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In natural water, as a rule, there is a violation of radioactive equilibrium in the chain ²³⁸U … → ²³⁴U → ²³⁰Th →. Groundwater usually has a ²³⁴U/²³⁸U ratio in the range of 0.8–3.0 (by activity). However, in some regions, the ²³⁴U/²³⁸U ratio reaches >10 and up to 50. Ultrahigh excesses of ²³⁴U can be explained by climatic variations. During a cold period, minerals accumulate ²³⁴U as a normal component of the radioactive chain, and after the melting of permafrost, it is lost from the mineral lattice faster than ²³⁸U due to its higher geochemical mobility. This hypothesis was tested using data on the isotopic composition of uranium in the chemo- and bio-genic formations of the World Ocean and large lakes, which are reservoirs that accumulate continental runoff. The World Ocean has the most significant ²³⁴U enrichments in the polar and inland seas during periods of climatic warming in the Late Pleistocene and Holocene. In the bottom sediments of Lake Baikal, the ²³⁴U/²³⁸U ratio also increases during warm periods and significantly exceeds the ²³⁴U excess of the World Ocean. Furthermore, the ²³⁴U/²³⁸U ratio in the water of Lake Baikal and its tributaries increases from north to south following a decrease in the area of the continuous permafrost and has a seasonal variation with a maximum ²³⁴U/²³⁸U ratio in summer. The behavior of ²³⁴U in large water reservoirs is consistent with the hypothesis about the decisive influence of permafrost degradation on the anomalies in ²³⁴U/²³⁸U ratios in groundwater. Full article

Theoretical studies of tungsten ions in plasmas are presented. New calculations of the radiative recombination and photoionization cross-sections, as well as radiative recombination and radiated power loss rate coefficients have been performed for 54 tungsten ions for the range W6+–W71+. The data are of importance for fusion investigations at the reactor ITER, as well as devices ASDEX Upgrade and EBIT. Calculations are fully relativistic. Electron wave functions are found by the Dirac–Fock method with proper consideration of the electron exchange. All significant multipoles of the radiative field are taken into account. The radiative recombination rates and the radiated power loss rates are determined provided the continuum electron velocity is described by the relativistic Maxwell–Jüttner distribution. The impact of the core electron polarization on the radiative recombination cross-section is estimated for the Ne-like iron ion and for highly-charged tungsten ions within an analytical approximation using the Dirac–Fock electron wave functions. The effect is shown to enhance the radiative recombination cross-sections by ≲20%. The enhancement depends on the photon energy, the principal quantum number of polarized shells and the ion charge. The influence of plasma temperature and density on the electron structure of ions in local thermodynamic equilibrium plasmas is investigated. Results for the iron and uranium ions in dense plasmas are in good agreement with previous calculations. New calculations were performed for the tungsten ion in dense plasmas on the basis of the average-atom model, as well as for the impurity tungsten ion in fusion plasmas using the non-linear self-consistent field screening model. The temperature and density dependence of the ion charge, level energies and populations are considered. Full article