Behavior of Niobium-95 in Techa River Water

Abstract

The study is aimed at reconstructing the parameters of the source from which the people living in 1949–1956 on the banks of the Techa River were irradiated. It deals with the migration of γ-emitting nuclides (GENs) and specifically ⁹⁵Nb in the river water. Some assumptions concerning the sources from which ⁹⁵Nb got into the river water and its migration with the river flow were made previously. It was noted that the ⁹⁵Nb sorbability is abnormally low and it is readily transported over long distances with the river water, whereas ⁹⁵Zr, on the contrary, is efficiently taken up by bottom sediments already at the point of the liquid radioactive waste (LRW) discharge and can migrate only in the form of suspended material or bed load. It was noted that ⁹⁵Nb could get into the river water both directly with the discharged LRW (external source) and via release from bottom sediments (BSs) after the ⁹⁵Zr decay (internal source). It was assumed that, after October 1951 when the LRW discharge into the river was reduced by 2–3 orders of magnitude, the internal source became the main source of the ⁹⁵Nb supply into the river water. This study additionally confirms the above assumptions. Archive results of daily measurements of the water specific activity in 1952 in the upper reaches of the Techa River are considered. The activity concentration of the sum of β-emitting nuclides (BENs), A_Σ (μCi/L), was determined by measuring the β-particle count rate with an end-window counter, and that of the sum of γ-emitting nuclides (GENs), М_Σ (μg-equiv Ra/L), by comparing the ionizing power of the test and reference (²²⁶Ra) sources. In determining the activity concentration of jth BEN, the β-radiation absorption coefficient in the layer of all the materials between the sample and the end-window counter, K_j, should be taken into account. Determination of the ratio R_j = М_j/А_j = m_j/K_j, where m_j is the gamma equivalent of jth GEN (μg-equiv Ra/μCi), was suggested for each jth GEN in the discharged LRW. Comparison of the experimental ratios R_Σ = М_Σ/А_Σ with the corresponding calculated values, R_j = m_j/K_j, furnishes new valuable information on the ⁹⁵Nb concentration in the river water. In the first half of the year 1952, the ⁹⁵Nb activity concentration in river water was more than 10 times higher than the A_Σ values measured with an end-window counter (practically, the ⁹⁵Nb activity was not detected). The main source of the ⁹⁵Nb supply into the river flow was the release of recoil nuclei from the BS surface layer upon ⁹⁵Zr decay (internal source). It decreased with time by an exponential law in accordance with the ⁹⁵Zr half-life. The ⁹⁵Nb activity concentrations in water at Metlino village and Nadyrov Most settlement (42 km downstream LRW discharge point) were virtually the same, confirming abnormally low sorbability of ⁹⁵Nb.