Nuclear Power Plant Containment Structures Research Articles

Seismic Fragility Assessment of NPP Containment Structure based on Conditional Mean Spectra for Multiple Earthquake Scenarios

A methodology to assess seismic fragility of a nuclear power plant (NPP) using a conditional mean spectrum is proposed as an alternative to using a uniform hazard response spectrum. Rather than the single-scenario conditional mean spectrum, which is the conventional conditional mean spectrum based on a single scenario, a multi-scenario conditional mean spectrum is proposed for the case in which no single scenario is dominant. The multi-scenario conditional mean spectrum is defined as the weighted average of different conditional mean spectra, each one of which corresponds to an individual scenario. The weighting factors for scenarios are obtained from a deaggregation of seismic hazards. As a validation example, a seismic fragility assessment of an NPP containment structure is performed using a uniform hazard response spectrum and different single-scenario conditional mean spectra and multi-scenario conditional mean spectra. In the example, the number of scenarios primarily influences the median capacity of the evaluated structure. Meanwhile, the control frequency, a key parameter of a conditional mean spectrum, plays an important role in reducing logarithmic standard deviation of the corresponding fragility curves and corresponding high confidence of low probability of failure (HCLPF) capacity.

Method for evaluation of concrete containment structure subjected to earthquake excitation and internal pressure increase

SummaryThis study evaluates capabilities of the VecTor4 computer program to assess the seismic performance of a concrete containment structure subjected to both design‐basis and beyond design‐basis earthquakes. Based on a detailed nonlinear finite element model of the nuclear power plant containment structure, the seismic performance in terms of stress and strain distributions, cracks, yielding of reinforcement bars and tendons, as well as overall failure mechanism, is thoroughly reviewed. In addition, the post‐seismic performance of the containment structure subjected to a subsequent pressure increase is also investigated. Consideration is also given to the time‐dependent parameters of creep, shrinkage, and relaxation of prestressing tendons. It is found that the time‐dependent parameters and earthquake have a non‐negligible impact on pressure‐induced structural performance but that this impact can be predicted for a given earthquake. The proposed method is useful for assessing the progressive failure behaviour of containment structure under chain events.

Dynamic soil-structure interaction: A three-dimensional numerical approach and its application to the Lotung case study

This paper presents a three-dimensional non-linear finite element (FE) approach to analyse the dynamic soil-structure interaction (SSI) phenomena observed at the Lotung Large-Scale Seismic Test (LSST) site. The numerical study is carried out in the time domain by a commercial FE code, taking into account the non-linear behaviour of soil and the multi-directional nature of real seismic events. The soil response is simulated by an isotropic hardening elasto-plastic hysteretic model (HSsmall) implemented in the material model library of the code. This model allows to describe the non-linear cyclic response ranging from small to large strain amplitudes and to account for the variation of the initial stiffness with depth.In the paper, the FE numerical approach is first validated through a series of parametric analyses simulating simplified cases (i.e. linear visco-elastic structures founded on a homogeneous linear visco-elastic soil deposit) for which analytical solutions exist. Then, it is adopted to back-analyse the behaviour of the 1/4-scale nuclear power plant containment structure constructed at the Lotung LSST site which was shook by several earthquakes of different intensities and frequency contents. The FE results are thus compared to the recorded in-situ free-field and structural motions, highlighting the satisfactory performance of the numerical model in replicating the observed response. The overall outcome of this research proves that nowadays complex dynamic SSI phenomena can be tackled by direct approach, overpassing the strong simplifications of the well-established substructure approaches.

Finite element modelling of a nuclear containment structure subjected to high internal pressure

The objective of this article is to investigate the response of a nuclear power plant containment structure to a much higher internal pressure that could be encountered during a severe accident. To predict the nonlinear response of the prestressed containment structure under internal pressure, finite element program ANSYS is employed. The major challenges for modelling prestressing tendon elements are to accommodate parameters affecting prestressing forces including prestressing losses. In this study, the prestressing tendons are modelled using discrete elements. This method is more appropriate to model the prestressing tendons when they are not located in a symmetrical fashion. The effects of prestressing system are added directly as initial strain on the prestressing tendons in the finite element model. The proposed approach enables capability of the finite element model to update the prestressing forces, and to accomodate the non-uniform losses of prestressing tendon forces. It is observed that the containment structure subject of this study meets the design requirement of the current standards. The containment structure behaves linearly in excess of 1.5 times the design pressure and the structural integrity limit for pressure capacity is estimated as 2.7 times the design pressure.

Seismically induced uplift effects on nuclear power plants. Part 1: Containment building rocking spectra

Current nuclear regulatory codes specify design considerations for extreme seismic scenarios, focusing primarily on the response of the containment structure of a nuclear power plant. However, in current state-of-practice and in most seismic regulations worldwide, the consideration of soil-structure interaction and potential development of geometrically nonlinear effects, such as rocking and sliding with uplift, is not taken into consideration. To explore this issue, a refined 3D finite element model of a typical nuclear power plant containment structure is developed, comprising solid elements for the soil and foundation, plus shell elements for the structure. The aim is identification of foundation-soil separation phenomena under a suite of ground motions with distinct frequency content. At first, harmonic excitations are used, for both cases of stiff sand and rock subsoil profiles, leading to rocking spectra that depict the displacement demand in connection with nonlinear separation. Clear influence zones can be distinguished, especially in the low frequency bands for the stiff sand case. Next, three subsets of 30 ground motion records are carefully selected and grouped in ensembles according to their frequency content, normalized to a PGA of 0.36g, which corresponds to the highest design acceleration in Europe. Ground motions with low mean frequency content are observed to lead to the onset of geometrically nonlinear phenomena, along with a higher displacement demand. The interplay between ground motion characteristics, dynamic properties of the containment structure and stiffness of the soil is also highlighted. More specifically, it is shown that stiff containment structures on soft soils are more prone to foundation uplift. This possibility is often neglected in design codes and the consequence is that under certain circumstances, damage may be caused to the internal power generation equipment.

Seismic Fragility Analysis of Seismically Isolated Nuclear Power Plant Structures using Equivalent Linear- and Bilinear- Lead Rubber Bearing Model.

In order to increase seismic performance of nuclear power plant (NPP) in strong seismic zone, lead-rubber bearing (LRB) can be applied to seismic isolation system of NPP structures. Simple equivalent linear model as structural analysis model of LRB is more widely used in initial design process of LRB than a bilinear model. Seismic responses for seismically isolated NPP containment structures subjected to earthquakes categorized into 5 different soil-site classes are calculated by both of the equivalent linear- and bilinear- LRB models and compared each others. It can be observed that the maximum displacements of LRB and shear forces of containment in the case of the equivalent linear LRB model are larger than those in the case of bilinear LRB model. From the seismic fragility curves of NPP containment structures isolated by LRB, it can be observed that seismic fragility in the case of equivalent linear LRB model are about 5~30 % larger than those in the case of bilinear LRB model.

납-고무받침에 의해 면진된 원전 격납구조물의 다중단계해석

In order to increase the seismic safety of nuclear power plant (NPP) structures in high seismicity regions, seismic isolation techniques can be adapted to NPP structures. In this paper, the applicability of multi-step analysis of seismically isolated NPP containment structures with lead-rubber bearings (LRB) is evaluated. The floor response spectrum of NPP containment structures with equivalent linear LRB and nonlinear LRB are compared. In addition, the force-displacement relationships for equivalent linear LRB and nonlinear LRB are compared.

Thermal-pressure loading effect on containment structure

Because the elevated temperature degrades the mechanical properties of materials used in containments, the global behavior of containments subjected to the internal pressure under high temperature is remarkably different from that subjected to the internal pressure only. This paper concentrates on the nonlinear finite element analyses of the nuclear power plant containment structures, and the importance for the consideration of the elevated temperature effect has been emphasized because severe accident usually accompanies internal high pressure together with a high temperature increase. In addition to the consideration of nonlinear effects in the containment structure such as the tension stiffening and bond-slip effects, the change in material properties under elevated temperature is also taken into account. This paper, accordingly, focuses on the three-dimensional nonlinear analyses with thermal effects. Upon the comparison of experiment data with numerical results for the SNL 1/4 PCCV tested by internal pressure only, three-dimensional analyses for the same structure have been performed by considering internal pressure and temperature loadings designed for two kinds of severe accidents of Saturated Station Condition (SSC) and Station Black-out Scenario (SBO). Through the difference in the structural behavior of containment structures according to the addition of temperature loading, the importance of elevated temperature effect on the ultimate resisting capacity of PCCV has been emphasized.

Analysis of impact of large commercial aircraft on a prestressed containment building

In this paper, the results of nonlinear dynamic analyses of a concrete containment building under extreme loads are presented. The impact of a large commercial B747 airliner is investigated as the extreme load, and a rigid wall impact test is performed using commercial nonlinear finite element codes. The impact forces exerted by the aircraft are verified compared with the time-dependent impact force provided by OECD/NEA (2002), which was calculated based on the so-called Riera method. The rigid wall impact analysis shows that the finite element model of a B747 is appropriate for the purpose of the aircraft crash analysis exposed to the external hazard of “Beyond Design-Basis Events” defined by U.S. Nuclear Regulatory Commission. Finally, the applicability of this methodology is further studied and verified by conducting parametric studies on the critical infrastructures of nuclear power plant containment structures.

강진지속시간 기준 개선을 위한 원전 격납구조물의 지진응답해석

이 논문은 원전구조물의 내진설계에 적용되는 인공지진파의 강진지속시간과 포락함수에 대한 현행 국내 설계기준의 개선과 보완을 위해서 필요한 기반연구에 관한 내용을 다루고 있다. USNRC와 ASCE 4-98에서 제안한 응답스펙트럼과 강진지속시간에 대한 규정이 현재 통상적으로 사용되고 있으며, 첫 번째로 두 기준에 대한 비교와 검토를 수행하였다. 다음으로 총 209개의 암반사이트에서 실제 계측된 규모 5.0 이상인 강진기록을 ASCE 4-98의 강진지속시간기준에 적용한 결과를 통계 처리하여 지진규모에 대한 함수로 표현되는 강진지속시간의 실험적 예측모델을 제시하였다. 마지막으로 강진지속시간이 원전구조물의 지진응답특성에 미치는 영향을 파악하기 위하여 6초에서 20초까지 약 2초 간격으로 강진지속시간을 달리하는 10가지 Case에 대한 인공지진파를 각 30개씩 작성하고, 이들을 적용하여 대만 Hualien 지진시험구조물과 국내 울진 원자력발전소 원자로 격납구조물에 대한 광범위한 지진응답해석을 수행하고 그 결과를 분석하였다. This paper addresses a fundamental study that is necessary to complement and improve the current domestic design specifications for the strong motion duration criterion and the envelope function of artificial accelerograms that can be applied to the earthquake-proof design of nuclear structures. The criteria for the design response spectra and strong motion duration suggested by USNRC and ASCE Standard 4-98 are commonly being used in the profession, and they are first compared with each other and reviewed. By applying 209 real strong earthquake records that are greater than 5 in magnitude at rock sites to the strong motion duration criterion in ASCE 4-98, an empirical regression model that predicts the strong motion duration as a function of the earthquake magnitude was then developed. Using synthetically generated earthquake time histories for the 10 cases whose strong motion durations varied from 6 to 20 seconds, extensive seismic analyses were finally conducted to identify the effects of the strong motion durations on the seismic responses of the nuclear power plant containment structures.

Design and implementation of a high-velocity projectile generator

An explosively formed projectile (EFP) is designed as a high-velocity projectile generator to be used in a field test in which an equivalent fragment impacts on an equivalent tactical ballistic missile warhead. The formed slug of the EFP is ≈45 g and has a velocity of around 2500 m/sec. EFP detonation and slug forging are numerically simulated using an LS-DYNA3D finite-element code to optimize the design parameters of the EFP. Tests with determining EFP velocity and mass are performed to confirm the preliminary design and evaluate the EFP performance. A flash X-ray facility is employed to measure the profile and the velocity of the EFP slug. The high-velocity projectile generator developed herein can be used in verifying the lethality of the antitactical ballistic missile warhead against tactical ballistic missile. It is also useful for research of the resistance of military fortifications and nuclear power plant containment structures to penetration.

Feasibility study of the IE-SASW method for nondestructive evaluation of containment building structures in nuclear power plants

The IE-SASW method, a combination of impact-echo (IE) acoustics with spectral analysis of surface waves (SASW), is proposed as a newly developed nondestructive testing method in concrete structures. This feasibility study examines the IE technique and uses elastic P-wave velocity data as measured from the SASW method on concrete members in nuclear power plant containment structures. It was shown that both the thickness of the concrete specimens used in this study and the depth of the introduced defects (i.e. voids) could be identified by the IE-SASW method. In contrast, the reinforced steel bar itself could not be identified by the IE-SASW method. Additionally, GPR (ground penetrating radar) techniques were used to examine the same specimens in order to establish some level of performance and reliability to compare with the performance of the IE-SASW method. The GPR method provides an objective and reliable image corresponding to the reinforced steel bars. The experimental studies show that it is more feasible to use the IE-SASW method rather than GPR to detect voids that were positioned beneath the steel reinforcing bars in the concrete specimens.

Inspection of nuclear power plant containment structures

Safety-related nuclear power plant (NPP) structures are designed to withstand loadings from a number of low-probability external and interval events, such as earthquakes, tornadoes, and loss-of-coolant accidents. Loadings incurred during normal plant operation, therefore, generally are not significant enough to cause appreciable degradation. However, these structures are susceptible to aging by various processes depending on the operating environment and service conditions. The effects of these processes may accumulate within these structures over time to cause failure under design conditions, or lead to repair. Steel and concrete containment structures in nuclear power plants are described and their potential degradation factors identified. Reported incidences of containment degradation are summarized. Current regulatory in-service inspection requirements are reviewed. Nondestructive examination techniques commonly used to inspect NPP steel and concrete structures to identify and quantify the amount of damage present are described, and their capabilities and limitations identified. Finally, areas where nondestructive evaluation techniques require development (i.e. inaccessible portions of the containment pressure boundary, and thick heavily reinforced concrete sections) are identified and prior research addressing these needed developments summarized.

Serviceability design load factors and reliability assessments for reinforced concrete containment structures

A reinforced concrete nuclear power plant containment structure is subjected to various random static and stochastic loads during its lifetime. Since these loads involve inherent randomness and other uncertainties, an appropriate probabilistic model for each load must be established in order to perform reliability analysis. The current ASME code for reinforced concrete containment structures are not based on probability concepts. The stochastic nature of natural hazard or accidental loads and the variations of material properties require a probabilistic approach for a rational assessment of structural safety and performance. The paper develops probability-based load factors for the limit state design of reinforced concrete containment structures. The purpose of constructing reinforced concrete containment structure is to protect against radioactive release, and so the use of a serviceability limit state against crack failure that can cause the emission of radioactive materials is suggested as a critical limit state for reinforced concrete containment structures. Load factors for the design of reinforced concrete containment structures are proposed and carried out the reliability assessments.

Leakage behavior of inflatable seals subject to severe accident conditions

Abstract The paper describes tests to determine the leakage behavior of inflatable seals when subjected to containment pressures that exceed the design basis. 2 Inflatable seals are used to prevent leakage around personnel and escape lock doors in about 10% of the commercial nuclear power plant containment structures in the United States. All of the installations are in either Pressurized Water Reactor (PWR) or Boiling Water Reactor (BWR) Mark-Ill type containments. This work is a part of an overall effort at Sandia National Laboratories to develop proven techniques for evaluating the performance of Light Water Reactor (LWR) containment buildings for beyond design basis loadings. Inflatable seals were tested at both room temperature and at elevated temperatures representative of postulated severe accident conditions. Parameters that were monitored and recorded during each test were the internal seal pressure and temperature, chamber (containment) pressure, leakage past the seals, and temperature of the test chamber and fixture to which the seals were attached. An empirically based, analytical method is presented to predict the containment pressure at which significant leakage past inflatable seals can be expected.

Statistical prediction of post-tensioning system relaxation based on the in-service surveillance

The purpose of this paper is to present and establish a procedure to predict the prestress forces during the service life of the system. The statistical approach of this procedure is using the in-service surveillance data of the post-tensioning system to develop a regression analysis. The regression analysis results can be used to check the prestress force at the end of the service life so that the remaining prestress force is equal to or exceeds the design requirement. Results from the surveillance program of a nuclear power plant containment structure were used to demonstrate the procedures.

Application of frequency domain analysis to transient response of nuclear containment structures

A combination of frequency domain and time domain analyses is proposed to obtain the dynamic responses of nuclear power plant containment structures. A soil-structure model of a boiling water reactor containment subjected to an assumed safety relief valve blowdown load is used as illustration. Linear time-invariant systems are analyzed using input forcing functions with varying frequency contents. Time domain analysis is performed using a synthesized input forcing function. The system characteristic function is generated in the frequency domain through Fourier transforms of the response time history and the synthesized input time history. The frequency response due to any other forcing function is obtained in frequency domain by using the system characteristic function, and the response time history is obtained by inverse Fourier transforms of the frequency response. The results obtained by the proposed method are in close agreement with the conventional time domain dynamic finite element analysis.

Research requirements for improved design of reinforced concrete containment structures

Reinforced concrete is a competitive material for the construction of nuclear power plant containment structures. However, the designer is constrained by limited data on the behavior of certain construction details which require him to use what may be excessive rebar quantities and lead to difficult and costly construction. This paper discusses several design situations where research is recommended to increase the designer's options, to facilitate construction, and to extend the applicability of reinforced concrete to such changing containment requirements as may be imposed by an evolving nuclear technology.

Vertical soil-structure interaction of nuclear power plants subjected to seismic excitation

The vertical soil-structure interaction problem is investigated by coupling an N-mass lumped mass structure to a two-dimensional elastic half space. This problem is formulated as an integral equation of the Volterra type. Numerical results are obtained by iteration for an idealized threemass two-mode model of a nuclear power plant containment structure. The effects of interaction are evaluated by comparing free-field acceleration spectrum response curves with similar curves determined from foundation motion.