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[en] The 2D/1D transport method is widely implemented for direct whole-core calculations because of its good balance between efficiency and accuracy. In the 2D/1D transport method, negative total source may occur for MOC calculation in iteration process, which will lead to iteration divergence. The negative total source in the MOC equation is studied and an improved leakage splitting method is proposed to improve stability for 2D/1D transport method in this paper. With the new method, the accuracy is preserved while the memory cost only increases slightly. Finally, high leakage cases, the C5G7 and VERA benchmark are tested to show the accuracy and performance of the proposed method.
[en] Highlights: • Gas-solid reaction led to the production of Ni-doped Mo2C. • The amount of nickel influenced the morphology of Mo2C particles. • The increase of the dopant content reduced the mean diameter of agglomerates. • Small amounts of dopant favors the densification of pelletized Mo2C.
[en] The 2D/1D fusion method (2D/1D method) is becoming a standard transport method for whole-core calculations, which reduces the group condense and assembly homogenization approximations in the conventional two-step reactor physics calculations. In most 2D/1D codes, a pin is chosen as a 1D calculation domain, which assumes that the axial leakage of the pin is flat on top/bottom surfaces. Similar to the axial leakage, the radial leakage of every 2D plane also introduces several approximations along axial direction for the 1D calculation. This paper studied the error introduced by both axial and radial leakages, and proposed a leakage reconstruction method according to the error analysis for the 2D/1D fusion method, in which MOC is applied to the radial 2D calculation and the Sn diamond difference method is used for the axial 1D calculation. Several cases are tested to show the performance of this leakage reconstruction method especially when the leakage term is significant.
[en] It is shown that heating of a fullerite film several monomolecular layers thick deposited onto single-layer graphene formed on a substrate of molybdenum carbide Mo2C at T = 700–800 K leads to the intercalation of C60 molecules under the graphene layer. The direct deposition of C60 molecules at T = 650 K also leads to the intercalation of C60 molecules under graphene; the maximal amount of fullerene accumulated under graphene is one single layer.
[en] Modified 9Cr-1Mo ferritic steel is extensively used in high temperature application in fossil-fired power plant and steam generator of fast reactors. The material derives its strength from its martensitic lath structure containing high dislocation density, grain and sub-boundaries decorated with chromium rich M_2_3C_6 carbide and Nb-V carbonitride precipitates. The steel under service conditions suffers from the coarsening of M_2_3C_6 carbide and recovery of martensitic structure. In the present investigation, microstructure of the modified 9Cr-1Mo (T91) steel has been refined through thermo-mechanical treatment (TMT). The finer as well as enhanced precipitation of M_2_3C_6 and MX precipitates were observed in the TMT processed T91 steel. Creep deformation behaviour of T91 steel and TMT processed T91 steel have been carried out at 923 K. The minimum creep rate of the TMT processed steel was significantly lowered as compared to T91 steel. The TMT processing of modified 9Cr-1Mo steel resulted in enhancement of creep strength due to presence of fine and relatively more number of precipitates which relatively delays the recovery of dislocation structure and coarsening of sub-boundaries than the steel in the normalized and tempered condition. (author)
[en] Several transition metal carbides (especially in the molybdenum--carbon system) are studied by means of field ion microscopy. The field ion micrographs of αMoC, αMo2C, as well as other carbides, are presented. A computer technique is used to simulate the ion image formation process for these carbides. Both the geometrical degree of protrusion of a surface atom on a field ion microscopic specimen and nearest neighbor interactions of that atom are considered in various simulation models. A best-fit model is then chosen by comparing the simulated patterns with the observed field ion micrographs. A mechanism of field ion image formation is proposed, and qualitative explanations are given for the characteristic features of the carbide images. (Diss. Abstr. Int., B)
[en] The catalytic transformation of the heavy aromatics in bitumen into lighter components is the key to the upgrading and refining of the oil sands. To understand the chemical bonding in molybdenum carbide nanoparticle (MCNP) catalysts and the chemisorption bonds between the MCNPs and unsaturated hydrocarbons, the topological analysis of the electron localization function was applied to various MCNPs and their complexes with unsaturated hydrocarbons. For some of the smaller complexes, comparisons are made with the atoms in molecules approach, including the calculation of delocalization indices. The results are interpreted in the Lewis bonding scheme. It was found that the Mo-C bonding can be highly ionic in cases like Mo8C12 and MoC but shows significant covalent character in Mo2C, Mo3C, and Mo28C14. The chemisorption bonds between hydrocarbons and the MCNPs involve electron sharing of various types with strong covalent character. The strong three- or four-center interactions determine the adsorption configurations of the hydrocarbons on the MCNPs. Derivatives of benzene show some different bonding features, which depend strongly on the substituent or the heteroatom. (author)
[en] Based on generalized coarse mesh re-balance (GCMR) method, this paper proposes a new acceleration method for the method of characteristics (MOC) in unstructured meshes: the generalized coarse-mesh finite difference (GCMFD) method. The GCMFD method, which applies equivalent width of coarse mesh to establish the finite difference discretization, can use unstructured coarse meshes composed of adjacent fine meshes to speed up the MOC iteration. The convergence property of the GCMFD method is controlled by width factor. However, the optimal width factor cannot be given a priori. Method of adjusting width factor automatically is proposed in this paper. Finally, the GCMFD method is adopted in the 3-D neutron transport MOC code TCM. Numerical tests show that the GCMFD, using generalized-geometry coarse meshes, can accelerate the MOC iteration with good speedup. (authors)
[en] A novel approach for the preparation of molybdenum carbide by solution combustion synthesis (SCS) combined with subsequent programmed heating of SCS products was proposed using ammonium heptamolybdate (AHM) and organic reducers (glycine, alanine, glucose, etc.) as precursors. It has been shown that SCS temperature and composition of the products are governed by changing the AHM-organic fuel ratio, the type of organic reducer, the rate of gaseous oxygen flow, and quantity of ammonium nitrate. A solution combustion synthesis method allowed to produce molybdenum carbide at the first stage only from the AHM-glycine system. In the other studied systems, carburization process was stimulated by the subsequent programmed heating of the SCS product, sometimes with addition of a certain amount of carbon source up to 1200 °C with Vh = 20–100°min−1. The catalytic activity and selectivity of Mo2C was tested on the model reaction of isopropyl alcohol conversion. A new phenomenon showing the temperature influence on the selectivity of either propylene or acetone formation was revealed. .
[en] Graphite is joined to graphite by employing both fine molybdenum powder as the brazing material and an annealing step that together produce a virtually metal-free joint exhibiting properties similar to those found in the parent graphite. Molybdenum powder is placed between the faying surfaces of two graphite parts and melted to form molybdenum carbide. The joint area is thereafter subjected to an annealing operation which diffuses the carbide away from the joint and into the graphite parts. Graphite dissolved by the dispersed molybdenum carbide precipitates into the joint area, replacing the molybdenum carbide to provide a joint of graphite