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Application of Natural and Artificial Isotopes in Groundwater Recharge Estimation
Published in M. Thangarajan, Vijay P. Singh, Groundwater Assessment, Modeling, and Management, 2016
The use of stable isotopes in hydrology depends on variations in natural waters. The variations result from isotope fractionation, which occurs during some physical and chemical processes. Examples of physical processes that could lead to isotopic fractionation are evaporation of water or condensation of vapor. During evaporation, the residual liquid is enriched in the heavier isotope molecule because the lighter molecule moves more rapidly and hence has a greater tendency to escape from the liquid phase, that is, there is a difference in the volatility between the two molecular species. Chemical fractionation effects occur because a chemical bond involving a heavy isotope will have a lower vibrational frequency than an equivalent bond with a lighter isotope. The bond with the heavy isotope will thus be stronger than that with the light isotope. Fractionation may occur during both equilibrium and nonequilibrium chemical reactions. During nonequilibrium or irreversible reactions, kinetic fractionation leads to the enrichment of the lighter isotope in the reaction product because of the ease with which the bond between lighter isotopes could be broken.
Refractive Index Measurements of Solid Hydrogen Isotopologues
Published in Fusion Science and Technology, 2023
Jiaqi Zhang, Akifumi Iwamoto, Keisuke Shigemori, Masanori Hara, Kohei Yamanoi
Isotopologue fractionation has two kinds of processes: equilibrium isotope effects and kinetic isotope effects.15 Equilibrium fractionation means a heavier isotope equilibrates into the compound or phase in which it is most stably bound. Thus, D2 was more inclined to be enriched in the solid phase. For kinetic fractionation, if reaction products become physically isolated from the reactants, this isotopologue fractionation is a unidirectional kinetic reaction. During H-D solidification, as the temperature decreased, D2 solidified first and precipitated at the bottom of the cell. Since heavier isotopologues move more slowly (KE = 1/2 mv2) leading to slower reactions, D2 was covered with new crystals before the exchange reaction with liquid H2 took place in time. Meanwhile, the H composition increased in the residual liquid H2-D2. As crystallization continued, the residual liquid solidified into new crystals with a higher H composition. The diffusion of H2 and D2 in the solid phase was slower, which eventually led to fractional crystallization. Based on isotopic fractionation in the solid H-D, it can be predicted that this phenomenon may also occur in the solid D-T fuel.