Environ. Sci. Technol. 2005, 39, 92-97
Pu and U Atom Ratios and Concentration Factors in Reservoir 11 and Asanov Swamp, Mayak PA: An Application of Accelerator Mass Spectrometry B Ø R R E T Z E N , P . , * ,† STANDRING, W. J. F.,† OUGHTON, D. H.,‡ DOWDALL, M.,† AND FIFIELD, L. K.§ Norwegian Radiation Protection Authority, P.O. Box 55, 1332 Østerås, Norway, Department of Plant and Environmental Science, Agricultural University of Norway, 1432 A° s, Norway, and Department of Nuclear Physics, RSPhysSE, Australian National University, Canberra, ACT 0200, Australia
Mayak Production Association, East Ural, Russia, was established to produce weapons-grade plutonium. Routine discharges and accidents at Mayak PA contaminated large areas, including the Techa River. The objectives of the present work were to study atom ratios for plutonium and, for the first time to our knowledge, uranium isotopes in water, soil, grass, and aquatic biota samples from Reservoir 11 and the Asanov Swamp, downstream from Mayak PA. Atom ratios (240Pu/239Pu, 236U/235U, 235U/238U) were determined using accelerator mass spectrometry to confirm radionuclide source characteristics and calculate activities and concentration factors for the studied samples. The lowest 240Pu/239Pu atom ratios were consistently found in Asanov Swamp samples (∼0.019), indicating a major contribution from early discharges of weapons-grade Pu. 240Pu/239Pu atom ratios in Reservoir 11 were higher, indicating influence from more recent civil reprocessing. The presence of 236U is usually indicative of fuel irradiation; 236U/235U ratios increase from weapons to civil sources. Our new data show that Asanov samples had lower 236U/ 235U ratios than Reservoir 11 samples (0.0005-0.0045 for Asanov compared with 0.0074-0.0153 for Reservoir 11) in agreement with Pu results. Pu and U concentration factors calculated for vegetation and biota samples at Mayak were comparable with corresponding values found in the literature.
Introduction Mayak Production Association (Mayak PA) is situated in East Ural, Russia, and was established in the late 1940s to produce weapons-grade plutonium. Early Mayak PA activities have contaminated large areas, through routine discharges (∼100 PBq directly into the Techa River from 1949 to 1956) and accidents: notably “Kyshtym”, where an on-site high-level * Corresponding author phone: (+47) 67 16 26 01; fax: (+47) 67 14 74 07; e-mail:
[email protected]. † Norwegian Radiation Protection Authority. ‡ Agricultural University of Norway. § Australian National University. 92
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liquid waste tank exploded in 1957 spreading ∼74 PBq of radionuclides over an area now called the East Ural Radioactive Trace (EURT); and “Karachay” in 1967 where sediments containing ∼22 TBq were dispersed by winds from a dried reservoir bed (1). Of the original seven military reactors, two remain in service producing radionuclides for civil and military use. A 1990 inventory of radionuclides accumulated at the Mayak site was estimated as 30 000 PBq of solid and liquid wastes, of which 10 500 PBq of high- and mediumlevel liquid wastes had been vitrified by 1996 (2). A cascade of artificial reservoirs has been constructed to store liquid wastes, designed to hold back radionuclide contamination and significantly reduce the amounts of radionuclides entering the Techa River (Figure 1). Reservoirs 10 (1956) and 11 (1963) are the largest. Russian total inventory estimates in 1993-1995 were 6.6 and 1.2 PBq in Reservoirs 10 and 11, respectively (1), and the reservoirs are expected to have been influenced by more recent discharges from civil reprocessing. However, Techa sediments and riverbank soils, notably in the boggy Asanov Swamp downstream from Reservoir 11, still contain artificial radionuclides from previous direct discharges: estimates of 40 TBq of 90Sr and 210 TBq of 137Cs in surface soils (1992 data) have been reported (3). Plutonium isotope ratios vary with reactor type, nuclear fuel burnup time, neutron flux, and energy; and from weapon type and yield after nuclear detonations. 240Pu/239Pu atom ratios enable weapons-grade Pu (ratios of 0.01-0.05) to be distinguished from civil reprocessing (ratios of 0.2-0.8) and global fallout (ratios of 0.17-0.19), allowing the study of Pu transported from different sources (2). Uranium has three naturally occurring isotopes: 234U, and 238U, with a natural isotopic abundance (atom %) of 0.00548, 0.7200, and 99.2745, respectively (4). The 235U/ 238U ratio in uranium of natural origin is therefore 0.00725. Trace amounts of 236U can be formed in uranium ore bodies by neutron capture of 235U, with 236U/238U ratios in the range (0.1-5) × 10-10 (5). Elsewhere in nature, the ratio is expected to be below 10-13. Emissions of U from anthropogenic nuclear sources can, however, significantly alter the ratios in environmental samples. Hence, ratios between U isotopes (e.g., 235U/238U, 236U/238U, and 236U/235U) give information regarding the source of the uranium. Uranium that is enriched in 235U is indicative of weapons manufacture, while low 235U/238U ratios indicate nuclear fuels that have been burnt up, reprocessed, or are waste from fuel enrichment processes (i.e., depleted U). 236U/235U ratios give information about nuclear fuel burnup time, lower ratios being indicative of weapons manufacturing sources. 235U,
The present work was designed to study atom ratios for Pu and U isotopes in water, soil, grass, and aquatic biota samples collected in 1994 and 1996 from Reservoir 11 and the Asanov Swamp area (Table 1). The study had three objectives: to confirm sediment and water 240Pu/239Pu atom ratios found in the literature for this area (2, 6) and supplement them with new ratios for biota samples; to generate new data in the form of 236U/235U atom ratios that could further clarify the nuclide source characteristics in the studied area; and to calculate concentration factors (CF) for the different biota in the studied area. Atom ratios were determined in the different samples using accelerator mass spectrometry (AMS). The very high sensitivity of AMS has proved a useful technique for studying radioactively contaminated environmental samples where limited amounts of sample material are available with variable activity levels (e.g., refs 7 and 8). 10.1021/es049618p CCC: $30.25
2005 American Chemical Society Published on Web 12/03/2004
FIGURE 1. Map showing Mayak PA (inset) and the Techa-Iset-Tobol-Irtysh-Ob river system that drains into the Kara Sea: O, approximate sampling locations in Reservoir 11 and Asanov Swamp. This figure is adapted from ref 3.
TABLE 1. Mayak PA Samples Used in AMS Experiments ID no.
station
matrix (family name)
info
date collected
1500 7013 7014 3208 3209 4011 4012 1532 6009 6010 6011 6014
Asanov Swamp Asanov Swamp Asanov Swamp Asanov Swamp Asanov Swamp Asanov Swamp Asanov Swamp Reservoir 11 Reservoir 11 Reservoir 11 Reservoir 11 Reservoir 11
water water plant (Apiaceae) water plant (Juncaceae) surface soil A surface soil B grass A grass B water mussel (Unionidae) pike bone (Esocidae) pike fillet (Esocidae) 5 roach (Cyprinidae)
0.45-µm fraction. Low-concentration factors from Asanov soil to grass indicate low mobility of Pu and U. Concentration factors calculated for biota were
Acknowledgments This study has been completed as part of the ADVANCE Project FIS5-1999-00353, funded by the European Community. The authors thank members of the JNREG for the collection of sample materials. Autoradiography was performed at the Isotope Laboratory, Agricultural University of Norway.
Literature Cited (1) JNREG, Sources contributing to radioactive contamination of the Techa River and areas surrounding the ‘Mayak’ production association, Urals, Russia. Programme of the Joint NorwegianRussian Investigations of possible impacts of the Mayak PA activities on radioactive contamination of the Barents and Kara Seas. Joint Norwegian-Russian Expert Group, Østerås, 1997 (ISBN 82-993079-6-1). (2) Oughton, D. H.; Fifield, L. K.; Day, J. P.; Cresswell, R. C.; Skipperud, L.; Di Tada, M. L.; Salbu, B.; Strand, P.; Drozcho, E.; Mokrov, Y. Plutonium from Mayak: Measurement of Isotope Ratios and Activities Using Accelerator Mass Spectrometry. Environ. Sci. Technol. 2000, 34, 1938-1945. (3) Standring, W. J. F.; Oughton, D. H.; Salbu, B. Potential Remobilization of 137Cs, 60Co, 99Tc, and 90Sr from Contaminated Mayak Sediments in River and Estuary Environments. Environ. Sci. Technol. 2002, 36, 2330-2337. (4) IUPAC, Isotopic Compositions of the Elements 1997, Rosman KJR, Taylor PDP. Pure Appl. Chem. 1998, 70, 217-235. (5) Berkovits, D.; Feldstein, H.; Ghelberg, S.; Hershkowitz A.; Navon, E.; Paul, M. 236U in uranium minerals and standards. Nuclear Instrum. Methods. Phys. Res., Sect. B 2000, 172, 372-376. (6) Beasley, T. M.; Kelley, J. M.; Orlandini, K. A.; Bond, L. A.; Aarkrog, A.; Trapeznikov, A. P.; Pozolotina, V. N. Isotopic Pu, U, and Np Signatures in Soils from Semipalatinsk-21, Kazakh Republic and the Southern Urals, Russia. J. Environ. Radioact. 1998, 39, 215230. (7) Marsden, O. J.; Livens, F. R.; Day, J. P.; Fifield, L. K.; Goodall, P. S. Determination of U-236 in sediment samples by accelerator mass spectrometry. Analyst 2001, 126, 633-636. (8) Fifield, L. K.; Cresswell, R. G.; di Tada, M. L.; Ophel, T. R.; Day, J. P.; Clacher, A. P.; King, S. J.; Priest, N. D. Accelerator mass spectrometry of plutonium isotopes. Nuclear Instrum. Methods Phys. Res. B 1996, 117, 295-303. (9) Christensen, G. C.; Romanov, G. N.; Strand P.; Salbu B.; Malyshev, S. V.; Bergan, T. D.; Oughton, D.; Drozhko, E. G.; Glagolenko Y. V.; Amundsen, I.; Rudjord, A. L.; Bjerk, T. O.; Lind, B. Radioactive contamination in the environment of the nuclear enterprise ‘Mayak’ PA. Results from the joint Russian-Norwegian field work in 1994. Sci. Total Environ. 1997, 202, 237-248. (10) Clacher, A. R. Development and application of analytical methods for environmental radioactivity. Ph.D. Thesis. University of Manchester, U.K., 1995. (11) Oughton, D. H.; Day, J. P.; Fifield, L. K. Plutonium measurement using accelerator mass spectrometry: methodology and applications. Pu in the environment. In Radioactivity in the Environment; Kudo, A., Ed.; Elsevier: Amsterdam, 2001; Vol. 1, pp 47-62 ( ISBN 0-08-043425-8). (12) Thompson, S. E.; Burton, C. A.; Quinn, D. J.; Ng., Y. C. Concentration Factors of Chemical Elements in Edible Aquatic Organisms. UCRL-50564, Rev. 1., University of California, Lawrence Livermore National Laboratory, Livermore, CA, 1972. (13) IAEA, Handbook of Parameter Values for the Prediction of Radionuclide Transfer in Temperate Environments; Technical Reports Series No. 364; International Atomic Energy Agency: Vienna, 1994 (ISBN 92-0-101094-X). (14) Børretzen, P. Mobility of Trace Metals and Radionuclides in Sediments and Bioavailability of Resuspended Particles. Doctor Scientarium Thesis, Department of Chemistry and Biotechnology, Agricultural University of Norway, 2001 (ISBN-82-5750447-5).
Received for review March 11, 2004. Revised manuscript received August 19, 2004. Accepted October 18, 2004. ES049618P VOL. 39, NO. 1, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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