[en] A nonlinear dynamic model of a one-dimensional photonic crystal nanocavity resonator is presented. It considers the internal tensile stress and the geometric characteristics of a photonic crystal with rectangular (and circular) holes. The solution of the dynamic model shows that the internal tensile stress can suppress the hardening and softening behaviors of the resonator. However, the stress can reduce the amplitude, which is not conducive to an improvement of the sensitivity of the sensor. It is demonstrated that with an optimized beam length, the normalized frequency drift of the beam can be stabilized within 1% when the optical power increases from 2 mW to 6 mW. When the hole size of the resonator beam is close to the beam width, its increase can lead to a sharp rise of the resonant frequency and the promotion of hardening behavior. Moreover, the increase in the optical power initially leads to the softening behavior of the resonator followed by an intensification of the hardening behavior. These theoretical and numerical results are helpful in understanding the intrinsic mechanism of the nonlinear response of an optomechanical resonator, with the objective of avoiding the nonlinear phenomena by optimizing key parameters.

[en] The structural, magnetic and transport properties of La_1+xK_1-xFe_1-yCo_yMoO₆ (0.0≤x≤0.1 and 0.1≤y≤0.2) series are studied. At room temperature, the crystal structure is a monoclinic system with space group P2₁/n. The antisite defect lowers with Co doping in LaKFe_1-yCo_yMoO₆ series. However, it increases with the substitution of K by La. Magnetizations increase with the increase in Co content (x=0) and with the La substitution for K, respectively. All compounds demonstrate semiconducting behavior. Their electrical resistivities increase with Co content for LaKFe_1-yCo_yMoO₆ and also increase with La for La_1+xK_1-xFe_1-yCo_yMoO₆. For the LaKFe_1-yCo_yMoO₆ the electrical transport behavior can be described by Mott variable range hopping model in the studied temperature range, whereas for the La_1+xK_1-xFe_1-yCo_yMoO₆ (x≠0 and y≠0) the electrical transport behavior follows the Mott and ES variable range hopping model in high and low temperature ranges, respectively. Each sample exhibits a large magnetoresistance effect. - Research Highlights: → Antisite defects decrease with Co doping in LaKFe_1-yCo_yMoO₆ series. → Antisite defects increase with substitution of K by La in La_1+xK_1-xFe_1-yCo_yMoO₆ series. → Electrical resistivities increase with Co content.

[en] For designing and optimizing the reactor core of modular pebble-bed fluoride salt-cooled high-temperature reactor (PB-FHR), it is of importance to simulate the coupled fluid and particle flow due to strong coolant-pebble interactions. Computational fluid dynamics and discrete element method (DEM) coupling approach can be used to track particles individually while it requires a fluid cell being greater than the pebble diameter. However, the large size of pebbles makes the fluid grid too coarse to capture the complicated flow pattern. To solve this problem, a two-grid approach is proposed to calculate interphase momentum transfer between pebbles and coolant without the constraint on the shape and size of fluid meshes. The solid velocity, fluid velocity, fluid pressure and void fraction are mapped between hexahedral coarse particle grid and fine fluid grid. Then the total interphase force can be calculated independently to speed up computation. To evaluate suitability of this two-grid approach, the pressure drop and minimum fluidization velocity of a fluidized bed were predicted, and movements of the pebbles in complex flow field were studied experimentally and numerically. The spouting fluid through a central inlet pipe of a scaled visible PB-FHR core facility was set up to provide the complex flow field. Water was chosen as liquid to simulate the molten salt coolant, and polypropylene balls were used to simulate the pebble fuels. Results show that the pebble flow pattern captured from experiment agrees well with the simulation from two-grid approach, hence the applicability of the two-grid approach for the later PB-FHR core design. (authors)

[en] Abstract Background: The Pebble-bed Fluoride-salt-cooled High-temperature Reactor (PB-FHR) is one type of next generation IV nuclear power plants. It combines two existing technologies to create a new reactor option:graphite-matrix, coated-particle fuels developed for helium-cooled high-temperature reactors and liquid-fluoride-salt coolant used in molten salt reactors. To proceed thermal-hydraulic analysis of the core of PB-FHR such as online refuelling, randomly packed bed with different porosity is usually required firstly. Purpose: In this study, an efficient algorithm to produce randomly packed pebble bed with mono-sized spheres and variable packing factor in cylindrical containers is proposed. Methods: The packing of the pebble bed is initially constructed by free falling of soft particles (Young's module much less than the real value) under the gravity environment using the discrete element method (DEM). During the free-falling process, different Young's modules and friction factors are used to control the overlaps of the packing. Then the packing expands with specific large Young's module and friction factor to eliminate the unrealistic large overlaps. In the expanding process, the time step is limited and the strategy of dissipating elastic energy is introduced to constrain the speed of expansion. Results: Low friction factor in two processes tends to produce the dense packing or vice versa. The computational burden depends on the Young's module in the free-falling process significantly. Conclusion: Through adjusting the friction factor and the Young's module in the free-falling process and the friction factor in the expansion process, the packing algorithm can generate the pebble bed with wide range of porosity and higher computational efficiency. (authors)

[en] Abstract Background: Thorium-based molten salt reactor with solid fuel (TMSR-SF) is one type of the fourth nuclear power plant which combines high-temperature graphite-matrix coated-particle pebble fuel for high temperature gas-cooled reactors and liquid salts developed for the molten salt reactors. The design of TMSR-SF requires detailed understanding of the packing structure of the pebble bed in reactor core. Purpose: This article is aimed to explore the effectiveness of refractive index matched scanning technique (RIMS) in the 3D-reconstruction of reactor core. Methods: The cross sections of the pebble bed are imaged by a charge-coupled device (CCD), then the threshold transformation, filter transformation and Hough transformation are used to process the images, and the coordinates of points on the spheres'edge are extracted. Then an algorithm is used to acquire the relative center coordinates of each sphere. Results: The result shows that excluding irrelevant distractors can improve the accuracy of the 3D-reconstruction, and creating region of interest (ROI) to make threshold transformation is a good method to enhance circles'edges, and when the circles in the images distort, minimum bounding rectangle (MBR) functions is more effective than Hough circle transformation. Conclusion: Our scheme of reconstruction can reconstruct the 3D packing structure of the pebble bed with the mean distance-overlapped which is 1.43 mm, and this precision needs to be improved later. (authors)

[en] [Background] The 10 MW thorium-based molten salt reactor with solid fuel (TMSR-SF), designed by Shanghai Institute of Applied Physics, is a pebble bed fluoride salt cooled high temperature reactor in the world. The three-dimensional random packing structure has an important influence on the reactivity and fuel management scheme in the neutron physics as well as the heat transfer in thermal-hydraulic analysis. [Purpose] This study aims at the simulation analysis of the structural characteristics of the random packing pebble bed in TMSR-SF by discrete element method (DEM). [Method] The particle flow code 3 dimensional (PFC3D) was employed to simulate the particle motion under buoyancy effects of particle size, friction coefficient, and number of particles on the steady state packing structures. Radial porosity distribution of stochastic accumulation pebble bed generated by DEM was verified. [Results and Conclusion] The simulation results show that packed bed with smaller particle diameter can weaken the oscillation of the axial and radial porosity, which will help to smooth the power distribution in the reactor core. By reducing the friction between particles, the vibration process can be well reflected. The relationship between the friction coefficient and packing porosity as well as the coordination numbers of the filler is in accordance with the negative exponential distribution. When the filled particles number exceeds 8000, the bed average porosity and coordination numbers tend to be constant with the value of 0.43 and 5.6 respectively, the influence of particle number was minimized. Then the packing bed with 8000 filled particles can be used as a representative of the full packed pebble bed to study the thermal-hydraulic numerical analysis of the natural circulation in TMSR-SF, which will help saving the computational resources. (authors)

[en] Highlights: • Natural and human environments are abundant of unused renewable energy. • Metamaterials are able to enhance local energy density for improved energy harvesting. • An overview on multi-field energy harvesting via multi-scale metamaterials is provided. • The goal is to spark interest of new investigators to metamaterial energy harvesting. • Multiscale metamaterial based energy harvesting will benefit the Internet of things. -- Abstract: Natural and human environments are abundant of unused renewable energy such as mechanical energy, acoustic energy, electromagnetic energy, thermal energy, etc. The idea of designing multi-scale metamaterials with super-normal functions on energy manipulation is utilized in multi-field renewable energy harvesting and absorbing. The metamaterials are able to enhance the local energy density by confining and focusing the energy before it to be harvested, leading to remarkable improvement of the output power and conversion efficiency. Leveraging the multi-scale metamaterials for renewable energy harvesting is an emerging direction to exploit the excess energy in the natural and man-made environments. This paper provides a brief overview of the studies published over the past decade on mechanical, acoustic, electromagnetic and thermal energy harvesting using the relevant metamaterials. The goal is to spark the interest of new investigators to this unconventional but fast-evolving branch of energy harvesting that will impact the Internet of things, smart cities and sustainable developments.

[en] Highlights: • A metamaterial and Helmholtz coupled resonator was proposed for high-density acoustic energy harvesting. • Vibroacoustic coupled analyses visualize the effects of sound pressure focusing and amplification. • Anechoic-room experiments demonstrate superiority of coupled resonator compared to metamaterial. • Field tests illustrates the effectiveness of coupled resonator in mechanical noise environment. High-density acoustic energy harvesting is one of the power solutions for wireless sensor network nodes in the Internet of Things. In this paper, we present a novel metamaterial and Helmholtz coupled resonator (MHCR) to enhance the sound energy density by energy focusing and pressure amplification. Metamaterial refers to a type of structural composite material, usually periodic. The local modification of the material by introducing a defect can make the wave at the defect band frequency be confined to the defect area to achieve acoustic energy focusing. The Helmholtz resonator is added to the defect of the metamaterial to amplify the focused sound waves. The variation in channel pressure causes the plug of the air in the neck to oscillate in and out, producing adiabatic compression and expansion of the air in the cavity to amplify sound pressure. The mathematical models of band structure, resonant frequency, vibration amplitude with vibroacoustic coupling and output voltage with electromechanical coupling are developed to design MHCR. The maximum voltage of the coupled energy harvester was about 3.5 times that of the maximum voltage of the metamaterial energy harvester. Field tests illustrated the effectiveness of the proposed MHCR with the maximum transmission ratio of 30.83 mV/Pa in mechanical noise environment, which was 48 times the maximum transmission ratio of the metamaterial energy harvester in the chirping of cicadas.

[en] Environmental quality data sets are typically imbalanced, because environmental pollution events are rarely observed in daily life. Prediction of imbalanced data sets is a major challenge in machine learning. Our recent work has shown deep cascade forest (DCF), as a base learning model, is promising to be recommended for environmental quality prediction. Although some traditional models were improved by introducing the cost matrix, little is known about whether cost matrix could enhance the prediction performance of DCF. Additionally, feature extraction is also an important way to potentially improve the model's ability to predict the imbalanced data. Here, we developed two novelty learning models based on DCF: cost-sensitive DCF (CS-DCF) and DCF that combines unsupervised learning models and greedy methods (USM-DCF-G). Subsequently, CS-DCF and USM-DCF-G were successfully verified by an imbalanced drinking water quality data set. Our data presented both CS-DCF and USM-DCF-G show better prediction performance than that of DCF alone did. In particular, USM-DCF-G shows the best performance with the highest F1-score (95.12 ± 2.56%), after feature extraction and selection by using unsupervised learning models and greedy methods. Thus, the two learning models, especially USM-DCF-G, were promising learning models to address environmental imbalanced issues and accurately predict environmental quality.

[en] Highlights: • The loading pattern of pebbles in the reactor is simulated in the model experiments. • The packing factors in reactor model with different packing heights and diameters were measured. • The geometry of the lower cone reflector may induce large variety of packing factor if the angle of the reflector is small. • The changes of the loading rate and the flow velocity in TMSR-SF have little effect on the packing factor. - Abstract: Many concepts of molten salt cooled pebble bed reactor have been developed in recent decades. The packing factor (PF) of the pebble bed in the molten salt reactor should be investigated because it is of great importance for reactor design. Model experiments based on the solid fuel thorium-based molten salt reactor (TMSR-SF) were performed. Experimental results show that the PF of the pebble bed in TMSR-SF (D = 21d, H = 18d, d is pebble diameter) is about 0.57 ± 0.02. The pebble bed in liquid environment is looser than that in dry condition. The PF increases with the diameter of the reactor core and the height of the pebble bed. The geometry of the lower cone reflector may induce large variety of packing factor if the angle of reflector is smaller than the angle of repose of the pebble bed. The loading rate and flow velocity in TMSR-SF are considered to be of little influence on PF. Results from the experiments will be of reference value for the design of reactors.