Zirconium-95 Distribution in Bottom Sediments of the Techa River in 1952–1955

The study is aimed at the source-term reconstruction for the exposure of people to ionizing radiation who lived in 1949–1956 on the Techa riverside. It deals with the migration of γ-emitting nuclides (GENs), and specifically ⁹⁵Zr and ⁹⁵Nb, in the river system. It is known that ⁹⁵Zr exhibits abnormally high sorption properties and is virtually completely and irreversibly sorbed onto bottom sediments (BSs) and suspended material already near the liquid radioactive waste (LRW) discharge site. Therefore, its further migration is possible only with the solid runoff. It was shown previously that the large-scale radioactive contamination of the river occurred in the relatively short period from July to October 1951, when the water flow rate in the upper section was varied from 8 to 30 m³/s. Under such water runoff conditions, the most contaminated BSs near the LRW discharge site undergo intense stirring-up (erosion), and the resulting suspensions are transferred with gradual sedimentation of the suspended material throughout the river stretch. After October 1951, when the LRW discharge into the river decreased by 2–3 orders of magnitude, the ⁹⁵Zr distribution in BSs was determined only by the radioactive decay and transfer of suspended matters and saltation loads. Specific features of methods used in the 1950s for measuring the specific activity of the sum of β-emitting nuclides (BENs, A_Σ, μCi/kg) and sum of GENs (М_Σ, μg-equiv Ra/kg) are discussed. The use of the ratio R_Σ = М_Σ/А_Σ as a criterion furnishes new information on the ⁹⁵Zr activity concentration in BSs and allows picking out BS samples with prevalent contributions of ⁹⁵Zr (М_Zr) и ⁹⁵Nb (М_Nb) to М_Σ from the whole set of the archive data. Analysis of the measured archive data on the specific activity of BS samples taken from different sections of the Techa River in 1952, 1954, and 1955 using the criterion R_Σ = М_Σ/А_Σ allowed estimation of М_Σ ≈ М_Zr + М_Nb and recalculation of these results taking into account the radioactive decay as of November 1, 1951. The results of M_Σ measurements performed in 1952, 1954, and 1955 and converted to the level of November 1, 1951 appeared to be close, although the absolute activity level decreased by a factor of up to ~10 mln owing to radioactive decay. The ⁹⁵Zr specific activity in the river BSs as of November 1951 remained virtually constant throughout the river stretch. These results provide a new approach to reconstruction of the external dose for the Techa riverside population.