Theoretical Study of Isotope Enrichment Caused by Nuclear Volume Effect

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Isotope separation is important in industry, such as uranium enrichment. It is also very important in geochemistry and cosmo-chemistry because heavy and light isotopes are gradually fractionated in global equilibrium processes. For isotopes of heavy elements such as uranium and gadolinium, this fractionation occurs mainly owing to the difference of nuclear charge radii between the isotopes. This is because electronic states of isotopologues are different because of the difference of nuclear charge radii. Nuclear volume terms (lnKnv) in the equilibrium constants of isotope fractionations can be calculated from the electronic energy difference of isotopologues, obtained by relativistic quantum chemical calculations with finite-size nuclear models. In this review, we summarize our theoretical results in uranium isotope separations, using U (III)-U (IV) and U (IV)-U (VI) redox reaction systems. Calculated nuclear volume term (lnKnv) is reasonably close to the ones estimated from experimental analysis of temperature dependence in equilibrium constants. We used the four-component Dirac-Coulomb Hartree-Fock method with the Fermi and Gaussian-type nuclear models in our calculations. We also briefly mention how the nuclear volume effect is important in geochemistry or cosmo-chemistry referring to recent relevant works.
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