Environ. Sci. Technol. 2000, 34, 1938-1945
Plutonium from Mayak: Measurement of Isotope Ratios and Activities Using Accelerator Mass Spectrometry D E B O R A H H . O U G H T O N , * ,† L. KEITH FIFIELD,‡ J. PHILIP DAY,§ RICHARD C. CRESSWELL,‡ LINDIS SKIPPERUD,† MARIANNE L. DI TADA,‡ BRIT SALBU,† PER STRAND,| EUGENY DROZCHO,⊥ AND YURI MOKROV⊥ Department of Chemistry and Biotechnology, P.O. Box 5026, Agricultural University of Norway, 1432 Aas, Norway, Department of Nuclear Physics, RSPhysSE, Australian National University, Canberra, ACT 0200, Australia, Department of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, U.K., Norwegian Radiation Protection Authority, P.O. Box 55, 1345 Østera˚s, Norway, and Mayak Production Association, Chelyabinsk, Russia
Accelerator mass spectrometry (AMS) has been used to measure Pu activities and 240Pu/239Pu isotope ratios in samples contaminated by releases from the Mayak nuclear installation. Determination of Pu isotopes in high-level samples indicated that the ratio of 240Pu/239Pu in waste has increased toward the present. The lowest 240Pu/239Pu atom ratios, 0.012-0.024, were found at the Asanov Swamp, where the primary source of contamination was discharge of intermediate-level radioactive waste between 1949 and 1951. The highest ratios, 0.06-0.29, were found in industrial reservoirs contaminated by various sources of waste up to the present day. Measurement of Pu isotopes in low-level samples collected from the Techa, Iset, and Ob Rivers showed that while activity levels decrease with distance from Mayaksfrom 2000 Bq/kg at 7 km downstream to less than 1 Bq/kg sediment at 250 kms240Pu/239Pu isotope ratios increase. Results suggest that most of the plutonium in the Upper Techa River originates from the early waste discharges, although enhanced atom ratios in surface sediments downstream (0.035-0.099) indicate a contribution from other sources. On the basis of procedural blanks, detection limits for AMS were below 1 fg of Pu.
Introduction The “Mayak” Production Association (Mayak PA) was established in the late 1940s to produce plutonium for the Soviet nuclear weapons program (1). The site is located at the head of the Techa River in the Southern Urals and comprises military and civil reactors as well as reprocessing and metallurgical plants (Figure 1). Accidental and routine * Corresponding author telephone: (+47) 64 94 82 50; fax: (+47) 64 94 83 59; e-mail:
[email protected]. † Agricultural University of Norway. ‡ Australian National University. § University of Manchester. | Norwegian Radiation Protection Authority. ⊥ Mayak Production Association. 1938
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 10, 2000
releases of radioactive waste, most notably the direct discharges of intermediate-level waste to the Techa River (1949-1956) and Lake Karachay (1951-), and the Kyshtym accident in 1957 have caused severe contamination of surrounding areas (Table 1). Between 1953 and 1967, more than 17 000 people were evacuated from contaminated areas, and many of these, along with inhabitants of villages along the Upper Techa River, have received significant radiation doses (2, 3). Today, the major source of radionuclides to the surrounding area is believed to be remobilization from previously contaminated soils and sediments (4). Seepage through the beds of industrial waste storage reservoirs is contaminating groundwater; contaminated sediments are transported downstream by Techa River waters during flooding; and chemically mobile radionuclides such as 90Sr are leached out of contaminated soils (1). Models of radionuclide transfer in the Mayak area need to take account of these processes, but assessments are confounded by the complex history of Mayak. Several sources have contributed to the contamination of the surrounding areas, and these vary in radionuclide composition, activity, radionuclide ratios, and physicochemical form (Table 2). Furthermore, the post-deposition behavior of radionuclides depends on both the source term and the environmental conditions. Plutonium does not represent the major health risk in the area at presentsactivities of 137Cs in Techa River soils and sediments and 90Sr and 60Co in surface and groundwaters are between 2 and 3 orders of magnitude higher than Pu (1). But Pu isotope ratios can act as a fingerprint for different sources, and information on these ratios can be useful both for identifying sources and for following the migration of Mayak-derived plutonium and contaminated soils, sediments, or water bodies. Plutonium isotope ratios are known to vary with reactor type, nuclear fuel burn-up time, neutron flux, and energy, and for fallout from nuclear detonations, weapon type and yield. Weapons-grade plutonium is characterized by a low content of the 240Pu isotope, with 240Pu/239Pu ratios usually less than 0.07 (6, 7) and is produced by leaving the nuclear fuel in the reactor for only a short time in order to minimize neutron activation of 239Pu. In contrast, both global weapons fallout and spent fuel from civil reactors have higher 240Pu/ 239Pu ratios (Table 3). In addition to 240Pu, both 238Pu and 242Pu may be produced in measurable quantities in nuclear reactors. Plutonium-238 is the product of neutron capture by 237Np, which is itself produced by two successive neutron captures from 235U or via the fast-neutron induced 238U(n,2n)237U reaction. Its shorter half-life (84.8 yr) makes it comparatively easy to detect with R-particle spectrometry. Plutonium-242 is produced by three successive neutron captures from 239Pu. Appreciable quantities of these two isotopes are further evidence of long irradiation times. In the present study, accelerator mass spectrometry (AMS) and R-spectrometry have been used to determine Pu activities and isotope ratios in samples collected from the Mayak area and further afield. The study had two objectives: first, to determine Pu isotope ratios in historical and potential sources to the Techa River; and, second, to measure Pu activities and isotope ratios in samples from the Techa, Iset, and Ob Rivers. The overall aim was to discern whether a comparison of isotope ratios in the Techa River with those seen in the different “source terms” could provide information on both the sources of the plutonium and its subsequent dispersal in the river system. 10.1021/es990847z CCC: $19.00
2000 American Chemical Society Published on Web 04/14/2000
TABLE 1. Brief History with Major Routine and Accidental Releases of β-Emitting Radionuclides from Mayak PAa June 1948 1948-1956 Aug 1949 1949-1955 19511956 1957 1963 1967 1977 1979 1987 1987-1991 1992a
first military uranium-graphite reactor operational; reprocessing and chemical/metallurgical plants completed discharge of 106 PBq intermediate-level waste to Techa River (98% from Dec 1949 to Nov 1951) high-purity metallic plutonium components produced for the first Soviet atomic bomb; exploded August 29, 1949, at Semipalatinsk, Kazakstan five more military reactors constructed discharge of 20 000 PBq intermediate-level waste to Lake Karachay dam 10/reservoir 10 complete accidental release and dispersal of 74 PBq radionuclides after an explosion in a high-level waste tanksKyshtym accident dam 11/reservoir 11 complete wind dispersal of 0.044 PBq contaminated sediments from Lake Karachay new reprocessing plant for civil fuel completed seventh reactor in operation first military reactor shut down after 39 years of operation production of weapons-grade plutonium ceased four other reactors shut down two remaining reactors producing radionuclides for civil and military use
PBq ) 1015 Bq.
TABLE 2. Composition of Historical and Present-Day Routine Discharges from Mayak PA (1)a discharge
date
Techa River (reservoir 3) reservoir cascadec (reservoirs 3-11) Lake Karachay
1949-1956
(reservoir 9) airborne
activity (PBq)
1956-1990 1993-1995 1951-1995
106 β 0.002 Rb 7 0.092/yr 20 000
1958-1972 1993-1995
700-1500/yr ∼24/yr
1948d-1993
0.140 β 0.002 R 3.0 × 10-6/yr β 5.2 × 10-7/yr R
1993-
composition intermediate-level waste (13 PBq of Cs and 12 PBq of Sr) low-level waste low-level waste intermediate-level waste (2600 PBq of 137Cs and 1800 PBq of 80Sr + 90Y) intermediate-level waste (22-55% 137Cs;
