Liquid phase separation in transition element high entropy alloys

Non-consumable arc melting was used to produce buttons of transition metal (Co-Cu-Fe) high entropy alloys (HEAs) with additions of Al, Cr, Mn, Ni, V or Ti. High resolution scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) capabilities were used for microstructural characterization. It was found that certain HEAs exhibit stable liquid miscibility gaps causing separation into two liquids, namely a Cu-lean liquid, L1, and a Cu-rich liquid, L2. Since, in stable liquid phase separation (LPS) involving Cu, the Cu-rich L2 has a higher density and a lower melting temperature, it sinks to the bottom of the casting. Secondary melt separation was also observed and resulted in small spheres of the minority liquid embedded in the matrix of the first liquid. Additions of Co, Al, Ti, and Ni were found to lower the miscibility gap temperature (T_MG), while additions of Cr, V, and Nb were observed to raise T_MG and enhance stable LPS. Thermodynamic calculations were used to support the experimental results.     

弥散燃料颗粒裂纹起源的有限元模拟分析

通过建立含多气泡的燃料颗粒模型,采用有限元方法分析了燃料颗粒在裂变气体气泡内压作用下的应力分布,统计了燃料颗粒内部气泡位置对气泡内壁处的最大拉应力的影响,并结合实验结果探寻了弥散燃料颗粒在辐照后退火时的裂纹起源。结果表明:当弥散燃料颗粒内部含有多个裂变气体气泡时,受气泡内压作用,气泡内壁径向应力为压应力,环向应力为拉应力;气泡位置距燃料颗粒心部越远,气泡内壁处的最大环向拉应力越大;表层气泡的最大环向拉应力远大于心部气泡的;燃料颗粒裂纹起源于表层气泡内壁。     

Investigation of BeO as a reflector for the low power research reactor

Calculations of the fuel burnup, core excess reactivity, and the reactivity worths of the top beryllium shim plates for two reflector types (beryllium and beryllium oxide (BeO)) in the Miniature Neutron Source Reactor (MNSR) have been presented in this paper using the GETERA and MCNP4C codes. The results showed that the reactor infinity multiplication factors were 1.7030 and 1.6824, the core unadjusted excess reactivities were 31.9 and 5.0 mk, and the reactivity worths of the top beryllium shim plates were 22 and 19 mk for the BeO and Be reflectors respectively. Finally, using the beryllium oxide instead of the existing Be reflector in the MNSR reactor increased the core excess reactivity and reactor operation time.     

AFM characterization of model nuclear fuel oxide multilayer structures modified by heavy ion beam irradiation

This work explored a potential new model dispersion fuel form consisting of an actinide material embedded in a radiation tolerant matrix that captures fission products (FPs) and is easily separated chemically as waste from the fuel material. To understand the stability of this proposed dispersion fuel form design, an idealized model system composed of a multilayer film was studied. This system consisted of a tri-layer structure of an MgO layer sandwiched between two HfO₂ layers. HfO₂ served as a surrogate fissile material for UO₂ while MgO represented a stable, fissile product (FP) getter that is easily separated from the fissile material. This type of multilayer film structure allowed us to control the size of and spacing between each layer. The films were grown at room temperature by e-beam deposition on a Si(1 1 1) substrate and post-annealed annealing at a range of temperatures to crystallize the HfO₂ layers. The 550 °C annealed sample was subsequently irradiated with 10 MeV Au³⁺ ions at a range of fluences from 5 × 10¹³ to 3.74 × 10¹⁶ ions/cm². Separate single layer constituent films and the substrate were also irradiated at 5 × 10¹⁵ and 8 × 10¹⁴ and 2 × 10¹⁶, respectively. After annealing and irradiation, the samples were characterized using atomic force imaging techniques to determine local changes in microstructure and mechanical properties. All samples annealed above 550 °C cracked. From the AFM results we observed both crack healing and significant modification of the surface at higher fluences.     

Co纳米线阵列的掠入射中子小角散射研究

本文采用掠入射中子小角散射（GISANS）技术,研究了沉积在多孔阳极氧化铝（AAO）模板中的Co磁性纳米线阵列的空间结构,利用计算机软件对GISANS实验进行了理论模拟,并与扫描电子显微镜（SEM）观察的结果进行比较。结果表明,GISANS实验结果、GISANS理论模拟结果和SEM实验结果三者相吻合。纳米线直径约为60 nm,纳米线中心平均距离约为121 nm,垂直于样品表面相互平行排列,在样品平面内呈现出二维局域六角有序排布。本工作证明了GISANS实验技术结合计算机理论模拟,是研究纳米线阵列及类似纳米体系的有力手段。     

Application of Multiband Method to Pin Cell Calculations

Abstract

In order to accurately calculate effective neutron cross sections in the resonance energy region, the multiband method has been applied to cell calculations. Cell calculations for UO² and MOX fuels of light water reactors have been performed and the results were compared with those of a continuous energy Monte Carlo code VIM and the conventional self-shielding method using the Dancoff factor.

The k∞values calculated by the multiband method agreed with those of the VIM calculations within 0.20% Δk for the UO² fuel cell and within 0.30% Δk for the MOX fuel cell, respectively, whereas the Dancoff factor method yielded about l.l%Δk errors for the two cells. The element- wise contribution to this error was investigated, and it was found that the effective microscopic cross sections, particularly those for the giant resonances of ²³⁸U, calculated by the multiband method were in good agreement with those of VIM. It was also found that interference effect between ²³⁸U and ²³⁵U resonances in the UO² fuel and that between ²³⁸U and ²³⁹Pu resonances in the MOX fuel made about 0.20%Δk contributions to k∞ in both fuel cells.     

Protective Oxide Film on Alloy X750 Formed in Air at 973 K

An effective pre-oxidation method for Alloy X750 was developed to reduce general corrosion in an oxygenated aqueous environment such as in BWR core water. The optimum condition of preoxidation in air at elevated temperatures was found to be 5–20 h at 973 K by considering the allowance condition of heat treatment for age-hardening.

Some characteristics of the corroded oxide film have been clarified by surface analyses with XMA, SIMS, AES, XPS etc. The film was composed of double oxide layers, namely a highly crystallized NiFe₂O₄ outer layer and a high Cr₂O₃ content inner layer. The passive property of the film has been recognized to be due to the nature of the oxides whereby NiFe₂O₄ restricts the dissolution of metals because of its low solubility and Cr₂O₃ restricts the diffusion of metal ions because of its high binding energy and low diffusion coefficient.     

Assessment of Fuel Coolant Interactions (FCIs) in the FBR Core Disruptive Accident (CDA)

Application of general behavior principles (GBPs) and consideration of relevant contact modes suggest that only incoherent small-scale fuel coolant interactions (FCIs) with negligible damage potential appear possible with the molten oxide fuel-liquid sodium system as the fuel disperses away from the core into a coolable non-critical array.

In contrast to the SPERT-1, BORAX-1 and SL-1 nuclear transients that ultimately led to energetic vapor or steam explosions, the presence of molten fuel and liquid sodium in the FBR core always requires the presence of solid cladding which separates the fuel and coolant and, hence prevents energetic FCIs prior to coolant escape.

Furthermore, unlike the CORECT-II experiments which examined dynamic re-entry of liquid sodium on molten fuel pools that resulted in unstable interfaces leading to significant sodium entrapment and relatively energetic FCIs, the prevailing contact mode in the FBR core disruptive accident (CDA) scenario is displacement of the lighter and less viscous liquid sodium by the heavier and more viscous molten fuel resulting in stable interfaces with no significant sodium entrapment and FCIs. Dynamic re-entry of liquid sodium into the core is not possible with the two-component steel vapor-liquid sodium system, since the interface contact temperature upon steel vapor condensation is well in excess of the sodium boiling temperature. A pressure reduction in the steel vapor region due to condensation is immediately compensated for by an equivalent pressure increase due to sodium evaporation.

Finally, considering that the molten oxide fuel-liquid sodium interface contact temperature is well below the sodium homogeneous nucleation temperature which in turn is well below the fuel melting temperature, not only eliminates the potential for large-scale vapor explosions as molten fuel streams are injected into liquid sodium pools, but also implies that small scale superheat explosions are possible which are consistent with the usually observed incoherent sharp pressurization events (amplitudes up to the order of 10 MPa and duration of the order of 1 ms). These general behavior characteristics are also consistent with complete fuel fragmentation with fragment sizes ranging from 100 to 1,000 μm, and the absence of significant or damaging FCIs.     

Electrochemical Study on the Reduction Mechanism of Uranium Oxide in a LiCl-Li2O Molten Salt

By means of a linear sweep voltammetry, a cyclic voltammetry and a chronopotentiometry, the electrolytic reduction of uranium oxide has been studied to establish the reduction mechanisms, which are based on a simultaneous uranium oxide reduction and a Li₂O electrowinning, and the formation and electrolysis of lithium uranate. From the voltammograms, the reduction potentials of the uranium oxide and Li₂O were obtained. From the chronopotentiometries based on the results of the voltammograms, the uranium oxide was reduced to uranium metal through the reduction mechanisms showing a more than 99% conversion. For a verification of the reduction mechanisms feasibility, basic data on the electrolytic reduction of the uranium oxide was obtained from the experiments and the characteristics of the closed recycle of Li₂O were discussed.