Modeling for the Effect of Iron Impurities on the Characteristics of Gadolinium Molten Pools During Electron Beam Heating Process

Abstract:

The isotope _¹⁶⁰Gd can be separated from natural gadolinium using atomic vapor laser isotope separation (AVLIS) technique; and then, ₁₆₀^{Gd can be used to produce the medic}al isotope ₁₆₁^Tb through irradiation. Metal evaporation process is one of the critical steps in AVLIS, and the introduction of iron impurities from the working environment can affect the characteristics of the molten pool and the vapor quality. In this study, a molten pool model for gadolinium containing iron impurities during electron beam heating is established by employing an “evaporation–activity–concentration gradient transport–VOF free surface” coupling model with considering the effects of metal vapor recoil pressure, surface tension, buoyancy force, transport due to species concentration gradient and activity. Based on this new model, a criterion for dividing the iron content distribution regions at the molten pool surface is established, and the distributions of temperature and iron content in the molten pool of a Gd-Fe system are obtained. In the parameter range studied in this paper, the modeling results show that: With the lasting of electron beam exposure duration, the temperature at the center point of the top surface of the metal ingot rises rapidly at first, and then stabilizes at approximately 2600 K, while the molten pool surface expands into an approximately elliptical shape, and the depth of the surface depression gradually increases to about 2 mm. The distribution of iron content at the molten pool surface can be divided into three regions, i.e., the central evaporation zone, transition zone, and unmelted zone, with the isotherm of 2200 K and the boundary of unmelted metal on the top surface of the metal ingot as a criterion. The initial iron content has significant influences on the iron content distributions in the transition zone and the purity of the gadolinium vapor; with the increase of the initial iron content from 5 to 15 wt%, the iron impurity content in the gadolinium vapor increases from 8.2 to 36.5 wt% corresponding to the surface temperature of 2200 K of the molten pool. There also exist obvious influences of the electron beam power on both the temperature distribution and iron content distribution on the top surface of the metal ingot. With increasing the electron beam power from 10 to 30 kW, the maximum surface temperature at the top surface of the metal ingot increases from approximately 2200 to 2600 K, while the iron content in the central evaporation zone decreases from about 0.8 to 0.2 wt%, and simultaneously, the boundary of the central evaporation zone and the outer boundary of the transition zone both expand outward noticeably. Based on the modeling results under different initial iron contents and electron beam powers, it is recommended that 10 wt% of the initial iron content in the Gd-Fe metal ingot can be set as the upper limit for obtaining high-quality gadolinium vapors in actual applications.