Response Of Nuclear Power Plants Research Articles

Dynamic soil-structure interaction analysis for base-isolated nuclear island building based on the method of external source wave motion

Soil-structure interaction (SSI) is crucial to the dynamic response of nuclear power plants (NPPs), and should be properly considered when analysing the seismic isolation performance of base-isolated NPPs. In this study, a three-dimensional numerical simulation program is developed for the time-domain dynamic analysis of base-isolated NPPs considering SSI effects. The seismic input is realised using the method of external source wave motion, and the nonlinear behavior of isolators is fully considered. Using this program, the SSI effects of the base-isolated nuclear island building under different sites are analysed and compared, including the acceleration, displacement, and floor response spectra. The numerical results show that the SSI effects change the spectral characteristics of the actual seismic input. When the shear wave velocity of the site is relatively large, the SSI effects can increase the dynamic response of base-isolated NPPs. Additionally, the isolation performance of the base-isolated NPPs is weakened when SSI effects are considered, and with a decrease in the shear wave velocity of the site, the isolation capacity is further reduced. The influence of the SSI effects should be fully considered in the base isolation design of NPPs.

Rocking induced axial response and performance of lead‐rubber bearings in seismically isolated nuclear plants

AbstractSeismic isolation using lead‐rubber (LR) bearings can be an effective earthquake mitigation technique to ensure the safety of Nuclear Power Plants (NPPs) against extreme earthquake events. Accurate estimation of the axial and rocking response of NPPs is critical to the design of the safety‐related secondary components, piping, and auxiliary system crossing the interface of the isolation system. The isolators along the periphery of the isolation interface are especially susceptible to buckling in compression and cavitation in tension due to the rocking motion of the structure. No study so far has looked into rocking response of base‐isolated NPP and its consequence on isolator's performance. This paper investigates the axial and rocking behavior of base‐isolated NPPs at a site of high seismic hazard. A numerical study is conducted on the different representations of NPP models with the varying height of the center of mass isolated using a range of isolation systems of different periods and characteristic strengths. The rocking responses are obtained using simplified and advanced isolator models, which consider the interaction between the axial and shear response. The results provide estimates of the rocking responses and their influence on the axial response of isolators. It was observed that the rocking response of a base‐isolated NPP increases with an increase in the isolation period and the characteristics strength of the isolation system. Rocking effects may amplify the axial forces in the lead‐rubber isolators by more than 100% and need to be considered for true performance assessment of isolators when the horizontal to vertical shaking intensity exceeds a ratio of 0.5.

Global method for nonlinear seismic analysis of embedded base-isolated nuclear island buildings in layered sites

Owing to the increasing scarcity of high-quality sites, the seismic analysis of base-isolated nuclear power plants (NPPs) under complex non-lithology sites has gained important engineering significance. In this study, a global method for nonlinear numerical analysis was developed for the seismic evaluation of base-isolated NPPs in complex layered sites. The seismic analysis of layered sites is realised using the equivalent linear method and one-dimensional wave propagation theory, the radiation damping of the infinite domain is modelled with a visco-elastic artificial boundary, and the nonlinear behaviour of the isolators is fully considered using a bilinear hysteretic model. Using this method, the influence of soil–structure interaction (SSI) and soil nonlinearity on the behaviour of base-isolated NPPs under the design basis earthquake and beyond design basis earthquake was analysed and compared, in terms of the floor response spectra (FRS) and inter-story displacement angle. In addition, the influence of embedment is also studied. The results show that in layered sites, the SSI significantly amplifies the response values for base-isolated NPPs, and as the ground motion input increases, the soil nonlinearity more effectively inhibits the response of base-isolated NPPs. In addition, embedding can significantly reduce the response, but it increases the FRS at some frequency domains. The isolation effect of base-isolated NPPs decreases significantly when SSI effects are considered, and the soil nonlinearity and embedment have little impact on the base isolation capacity. The method proposed in this study can be easily applied to study base-isolated NPPs in layered sites.

Time-series forecasting of a typical PWR system response under Control Element Assembly withdrawal at full power

To expedite the decision-making process under Nuclear Power Plant (NPP) accident conditions, at a reduced computational cost, a Machine Learning (ML) time-series meta-model is proposed to map the relationships between the NPP's real-time parameters to its transient response and hence forecast its future response. To this end, three variations of Recurrent Neural Networks (RNNs), namely: the Long-Short-Term-Memory (LSTM), Gated Recurrent Unit (GRU) and a series combination of a convolutional neural network (CNN) with LSTM were developed. The NPP response was simulated using the thermal hydraulics best estimate code, MARS-KS, and cross-validated against the Design Control Document (DCD) of APR1400. The time-series database required to train, test, and validate the time-series Machine Learning (ML) expert algorithm was acquired by coupling DAKOTA software with the best estimate thermal hydraulics system code, MARS-KS to conduct a Best Estimate Plus Uncertainty (BEPU) analysis. The Monte-Carlo random propagation approach and the Latin Hypercube Sampling technique were used to sample and propagate key uncertain parameters until a statistically significant sample of the NPP transient responses was achieved. All three RNN models were developed and optimized with the help of the Talos tool, and were found to exhibit reasonable performance in forecasting the key most probable NPP transient responses, Furthermore, the uncertainty of the ML predictions was assessed by integrating a Bayesian Neural Network (BNN) with the LSTM model. The prediction accuracy ranged from 86% to 96%, when evaluated on an unseen test dataset. Based on a voting system with five performance metrics (MAE, MSE, RMSE, R2 and Accuracy) the LSTM model and its derivative CNN+LSTM outperformed the GRU model; while the CNN +LSTM is found to be the most computationally efficient.

Shaking Table Test for Seismic Response of Nuclear Power Plant on Non-Rock Site

In order to compare and analyze the seismic response characteristics of a safety-related nuclear structure on a non-rock site in the condition of raft and pile group foundations under unidirectional and multidirectional seismic motion input, a large-scale shaking table test of the soil-nuclear structure system was carried out in this paper. In the test, the soil was uniform silted clay, and the shear wave velocity was 213 m/s. Considering the similarity of the superstructure natural frenquency, the actual nuclear power structure was simplified to a three-story frame shear wall structure model. The annular laminated shear model box was used to take the boundary effect of soil into consideration; the seismic motions = were input in only one horizontal direction or three directions at the same time for the shaking table test, and the results were analyzed. The results of the test show that the acceleration response of the safety-related nuclear plant is affected by the directions of input seismic motion and the forms of the foundation. When the seismic motion is input simultaneously in three directions, the acceleration responses of the horizontal motion and vertical rocking of the safety-related plant are larger than those of the single-direction input. The acceleration response of the horizontal motion and vertical rocking of the safety-related structure with the pile group foundation is smaller than that with the raft foundation. The values of most frequency bands in the horizontal acceleration Fourier amplitude spectrum at the top of the pile-foundation structure are smaller than that at the top of the raft-foundation structure, while the displacement is basically the same as that of the raft-foundation structure. This is related to the relation between the frequency component of input seismic motion and the natural frequency of the structure system. Therefore, it is more reasonable to use three-dimensional seismic input in the seismic response analysis of nuclear power plants. The seismic performance of nuclear power plants can be enhanced by using pile group foundations.

The influence of boundary conditions on nonlinear time domain site response analysis using LS-DYNA

The dynamic response of nuclear power plants and other critical infrastructure under seismic conditions is a major issue, and the earthquake-resistant capacity design of such facilities and their interior equipment should be based on prediction of dynamic response. Therefore, field geological investigations and corresponding site response analyses facilitate the evaluation of seismic site effects for the entire construction process, including site selection and detailed design. During site response analysis, soil parameters are usually obtained from laboratory tests using specimens retrieved from boreholes or from empirical equations. Conventional site response analysis software assumes horizontal layering in a 1D soil model, though the actual conditions of underground stratification are far more complicated. Some research efforts have undertaken 3D site response and soil-structure interaction analysis using LS-DYNA; however, the influence of different boundary conditions in these studies has been rarely discussed. The research reported on in this paper utilized LS-DYNA to perform nonlinear time-domain site response analysis and evaluated the difference between 1D and 3D model results. The results of both 1D and 3D site response analyses conducted using LS-DYNA were comparable to those of traditional approaches using a constrained boundary. The 3D models that adopt a non-reflective boundary condition can be affected by energy attenuation near the model boundary, causing a significant difference in the results. We suggest that the constructed model should have an area of at least 600 m × 600 m (plan view) to ensure accuracy in a 200 m × 200 m area in the center of the 3D simulation model using non-reflecting boundary conditions. Furthermore, four soil profile from real sites have been verified that the suggestion is also applicable in multilayered scenario.

Stochastic dynamic and reliability analysis of AP1000 nuclear power plants via DPIM subjected to mainshock-aftershock sequences

Earthquakes usually consist of a mainshock and a series of aftershocks, both of which are random. The mainshock can damage structures, and the subsequent aftershocks may cause further damage. However, no studies have considered the effects of stochastic seismic sequences on nuclear power plants (NPPs). We propose a new framework to analyze the stochastic dynamic response and dynamic reliability of AP1000 NPPs. First, stochastic mainshock-aftershock sequences are generated by combining a physical random function model, the narrowband harmonic group superposition method and the copula function. Then, the effects of aftershocks on NPPs are highlighted using acceleration and displacement as response indices and the tensile damage ratio (TDR) and plastic strain as damage indicators. Finally, probabilistic information and reliability of NPPs are obtained based on the direct probability integration method (DPIM) and the absorbing condition method (ACM). The results show that the dynamic response is larger for seismic sequences than a single mainshock, and seismic sequences cause more damage to the NPPs. When the peak ground acceleration (PGA) increases, the effect of the aftershock becomes more pronounced, and the response of the NPP becomes more random. Moreover, aftershocks reduce the reliability of NPPs, and the degree of reduction is related to the thresholds.

Influence of various parameters in the seismic soil-structure interaction response of a nuclear power plant

This paper presents a numerical study of the influence of various parameters on the seismic response of the CAREM-25 nuclear power plant (NPP) located in Argentina. The influence of soil stiffness, structure-soil-structure interaction (3SI), and infill soil stiffness are studied. Considering the available analysis capabilities, a full 3D methodology was adopted (direct method). Two soil profiles were used in order to quantify the influence of the soil on the structural response. An engineered rockfill was also used in order to study the influence of infill soil stiffness. Three artificial records with three components each were used as free field input. These records were obtained from the site design spectra. The turbo-group building was approximately modelled in order to consider 3SI. Finally, all live loads as well as the water in pools were included in the model. Design guides for this type of problem are given for all the elements of the model.When comparing flexible soil with stiff soil, important reductions of the maximum absolute acceleration value are obtained by flexible soil. Moreover, the horizontal maximum accelerations are remarkably amplified in height in the case of stiff soil. The influence of the proximity of the turbo-group building (3SI) on the dynamic response of the reactor building is negligible. Finally, the influence of infill soil stiffness is also negligible.

Research on Floor Response Spectrum of Shielded Building Structure under Seismic Loading

As a clean energy source, nuclear power has broad prospects for development. It is of great significance to study the seismic response of nuclear power plants. PLAXIS software is used to establish the whole finite element model of AP1000 nuclear power plant shielding plant structure. Considering the foundation embedding and soil-structure interaction, the seismic response analysis of nuclear power plants on different inclined soft rock sites was carried out under seismic loading. By analyzing and comparing the calculated results of the floor response spectrum amplification factor, the peak frequency of the floor response spectrum at the important position is smaller than the peak frequency of the design response spectrum, the inclination of the inclined site will affect the peak frequency and cause the amplification effect.

Validation of WWER-440/213 Thermohydraulic Model for TRACE Computer Code Based on RNPP-1 Incident Data

This paper describes the main steps and results of preparing WWER-440/213 model for TRACE system thermohydraulic code to be used in further deterministic analysis calculations. As the basis for developing the model, the previously developed RELAP5/Mod.3.2 input deck was used. As a first step, the thermohydraulic components of input deck were automatically converted to TRACE format using SNAP utility. Then, necessary manual adjustments and additions were implemented to incorporate the safeguards actuation logic and operation of various plant controllers into the model.
 A set of validation calculations was performed for various transients (abnormal operation events) that had actually occurred at Rivne NPP Unit 1 to confirm that the TRACE model is capable of reproducing adequately the response of nuclear power plant with WWER-440/213 reactor. The results for one of validation calculations (Loss of normal power supply with reactor scram and additional failures) are provided as an example. Comparison of calculation results with plant measured data demonstrate acceptable agreement for the main plant parameters.
 To extend the scope of model validation, the cross-code comparison between TRACE and RELAP5 results is planned to be performed for a number of representative design-basis accident scenarios.

A case study of time-dependent risk informed integrated safety assessment under complex accident sequences

A time-dependent risk informed integrated safety assessment methodology is suggested to evaluate the response of nuclear power plants under complex accident sequences caused by failures of multiple safety features. Event propagation is modeled based on the combination of deterministic and probabilistic safety assessment methods, with results representing changes in plant risk over time. Operators can utilize the time-dependent risk information as a quantitative basis, and results can determine the degree of safety enhancement by the improvement of emergency operator action procedures or by strengthening the design of safety features. The concepts of consequential failure probability and point-estimate failure time are introduced. The consequential failure probability, calculated from central limit theorem, identifies the key safety system and provides a precise risk calculation. The point-estimate failure time, using non-parametric order statistics, justifies the crediting of emergency operator actions. As a case study, risk of large early radioactive release due to pressure-induced multiple steam generator tube rupture following main steam line break accident in Advanced Power Reactor 1400 is considered. Results are used to discuss the credibility of operator mitigation actions, and a remedy to enhance plant safety to satisfy safety goals is suggested.

Chromia coating with nanofluid deposition and sputtering for accident tolerance, CHF enhancement

In Fukushima accident, zirconium cladding was rapidly oxidized with high temperature steam, which ultimately led to hydrogen explosion. To overcome materialistic limitation, accident tolerant fuel (ATF) was suggested to improve safety response of nuclear power plants during accidents by modifying cladding surface with various coating materials. When chrome was coated on cladding surface, it showed fewer weight gain by high temperature oxidation compared to bare zirconium cladding. Chrome forms chrome oxide or chromia (Cr2O3) when oxidized, and this layer prevents further oxidation thus protecting inner material from oxidizing. However, previous studies indicated that implementation of chrome containing alloys have major drawbacks such as excessive coating thickness or degraded critical heat flux (CHF). Instead, direct coating of chromia was suggested in this study with the expectation of CHF enhancement compared to other chrome alloy coatings. Chromia nanoparticles were coated on nichrome wire surface with boiling deposition of chromia nanofluid. Another method was applying RF sputtering with chromia target. Chrome coating with DC sputtering were also tested for comparison. Verification of chromia coating was conducted by three steps: CHF measurement with wire pool boiling, high temperature oxidation in furnace to compare the oxidation resistance of specimens, and surface investigation. Surface characteristics investigation were conducted with measurement of contact angle by sessile drop method, capillary wicking height, and scanning electron microscope image. Experimental results show that chromia coating significantly increased CHF. Weight gain by oxidation indicate chromia nanofluid coating had improved oxidation resistance property.

Extreme earthquake response of nuclear power plants isolated using sliding bearings

Horizontal seismic isolation is a viable approach to mitigate risk to structures, systems and components (SSCs) in nuclear power plants (NPPs) under extreme ground shaking. This paper presents a study on an NPP seismically isolated using single concave Friction Pendulum™ (FP) bearings subjected to ground motions representing seismic hazard at two US sites: Diablo Canyon and Vogtle. Two models of the NPP, five models to describe friction at the sliding surface of the FP bearings, and four levels of ground shaking are considered for response-history analysis, which provide insight into the influence of 1) the required level of detail of an NPP model, 2) the vertical component of ground motion on response of isolated NPPs, and 3) the pressure-, temperature- and/or velocity-dependencies of the coefficient of friction, on the response of an isolated NPP. The isolation-system displacement of an NPP can be estimated using a macro model subjected to only the two orthogonal horizontal components of ground motion. The variation of the coefficient of friction with temperature at the sliding surface during earthquake shaking should be accounted for in the calculation of isolation-system displacements, particularly when the shaking intensity is high; pressure and velocity dependencies are not important. In-structure floor spectra should be computed using a detailed three-dimensional model of an isolated NPP subjected to all three components of ground motion.

Analysis of Seismic Soil-Structure Interaction for a Nuclear Power Plant (HTR-10)

The response of nuclear power plants (NPPs) to seismic events is affected by soil-structure interactions (SSI). In the present paper, a finite element (FE) model with transmitting boundaries is used to analyse the SSI effect on the response of NPP buildings subjected to vertically incident seismic excitation. Analysis parameters that affect the accuracy of the calculations, including the dimension of the domain and artificial boundary types, are investigated through a set of models. A numerical SSI analysis for the 10 MW High Temperature Gas Cooled Test Reactor (HTR-10) under seismic excitation was carried out using the developed model. The floor response spectra (FRS) produced by the SSI analysis are compared with a fixed-base model to investigate the SSI effect on the dynamic response of the reactor building. The results show that the FRS at foundation level are reduced and those at higher floor levels are altered significantly when taking SSI into account. The peak frequencies of the FRS are reduced due to the SSI, whereas the acceleration at high floor levels is increased at a certain frequency range. The seismic response of the primary system components, however, is reduced by the analysed SSI for the HTR-10 on the current soil site.

Investigation on sloshing and vibration mitigation of water storage tank of AP1000

AP1000 is a standard design developed by Westinghouse for an advanced generation III+ nuclear power plant (NPP) utilizing passive features. Water storage tank of AP1000 plays an important role in cooling down the temperature of the inside of the NPP. However, the water sloshing of the water tank may have significant effects on the seismic performance of the NPP when the structure is subjected to earthquake. The purpose of the present study is to numerically investigate the influence of fluid–structure interaction (FSI) on the dynamic behavior of water tank and effects of water sloshing in reducing seismic response of the shield building. For that purpose, a FSI algorithm of finite element technique is employed for the seismic analysis of water storage tank for AP1000. To evaluate the positive effect of FSI during the process of reducing seismic response, six cases of water height have been studied and also compared with that of the empty water tank. The Arbitrary Lagrangian Eulerian (ALE) algorithm is used to simulate the fluid sloshing and oscillation of water tank under a typical earthquake occurred at NPP. Furthermore, comparison is performed among parameters obtained from simulation for various heights of water and oscillation frequencies, with respect to seismic response and fluid surface. The results clearly show that the water level height has an important influence on the dynamic response of nuclear power plant (NPP), but case 3 of water level can effectively mitigate seismic response and sloshing oscillations of the structure, which can improve the design of seismic performance for AP1000 or CAP1400 in future.

Response of base-isolated nuclear structures to extreme earthquake shaking

Seismic isolation using low damping rubber and lead–rubber bearings is a viable strategy for mitigating the effects of extreme earthquake shaking on safety-related nuclear structures. The mechanical properties of these bearings are not expected to change substantially in design basis shaking. However, under shaking more intense than design basis, the properties of the lead cores in lead–rubber bearings may degrade due to heating associated with energy dissipation, some bearings in an isolation system may experience net tension, and the compression and tension stiffness may be affected by the lateral displacement of the isolation system. The effects of intra-earthquake changes in mechanical properties on the response of base-isolated nuclear power plants (NPPs) are investigated using an advanced numerical model of a lead–rubber bearing that has been verified and validated, and implemented in OpenSees.A macro-model is used for response-history analysis of base-isolated NPPs. Ground motions are selected and scaled to be consistent with response spectra for design basis and beyond design basis earthquake shaking at the site of the Diablo Canyon Nuclear Generating Station. Ten isolation systems of two periods and five characteristic strengths are analyzed. The responses obtained using simplified and advanced isolator models are compared. Strength degradation due to heating of lead cores and changes in buckling load most significantly affect the response of the base-isolated NPP.

Development of rapid atmospheric source term estimation system for AP1000 nuclear power plant

The development of rapid atmospheric source term estimation system (RASTES) for AP1000 nuclear power plant (NPP) is described. The system is designed to give rapid estimation of the amount of fission products released to the atmosphere, and to provide necessary input for fission products atmospheric dispersion simulation and public dose calculation, during a nuclear accident. The estimation method in RASTES mainly refers to NUREG-1228. However, some adjustments are made based on an investigation of studies in nuclear emergency response and design features of AP1000 NPP during development of RASTES. These adjustments enable the system to be more flexible and realistic in atmospheric source term estimation, and more applicable to AP1000 NPP. These adjustments include modifications and validations of aerosol removal factors in reactor containment in both natural process and spray process, as well as addition of aerosol removal factors in passive containment cooling process. Besides, three reactor core damage assessment methods are integrated into atmospheric source term estimation. In addition, containment radiation levels for specified fuel damage which are used in core damage assessment are recalculated. The calculation function of RASTES is confirmed through comparison with RASCAL. Atmospheric source terms in different fission product removal processes are also calculated and compared. The result shows that passive containment cooling is able to further reduce aerosol release by about 50% compared with natural process. Development of RASTES will enhance the capability of nuclear emergency response of AP1000 NPP.

Evaluation of Soil Stiffness Variability Effects on Soil-Structure Interaction Response of Nuclear Power Plant Structure

This study investigated the influence of probabilistic variability in stiffness and nonlinearity of soil on response of nuclear power plant (NPP) structure subjected to seismic loads considering the soil-structure interaction (SSI). Both deterministic and probabilistic methods have been employed to evaluate the dynamic responses of the structure. For the deterministic method, SRP min method given in USNRC SRP 3.7.2(2013) (envelope of responses using three shear modulus profiles of lower bound(G LB ), best estimate(G BE ) and upper bound(G UB )) and SRP max method (envelope of responses by more than three ground profiles within range of G LB ≤ G≤ G UB ) have been considered. The probabilistic method uses the Latin Hypercube Sampling (LHS) that can capture probabilistic feature of soil stiffness defined by the median and the standard deviation. These analysis results indicated that 1) number of samples shall be larger than 60 to apply the probabilistic approach in SSI analysis and 2) in-structure response spectra using equivalent linear soil profiles considering the nonlinear behavior of soil medium can be larger than those based on low-strain soil profiles.

반복법을 이용한 면진적용 원전구조물의 지반-구조물 상호작용 해석

The nuclear accident due to the recent earthquake in Japan has triggered awareness of the importance of safety with regard to nuclear power plants (NPPs). An earthquake is one of the most important parameters which governs the safety of NPPs among external events. Application of a base isolation system for NPPs can reduce the risk for earthquakes. At present, a soil-structure interaction (SSI) analysis is essential in the seismic design of NPPs in consideration of the ground structure interaction. In the seismic analysis of the base-isolated NPP, it is restrictive to consider the nonlinear properties of seismic isolation devices due to the linear analysis of the SSI analysis programs, such as SASSI. Thus, in this study, SSI analyses are performed using an iterative approach considering the material nonlinearity of the isolators. By performing the SSI analysis using an iterative approach, the nonlinear properties of isolators can be considered. The difference between the SSI analysis results without iteration and SSI with iteration using SASSI is noticeable. The results of the SSI analysis using an effective linear (non-iterative) approach underestimate the spectral acceleration because the effective linear model cannot consider the nonlinear properties of isolators. The results of the SSI analysis show that the horizontal response of the base-isolated NPP is significantly reduced.

A probabilistic seismic risk assessment procedure for nuclear power plants: (II) Application

This paper presents the procedures and results of intensity- and time-based seismic risk assessments of a sample nuclear power plant (NPP) to demonstrate the risk-assessment methodology proposed in its companion paper. The intensity-based assessments include three sets of sensitivity studies to identify the impact of the following factors on the seismic vulnerability of the sample NPP, namely: (1) the description of fragility curves for primary and secondary components of NPPs, (2) the number of simulations of NPP response required for risk assessment, and (3) the correlation in responses between NPP components. The time-based assessment is performed as a series of intensity-based assessments. The studies illustrate the utility of the response-based fragility curves and the inclusion of the correlation in the responses of NPP components directly in the risk computation.