Nuclear Containment Structure Research Articles

Seismic Response Evaluation of Seismically Isolated Nuclear Power Plant Structure Subjected to Gyeong-Ju Earthquake

The Gyeong-Ju earthquake in the magnitude of 5.8 on the Richter scaleoccurred in September 12, 2016. Because there are many nuclear power plants (NPP) near the epicenter of the Gyeong-Ju earthquake, the seismic stability of nuclear power plants is becoming a social problem. In order to evaluate the safety of seismically isolated NPP, the seismic response of a NPP subjected to the Gyeong-Ju earthquake was compared with those of 30 sets of artificial earthquakes corresponding to the nuclear standard design spectrum (NSDS). A 2-node model and a simple beam-stick model were used for the seismic analysis of seismically isolated NPP structures. Using 2-node model, the effect of internal temperature rise, decrease of shear stiffness, increase of lateral displacement and decrease of vertical stiffness according to nonlinear behavior of lead-rubber bearing (LRB) were evaluated. The displacement response, the acceleration response, and the shear force response of the seismically isolated nuclear containment structure were evaluated using the simple beam-stick model. It can be observed that the seismic responses of the isolated nuclear structure subjected to Gyeong-Ju earthquake is significantly less than those to the artificial earthquakes corresponding to NSDS.

GOTHIC 8.1 Benchmark to ThAI Facility Experiment with Steam-Helium Stratification

The Thermal hydraulics, Aerosols and Iodine (ThAI) facility, located in Eschborn, Germany, is a 60-m3 steel test vessel designed to simulate operational and accident conditions in a nuclear containment structure. The ThAI facility provides experimental data used for validation of thermal-hydraulic codes. The test performed at this facility has been modeled using the GOTHIC 8.1(QA) software package for the purpose of validating both physical models and modeling techniques.The test analyzed is from step 2 of the International Standard Problem 47 test performed at the ThAI facility. This test consisted of three injection ports for steam and helium to enter the vessel off-axis. The off-axis injection locations along with the compartmented geometry of the facility provide a complex coupling of physics that would be present in an accident transient inside the containment of a typical light water reactor. Key considerations of this analysis are stratification of the steam and helium, condensation deposition, and flow patterns within the vessel.

Response analysis of a nuclear containment structure with nonlinear soil–structure interaction under bi-directional ground motion

Design of critical facilities such as nuclear power plant requires an accurate and precise evaluation of seismic demands, as any failure of these facilities poses immense threat to the community. Design complexity of these structures reinforces the necessity of a robust 3D modeling and analysis of the structure and the soil–foundation interface. Moreover, it is important to consider the multiple components of ground motion during time history analysis for a realistic simulation. Present study is focused on investigating the seismic response of a nuclear containment structure considering nonlinear Winkler-based approach to model the soil–foundation interface using a distributed array of inelastic springs, dashpots and gap elements. It is observed from this study that the natural period of the structure increases about 10 %, whereas the force demands decreases up to 24 % by considering the soil–structure interaction. Further, it is observed that foundation deformations, such as rotation and sliding are affected by the embedment ratio, indicating an increase of up to 56 % in these responses for a reduction of embedment from 0.5 to 0.05× the width of the footing.

Damage assessment of nuclear containment against aircraft crash

The behavior of nuclear containment structure has been studied against aircraft crash with an emphasis on the influence of strike location. The impact locations identified on the BWR Mark III type nuclear containment structure are mid-height, junction of dome and cylinder, crown of dome and arc of dome. The containment at each of the above locations has been impacted normally by Phantom F-4, Boeing 707-320 and Airbus A320 aircrafts. The loading of the aircraft has been assigned through the corresponding reaction-time response curve. ABAQUS/Explicit finite element code has been used to carry out the three-dimensional numerical simulations. The concrete damaged plasticity model was used to simulate the behavior of concrete while the behavior of steel reinforcement was incorporated using the Johnson–Cook elasto-viscoplastic material model. The mid-height of containment has been found to experience most severe deformation against each aircraft. Phantom F4 has been found to be most disastrous at each location. The results have been compared with those of the available studies with respect to the containment deformation.

Assessment of in-situ concrete creep: Cylindrical specimen and prototype nuclear containment structure

The paper is dedicated to develop an efficient methodology that assess and segregate the creep, instantaneous and shrinkage strains on small scale: concrete cylindrical specimens and large proto type structure: BARC Containment (BARCOM) test model. The evaluation of concrete creep, shrinkage and instantaneous strains helps in analyzing the long term behavior of the most critical safety barrier the concrete containment structure. The main purpose of the nuclear pre-stressed concrete containment is to prevent the radioactive leakage to the environment in the case of an internal design basis and beyond design basis accidents. During the initial design, the concrete creep is most important input parameter to be considered for the performance and safety evaluation of containment structure, and hence this work mainly focuses on creep studies.The creep, instantaneous and shrinkage strain in concrete cylindrical specimens and BARCOM test model is experimentally computed from the installed embedded sensors. BARCOM test model consists of cylindrical wall portion headed with dome. The concrete cylindrical specimens and dome of BARCOM test model was cast with identical concrete mix in similar duration and therefore indistinguishable behavior were expected. The identical instantaneous and shrinkage strains are found in concrete cylindrical specimens and dome of BARCOM test model. However due to the aging effect, the dissimilarity in the experimental creep strain for concrete cylindrical specimen and dome of BARCOM test model is observed. The influence of elevation and boundary effect on creep strains of cylindrical wall portion of BARCOM test model is also investigated in this paper.Further, the experimentally observed creep and instantaneous strain are validated with the famous B-3 model for concrete cylindrical specimens and BARCOM test model. The experimentally observed results and predicted through B-3 model are found to be in excellent agreement. A sufficient realistic data on concrete creep for long-time serviceability, durability, long-time stability, safety against collapse is useful for performing consistent non-linear analysis of BARCOM test model is explored. Thus the present work deals with the in-situ creep behavior of concrete structures to develop understanding of the mechanisms of creep, instantaneous and shrinkage behavior on concrete structures. The present formulated study is limited to linear basic creep in which the influence of humidity, temperature effect on creep parameters and superimposition of the cracking strain are not introduced.

Evaluation of seismic behavior of soils under nuclear containment structures via dynamic centrifuge test

To evaluate the earthquake loads for the seismic design of a nuclear containment structure, it is necessary to consider the soil–foundation-structure interaction (SFSI) due to their interdependent behavior. Especially, understanding the effects of soil stiffness under the structure and the location of control motion to SFSI are very important. Motivated by these requirements, a series of dynamic centrifuge tests were performed with various soil conditions from loose sand to weathered rock (WR), as well as different seismic intensities for the bedrock motion. The different amplification characteristics in peak-accelerations profile and effects of soil-nonlinearity in response spectrum were observed. The dynamic behaviors were compared between surface of free-field and foundation of the structure for the evaluation of the control motion for seismic design. It was found that dynamic centrifuge test has potentials to estimate the seismic load considering SFSI.

A four-scale homogenization analysis of creep of a nuclear containment structure

A four-scale approach is proposed to predict the creep behavior of a concrete structure. The behavior of concrete is modeled through a numerical multiscale methodology, by successively homogenizing the viscoelastic behavior at different scales, starting from the cement paste. The homogenization is carried out by numerically constructing an effective relaxation tensor at each scale. In this framework, the impact of modifying the microstructural parameters can be directly observed on the structure response, like the interaction of the creep of concrete with the prestressing tendons network, and the effects of an internal pressure which might occur during a nuclear accident.

Nuclear containment structure subjected to commercial and fighter aircraft crash

The response of a boiling water reactor (BWR) nuclear containment vessel has been studied against commercial and fighter aircraft crash using a nonlinear finite element code ABAQUS. The aircrafts employed were Boeing 747-400, Boeing 767-400, Airbus A-320, Boeing 707-320 and Phantom F4. The containment was modeled as a three-dimensional deformable reinforced concrete structure while the loading of aircraft was assigned using the respective reaction–time curve. The location of strike was considered near the junction of dome and cylinder, and the angle of incidence, normal to the containment surface. The material behavior of the concrete was incorporated using the damaged plasticity model while that of the reinforcement, the Johnson–Cook elasto-viscoplastic model. The containment could not sustain the impact of Boeing 747-400 and Boeing 767-400 aircrafts and suffered rupture of concrete around the impact region leading to global failure. On the other hand, the maximum local deformation at the point of impact was found to be 0.998m, 0.099m, 0.092m, 0.089m, and 0.074m against Boeing 747-400, Phantom F4, Boeing 767, Boeing 707-320 and Airbus A-320 aircrafts respectively. The results of the present study were compared with those of the previous analytical and numerical investigations with respect to the maximum deformation and overall behavior of the containment.

Numerical simulation of aircraft crash on nuclear containment structure

Numerical simulations were carried with ABAQUS/Explicit finite element code in order to predict the response of BWR Mark III type nuclear containment against Boeing 707-320 aircraft crash. The load of the aircraft was applied using Riera (1968) and Riera et al. (1982) force history curve. The damaged plasticity model was used to predict the behavior of concrete while the Johnson–Cook elasto-viscoplastic material model was used to incorporate the behavior of steel reinforcement. The crash was considered to occur at two different locations i.e., the midpoint of the cylindrical portion and the junction of dome and cylinder. The midpoint of the cylindrical portion experienced more deformation. The strain rate in the material model was varied and found to have a significant effect on the response of containment. The results of the present investigation were compared with those of the studies available in literature and a close agreement with the previous results was found in terms of maximum target deformation.

Crack width for thick offshore plates

Using thick concrete covers in offshore and nuclear containment applications is increasing because it is a durability issue. Most crack width models indicate that increasing concrete covers results in increased crack spacing and hence increased crack width this means that thick concrete covers are detrimental to crack control. In this paper, tests were conducted on two groups of thick plates. Group I, included five specimens that had two concrete covers, 60 and 70 mm. Group II, included four thick heavy reinforced specimens; all specimens in this group had a clear concrete cover of 70 mm. Using thick concrete covers is a common practice in offshore and containment structures for nuclear power generation. The objective of testing both groups is to measure flexural crack widths under different load levels and, most importantly, under service loads. Group I was intended primarily to investigate the effect of increasing the cover and bar spacing on the crack width. Group II represents a unique experimental investigation in assessing the magnitude of crack width in full-scale thick plates under service loads. An analytical investigation is presented in this work. The main focus of this study is to evaluate the available codes’ models for estimating the crack width of thick concrete plates having thick concrete covers used for offshore and nuclear containment structure applications. It was concluded that crack control can still be achieved by limiting the spacing of the reinforcing steel despite using thick concrete covers.

Damage prediction of RC containment shell under impact and blast loading

There is world wide concern for safety of nuclear power installations after the terrorist attack on World Trade Center in 2001 and several other civilian structures in the last decade. The nuclear containment structure in many countries is a double shell structure (outer shell a RCC and inner a prestressed concrete). The outer reinforced concrete shell protects the inner shell and is designed for external loading like impact and blast. A comparative study of non-linear response of reinforced concrete nuclear containment cylindrical shell subjected to impact of an aircraft (Phantom) and explosion of different amounts of blast charges have been presented here. A material model which takes into account the strain rate sensitivity in dynamic loading situations, plastic and visco-plastic behavior in three dimensional stress state and cracking in tension has been developed earlier and implemented into a finite element code which has been validated with published literature. The analysis has been made using the developed software. Significant conclusions have been drawn for dissimilarity in response (deflections, stresses, cracks etc.) of the shell for impact and blast loading.

Non-linear response of reinforced concrete containment structure under blast loading

The paper demonstrates the effect of an external explosion on the outer reinforced concrete shell of a typical nuclear containment structure. The analysis has been made using appropriate non-linear material models till the ultimate stages. The generation and the propagation of blast wave and its effect on a cylindrical structure are discussed. Parametric studies have also been performed for surface detonations of different amount of blast charges at a distance of 100 m from a nuclear containment shell. Critical distances have been evaluated for different amount of blast charges for nuclear containment shell.

Creep prediction of concrete for reactor containment structures

Since the biggest time-dependent prestress loss of a prestressed concrete nuclear reactor containment structure is due to the creep of concrete, creep is one of the most important structural factors to be considered for the safety of a reactor containment structure during design, construction and maintenance. Creep in concrete has also recently been considered in evaluation of the crack resistance of concrete at an early-age in the durability examination of massive concrete structures like reactor containment structures. Existing empirical formulas on creep prediction show errors in their predictions due to simplified consideration of mixture proportions, and they also show large discrepancy among their predictions. In addition, they do not consider early-age behaviors of concrete and thus are mainly for the prediction of long-term creep at hardened concrete. In this paper, the creep characteristics of the reactor's both early-age and hardened reactor concrete made of type V cement are examined by carrying out both early-age and long-term creep tests. Then, the creep of the reactor concrete is predicted by using major creep-prediction equations of the AASHTO LRFD design specification, the Japanese standard specification for concrete structure, the ACI Committee 209 and the CEB/FIP model code and the Bazant and Panula's model, and the predicted results are compared with the test results. From the comparison, the applicability of the creep-prediction equations for the concrete of a reactor containment structure at both early-age and hardened stages is discussed.

Time history nonlinear earthquake response analysis considering materials and geometrical nonlinearity

A time history nonlinear earthquake response analysis method was proposed and applied to earthquake response prediction analysis for a Large Scale Seismic Test (LSST) Program in Hualien, Taiwan, in which a 1/4 scale model of a nuclear reactor containment structure was constructed on sandy gravel layer. In the analysis both of strain-dependent material nonlinearity, and geometrical nonlinearity by base mat uplift, were considered. The ‘Lattice Model’ for the soil–structure interaction model was employed. An earthquake record on soil surface at the site was used as control motion, and deconvoluted to the input motion of the analysis model at GL-52 m with 300 Gal of maximum acceleration. The following two analyses were considered: (A) time history nonlinear, (B) equivalent linear, and the advantage of time history nonlinear earthquake response analysis method is discussed.

SSI Effects on Ground Motion at Lotung LSST Site

A 3D finite-element model is developed to study soil-structure interaction (SSI) effects at a Large-Scale Seismic Test (LSST) site in Lotung, Taiwan, during the earthquake of May 20, 1986. Analyses are carried out by direct method incorporating a 1/4-scale nuclear plant containment structure. The containment structure is modeled as a linearly elastic material, while the subsoil is modeled as an elastoplastic continuum material that deforms plastically according to a bounding surface plasticity theory with a vanishing elastic region. Eigenvalue analyses are performed to see how the presence of the structure affects the fundamental frequencies and modes of vibration of the system in the limit of elastic response. SSI effects are shown to be partly responsible for the reduced peak north-south ground surface acceleration recorded by a downhole array near the containment structure. Eigenvalue studies suggest that the local effect of the containment structure is to generate rocking and torsional vibration modes, in addition to the usual lateral and vertical modes. However, results of time domain studies indicate that the former modes (rocking and torsional) were not triggered by the 1986 earthquake.

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.

Reliability-based assessment of integrity of bonded prestressed concrete containment structures

The main function of a nuclear containment structure is to prevent the leakage of radioactive materials from the reactor in the event of a serious failure in the process system. To maintain a high level of leak integrity, prestressed concrete is widely utilized in containment construction. In bonded prestressing systems, excessive prestressing losses caused by unexpected material deformations and degradation of tendons could result in the loss of leak integrity under an accident. To safeguard against this, the Canadian Standard, CSA N287.7 (1995), recommends periodic inspection and evaluation of prestressing systems of CANDU containments. As bonded tendons are not amenable to direct inspection, the evaluation is based on the testing of a set of beams with features identical to the containment. The paper presents a quantitative reliability-based approach to evaluate the containment integrity in terms of the condition of bonded prestressing systems. The proposed approach utilizes the results of lift-off, destructive, and flexural tests to update the probability distribution of prestressing force, and to revise the calculated reliability against through-wall cracking of containment elements. An acceptable criterion for the results of beam tests is established on the basis of maintaining adequate reliability throughout the service life of the containment.

Uplifting-sliding response of flexible structures to seismic loads

This study presents the development of a BEM-FEM methodology to analyze flexible structures on an elastic halfspace allowed to simultaneously uplift and slide under seismic excitations. The methodology combines the Boundary Element Method (BEM) applied to model the semi-infinite soil medium and the Finite Element Method (FEM) to model the foundation and the super-structure. The two methods are combined with appropriate force equilibrium and displacement compatibility requirements through the utilization of FEM interface elements at the foundation-soil interface. All four modes of interface deformations, i.e., stick, debonding, rebonding and sliding, are accounted for to accurately simulate uplift and sliding of the foundation-structure system from the underlying soil medium. The sliding mode of deformation is associated with Coulomb friction at the soil-structure interface. The methodology is employed to investigate the response of a nuclear containment structure subjected to the El Centra earthquake of 1940. The results revealed that the base shear reduces significantly if the structure is allowed to slide. Further, parametric studies for various values of the friction coefficient are conducted in order to investigate the structure response under varying friction conditions.

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.

Time domain BEM-FEM seismic analysis including basemat lift-off

Strong earthquake lateral forces can induce base overturning moments that exceed the available overturning resistance due to gravity loads causing uplift of the basemat. To address the problem of uplift, a time domain BEM-FEM methodology for a two-dimensional plane strain soil-structure interaction problem is presented. The boundary element method (BEM) is applied to the soil medium and the finite element method (FEM) is applied to the foundation and the superstructure. FEM interface elements are utilized to effectively simulate foundation uplift from the underlying soil medium. The two methods are combined through displacement compatibility and force equilibrium considerations at the soil-foundation interface. A representative problem of a nuclear containment structure subjected to the El Centro earthquake of 1940 is analysed using the rigorous BEM-FEM and two approaches employing discrete models in the time and frequency domains. The results obtained from the approximate models are critically compared with those obtained from the BEM-FEM approach. The results indicate that the base shear at the foundation-structure interface can cause malevolent or benevolent effects depending on various structural and soil parameters. Further, it is concluded that the effects of uplift on the structure response cannot be predicted from the linear behaviour under bilateral contact conditions.