery varied from 30% to 75% that is in good agreement with the literature. The variation observed is related to the lost of plutonium by volatilization as well as to the interference in the plutonium deposition. It can be pointed out that even samples with low levels of plutonium, such as 0.3 Bq kg -1 can be analysed by this method, presenting errors of about 14%. We have also participated in a comparison analysis by the International Atomic Energy Agency (IAEA). The results obtained for the analysis of the IAEA-384 sample (Fangataufa sediment sample) and a mineral matrix (IAEA) are presented in Table II. It can be verified that the values obtained by our laboratory are in good agreement with the certified values, indicating that the methods developed here can be applied to the analysis of 137 Cs, 90 Sr and Pu. Table II. Radionuclide levels in intercomparison analysis Material (IAEA) Certified value (Bg kg -1 ) Our result (Bg kg -1 ) 137 Cs IAEA-384 (marine sediment) 90 Sr 48B5 sample 0.31 ( ) (1) 0.57 ± ± ± (mineral matrix) 48B6 sample ± ± (mineral matrix) Pu IAEA-384 (marine sediment) ( ) (1) 95 ± 8 EML ± ± 0.4 (soil) 2 1) confidence interval 2) IRD Instituto de Radioproteção e Dosimetria.CNEN-Rio de Janeiro-Brazil. 3.2 Radionuclide Levels Figure 1 shows the steps employed to the analysis of artificial radionuclides in sediments. Table III presents the radionuclide levels in sediment samples collected at the southern coast of Brazil. Memorias CIC Cancún 2004 en CDROM 6/11 Proceedings IJM Cancun 2004 on CDROM

7 Congreso Internacional Conjunto Cancún 2004 LAS/ANS-SNM-SMSR /International Joint Meeting Cancun 2004 LAS/ANS-SNM-SMSR MARINE SEDIMENT SAMPLE Determination of 137 Cs Determination of 239,240 Pu and 238 Pu Determination of 90 Sr Gamma spectrometry g of sample g of sample Pu or 242 Pu tracer - 8 M HNO 3 and H 2 O 2 - leaching for 40 h or microwave oven digestion Supernatant Ionic exchange - co-precipitation of Pu with Fe(OH) 3 - dissolution of Fe(OH) 3 and addition of NaNO 2 - AG-1X8 (8x100 mm) ionic exchange column g of sample - 85 Sr tracer mg Sr carrier - 8 M HNO 3 and H 2 O 2 - leaching for 16 h Supernatant Supernatant -interferents elimination: Fe(OH) 3 - equilibrium time Iodine elimination -Am elution (8 M HNO 3 ) - U, Th and Fe elution (10 M HCl); -Pu elution (10 M HCl g - HNO 3conc and heating to dryness - 1 ml of Na 2 SO 4 addition - 0,3 ml of H 2 SO 4 addition - ph to 2 with NH 4 OH conc Y(OH) 3 (ppt) - 20 mg of Y carrier - addition of NH 4 OH - purification of Y(OH) 3 using HCl conc and NH 4 OH conc - 3 ml of 1 M H 2 C 2 O 4 addition Eletrodeposition on polished steel disk, 75 min, 1.8 A Alpha counting Y 2 (C 2 O 4 ) 3.9H 2 O (ppt) Beta counting Figure 1: Methodology for 137 Cs, 239,240 Pu and 90 Sr analyses in marine sediment sample. Memorias CIC Cancún 2004 en CDROM 7/11 Proceedings IJM Cancun 2004 on CDROM

8 Figueira, R. C. L. et al, Methodologies and Levels of Cs-137,Sr-90 and Pu-239 in Marine Sediments... Table III. Levels of 137 Cs, 90 Sr and 239 Pu in marine sediments from Brazilian Southern Coast. Sample Year Depth (m) Latitude Longitude 137 Cs (Bq kg -1 ) Pu (mbq kg -1 ) 90 Sr (Bq kg -1 ) o o ± ± ± M o o ± ± ± M o o ± ± ± M o o ± ± 10 < MDC o o ± ± 5 < MDC 1021M o o ± ± 3 < MDC o o ± ± ± M o o ± ± ± M o o ± ± ± o o ± ± ± o o ± ± ± o o ± 0.12 < MDC 1.5 ± o o ± ± ± o o ± ± ± o o ± ± ± o o ± ± ± o o ± ± 9 < MDC o o ± 0.12 < MDC < MDC o o ± ± ± o o ± ± 10 < MDC o o ± ± ± o o ± ± ± o o ± ± ± o o ± ± ± o o ± 0.15 < MDC < MDC Memorias CIC Cancún 2004 en CDROM 8/11 Proceedings IJM Cancun 2004 on CDROM

9 Congreso Internacional Conjunto Cancún 2004 LAS/ANS-SNM-SMSR /International Joint Meeting Cancun 2004 LAS/ANS-SNM-SMSR 137 Cs levels in Brazilian coastal sediments ranged from 0.37 to 2.03 Bq kg -1, for 90 Sr from MDC to 8.2 Bq kg -1 and for Pu from MDC to 227 mbq kg -1. The minimum detectable concentration values (MDC) for 90 Sr and Pu were of 0.3 Bq kg -1 and 4.2 mbq kg -1, respectively. The concentrations of the radionuclides in Brazilian sediments are very low and are typical values due to fallout deposition. The values obtained are similar to values found in sediments from the Guatemala Coast or Syria Coast. However, they are lower as compared to areas influenced by inputs from discharge of nuclear reprocessing plants or Chernobyl accident, like United Kingdom or Black Sea (Table IV). Table IV: Levels of the artificial radionuclides in Brazilian coast, in Bq.kg -1, compared to different areas of the world [4-14]. Area Author Sampling depth (m) 137 Cs 90 Sr Pu Brazilian southern coast Algeria Bay Guatemala Coast This work Benamar et al.[4]; Noureddine and Baggoura [5] Pérez-Sabino et al. [6] bottom surface (10 500) < < [4] (core) ( cm) [5] bottom surface 1.09 (1) (1) Syria Coast Othman et al. [7] bottom surface Ireland Sea Mitchell et al. [8] bottom surface Black Sea Kara and Barents Sea Mediterranean Sea IAEA [2] ; Strezov et al. [9]; Bologa et al. [10] Baskaran et al. [11]; Krosshavn et al. [12] IAEA [2]; Marzano et al. [13] bottom surface (0 70) bottom surface (0 3) bottom surface < United Kingdom McDonald et al. [14] bottom surface (1) Pacific Ocean area Memorias CIC Cancún 2004 en CDROM 9/11 Proceedings IJM Cancun 2004 on CDROM

10 Figueira, R. C. L. et al, Methodologies and Levels of Cs-137,Sr-90 and Pu-239 in Marine Sediments CONCLUSIONS Results obtained for radionuclides analysis in participation at intercomparison exercices and in reference materials are in good agreement with certified values. Methodologies here described provide effective methods for radionuclide analysis present in low concentration and it will be used on a routine monitoring program aiming at the determination of anthropogenic radionuclides in Brazilian marine samples. Data represent reference values for our country and they will be used to estimate the radionuclide inventory in marine sediment. These estimates are necessary as a baseline for detection of any future contamination. ACKNOWLEDGEMENTS The authors thank to FAPESP, CNPQ, IPEN, UNICSUL for the finantial support and to the Oceanographic Institute (University of São Paulo) for sampling. REFERENCES 1. Aarkrog, A."Worldwide data on fluxes of Pu-239, Pu-240 and 238 Pu to the oceans". In: Inventory of selected radionuclides in oceans. IAEA, Vienna, Austria (1988). 2. IAEA International Atomic Energy Agency. Sources of radioactivity in the marine environment and their relative contributions to overall dose assessment from marine radioactivity (MARDOS). IAEA, Vienna, Austria (1995). 3. EG& ORTEC. Maestro II Software Model A64-BI operator s manual. USA (1992). 4. Benamar, M.A.; Zerrouki, A; Tobbeche, S. Radioactivity levels in superficial sediments of Algiers Bay. Centre de Développement des Téchniques Nucléaires, France (1996). 5. Noureddine, A.; Baggoura, B. "Radioactivity of some alpha, beta and gamma emitting radionuclides in surface marine sediments of different bays in Algeria. Proceedings of International Symposium on Marine Pollution, Monaco, 5-9 October, p. 581 (1998). 6. Perez-Sabino, J.F.; Oliva de Sandoval, B. E.; Orozco-Chile, R. M.; Aguilar-Sandoval, E. Radioactive contamination of the Guatemalan marine environment. Proceedings of International Symposium on Marine Pollution, Monaco, 5-9 October, p (1998). 7. Othman, I; Yassine, T.; Bhat, I.S. The measurement of some radionuclides in the marine coastal environment of Syria. Sci.Total Environ., 153, p (1994). 8. Mitchell, P.I.; Battle, J.V.I.; Downes, A.B.; Condren, O.M.; Vintró, L.L.; Sánchez-Cabeza, J.A. "Recent observations on the physico-chemical speciation of plutonium in the Irish Sea and the western Mediterrranean". Appl.Radiat.Isot., 46, n. 11, p (1995). 9. Strezov, A.; Yordanova, I.; Pimpl, M.; Stoilova, T. Natural radionuclide and plutonium content in Black Sea bottom sediments. Health Phys. 70, n. 1, p (1996). 10. Bologa, A.S.; Apas, M.; Cociasu, A.; Cuingioglu, E.; Patrascu,V.; Pecheanu, I.; Piescu, V.; Popa, L. "Present level of contaminants in the Romania Black Sea sector". Proceedings of International Symposium on Marine Pollution, Monaco, 5-9 October, p (1998). 11. Baskaran, M.; Asbill, S.; Santschi, P.; Davis, T.; Brooks, J.; Champ, M.; Makeyev, V.; Khlebovich,.V. "Distribution of 239,240 Pu and 238 Pu concentrations in sediments from Ob and Yenisey rivers and the Kara Sea". Appl.Radiat.Isot., 46, n. 11, p (1995). Memorias CIC Cancún 2004 en CDROM 10/11 Proceedings IJM Cancun 2004 on CDROM

11 Congreso Internacional Conjunto Cancún 2004 LAS/ANS-SNM-SMSR /International Joint Meeting Cancun 2004 LAS/ANS-SNM-SMSR 12. Krosshavn, M.; Engoy, T.; Carroll, J.; Johnson, D., King, S.E. Varskog, P. "Transport of radionuclides in the Kara Sea abd Eastern Barents Sea". Proceedings of International Symposium on Marine Pollution, Monaco, 5-9 October, p (1998). 13. Marzano, N.; Triulzi, C. " 90 Sr, 137 Cs, 238 Pu, 239,240 Pu and Am-241 in matrices of the Terra Nova Bay ecosystem (Antarctica)". Proceedings of International Symposium on Marine Pollution, Monaco, 5-9 October, p (1998). 14. McDonald, P.; Cook, G.T.; Baxter, M.S. Natural and Artificial Radioactivity in Coastal Regions of UK. Scottish Universities Research and Reactor Centre, Glasgow, UK and IAEA - International Laboratory of Marine Radioactivity, Monaco, (1991). Memorias CIC Cancún 2004 en CDROM 11/11 Proceedings IJM Cancun 2004 on CDROM