电子束加热过程中铁杂质对钆熔池特性影响的数值模拟研究

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

摘要:

原子蒸气激光同位素分离(AVLIS)技术可从天然钆中分离出₁₆₀^Gd同位素,而₁₆₀^Gd经辐照可制得医用同位素_1⁶¹Tb。其中,金属蒸发过程是AVLIS的一个重要环节,而从工作环境中引入的铁杂质将影响熔池状态和蒸气品质。本研究考虑了金属蒸气反冲压力、表面张力、浮力、浓度梯度输运、活度等因素，将“蒸发–活度–浓度梯度输运–VOF自由液面”耦合用于Gd-Fe体系，建立了电子束加热含铁杂质钆金属的熔池模型，给出了金属锭上表面铁含量分布区域划分的判据，得到了含铁杂质钆金属在电子束加热过程中的温度与铁含量等参数的空间分布。计算结果表明：在本文研究的参数范围内，随着电子束作用时长的增加，金属锭上表面中心处温度先迅速上升后稳定在约2600 K，熔池表面扩张为近似椭圆形，熔池表面凹陷深度逐渐增加至约2 mm；基于金属锭上表面2200 K等温线和未熔化金属边界，可将熔池表面的铁含量分布划分为中心蒸发区域、过渡区域与未熔化区域三个区域；初始铁含量的增大对过渡区的铁含量分布和所形成的钆金属蒸气纯度有显著影响，当初始铁含量从5 wt%升高至15 wt%时，对应于熔池表面温度2200 K时的钆蒸气中铁杂质含量从8.2 wt%升高至36.5 wt%；电子束功率对金属锭上表面温度与铁含量分布亦有显著影响，当电子束功率从10 kW升高至30 kW时，熔池上表面最高温度从约2200 K上升至约2600 K，中心蒸发区域的铁含量从约0.8 wt%降低至约0.2 wt%，且中心蒸发区域边界和过渡区域外边界明显向外扩张。基于不同初始铁含量和电子束功率的计算结果，为了获得高品质的钆蒸气，建议在实际生产中将10 wt%作为金属锭初始铁含量上限。

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.