Characterization of Gamma-Ray Source-Term in the Vicinity of Metlinsky Pond at the Techa River. Radionuclide Composition of the Source

The paper is aimed at reconstruction of a source term for external exposure of population that inhabited the banks of the Techa River from 1949 to 1956. The composition of γ-emitting nuclides contributing to radiation environment of the river banks and floodplain is also studied. The γ-emitting nuclides entered the river system both in solutions and as industrial and natural suspended matter. The paper considers archive data on measured parameters of radiation contamination of the Techa bottom sediments and riverside area. Results of the present-day radiation surveys of the Techa River show that the distribution of the exposure dose rate has a pronounced spotted and non-monotonic character along the entire river stretch and is due to sedimentation of suspended particles contaminated with the γ-emitting nuclides on the soil surface. The activity of γ-emitting nuclides accumulated in the upper reaches of the river due to sorption of water-soluble radionuclides onto bottom sediments and to sedimentation of contaminated suspended matter on the surface of bottom sediments was calculated using a stationary sorption model and a hydrodynamic model of settling ponds. Until July 1951, when the river runoff did not exceed 1 m³/s, practically all industrial and natural suspended particles settled to and accumulated at the bottom of Metlinsky Pond. Only a small portion of clay particles could enter the river below the dam of Metlinsky Pond. ⁹⁵Zr and ⁹⁵Nb are of high importance for reconstruction of the radiation environment in the Techa riverside area, because before 1953 the activity of the bottom sediments in all segments of the river was predominantly determined by these γ-emitting nuclides. There is practically no sorption of ⁹⁵Nb onto bottom sediments and suspended matter, and the river flow carries predominantly soluble ⁹⁵Nb species. ⁹⁵Zr, on the contrary, is strongly sorbed onto bottom sediments and can be transferred for long distances only as suspended matter or bed silt. Until October 1951, the radiation environment along the entire river stretch below Metlinsky Pond was contributed predominantly by ⁹⁵Nb and was characterized by uniform and stable contamination level in all swampy areas of the floodplain. Vast contamination of the bottom sediments and the floodplain along the entire river stretch by other γ-emitting nuclides (⁹⁵Zr, ¹³⁷Cs, ^141,144Ce, ^103,106Ru, etc.) could result only from “washing” of Metlinsky Pond in October 1951, when the flow rate in the upper reaches of the river was up to 30 m³/s. This was accompanied by intensive stirring-up of the bottom sediments and activity transfer out of Metlinsky Pond.