Refractive index measurement of hydrogen isotopologue mixture and applicability for homogeneity of hydrogen solid at cryogenic temperature in fusion fuel system

Deuterium (D)-Tritium (T) nuclear fusion reaction has potential as an energy source in the future. In both magnetic confinement and inertial confinement fusion reactors, solid D-T will generally be supplied as fusion fuel. The efficiency of the nuclear fusion reaction depends on the quality of solid D-T fuel, which is related to the composition, homogeneity, helium-3 (³He) content, and so on. However, there is no technique for in-situ examination of solid D-T fuel. In this study, we consider a simple and precise method for the characterization of solid hydrogen isotopologues at cryogenic temperature using refractive index measurement, and evaluate the distribution of hydrogen isotopologue composition and homogeneity. To evaluate without the effect of tritium decay, the homogeneity of the hydrogen (H₂)-deuterium (D₂) mixture is measured at first. By the in-situ refractive index measurement at cryogenic temperature, the homogeneity of solid H₂-D₂ mixture is roughly quantified. The phase diagram of the H₂-D₂ mixture shows a solid solution type. D₂-rich crystal first appears from the liquid phase as a primary crystal. The composition of D₂ in liquid phase ias homogeneous, whereas it reduces by obeying the liquidus line in the phase diagram with the crystallization. On the other hand, the composition of the H₂-D₂ mixture in solid phase is inhomogeneous because the mobility of H₂ and D₂ in solid phase was too slow to be homogeneous and solid. The compositions of H₂-D₂ mixture in liquid and solid phases could be evaluated by the in-situ refractive index measurement in time. Consequently, the refractive index measurement shows great potential as an inspection method of solid D-T fuel in fusion reactors.