Uranium and plutonium atom ratios and concentration factors in Reservoir 11 and Asanov Swamp, Mayak PA

Mayak Production Association, East Ural, Russia was established in the late 1940s to produce weapons-grade plutonium. Routine discharges and accidents at Mayak PA contaminated large areas with radionuclides, including the Techa River. When the scale of contamination became apparent a cascade of artificial reservoirs was constructed to store liquid wastes, hold back contamination and significantly reduce the amounts of radionuclides entering the Techa River. Reservoirs 10 and 11 (dams constructed in 1956 and 1963, respectively) are the largest: Russian total inventory estimates in 1993-1995 were 6.6 PBq and 1.2 PBq for Reservoirs 10 and 11, respectively. These 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, are expected to contain artificial radionuclides from previous direct discharges associated with the production of weapons-grade plutonium. The present work was designed to study atom ratios for U and Pu isotopes in water, soil, grass and aquatic biota samples collected in 1994 and 1996 from Reservoir 11 and the Asanov Swamp area. The study had three objectives: to generate new data in the form of 236U/235U atom ratios to help clarify the nuclide source characteristics in the studied areas; to confirm sediment and water 240Pu/239Pu atom ratios found in the literature for Mayak PA and supplement them with new ratios for biota samples; and lastly, to estimate concentration factors (CF) for the different biota in the studied area. Atom ratios (236U/235U, 235U/238U and 240Pu/239Pu) were determined using Accelerator Mass Spectrometry (AMS) to confirm radionuclide source characteristics and calculate activities and concentration factors for the studied samples. 236U/235U ratios give information about nuclear fuel burn-up time; 236U/235U ratios increase from weapons to civil sources. Plutonium isotope ratios vary with reactor type, nuclear fuel burn-up time, neutron flux and energy; and from weapon type and yield after nuclear detonations. 240Pu/239Pu atom ratios enable weapons grade Pu (ratio of 0.01-0.05) to be distinguished from civil reprocessing (ratio of 0.2-0.8) and global fallout (ratio of 0.17-0.19). Our new data shows 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 indicating that Asanov samples are affected by early discharges from weapons-grade Pu production. Results for Pu support this: The lowest 240Pu/239Pu atom ratios were consistently found in Asanov Swamp samples (about 0.019) while 240Pu/239Pu atom ratios in Reservoir 11 were higher, indicating influence from more recent civil reprocessing. Uranium and plutonium concentration factors calculated for vegetation and biota samples at Mayak were comparable with corresponding values found in the literature