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A versatile reactor system analysis code RESYS for advanced nuclear reactor was developed by using object-oriented language C++. Based on RESYS code, the model of thermionic nuclear reactor TOPAZ-Ⅱ developed in the former Soviet Union was established, and its start-up process was simulated. The established TOPAZ-Ⅱ reactor system model included reactor core thermal model, thermionic conversion system model and the radiator model. The current density model of cesium thermionic converter used model by Rasor. And the 6 groups of delayed neutron point reactor kinetic model was used to consider the influence of various structural components on reactivity. The point reactor equation with strong rigidity was solved using the Gear algorithm in the RESYS code. The reactivity feedback model of TOPAZ-Ⅱ reactor took into account the Doppler reactivity feedback of the fuel, the reactivity feedback of the electrodes, the reactivity feedback of the moderator and reflector, and the introduction of reactivity control drums. The steady state and start-up transient were simulated based on established thermal-hydraulic model and RESYS code. At steady state, the maximum temperature on the inner surface of the central thermionic fuel element is 2 291 K, the maximum temperature on the outer surface is 2 066.55 K, the maximum temperature on the emitter is 1 961 K, the maximum temperature difference between the electrodes is close to 1 200 K, the temperature at the coolant inlet is 743 K, and the temperature at the core outlet is 837 K. The comparison between the steady-state operating conditions calculated by the RESYS code and the TOPAZ-Ⅱ design values was carried out. The calculated steady-state electric power output is consistent with the calculation results of TITAM code. The calculation results verify the correctness of the developed RESYS code and the established TOPAZ-Ⅱ system model. During the startup process, the reactivity feedback of the moderator and reflection layer is positive feedback, which dominates all feedback effects. The overall reactivity feedback of the TOPAZ-Ⅱ reactor core is positive and does not have a self-stable negative feedback effect, which increases the difficulty of reactor control. During the start-up process of TFE, due to changes in the gas thermal conductivity of the electrode gap, the temperature of the fuel pellets and emitter rapidly increases. At this time, some of the thermal energy generated by fission is stored in the fuel pellets, causing a decrease in coolant temperature and the power of the radiation radiator.

In this work, the distribution and segregation behaviors of helium (He) and the effect of yttrium (Y) on these behaviors in the symmetrical tilt tungsten (W) Σ5(310)/[001] grain boundary (GB) region were studied using first-principles calculations. The results revealed that the GB has a significant impact on the behaviors of He in W. The solution and segregation energies of He in the W Σ5(310)/[001] GB region increase with increasing the distance from He to the GB and are inversely proportional to the effective electrons of He. The density of states analysis showed that the GB can suppress partial hybridization between He and W atoms. In addition, we find that the strengthening element Y facilitates the dissolution of He in the W GB region. [ABSTRACT FROM AUTHOR]

The fuel assembly and fuel rod bundle of sodium-cooled fast reactor are arranged in a triangle form. The fuel assembly adopts closed structure deign without mixing of coolant between assemblies, which is cooled by the sodium coolant through the distribution header on the bottom of the core to accomplish the matching of coolant and nuclear power and flattening the outlet temperature of the reactor core. It is of great significance for the core design and flow distribution scheme to carry out the whole core pinbypin refinement power distribution and neutronicthermal coupling calculation. There are many internal components and complex structures in the pool type fast reactor container including the core, the internal shielding structure, the primary main pump, intermediate heat exchanger, independent heat exchanger and loading and unloading elevators which are arranged around the core. The design of neutron extraction channel in the direction of loading and unloading lifting devices leads to the asymmetry of the structure in the reactor container. It causes obvious anisotropic scattering in the process of neutron transport calculation, which is also a typical deep penetration problem, so more reliable and accurate calculation results require threedimensional calculation with the whole range of the reactor container. The above problems pose challenges to modeling and neutron transport calculation. The highperformance simulation system of sodiumcooled fast reactor adopts advanced modeling technology, largescale parallel computing technology, transportburnup coupling function, coreassemblylattice multilevel mapping and conversion technology to realize the full range threedimensional fine modeling and multiphysical coupling calculation, including the shielding calculation of the container of pool structure fast reactor, the deep penetration calculation of neutron transport of detectors outside the reactor, pinbypin refined power distribution calculation, transport burnup coupling calculation and full core neutronic-thermal sub-channel coupling calculation with the parallel computation scale, and reaches thousands of cores. Through CAD preprocessing modeling, mesh generation, highperformance calculation and visualization, the threedimensional calculation results of large pool type fast reactor can be given more comprehensively and finely, and a clearer physical image of the core or reactor container can be obtained. The comparison between the local key position and the existing calculation program verifies that the highperformance simulation system of fast reactor is reasonable and feasible which provides support for further reducing the conservative margin of design and local structure optimization.

Photocatalytic CH4 coupling into high-valued C2H6 is highly attractive, whereas the photosynthetic rate, especially under oxygen-free system, is still unsatisfying. Here, we designed the negatively charged metal supported on metal oxide nanosheets to activate the inert C–H bond in CH4 and hence accelerate CH4 coupling performance. As an example, the synthetic Au/ZnO porous nanosheets exhibit the C2H6 photosynthetic rate of 1,121.6 µmol gcat−1 h−1 and the CH4 conversion rate of 2,374.6 µmol gcat−1 h−1 under oxygen-free system, 2 orders of magnitude higher than those of previously reported photocatalysts. By virtue of several in situ spectroscopic techniques, it is established that the generated Auδ− and O− species together polarized the C–H bond, while the Auδ− and O− species jointly stabilized the CH3 intermediates, which favored the coupling of CH3 intermediate to photosynthesize C2H6 instead of overoxidation into COx. Thus, the design of dual active species is beneficial for achieving high-efficient CH4-to-C2H6 photoconversion. [ABSTRACT FROM AUTHOR]

We have systematically investigated the solution and aggregation behaviors of yttrium (Y) on symmetrically inclined tungsten (W) grain boundary (GB) Σ 5 (310)/[001] by first-principles simulation. It is found that the most stable site for Y is located on the GB plane, and the solution energy of the Y substitutional site increases with increasing the distance from the GB plane. The charge redistribution of Y atoms is positively correlated with the solution energy, i.e., the deviation of the electrons of Y atoms is beneficial to its solution in the W–GB system. Further, the segregation of multiple Y atoms in the W–GB is clearly observed, where the Y atoms preferentially occupy the GB plane substitutional sites. The alloying Y atoms at the GB plane sites can effectively strengthen the GB when the concentration is less than 3.367%. The stronger binding energy of W–Y than the binding energy of W–W in the GB region is the main factor for the strengthening of the GB. [ABSTRACT FROM AUTHOR]