NPP Structures Research Articles

Seismic response of combined piled raft foundation using advanced liquefaction model

The design of earthquake resistant structures founded on combined piled raft foundation (CPRF) mainly depends on raft-pile-soil interaction (RPSI). This interaction become more complex for these heavy structures, if founded on the liquefiable soil. The CPRF is considered to be useful foundation in practice to support such heavy structure; in which both raft and pile increase the bearing capacity of the soil. Where raft reduces the liquefaction induced settlement and piles help to transfer the load of the structure to the non-liquefiable soil layer. The aim of this paper is to carry out the 3D nonlinear finite difference analysis of CPRF considering the RPSI. A recently developed constitutive model i.e. CycLiq for the liquefaction modelling of the soil is utilized to represent the behaviour of sand in inelastic range which consider the cyclic behaviour and large post-liquefaction shear deformation of sand. The validation of the model is carried out with the centrifuge data. The effect of CPRF over the raft and pile group is carried out for harmonic excitation with varying frequency and earthquake motion with varying PGA. The study provides important observations and insights that were not previously evident. The findings emphasize the need to consider frequency effects, geometrical nonlinearity, and material nonlinearity in the design of NPP structures. Additionally, the study introduces the application of the CycLiq liquefaction model and the development of a user-defined material for FLAC3D. Further, the outcome shows the useful conclusion on the importance of CPRF foundation over the conventional foundation i.e. raft and pile group in the liquefiable soil. It is found that excitation frequency, ground motion PGA, porosity of sand and pile spacing of CPRF have great effects on the response.

Concrete corrosion in nuclear power plants and other nuclear installations and its mitigation techniques: a review

Abstract Nuclear power plants (NPPs) have been affected by various failures through corrosion which causes economic losses, increased chance of the radiation exposure and environmental risk. One of the major durability issues in reinforced concrete is the corrosion of reinforcement which significantly reduces the life of reinforced concrete. Considering the increasing demand for longer service lives of NPPs, and the high cost involved in building and maintaining it, adequate preventive measures should be followed to minimize the corrosion. This review majorly discusses about the mechanism of corrosion of steel in NPP structures with emphasis on the mechanisms relevant to NPPs, possible reasons for the concrete corrosion as well as potential failure happening in NPPs. The majors reason for the concrete corrosion in nuclear power plants are mainly corrosive external and internal environment, thermal and mechanical stress, moisture content, microorganisms and stray electrical currents. The corrosion of NPPs may result in loss of structural integrity and leakage of radioactive material. The review also discusses about various corrosion prevention and protection techniques against concrete corrosion and concludes with an overview of present methods and possible future perspectives used to enhance the efficiency of concrete corrosion mitigation methods with focus of NPPs.

Design of reinforced concrete structures with loop joints of rebar in accordance with State Standard R 70447-2022

Introduction. Currently, loop overlapping joints of reinforcing bars are used in reinforced concrete structures. These types of working rebar connections are most widely used in NPP structures for the construction of slabs and walls made of monolithic reinforced concrete. Loop joints in these structures are used to connect adjacent three-dimensional reinforced blocks that are assembled in advance and installed in the design position before concreting. This type of connection leads to an increase in the manufacturability of construction and installation works, as it significantly reduces labor costs on the construction site when installing reinforcement frames.The relevance of the development of the standard is due to the fact that the current regulatory framework did not contain detailed instructions on the design requirements for such connections. The absence of these provisions led to significant difficulties in the design of such structures, which led to excessive material and time costs.The aim of the work is to improve the system of urban planning activities in terms of clarifying and supplementing existing regulatory documents on the design of reinforced concrete structures.Materials and methods. The development of the standard was carried out taking into account modern requirements, as well as an analysis of the results of recent scientific research. Additionally, the practical domestic experience of using loop joints in the construction of nuclear power facilities was analyzed.Results. The result of the work is the State Standard R 70447-2022 standard, the provisions of which include new calculation methods, as well as clarifications of existing calculation and design methods, and updated requirements. The draft standard passed public discussions, as well as the necessary expertise and was put into effect.Conclusions. The standard State Standard R 70447-2022 "Reinforced concrete structures with loop joints of reinforcement for nuclear power facilities" has been developed, the provisions of which will allow developing reliable structural solutions for such structures, including achieving a certain economic effect. At the same time, certain areas for improving the provisions of this standard are noted.

Digital acoustic model of an NPP pressurizer with WWER

It is experimentally proven that the main cause of vibration excitation of equipment and internals of the main circulation circuit (MCC) are acoustic standing waves (ASW) and main circulation pumps (MCP). Usage of an interdisciplinary approach made it possible to create a digital acoustic model of pressurizer system with pipelines attached to it and prove that it is a self-oscillating system capable of generating acoustic standing waves (ASW) similar to simultaneous operation of several Helmholtz resonators. Each Helmholtz resonator in pressurizer is able to suppress a certain frequency of ASW generated by the reactor, which depends on coolant temperature. Determination of natural frequencies of coolant pressure oscillations (NFCPO) in pressurizer using Thomson formula is based on method of electroacoustic analogy. Acoustic compliance of pressurizer and acoustic mass of various combinations of pipelines connected to it make it possible to calculate NFCPO, in the system under consideration, according to the formula presented in the form of acoustic analogues of inductance and capacitance. Acoustic parameters of a system of Helmholtz resonators formed by a pressurizer and a connecting pipeline with hot leg of the third loop in nominal operating mode of NPP with WWER-1000 are calculated. A technology has been developed and verified, advantage of which is usage of a pressurizer for damping ASW, frequencies of which fall within vibration bandwidth of primary circuit structural elements. Calculation and experimental proof of previously unknown ability of pressurizer to damp ASW generated by the reactor in a wide frequency range is given, and possibility of preventing resonances with vibrations of equipment and NPP structures is substantiated.

Seismic design of NPP structures taking into account foundation slab compliance

The present article proposes a mathematical method for factoring in the compliance of foundation slabs in NPP structures under dynamic loading. In many cases, such an approach allows the analysis results to be significantly improved, whereas sometimes it is simply a necessary part of the procedure, i.e., when structures having “detached” exterior walls are exposed to an air shock wave generated by an aircraft crash.The presented method applies soil springs and dampers as per ASCE 4-16, specially distributed along the foundation slab bottom of a building.The conclusion presents the results of calculating the integral characteristics of soil springs and dampers according to the realistic (saddle-shaped) law of their distribution along the foundation slab bottom of a typical building.

Reengineering of organizational and technological processes of NPP construction

Nuclear power construction is characterized by high capital costs, a significant number of participants in the design and construction processes. One of the most significant factors of the efficiency of NPP projects implementation is the adopted technology for the construction. An analysis of the Soviet and modern experience in the NPPs construction allows to assert that the piece reinforcing application leads to an increase in the project implementation time. The technology of large-block assembling of NPP structures and equipment have been widely used by the world leaders in NPPs construction, but the implementation of this technology required significant costs for the organization of the necessary infrastructure. In the context of the rapid development of computing systems, instrumentation and mechanical engineering, it is proposed to fundamentally rethink the processes of NPP construction. It is proposed to return to the technologies of large-block installation of NPP structures by switching from rail lifting mechanisms to pneumatic wheels, by robotizing the processes of enlargement and installation of structures and equipment. The functions of managing and making organizational and technological decisions can be partially transferred to a specially developed neural network that has access to an array of data on previously implemented projects. This approach allows us to consider the NPP construction project as a cyber-physical system and opens the way to a whole range of areas of fundamental and applied research in the field of technology, organization and management of the construction of NPP.

Algorithm for constructing an organizational and technological model for dismantling NPP buildings

In coming decade the practical implementation of work on the decommissioning of nuclear power plants will begin. Failure to take into account the decommissioning stage for NPPs of the first generations, the presence of a radiation component in the building structures of NPP buildings and structures, the need to destroy protective barriers are the main problems in the issue of dismantling works. The destruction of barriers and the specifics of dismantling technological processes can lead to the release of radioactivity into the environment, which will lead to an increase in the radiation load on personnel and the population. Radioactive contamination of structures leads to the formation of a large volume of radioactive waste, the amount of which can be significantly reduced due to the competent separation of waste into classes, taking into account the holding time. In this study the design of the reactor building for unit 1 of the Novovoronezh NPP was considered. Based on the analysis of the data of the comprehensive engineering and radiation surveys (CERS), which was completed in 2004, a classification of buildings into three groups was proposed. Based on the results of CERS, the dependences of the total activity of the main radionuclides and the cost of maintaining the block under observation from time are presented. As a result of the performed studies, an algorithm for constructing an organizational and technological model for dismantling buildings and structures of NPP that are being decommissioned is proposed.

The effectiveness of the implementation of pile foundations on the example of the reactor building of a nuclear power plant with VVER-1000

Aims. The purpose of this work is to justify the reliability of the developed models of pile foundations on the data of field observers and to demonstrate the feasibility of using pile foundations to increase the stability and bearing capacity of the NPP structures foundation using the example of reactor building (RB). Methods. The data of field observers for the settlement of the RB are presented. The expressions for calculation of the static stiffness of the contact surface of the slab bottom and soil medium, as well as for pile foundations taking into account the effects of interaction of grillage and pile field with the soil media with a general character of displacement of a circular cross section pile are submitted. A spatial three-dimensional finite element static model of RB was developed together with the soil base. Isolines of vertical movements of RB fundamental slabs for natural foundations, as well as for pile foundations for normal operation are shown. A comparative analysis of the data of field observers with the calculation results allows us to justify the reliability of the developed model of the pile foundation. Results. The calculations of the foundation yield correspond to all the period of operation including construction, and the results of observations relate only to the period of operation that explain the difference. A comparative analysis of the data of field observers with the calculation results allows us to justify of the strong effectiveness of realization of pile foundation for the massive structures on soft soil bases.

Application Peculiarities of the SMA Method in the Estimation of NPP Structures, Systems and Elements by Means of Computational Complexes

Application Peculiarities of the SMA Method in the Estimation of NPP Structures, Systems and Elements by Means of Computational Complexes

Experience Obtained and Lessons Learnt from the Transition of Ukrainian NPPs to Long-Term Operation

Nuclear energy is a strategic Ukrainian industry that produces more than 50% of electricity at present. Over the last decade during the operation of Ukrainian NPPs, the set of technical matters have been found and corresponding tasks shall be decided with engaging of scientific potential of the country. One of such matter is a safety justification of NPP components and structures for the period that is bigger than an initial design one and that is defined as transition from design operation to the long-term operation (LTO).
 From the end of 2010 to the beginning of 2020, the State Nuclear Regulatory Inspectorate of Ukraine granted 11 licenses for 11 power units based on positive results of the state assessment and review of Periodic Safety Review Reports considering mandatory safety review each ten years.
 These results became possible due to the coordinated, conscientious and longstanding work of thousands of highly qualified nuclear experts, which led to the fact that Ukraine now has a unique experience in preparation for a transition of various types of WWER-1000 and WWER-440 reactors to LTO. The summarized results and lessons learnt from the practice of transition to LTO are presented in this paper.

Seismic Response Evaluation of NPP Structures Considering Different Numerical Models and Frequency Contents of Earthquakes

본 연구의 목적은 원자로 1400(APR 1400) 원자력 발전소(NPP)의 원자로 격납건물(RCB) 내진성능에 대해 상이한 수치모델과 지진 주파수 성분의 영향을 평가하는 것이다. 집중 질량 막대 모델(lumped-mass stick model, LMSM)과 3차원 유한요소모델(three- dimensional finite element model, 3D FEM)의 두 가지 수치 모델이 시간이력해석을 수행하기 위해 개발되었다. LMSM은 기존의 집중 질량 보-요소를 사용하여 SAP2000으로 구성하였으며, 3D FEM은 각기둥 입체-요소를 사용하여 ANSYS로 작성되었다. 저주파수 및 고주파수 성분을 고려한 두 그룹의 지진파를 시간이력해석에 적용하였다. 저주파수 지진파의 응답스펙트럼을 NRC 1.60의 설계 스펙트럼과 일치되도록 조정하여 작성하였으며, 고주파수 지진파는 10Hz ~ 100Hz의 고주파수 범위를 갖도록 생성하였다. RCB의 지진응답은 다양한 높이에서 층응답스펙트럼으로 검토하였다. 수치해석 결과, 저주파수 지진에 의한 구조물의 FRS 결과는 두 수치 모델에서 매우 유사한 결과를 보였다. 하지만, 고주파수 지진에 의한 LMSM의 FRS 결과는 고차 고유 주파수 영역에서 3D FEM과 큰 차이를 보였으며, RCB의 낮은 높이에서 명확한 차이를 보였다. 3D FEM이 정확한 구조물의 응답을 나타내는 것으로 가정한다면, RCB의 LMSM은 고주파수 지진에 의한 FRS 결과의 고차 고유 주파수 영역에서 일정 수준의 불일치성을 내포하고 있다.

Nonlinear analysis of extreme snow fragility of reactor hall frame

This paper gives the results of the safety analysis of the nuclear power plant structures in Slovakia in case of the extreme climatic loads. The linear and nonlinear analysis of NPP structures with the reactor VVER 440/213 in the case of the extreme external even is presented. On the base of the meteorological monitoring of the locality the extreme snow load was defined for the return period 104 years. There is showed summary of calculation models and calculation methods for the nonlinear analysis of the structural resistance in system ANSYS. The fragility curves of the critical structure elements were calculated using the FORM method.

Mathematical model for prediction of droplet sizes and distribution associated with impact of liquid-containing projectile

Abstract After the events of 9/11, the impact of fast flying commercial aircraft is considered as major hazard threatening the Nuclear Power Plant's (NPP) safety. The study of fuel spillage phenomenon and fireball formation is important to understand fire hazards due to burning of dispersed aviation fuel. The detailed analysis of fuel dispersion is very difficult to deliberate because both, large NPP structures and the large size of commercial aircrafts. Sandia National Laboratories, USA conducted impact tests using cylindrical projectiles filled with water to measure the associated parameters. Due to combustion properties and volatile nature of hydrocarbon fuels, the obtained parameters from impact studies using water are incomplete in fire analysis of flammable droplet clouds. A mathematical model is developed for prediction of droplet sizes and distribution associated with the impact of a liquid-containing projectile. The model can predict the transient behavior of droplet cloud. It is validated with experimental data available in literature. In the present study, the analysis has been performed using water and kerosene. The data obtained can be utilized as boundary and initial condition for CFD analysis. This information is useful for fire hazard analysis of aircraft impacts on NPP structures.

Actual Problems of the Safety and Reliability of the NPP Structures in Slovakia

The last accidents of the NPP in Chernobyl and Fukushima give us the new inspiration to verify the safety level of the NPP structures. This paper presents the new requirements to test of the safety and reliability of the NPP structures due to the last accidents in the world. The risk assessment to verify of the safety and reliability of the NPP structures based on probabilistic and nonlinear analysis is presented. The uncertainties of the loads level (long-time temperature and dead loads), the material model (concrete cracking and crushing, behaviour of the reinforcement and liner), degradation effects and other influences following from the inaccuracy of the calculated model and numerical methods were taken into account in the LHS method. The results of the deterministic and probabilistic analysis of the NPP structures are presented.

Geoecological analysis of short-term trends changes in the structure of NPP "Slobozhanskiy" territory based on satellite imagery PlanetScope

В статье представлены возможности анализа ландшафтной структуры и применения методов совместного использования спутниковой информации и полевой съемки на основе ГИС-технологий. Исследование раскрывает возможности использования данных космической съемки для нужд мониторинга уникальных водно-болотных угодий на территории национального природного парка «Слобожанский». Обоснована необходимость использования данных со спутника PlanetScope и раскрыты возможности инструментов ArcGIS для дешифрирования космических снимков, а также дальнейшей постклассификационной обработки (в частности генерализации результатов) и проведения оверлейного анализа. Изучение современного состояния и дальнейший мониторинг таких уязвимых и незначительных по площади водно-болотных объектов требует проведения детальных крупномасштабных исследований. Проведение таких исследований становится возможным при работе на местности, а также с использованием космических снимков высокого разрешения. В качестве таких снимков предложено использовать данные со спутника PlanetScope, имеющие разрешение до 3м. В ходе исследования было выявлено, что несмотря на динамические колебания сезонного характера, наличие трендов негативных изменений площади водного зеркала болот и озер, а также изменение растительного покрова в следствие постоянного высыхания болот.

Nonlinear Dynamic Analysis of the Aircraft Impact to the NPP Structures

This paper presents the nonlinear analysis of the reinforced concrete buildings of nuclear power plant under the aircraft attack. The response from the nonlinear analysis was considered taken the deterministic calculation procedures. The dynamic load is defined in time on base of the airplane impact simulations considering the real stiffness, masses, direc-tion and velocity of the flight. The dynamic response is calculated in the system ANSYS using the transient nonlinear analysis solution method. The damage of the concrete wall is evaluated in accordance with the standard NDRC consid-ering the spalling, scabbing and perforation effects. The simple and detailed calculations of the wall damage are compared.

Damping in the platform models for soil-structure interaction problems: Rayleigh damping options and limitations in modal analysis

Modal approach is widely used in seismic analysis. Moreover, spectral theory used in seismic codes all over the world is based on this modal approach. However, modal approach has an implicit limitation – relatively small or homogeneous damping. Otherwise the equations of motion in the ‘main coordinates’ remain coupled through the damping matrix even after decoupling through inertia and stiffness matrices. Due to this coupling, the results obtained using decoupled 1D modal equations of motion are incorrect. Practical example of such a situation is a platform model of the soil-structure system used for the analysis of soil-structure interaction (SSI) controlling seismic response of main NPP structures. Here, we see considerable damping in the ‘soil’ dashpots, completely different from structural damping. The difficulty with modal damping in SSI problems was realized in 1970s, and several suggestions have been made in order to save conventional modal approach. The author discusses these suggestions using a very simple 1D model and proposes to address the difficulties using combined asymptotic method (CAM). In CAM, a new platform model with homogeneous damping is analysed (thus enabling modal approach), and the difference between ‘true’ damping and artificial ‘platform’ damping is accounted for in the modified platform excitation.

Seismic Margin Assessment Methodology for NPP Buildings, Structures, and Equipment

The conditions for using the SMA method based on foreign databases on seismic classification are examined. In contrast to the initial SMA methodology a modified approach does not presume rejection of computationalexperimental evaluation of seismic safety under real conditions. However, the methodology presented examines the possibility of validated reduction of work, including rejection of computational-experimental checking when using the domestic database now under development. Evaluations of the seismic safety margin are used in the validation of service life extension for NPP, reexamination of safety, and probability analysis of safety as well as in establishing the technical specifications for equipment, modernization and reconstruction, and improving safety.

External Extreme Impacts on NPP Constructions – Methodology of Computational Simulation

In accordance with results of analysis of state-of-the-art theoretical and experimental research works, analysis and synthesis of data on the impact of extreme impacts on structures of existing nuclear power plants (including NPP "Fukushima-1", Japan), and other unique objects, the existing regulations, corresponding design codes and procedures do not fully take into account the specific definition of extreme (special) external loads and impacts on the main building of NPP, and, therefore, require clarification and development. The distinctive paper is devoted to advanced methods of analysis, choice of appropriate and correct methods and software application packages for numerical simulation of external loads and impacts on the basic structures of nuclear power plants (wind hurricane, tornado, plane crash, the impact of the shock wave, seismic and tsunami effect). Results of development, adaptation and verification (with the use of universal software application packages) within representative number of sample problems, dealing with advanced methods of analysis, definition of loads and impacts on the basic structures of nuclear power plants are presented. The results of analysis showed correctness and effectiveness of the developed numerical methods of advanced analysis and definition of extreme external loads and impacts on NPP structures of a new generation.

Виброакустическая динамика гетерогенных сред и конструкций ЯЭУ

Виброакустическая динамика гетерогенных сред и конструкций ЯЭУ