Rao, C. V. S. BrahmanandaNarasimhan, T. S. LakshmiTrinadh, V. V.Manikandan, P....
8页
查看更多>>摘要：High temperature vaporisation thermodynamic studies over < UAl2(cr) + U(cr)> two-phase region of U-Al system were carried out by employing Knudsen Effusion Mass Spectrometry (also known as High Temperature Mass Spectrometry) in the temperature range of 1146-1327 K. The vaporisation reaction occurs incongruently and Al(g) was the neutral vapour species observed in the equilibrium vapour. Partial pressure-temperature relation of Al(g) was derived as log(p(Al)/Pa) = ( -19809 +/- 209)/T + (12.58 +/- 0.17). Using the p ?T relation, delta H-r(298. 15)& nbsp; of the following reaction: UAl2(cr) = 2Al(g) + U(cr) was evaluated by second and third law methods. Subsequently, enthalpy and Gibbs energy of formation of UAl2(cr) were deduced as delta fH298.15 & nbsp; & nbsp;= -89.9 +/- 24.1 kJ mol (-1) and delta fG(298.15)& nbsp; = -89.9 + 24.1 kJ mol(-1), respectively. The vaporisation studies on this two phase region are being reported for the first time. (c) 2022 Elsevier B.V. All rights reserved.
Bull, Sarah K.Champ, TheodoreRaj, SaiWeimer, Alan W....
12页
查看更多>>摘要：Density functional theory was used to screen eleven refractory materials - two pure metals, six nitrides, and three carbides-as high-temperature hydrogen permeation barriers to prevent hydrogen embrittle-ment. Activation energies were calculated for atomic hydrogen (H) diffusion into the first subsurface layer from the lowest energy surface of the high-temperature phase of each candidate material. The candidate barrier materials with the highest activation energies are h-BN, c-BN, HfN, and ZrN with predicted barri-ers of 3.25 eV, 3.23 eV, 3.14 eV, and 2.76 eV, respectively. Strain energies, Bader charges, and density of states were calculated for the diffusing H at the relaxed initial state and the transition state to provide insight into contributing factors to high energy barriers. The diffusing H atom in materials with the high -est predicted barriers are protic. In addition, interstitial H atoms induce mid-gap states in the density of states of both BN polymorphs. The nitrogen retention of each nitride material at high temperatures was predicted using nitrogen vacancy formation energies with respect to gaseous nitrogen. Experimental eval-uation of nitrogen retention in h-BN, ZrN, and TiN confirmed their resistance to nitrogen loss at 1773 K. However, of these nitrides, TiN is predicted to be the least stable. This work identifies multiple promising materials that are predicted to be effective hydrogen barriers at high temperatures and that are stable at temperatures above 2700 K, with BN predicted to perform best.(c) 2022 Elsevier B.V. All rights reserved.
查看更多>>摘要：Li2TiO3 ceramic pebbles ought to have high crush strength and uniform microstructure for safety operation and efficient tritium release. Fabrication techniques should be optimized to achieve the desired properties. In our previous work, Li-rich Li2TiO3 ceramic pebbles with a satisfied crush load of 41 N were fabricated by a productive rolling method. Based on this work, effects of raw materials selection, Li/Ti ratio, and sintering atmosphere on Li-Ti-O phase formation were investigated. The pebbles with homogeneous microstructure and enhanced crush load of 92 N were obtained by optimizing the Li/Ti ratio and sintering procedure. The optimal Li/Ti ratio is determined to be 2.5, and the pebbles should be first sintered in air to achieve a high crush strength and then annealed in vacuum to remove the carbonate impurities. (C)& nbsp;2022 Elsevier B.V. All rights reserved.
查看更多>>摘要：The retardation effect of radionuclide Sr on geopolymerisation was investigated by designing four comparable experimental groups (M1/M2, M3/M4). A spot of Sr2+ in the geopolymer slurry extended the setting time but only had a limited negative effect on geopolymerisation (M1). When excessive Sr2+ was introduced into the slurry, carbonisation occurred, resulting in the failure of geopolymerisation (M3). Zeolite A in M1 effectively restricted the movement of Sr2+ and fundamentally relieved the negative retardation effect on geopolymerisation, which proved the remarkable advantage of geopolymer-zeolite A composites for immobilising radioactive waste.& nbsp;(c) 2022 Elsevier B.V. All rights reserved.
查看更多>>摘要：The behavior of oxygen in uranium oxycarbide (UCO) has been investigated using density functional theory. To assess the role of carbon on the stability of oxygen point defects, we first determined the formation energies of oxygen vacancy and interstitial for different carbon configurations. Subsequently, we evaluated the barriers for migration of various oxygen defects in the UCO matrix. Our results show that carbon atoms create strong attraction centers for oxygen atoms, as a consequence the spontaneous formation of oxygen Frenkel pairs in the fluorite structure is promoted. Moreover, the strong affinity of oxygen atoms for sites in the first coordination shell of a carbon center leads to the formation of CO, and thus reducing the mobility of oxygen atoms in the UCO fuel. Upon formation, the CO remains fixed at the carbon site due to significant hybridization with uranium atoms, therefore behaving as trap sites for oxygen atoms. Our findings indicate that the formation of CO molecules increases the retention of oxygen atoms inside the UCO matrix. Consequently, carbon atoms in the UCO matrix contribute to mitigating the deleterious effects that oxygen release from the fuel kernel has on the structural integrity of the silicon carbide layer in tristructural isotropic (TRISO) particles.Crown Copyright (c) 2022 Published by Elsevier B.V. All rights reserved.
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