Subway subgrades typically consist of alternating deposits of soil layers with significantly different physical and mechanical properties. However, the overall dynamic characteristics and the evolution of micro-porous structures in stratified soils is often overlooked in current studies. In this study, cyclic triaxial tests were conducted on homogeneous sand, silt and stratified soils with different height ratios, and nuclear magnetic resonance (NMR) was used to investigate the changes in pore structure and moisture content. The dynamic behavior and macroscopic deformation mechanisms were systematically investigated in terms of stress amplitude, confining pressure, and layer height ratio (the ratio of sand to silt height). The results show that as the sand height ratio increases, the axial strain and pore water pressure first increase and then decrease, reaching the maximum when h(Sand): h(Silt) = 2:1. When the confining pressure is 100 kPa, the axial strain of h(Sand): h(Silt) = 2:1 is 181.08 % higher than that of silt. Under the dynamic loading, the stratified soils form a dense skeletal structure near the stratification plane, which hinders the flow and dissipation of pore water, so that the pore water agglomeration phenomenon occurs near the stratification plane, which aggravates the accumulation of residual pore pressure and reduces the deformation resistance. However, when h(Sand): h(Silt) = 4:1, the influence of the stratification planes is significantly reduced, and the deformation characteristics approach homogeneity. This study reveals the dynamic characteristics of stratified soils by comparing and analysing homogeneous samples.

This paper proposes a frequency wavenumber-finite element hybrid method with kinetic source model for dynamic analysis of pile founded nuclear island from fault to structure. This method benefits from the effective synthesis of broadband ground motions by the fault source model, the realism of frequency wavenumber for earthquake simulation from fault to the site and the mesh refinement capabilities of the finite element in modeling the nuclear structure and the near soil. This method achieves the expression of source rupture, wave propagation, site response, soil-structure interaction, soil nonlinearity and structure response accurately, which solves the multi-scale problem from crustal layer to nuclear structure. Under finite-fault excitation, the correctness of the proposed method is validated by comparing with the frequency wavenumber method. Then, a full process seismic simulation of a pile founded nuclear island built on a non-rock site is conducted. The influence of source parameter and soil-structure interaction is studied. Results indicate that the change of source parameter can lead to difference nuclear island failure direction. With the increase of dip angle, the appearance of maximum stress is in advance. The soil nonlinearity could greatly amplify the soil-structure interaction effect and the loads on piles. The connection between the containment vessel and the raft is vulnerable and the piles on the edge of the raft is prone to damage. This hybrid method could accomplish an appropriate seismic evaluation of the nuclear structures and the conclusions may provide reference for seismic design of nuclear structure.

Resonance can significantly amplify a structure's response to seismic loads, leading to extended damage, especially in critical infrastructure like nuclear power plants. Thus, this study focuses on the resonance effects of the dynamic interaction between layered soil, pile foundations, and nuclear island structures, which is particularly important given the limited availability of bedrock sites for such facilities. Specifically, this study explores the resonance behavior of nuclear islands under various seismic conditions through large-scale shaking table tests by developing a dynamic interaction model for layered soil-pile-nuclear island systems. The proposed model comprises a 3 x 3 pile group supporting the upper structure of a nuclear island embedded within a three-layer soil profile. Sinusoidal waves of varying frequencies identify the factors influencing the system's resonance response. Besides, the resonance effects are validated by inputting seismic motions based on compressed acceleration time histories. Furthermore, the impact of non-primary frequency components on structural resonance is assessed by comparing sinusoidal wave components. The findings reveal that resonance effects increase as the amplitude of the input seismic motion increases to a certain threshold, after which the effect stabilizes. This trend is particularly pronounced in the bending moment response at the pile head. Additionally, an independent resonance phenomenon is observed in the superstructure, suggesting that its resonance effects should be considered separately in nuclear island design. Similar resonance effects are observed when the predominant frequency of sinusoidal waves closely matches the compressed seismic motions, suggesting that sinusoidal inputs effectively simulate structural resonance during seismic design testing.

Nuclear facility sites built on soft deposits often adopt a combined piled cushion raft foundation (CPCRF) to enhance bearing capacity. However, separation and slip at the raft-bottom interface is inevitable in refined seismic simulations of weakly anchored nuclear island buildings (NIBs). Multiple factors related to both the structure and foundation influence the interface behavior. To address this, a structure-interface-soil nonlinear interaction model was developed, incorporating interfacial discontinuity characteristics, tri-directional wave inputs, and a stable semi-unbounded condition. The validity of the wave-field simulation method and the interface model were confirmed through theoretical comparisons. Using the AP1000 NIB at a specific CPCRF site as an example, the practicability of the model was validated, and key behavioral patterns were identified. In the static-seismic process, correlations between interface behavior, pile damage, and structural vibration were quantitatively elucidated. When seismic intensity exceeded design limits, the minimum instantaneous grounding ratio decreased rapidly. Structural vertical acceleration nearly doubled, and the frequency band of peak horizontal vibration shifted to higher frequencies. Interface behavior strongly correlated with slip stability and pile body damage. These findings indicate that interfacial discontinuities at the raft's bottom pose safety risks warranting further investigation.

change of unfrozen water content in pores of rock during freeze-thaw process is one of the key factors affecting its mechanical properties. In this paper, the sandstone is taken as the research object, and the pore water content of rock during freeze-thaw process (20, 0, -2, -4, -6, -10, -15, -10, -6, -4, -2, 0, 20 degrees C) is monitored by low-field nuclear magnetic resonance system (NMR), and the evolution law of unfrozen water content with temperature is analyzed. The influence of the evolution of unfrozen water content on the mechanical properties of rock during freeze-thaw process is also discussed. The research findings show that the pore water in rocks during the freezing-thawing process is significantly influenced by temperature, passing through five stages: supercooling, rapid freezing, slow freezing, slow melting, and accelerated melting. A distinct hysteresis phenomenon is observed in the rock during thawing. At identical temperatures, the unfrozen water content during freezing is notably higher than during thawing. Consequently, the peak intensity and elastic modulus during thawing are significantly greater than during freezing. The relationship between uniaxial compressive strength, rock elastic modulus, and unfrozen water content in freeze-thaw process can be expressed by exponential function. At the beginning of freezing, the change of rock mechanical parameters is mainly affected by the increase of pore ice content and the cementation effect of pore ice on rock particles. With the further decrease of temperature, the thickness of adsorbed water film decreases, and the adsorption capacity increases, so that the integrity between pore ice and rock particles is enhanced, and rock mechanical parameters further change.

Low cohesion and poor scour resistance make sandy bank slopes in the lower reaches of rivers susceptible to instability and damage. Soil stabilization is one of ecological flexible bank protection technologies, which not only pays attention to the function of flood control, but also pays attention to the function of ecological and environmental protection. This study conducts a series of mechanical property test, nuclear magnetic resonance test (NMR), computed tomography test (CT), and scanning electron microscope test (SEM) on hydrogel-stabilized sand to highlight the link between pore-scale and macroscopic properties. The results of the mechanical tests indicate a linear increase in unconfined compressive strength, flexural strength, tensile strength, and cohesion with increasing hydrogel content. Conversely, the internal friction angle appears to be less impacted by fluctuations in hydrogel content. The specimen with 1 % hydrogel content exhibited a multi-peak T2 curve, and the specimens with 2 %, 3 %, and 4 % hydrogel contents share a similar three-peak spectrum shape. The start-end relaxation times, peak widths, and amplitudes of peaks decreased with the increase in hydrogel content. As the hydrogel content increased, there was a progressive increase in accumulated porosity, ranging from 1.0 % to 3.5 %. As the hydrogel content increased, the volume occupied by the hydrogel increased, and the spatial distribution of the hydrogel became more homogeneous as the hydrogel content increased and more hydrogel-sand aggregates formed. The number, length, and width of cracks decreased significantly and accordingly.

Vegetation barriers are an important environmental characteristic of spent fuel road transportation accidents. Spent fuel vessels may be affected by force majeure factors during transportation, which leads to damage to spent fuel assemblies and containers and can cause radionuclides to gradually release from assemblies to vessels to the external environment. In this work, considering the growth periods of coniferous vegetation barriers and vessel type, a radionuclide dispersion model based on computational fluid dynamics (CFD) was established by adding a decay term and a pressure loss term. The simulations showed that, first, compared to the small (Type-II) vessel, the effects of fluid flow around the large vessel (Type-I) have a more significant impact on radionuclide dispersion. The backflow around the Type-I vessel causes leaked radionuclides to disperse towards the vessel, and the larger the vessel is, the more significant the rise of the leaked radionuclide plume tail will be due to the increased negative pressure gradient area. Moreover, the area contaminated exceeding the maximum allowable concentration by radioactivity for the Type-I vessel is reduced gradually with the growth of coniferous vegetation barriers due to the weakening of the backflow effect by growing vegetation. Second, compared to vegetation barriers of 15 years and 23 years, the horizontal distance exceeding the maximum allowable concentration of the leaked I-131 dispersion from Type II vessels near vegetation barriers for 12 years is the longest. The older the vegetation barrier is, the shorter the horizontal dispersion range, and the shape of radionuclide dispersion gradually transforms from flat to semicircular with vegetation barrier growth, but this could cause a greater radioactive accumulation effect near the leakage point, and the maximum concentration of leaked I-131 reached 0.54 kBq center dot m(-3) for leaked radionuclides from the Type II vessel under vegetation barriers of 23 years. In addition, improvement suggestions based on the proposed method are presented, which will enable the Standards Institutes to apply the research methodologies described herein across various scenarios. Environmental Implication: Compared to nonradioative pollutants, radioactive pollutants are intercepted by vegetation barriers and then migrate to the soil through leaves, stems, and roots, which can contaminate the surrounding environment. Considering the effects of vessel type and coniferous vegetation growth, a radionuclide dispersion model based on CFD was established. Suggestions for decontaminating radioactive pollution areas have been proposed based on the simulation results of hypothetical scenarios. The scenario applicability improvements based on the proposed model could assist relevant Standards Institutes to making improving measures.

This study investigates the pile foundation of a nuclear power plant situated on medium-soft soil. It employs an improved viscoelastic artificial boundary unit to accurately simulate the boundary conditions of the calculation area. The research utilizes a constitutive model of concrete damage plasticity for the pile foundation and an equivalent linearized model for the soil layer. Through large-scale shaking table experiments and numerical simulations, we explore the internal force distribution within the nuclear power structure's pile foundation and assess the extent of the damage. The results indicate that damage primarily occurs in the medium-soft ground, concentrating in the upper part of the pile and affecting the entire cross-section. Subsequent numerical analyses were conducted after reinforcing the soil layer around the top of the pile. The findings demonstrate that this reinforcement leads to a more uniform and rational distribution of internal forces along the pile, significantly reducing damage. Notably, there is no severe damage extending across the entire cross- after reinforcement. This outcome highlights the potential for improving the force distribution in the pile foundations of nuclear power structures through appropriate soil layer reinforcement. The insights gained from this study provide valuable guidance for the seismic design of nuclear power structures.

Introduction Gas migration in low-permeability buffer materials is a crucial aspect of nuclear waste disposal. This study focuses on Gaomiaozi bentonite to investigate its behavior under various conditions.Methods We developed a coupled hydro-mechanical model that incorporates damage mechanisms in bentonite under flexible boundary conditions. Utilizing the elastic theory of porous media, gas pressure was integrated into the soil's constitutive equation. The model accounted for damage effects on the elastic modulus and permeability, with damage variables defined by the Galileo and Coulomb-Mohr criteria. We conducted numerical simulations of the seepage and stress fields using COMSOL and MATLAB. Gas breakthrough tests were also performed on bentonite samples under controlled conditions.Results The permeability obtained from gas breakthrough tests and numerical simulations was within a 10% error margin. The experimentally measured gas breakthrough pressure aligned closely with the predicted values, validating the model's applicability.Discussion Analysis revealed that increased dry density under flexible boundaries reduced the damage area and influenced gas breakthrough pressure. Specifically, at dry densities of 1.4 g/cm(3), 1.6 g/cm(3), and 1.7 g/cm(3), the corresponding gas breakthrough pressures were 5.0 MPa, 6.0 MPa, and 6.5 MPa, respectively. At a dry density of 1.8 g/cm(3) and an injection pressure of 10.0 MPa, no continuous seepage channels formed, indicating no gas breakthrough. This phenomenon is attributed to the greater tensile and compressive strengths associated with higher dry densities, which render the material less susceptible to damage from external forces.

This study examines how acid rain affects the microstructure and mechanical properties of cement-amended loess, crucial for ensuring the safety of engineering projects. We aimed to investigate how acid rain influences the micro-mechanical behavior of cement-amended loess and its damage characteristics under combined acid rain and loading conditions. Cement-amended loess samples were exposed to artificial acid rain with varying pH levels, and changes in their strength and microstructure were analyzed using unconfined compression tests, SEM, NMR, and XRD techniques. Our findings reveal that acid rain erosion of cement-amended loess triggers hydration and erosion reactions. As acid rain concentration increases, the unconfined compressive strength of the amended soil gradually decreases, accompanied by an expansion of pore spaces from small to large-medium pores. Additionally, particle contacts shift from face-to-face and side-to-side to point-to-point and side-to-side configurations. Furthermore, prolonged erosion time exacerbates pore space expansion, indicating a time-dependent effect on soil integrity. To characterize these effects, we developed a constitutive equation within the framework of damage mechanics that incorporates both erosion and loading. This equation successfully aligns with experimental data, providing a comprehensive understanding of the coupled effects of acid rain erosion and mechanical loading on cement-amended loess. These insights are pivotal for designing resilient engineering solutions in environments prone to acid rain erosion.