Nuclear Power Plant Structures Research Articles

Active Infrared Thermography applied for concrete structures inspection in Nuclear Power Plants

Due to the new requirements that are being demanded of the Nuclear Power Plants for their License Renewal (LR), Life Extension (LE) and Long Term Operation (LTO), it is necessary to verify the integrity of several civil structures, such as the containment building, used fuel building and cooling towers, among others. In general, the objectives of Non-Destructive Test methods (NDT) for the inspection of concrete structures in nuclear power plants can be arranged into: location of the reinforcement and tendons, determination of the state of the concrete matrix, determination of the corrosion of the metallic armor and detection of cracks, voids, inclusions and delaminations in the matrix of the structure. For this, Tecnatom, based on the experience acquired in the field of NDT, has developed a set of techniques for determining the structures condition including surface hardness testing, ultrasound (transverse wave pulse-echo and transmission speed), electromagnetic methods (Ground Penetrating Radar - GPR) and non-contact techniques such as Infrared Thermography (IRT). The method and technique best suited for a given inspection has to be selected depending on the objective sought, the typical defectology of the component, the characteristics of the structure and the conditions of the inspection itself. In this work it is presented an example of the development and in-field application of the IRT method for the inspection of a concrete structure in a nuclear power plant to detect possible delaminations caused by the corrosion of the metallic reinforcement.

Settlement of combined piled raft foundation of a nuclear power plant in non-liquefiable and liquefiable soils

The settlement analysis of a nuclear power plant (NPP) resting on the combined pile raft foundation (CPRF) under static and seismic loads is very important. However, it has not yet been reported in the literature as most NPPs are founded on raft foundation rather than on CPRF. In this paper, the settlement has been examined in detail. The geometrical nonlinearity at the interface of raft-soil and pile-soil is considered using the coulomb friction. The piles are rigidly connected to the raft. The nonlinear behaviour of the soil in the non-liquefiable soil is modelled using elasto-plastic Drucker Prager cap model and CycLiq model for the liquefiable soil. For this, user defined material (UDM) is implemented in the FLAC3D material library. Thus, a complete 3D finite difference model of an NPP considering raft-pile-soil-structure interaction (RPSSI) for nonlinear settlement analysis is developed. After successful validation of the present CPRF, numerical model with the literature, it has been used for further study under real earthquake acceleration–time history. Next, CPRF with NPP structure is studied under static loading, which is further extended for the nonlinear time domain settlement analysis. The effects of variation in various parameters i.e., spacing between piles, cohesion, friction angle, and Poisson's ratio of soil on the settlement, are examined and found to be significant. The dominant frequency of the earthquake significantly affects the response of the CPRF. It was inferred that the interface nonlinearity has greater significance in the liquefiable soil as compared to the non-liquefiable soil. Higher differential settlement in the liquefiable soil is attributed to the reduction in the stiffness of the soil. The findings of this study could be useful to frame the general guidelines for the economical design of NPP-CPRF systems.

Passive mitigation of condensation-induced water hammer by converging-diverging structures for offshore nuclear power plants

The equal-height-difference passive heat removal system (EHDPHRS) utilizes the ocean environment as an infinite heat sink that can better suit ocean nuclear power plants (ONPPs). However, a structure with an equal-height-difference can easily cause reverse flow of subcooled water, leading to serious condensation-induced water hammer (CIWH). In this paper, a fully visual experimental facility with same geometric characteristics as the EHDPHRS was established with the aim of studying the CIWH phenomenon. First, two kinds of CIWH modes were observed by the results of the visual experiment, namely the intermittent CIWH and the continuous CIWH. The typical feature of the intermittent CIWH is that the single-phase natural circulation occurs intermittently. Then, the converging-diverging structure (CDS) was designed to avoid the CIWH events in the EHDPHRS. The effectiveness of CIWH prevention was evaluated employing the velocity field, which was obtained by the PIV experimental system. It was found that the CDS can effectively inhibit the further reverse flow of subcooled water and thus eliminate the triggering mechanism of CIWH events. Finally, a dimensionless number was used to balance CIWH prevention and natural circulation capacity. It was shown that when the dimensionless number is in the range of 0.67–0.80, the CDS can eliminate the CIWH events while ensuring the natural circulation capacity.

Interactive iterative methodology for seismic analysis of pile-raft supported nuclear power plant structure in a conventional framework

Due to the fast growth of the nuclear-powered industry, nuclear power plants (NPP) are being planned on soil sites, especially in active seismic zones. In such cases, a combined pile-raft foundation (CPRF) is considered a preferred choice for designers to support massive and stiff nuclear structures. As such, no study has been stated on the seismic design approach for NPP supported on the CPRF system. In the present study, an interactive iterative practical seismic design approach is proposed to estimate the dynamic response of a NPP resting on the CPRF considering continuum-based geotechnical and spring-supported structural models. The pseudo-static response of the continuum-based model is used to determine the soil-pile system stiffness for subsequent seismic analysis in a spring-supported structural model. A step-by-step algorithm is proposed to arrive at the final converged state of the combined system through an iterative procedure. A reasonably good agreement is achieved between the outcomes obtained from the suggested approach and the results of the experimental testing. The proposed approach is subsequently implemented on a typical NPP structure, and a good match is obtained with the results of the coupled time domain direct analysis, thus showing the reliability and practical applicability of the suggested approach. The suggested procedure can be used for the seismic analysis of NPP structures supported by the CPRF system.

Performance-based drift prediction of reinforced concrete shear wall using bagging ensemble method

Reinforced Concrete (RC) shear walls are one of the civil structures in nuclear power plants to resist lateral loads such as earthquakes and wind loads effectively. Risk-informed and performance-based regulation in the nuclear industry requires considering possible accidents and determining desirable performance on structures. As a result, rather than predicting only the ultimate capacity of structures, the prediction of performances on structures depending on different damage states or various accident scenarios have increasingly needed. This study aims to develop machine-learning models predicting drifts of the RC shear walls according to the damage limit states. The damage limit states are divided into four categories: the onset of cracking, yielding of rebars, crushing of concrete, and structural failure. The data on the drift of shear walls at each damage state are collected from the existing studies, and four regression machine-learning models are used to train the datasets. In addition, the bagging ensemble method is applied to improve the accuracy of the individual machine-learning models. The developed models are to predict the drifts of shear walls consisting of various cross-sections based on designated damage limit states in advance and help to determine the repairing methods according to damage levels to shear walls.

Fabricated of the SCC Reference Specimen for Increasing of In-Service Inspection Accuracy in Nuclear Power Plant

To achieve the safety assurance of the nuclear power plants, in-service inspection have were enforced periodic for nondestructive test in weldment. Among them, the ultrasonic evaluation is widely used for the measurement of the defects. Generally, non-destructive inspectors have calibrated a UT machine using the reference specimen in the artificial cracks such as EDM, mechanical notch. As the reflected ultrasonic signal of real crack was detected amplitude value 50% lower than artificial cracks. Therefore, domestic ultrasonic inspectors have a little experience in measuring actual cracks generated in the nuclear power plant field, such as SCC(Stress corrosion cracks, below SCC) and thermal fatigue cracking. Accordingly, in order to improve the inspection accuracy of non-destructive inspectors during operation inspection of nuclear power plant structures, it is necessary to have a lot of experience in inspecting actual cracks.In this study, non-penetration SCC for using in educational specimen of the NDT inspector were fabricated by using the simulated nuclear environment system in base metal of STS 304 pipe. The size of the used pipe were 8 inch(80 sch., thickness 12.7mm). The end of the pipe was used a flange to simulating a fixed environment of the nuclear power plant structure. To accelerate the fabrication of SCC, a corrosive environment was composed by used a blending corrosion solution of the NaOH 1M and Na2S9-H2O 0.1M inside the pipe. The potential of hydrogen of the caustic solution was 14. The fabricated SCC was confirmed for crack size and shape by used UT and RT.

Influence of heavy concrete on seismic response of transfer and purging rooms of nuclear power plant

The high-density characteristic of heavy concrete is the main factor affecting its excellent radiation protection performance. The heavy concrete commonly used in the nuclear power plant structure are hematite concrete and barite concrete. Based on the finite element method, the finite element models of three transfer and purging rooms of nuclear power plant were established. The models used C40 ordinary concrete, C40 hematite concrete and C40 barite concrete respectively. Modal analysis and seismic response time history analysis were carried out for the three models to discuss the dynamic response characteristics and seismic performance of the transfer and purging rooms, and to analyze the influence of heavy concrete on the natural vibration characteristics and seismic response of the transfer and purging rooms. The results show that the structural stiffness of the transfer and purging rooms is large, the natural vibration period is short, the response to the ground motion is small, and the structure is still working in an elastic state, and the hematite concrete has little effect on the natural vibration characteristics and seismic response of the transfer and purging rooms, while the barite concrete can significantly prolong the natural vibration period of the transfer and purging rooms and amplify the seismic response of the structure. The structure of transfer and purging rooms has a sufficient safety margin, and the transfer and purging rooms using heavy concrete can fully meet the requirements of structural seismic performance.

Fragility analysis of nuclear power plant structure under real and spectrum-compatible seismic waves considering soil-structure interaction effect

Both the real and spectrum-compatible seismic waves are widely used as input loads in the seismic assessment of critical structures in engineering applications, which are also allowed by international codes. However, the effect of the real seismic waves and spectrum-compatible seismic waves generated by different methods on the dynamic responses of structures has not been thoroughly emphasized. This study focused on quantitatively comparing the dynamic responses and fragility of the AP1000 nuclear power plant under real and spectrum-compatible seismic waves. Real and spectrum-compatible seismic waves generated by the modification and synthetic methods were used to perform the time history analysis. The effects of three input motion types on a high-accuracy soil–structure interaction system model were investigated based on various dynamic response indexes. The fragility assessment and maximum tension strain contours of the AP1000 nuclear power plant subjected to the three types of seismic waves were also presented to evaluate the influence of the input motions. The results revealed that spectrum-compatible seismic waves can cause larger dynamic responses than real seismic waves at the same peak ground acceleration level. Additionally, although some response indexes, such as strain energy, are sensitive to the input motions when the nuclear power plant is in a nonlinear state, the generation methods of spectrum-compatible seismic waves have insignificant effects on most dynamic response indexes, fragility assessment, and damaged zone locations.

Temperature Field and Stress Analysis of the Heavy-Concrete Transfer-Purge Chamber of the Nuclear Power Plant.

A transfer-purge chamber (TPC) is a double-steel-plate, heavy-concrete, curved-surface composite structure composed of steel plates, heavy concrete, and shear connectors. It is an important facility in the external refueling system of a nuclear power plant (NPP), providing a safe and reliable biological shielding space for reactor refueling operations. Temperature load is one of the most important factors that must be considered in the design of NPP structures. The temperature loads experienced by the TPC during its life cycle include those encountered in both normal and abnormal operation, which are distinct. In this study, we investigated the steady state and transient-state temperature fields and stresses of a TPC structure under normal operation and after 48 h of abnormal operation, respectively, which were calculated using Abaqus finite element software and the directly coupled method. During normal operation, the temperature field of the structure shows relatively uniform changes, and the temperature gradient of the internal concrete in the direction of its thickness has a constant value of 0.245 °C/cm. At the junction between the transfer and purge sub-chambers of the TPC, under the influence of wall curvature and deformation constraints, the maximum tensile strain of heavy concrete is 8.84 × 10-3, the maximum compressive strain is 2.04 × 10-3, the peak stress of the steel plate is 98.305 MPa, and the peak stress of the stud is 306.725 MPa. After 48 h of abnormal operation, the temperatures of the inner surface of the heavy concrete of the wall, the inner steel plate of the wall, the outer surface of the heavy concrete of the wall, and the inner steel plate of the wall increased by 8.12, 8.11, 0.31, and 0.30 °C, respectively. The tensile strain of the heavy concrete of the wall increased significantly by 52.64%, and the compressive strain of the concrete increased by 67.33%, whereas the stresses of the studs and steel plates increased by only 1.57% and 6.79%, respectively. These results show that the change in the temperature field greatly influences the stress and strain on the TPC structure. As measures for mitigating the development of this unfavorable situation of temperature stress concentration, the temperature operating range should be rationally controlled or the junction structure between the transfer and purge sub-chambers of the TPC optimized accordingly. The results of our study can provide basic data for a dynamic analysis of the TPC under conditions of combined earthquake and temperature loads.

Deformation parameters of high-strength steel fiber concrete under long-term load application

This article contain an experimental study of the rheological characteristics of high-strength steel fiber concrete (SFRC) that is used for the manufacture of permanent formwork. Permanent SFRC formwork is used in the construction of reinforced concrete structures of new generation nuclear power plants (NPP), where due to improved physical and mechanical characteristics it is used as a load bearing element. As a result, a structure with combined reinforcement is obtained, requiring improvement of existing calculation methods, including continuously acting load. The use of existing models of concrete behavior under conditions of continuous loading using SFRC is limited and requires experimental refinement. Studies in the field of nonlinear creep, where, according to a number of studies, the use of steel fibers is most effective are of a particular interest. The high labor intensity and duration of creep tests limited the research to two compositions of SFRC recommended for the manufacture of formwork and fine-grained matrix. Based on the established rheological characteristics at loading levels 0.3, 0.8, 0.85 and 0.9, analytical dependences that allow predicting the values of creep strain at different loading levels are obtained. The presented dependences are recommended for the calculation of reinforced concrete structures with non-removable formwork made of SFRC under a long-term load.

Application of Effective Earthquake Force by the Boundary Reaction Method and a PML for Nonlinear Time-Domain Soil-Structure Interaction Analysis of a Standard Nuclear Power Plant Structure

Application of Effective Earthquake Force by the Boundary Reaction Method and a PML for Nonlinear Time-Domain Soil-Structure Interaction Analysis of a Standard Nuclear Power Plant Structure

Ownership structure of nuclear power plants for fair competition and efficiency: private low-cost access versus public ownership

Ownership structure of nuclear power plants for fair competition and efficiency: private low-cost access versus public ownership

Analysis of the Influence of Structure–Soil–Structure Interaction on the Seismic Response for Nuclear Power Plant ASTS

After the Fukushima nuclear accident in Japan, whether the structural safety of the nuclear power plant (NPP) can be guaranteed under an earthquake has been of wide concern. Automatic seismic trip systems (ASTS) have been deployed in NPPs. There are generally two units or more double units on the one NPP site, and the vibration energy of the structure can certainly affect its adjacent structures through the soil, and there is energy transfer and conversion between adjacent structures. At present, the distance between two reactors of NPPs of different reactor types is generally 100–200 m (this distance is referring to the distance between the centers of two reactors, which is slightly different for different reactor types). In the past, seismic instruments were used in the ASTS and the setting of shutdown threshold, so the impact of only one unit was considered in the structural analysis of NPPs, and the interaction of two reactors through site conditions was not considered. In addition, as the site conditions of NPPs become more and more complex, it is necessary to consider the impact of one reactor structure on another reactor structure through the soil on the same site under an earthquake. In order to analyze the influence of structure–soil–structure interaction (SSSI), a three-dimensional refined finite element model of NPPs is established in this paper. The soil–structure interaction (SSI) is considered by using viscoelastic boundary. The seismic responses of different positions of the raft foundation and NPP structure, under the conditions of considering SSI effect (one reactor) and SSSI effect (two reactors), are compared. The influence of SSSI effect on the seismic responses of the raft foundation and NPP structure is revealed. It is proposed that SSSI effect should be reasonably considered according to the site conditions for the structural analysis of NPPs for the NPP ASTS.

Experimental and numerical studies on shield building on non-rock site

Fluid-structure interaction (FSI) and soil-structure interaction (SSI) affect the seismic response of third-generation pressurized water reactor (PWR) nuclear power plants on non-rock site. In this paper, two 1/50 scaled simplified models were designed and a series of shaking table tests were carried to study the seismic responses of models considering FSI and SSI effect. Dynamic characteristics, acceleration, strain, hydrodynamic pressure and experimental phenomenon of the two models were analyzed. Moreover, the numerical analysis was performed and numerical results were compared with test data. The results show that the presence of model soil reduces the system’s frequency, and the presence and sloshing of water reduce the seismic response of model structure. The reasonableness and feasibility of the method used for the numerical simulation is verified. This study could provide a reference for analyzing the seismic response of prototype structure of nuclear power plant.

Soil–structure interaction analysis of nuclear power plant considering three-dimensional surface topographic irregularities based on automatic octree mesh

Many nuclear plants are located near the shoreline and distant from populated areas to dissipate the enormous residual heat and regulate radioactive risk. However, the topography of the shoreline, including hills and valleys near the site where a nuclear power plant (NPP) is built, may affect the dynamic characteristics of NPP facilities and structures, as confirmed by numerical simulations and field observations. Partly due to the lack of efficient mesh generation and calculation methods for sites with three-dimensional surface topographic irregularities, numerical soil–structure interaction (SSI) analyses of NPPs considering topographic irregularities are rare. Moreover, if a simplified SSI analysis is performed, an inaccurate simulation of the real situation is inevitable. In this study, an efficient and universal SSI method that can conveniently consider topographic irregularities based on the geographic information system, octree mesh, scaled boundary finite element method, and direct method was established for efficient dynamic analysis in the time domain. The AP1000 nuclear island structure and a certain site with hills and valleys were selected as examples to confirm the practicability of this model. The dynamic response of the site showed that the acceleration response tended to be amplified in the hills and minified in the valleys. The dynamic response of the NPP structure confirms that ignoring topographic irregularities may not lead to a conservative assessment of the dynamic response of the NPP in the SSI analysis.

Degradation of Concrete Structures in Nuclear Power Plants: A Review of the Major Causes and Possible Preventive Measures

Concrete, an integral part of a nuclear power plant (NPP), experiences degradation during their operational lifetime of the plant. In this review, the major causes of concrete degradation are extensively discussed including mechanisms that are specific to NPPs. The damage mechanism could be chemical or physical. The major causes of chemical degradation include alkali–aggregate reactions, leaching, sulfate attack, bases and acids attack, and carbonation. Physical degradation is a consequence of both environmental and mechanical factors combined. These factors are mainly elevated temperature, radiation, abrasion and erosion, salt crystallization, freeze–thaw distortions, fatigue and vibration. Additionally, steel reinforcements, prestressing steels, liner plates, and structural steel also experience degradation. The prospective areas in the structural components of the NPP where the degradation could occur are mentioned and the effective solutions to the causes of degradation are highlighted. These solutions are designed to enhance the physical and chemical characteristics of concrete. Some of the major recommendations include addition of mineral substitutes, use of low water-to-cement ratio as well as low water-to-binder ratio, use of low alkali cement, use of special aggregates and fibers, use of corrosion inhibitors, use of cathodic protection, etc. The review concludes with an overview of present methods and possible recommendations used to enhance the quality of concrete towards preventing concrete degradation and increasing the lifetime of NPPs.

Assessment and design of nuclear power plant structures under external explosions

With the increasing concerns over the safety of the nuclear power plants (NPP), the blast-resistant assessment and design of NPP structures have been emphasized both domestically and internationally. Aiming to efficiently assess and design the NPP structures under external explosions, the flexural SDOF method and the checking method of direct shear failure were firstly established, and the critical parameters concerning the blast loadings and structural resistance function, were determined. Then, a general assessment and design method for blast-loaded RC structures was proposed, and validated by comparisons with the experimental maximum lateral deflections and whether a direct shear failure occurs. Finally, a practical response surface method (RSM)-based design formula was proposed for the typical NPP structures. It is found that, (i) the pressure-based SDOF method has a higher prediction accuracy compared with the velocity-based SDOF method; (ii) with the increase of the slab thickness and rebar diameter, both the values of reflected pressure and impulse asymptotes increase; (iii) the typical NPP structure remains intact under blast loads specified by the Chinese Load Code for the Design of Nuclear Power Plants Building Structures; (iv) increasing the steel and concrete strength can enhance the blast resistance of the NPP structure at the plastic state attributed to the increased ultimate structure resistance, but not at the elastic state; (v) the design charts for typical NPP structures under the blast loads regulated in Safety Reports Series No. 87, Safety Aspects of Nuclear Power Plants in Human Induced External Events: Assessment of Structures, are given.

STRENGTH AND STABILITY ASSESSMENT OF NPP REINFORCED CONCRETE STRUCTURES REGARDING THE RESULTS OF VIBRATION ANALYSIS

Introduction . The article outlines methodological stages when assessing the resource of nuclear power plant (NPP) operation units with regard to degradation in the dynamic characteristics of reinforced concrete structures obtained by means of vibration analysis methods on the example of hot cells of nuclear installations. Aim : to improve the method currently used for assessing the stress-stain behavior and strength of NPP structures regarding the aging process of reinforced concrete elements under various environmental exposures. Materials and methods . A methodology is proposed for assessing the strength (loading capacity) and stability of NPP structures where significant variations in the properties of reinforced concrete elements have taken place under the action of continuous operation and thermal mode disturbances, thus weakening the element stiffness and, as a result, reducing their strength and stability. Therefore, in order to evaluate the stress-stain behavior and strength of hot cells, calculations should be carried out taking into account the history of structure loading and the process of crack development over the entire operation period. However, no analysis of the history of thermal loading has been performed during the operation period of the NPP under study. In addition, there is a lack of suitable software packages for nonlinear dynamic analysis certified by Rostechnadzor capable of considering the loading history and crack development in reinforced concrete structures. In order to eliminate the mentioned shortcomings, a vibration analysis of the walls and floors of hot cells should be performed to determine the following dynamic characteristics: natural vibration frequency and mode, logarithmic decrement of damping, deformation modulus and Poisson ratio for each wall and floor. Further, based on the obtained experimental data, the stress-stain behavior and strength of hot cells should be calculated using experimental data as calculation input information. The calculation methodology is based on a nonclassical method of modal superposition using the ABAQUS, ANSYS, and Nastran software packages. In order to verify the correctness of the results obtained by vibration analysis, an assessment of floor dynamic characteristics at the point +13.450 in the “П”–“Ж” rows was conducted. Results . The values of deformation modulus and natural vibration frequency obtained in the conducted experiment were found to agree with the calculated values. Conclusions . A methodology for assessing the strength of NPP structures regarding the aging of reinforced concrete under various environmental exposures was presented. The validity of dynamic characteristics obtained using vibration analysis was evaluated.

The Influence of Near- and Far-field Earthquakes on the Seismic Performance of Base-Isolated Nuclear Power Plant Structures

This study aims to investigate the influence of near- and far-field earthquakes on the seismic performance of base-isolated Nuclear Power Plant (NPP) structures. Two earthquake motion groups of near-field and far-field characteristics are selected for fragility evaluation analysis. A base-isolated advanced reactor power 1400 (APR-1400) is employed for numerical analysis. A set of fragility curves are derived for various limit states based on the maximum likelihood estimation. The limit states are defined in terms of Lead Rubber Bearing (LRB) deformation capacity. The numerical results reveal that the median maximum deformations of LRBs were smaller for far-field ground motions than for near-field motions. Also, the comparison of fragility curves demonstrates that the probability of failure of base-isolated NPP structures is higher for near-field ground motions than far-field motions. It is crucial to select earthquake ground motions with both near- and far-field motions for the seismic evaluation of NPP structures.

Earthquake response analysis of nuclear facilities subjected to incoherent seismic waves based on the random-vibration-theory methodology

In this study, based on the random-vibration-theory (RVT) methodology, an earthquake response analysis of a nuclear facility with a rigid foundation on rock/soil sites is considered when incoherent seismic waves are incident on the system. Using the coherency function of seismic waves, the power spectral density (PSD) matrix for six-degree-of-freedom motions of the rigid foundation is calculated. The stochastic earthquake responses of the soil–structure interaction system are obtained, and the PSD function for the pseudo-acceleration of equipment on one floor of the structure is derived. Peak factors of random vibrations are employed to estimate the spectral acceleration or in-structure response spectrum (ISRS) of the equipment. The accuracy of the proposed RVT methodology is verified for nuclear power plant structures on hard rock, soft rock, and layered soil acted upon by coherent and incoherent seismic waves. Because the ISRS outcomes are found to decline at high frequencies, reduction factors due to incoherency are obtained from the proposed RVT methodology. These factors can be applied to earthquake responses to coherent ground motions to consider the effects of incoherent seismic waves.