Deep insight of unique phase transition behaviors and mechanism in Zr_(2)Co-H isotope system with ultra-low equilibrium pressure

Efficient capture,safe storage and release of tritium from the international thermonuclear experimental reactor(ITER) reaction exhaust gas is a perplexing problem,and the development of an efficient tritium-getter material with ultra-low hydrogenation equilibrium pressure is considered as a reliable way.In this work,Zr_(2)Co alloy was selected as a tritium-getter material and prepared through induction levitation melting.Fundamental performance test results show that Zr_(2)Co exhibits an ultra-low hydrogenation equilibrium pressure of 3.22 × 10^(-6) Pa at 25℃ and excellent hydriding kinetics under a low hydrogen pressure of 0.005 MPa.Interestingly,unique phase transition behaviors were presented in Zr_(2)Co-H system.Specifically,Zr_(2)CoH_(5) formed by Zr_(2)Co hydrogenated at room temperature is initially decomposed into ZrH_(2) and ZrCoH_(3) at200 ℃.With the temperature increasing to 350 ℃,ZrCoH_(3)is dehydrogenated to ZrCo,and then ZrCo further reacts with ZrH_(2) at 650 ℃ to reform Zr_(2)Co and hydrogen.Among the staged phase transition pathways during dehydrogenation,the decomposition of Zr_(2)CoH_(5) occurs preferentially,which is well accordance with both the smallest reaction energy barrier and the maximum reaction spontaneity that are determined respectively from kinetics activation energy and thermodynamics Gibbs free energy.Furthermore,first principles calculation results indicate that the stronger binding of hydrogen in interstitial environments of ZrCoH_(3)and ZrH_(2) triggers the hydrogen-stabilized phase transformation of Zr_(2)CoH_(5).The unique phase transition mechanisms in Zr_(2)Co-H system can shed light on the further exploration and regulation of analogous staged phase transition of hydrogen storage materials.

Numerical investigations on performance improvement mechanism of a high-power vertical centrifugal pump with special emphasis on hydraulic component matching

The purpose of this paper is to investigate the performance improvement mechanism of a high power vertical centrifugal pump by using numerical calculations.Therefore,a comparative study of energy losses and internal flow characteristics in the original and optimized models was carried out with special attention to the hydraulic component matching.The optimized model(model B)was obtained by optimizing the vaned diffuser and volute based on the original model(model A),mainly the diffuser inlet diameter,diffuser inlet vane angle,volute channel inlet width and volute throat area were changed.Firstly,the comparative results on performance and energy losses of two models showed that the efficiency and head of model B was significantly increased under design and part-load conditions.It is mainly due to the dramatic reduction of energy loss PL in the diffuser and volute.Then,the comparisons of PL and flow patterns in the vaned diffuser showed that the matching optimization between the model B impeller outlet flow angle and diffuser inlet vane angle resulted in a better flow pattern in both the circumferential and axial directions of the diffuser,which leads to the PL3 reduction.The meridian velocity Vm of model B was significantly increased at diffuser inlet regions and resulted in improvements of flow patterns at diffuser middle and outlet regions as well as pressure expansion capacity.Finally,the comparisons of PL and flow characteristics in the volute showed that the turbulence loss reduction in the model B volute was due to the flow pattern improvement at diffuser outlet regions which provided better flow conditions at volute inlet regions.The matching optimization between the diffuser and volute significantly reduced the turbulence loss in volute sections 1–4 and enhanced the pressure expansion capacity in sections 8–10.

Electronic structure and optical properties of plutonium dioxide from first-principles calculations

The electronic structure and optical properties of plutonium dioxide were calculated using the generalized gradient approximation with a Hubbard parameter U （GGA ＋ U） for considering the strong coulomb correlation between localized Pu of electrons based on the first-principles density functional theory. The calculated results show that PuO2 is a semiconductor material with the band gap of 1.8 eV, which is in good agreement with the corresponding experimental data. Furthermore, the dielectric function, reflectivity, refractive index, and extinction coefficient were calculated and analyzed using the Kramers-Kronig relationship for PuO2. The calculated results were compared with the experimental data from the published literature.