Oleksii Ishchenko - Academia.edu (original) (raw)

Papers by Oleksii Ishchenko

Procedia structural integrity, 2024

This article presents adaptation of the Wittrick-Williams algorithm to calculate the natural vibr... more This article presents adaptation of the Wittrick-Williams algorithm to calculate the natural vibration frequencies of cylindrical shells. Initially, the “algorithm” was proposed and used to find the natural frequencies of branched piping systems (trusses) in structural mechanics problems. We expanded it for shells, where a more difficult system of main differential equations is solved, with increasing calculation effort. The Wittrick-Williams algorithm was chosen to find vibration frequencies, because it is the only one exact solution for the transcendental eigenvalue problem. The “algorithm” gives the number of natural frequencies below a trial value of frequency, therefore, using bisection method, as the simplest, it is possible to find frequencies with sufficient accuracy. It can been used, for example, in a dynamic analysis, where it is important to find accurately the lower frequencies, and to distinguish between coupled frequencies. For solution the shell must been divided into a certain number of parts; convergence graphs are given, according to which the optimal number of partitions is selected. Obtained results show good convergence with experimental and literature data for different boundary conditions. Several special cases have been considered for shells with different values of elastic boundary conditions. The method has proven to be practical and can been used for modal, harmonic and seismic analysis.

This paper presents a results of brittle fracture (BF) assessment of WWER-1000 reactor pressure v... more This paper presents a results of brittle fracture (BF) assessment of WWER-1000 reactor pressure vessel (RPV) subjected to pressurized thermal shook (PTS). Work is focused on the two factors affected on the BF safety margin. The first one is the definition of the heat transfer conditions from outer surface of RPV, which is commonly not considered due to the assumption of conservatism. The second one is the warm pre-stressing (WPS) of RPV metal during PTS. The procedure for the formation of thermal boundary conditions (BC) for the RPV (i.e. lower nozzle forging, cylindrical part and elliptical bottom), reactor supports, as well as for the reactor thermal insulation, is developed. This procedure takes into account the heat transfer due to radiation in the GA-301 room and the presence of forced supply of “cold” air by the ventilation system. A brief history of WPS implementation in Ukraine is presented. Also, the comparative review of the WPS experience in PWR countries and WPS approaches is made. The results of fracture mechanics calculations for the three most severe PTS scenarios of the Unit 3 RPV of South-Ukrainian NPP are presented. Wherein, both type of results is demonstrated: with and without consideration of the RPV outer surface heat transfer due to the air-cooling ventilation system. It is shown that the RPV air cooling consideration: leads to decreasing of the stress intensity factor; provides to satisfaction of the cladding integrity criterion according to VERLIFE-2008 methodology; can justify lifetime of the RPV nozzle region in terms of not to exceed the upper shelf of fracture toughness 200 MPa·m0.5. At the same time, consideration of the RPV air cooling alongside with the WPS approach, depending on WPS approach considered, can lead as to the decreasing the conservatism of RPV BF assessment, as to the increasing it. Also, concluded that Ukrainian WPS approach needs to be refined more carefully with consideration of experimental nature of pre-stressing or harmonized with some of the modern ones.

Procedia structural integrity, 2023

The assessment of seismic and dynamic strength for the VVER-1000 reactor with emphasis on fuel as... more The assessment of seismic and dynamic strength for the VVER-1000 reactor with emphasis on fuel assemblies interaction is carried out based on experience for Ukrainian NPPs. Evaluation of fuel assembly structural response to externally applied forces is based on NUREG-800 “Standard Review Plan”, item 4.2 “Design of fuel systems”. The analysis considered two types of events: Safety Shutdown Earthquake (SSE) and Large break Loss Of Coolant Accident (LB LOCA). As LB LOCA for the VVER-1000 – “Maximum design accident” Recirculation Line Break is considered. For the analysis of the initial dynamics, the 3D CFD model (ANSYS CFX) of the VVER-1000 was adapted to account for the two-phase flow. To take into account the transience of the phase transition during depressurization, a special model of interphase exchange is proposed, which takes into account not only the nonequilibrium temperature, but also the features of the pressure change by analogy with cavitation. For full dynamic analysis, it is necessary to develop an analytical and/or FEM model of the reactor, which will include reactor elements, their masses stiffness and inertia characteristics. The developed model is validated against experimental data of natural frequencies of fuel assembles, for uncompressed and compressed working position, also with and without fluid influence. The seismic analysis consider the available response spectra for NPP site with 3-component seismic impact of SSE level. Another important issue is to analyze reactor elements interaction and in case of need, to account for the non-linear behavior of the model. The seismic and LOCA impact at the location of fixation of the fuel assemblies (lower and upper support plates) is calculated by taking into account the dynamic response of the reactor system as a whole. After demonstrating the seismic/dynamic stability of the fuel assemblies and the reactor internals, a justified conclusion is provided on preserving the safety factor for fuel assembles at both loadings.

This article focuses on the dynamic behavior of the Pressurized Water Reactor (PWR) during the Lo... more This article focuses on the dynamic behavior of the Pressurized Water Reactor (PWR) during the Loss Of Coolant Accident (LOCA) which cause the significant acoustic loads on the Core Shrouds. The finite element analysis of a PWR was performed to obtain the acoustic response to the LOCA event. We have performed dynamic stress and strain calculations in the frequency domain for the Core Barrel, according to classical shell theories. The Duhamel integral was used to calculate the transient response of a shell to the transient load caused by the water hammer event. The results obtained were used for fracture mechanics evaluations for flaws, which may occur between inservice inspections.

Procedia Structural Integrity, 2020

This paper presents a simple semi-analytical approach for free vibration of cylindrical shell wit... more This paper presents a simple semi-analytical approach for free vibration of cylindrical shell with initial prestress based on equivalent load method and the Donell-Mushtari theory. In most practical applications, shells are subjected to static loadings causing internal stress field. The presence of such initial forces like internal pressure, axial force, centripetal force and torque moment significantly affects the natural frequency spectra. According to Calladin's equivalent load method initial stress field create additional curvatures and can be added as additional terms to the basic equations. The results of presented method agree well with experimental data found in the literature. Effects of elastic support stiffness, the shell length and radius to thickness ration on natural frequencies are investigated.

Procedia Structural Integrity

Mechanics and advanced technologies, Dec 29, 2022

International Scientific and Technical conference "The Progressive Technics, Technology and Engineering Education", 2021

In works of extending lifetime of WWER-type reactors, it is necessary to obtain the brittle fract... more In works of extending lifetime of WWER-type reactors, it is necessary to obtain the brittle fracture resistance (BF) of the reactor pressure vessel (RPV). Implementation calculations for brittle fracture resistance is regulated by the technical PNAE of Ukraine. The objective of these calculations is to prevent catastrophic brittle destruction of the reactor pressure vessel, pipelines and pressure vessels from the existence crack-like defects for all operating regimes, including emergency situations (ES). The paper considers the most dangerous postulated emergency situation in operation is "large" and "small" leak in the RPV NPP. Calculations with Warm Pre-Stressing effect (WPS) of the RPV for the most dangerous scenarios have been presented, and an assessment of the brittle strength of RPV NPP is taking into account with WPS. The results of studies with factor of brittle strength safety are also presented without taking into account the Warm Pre-Stressing, compar...

Simple accurate formulas for the natural frequencies of circular cylindrical shells are presented... more Simple accurate formulas for the natural frequencies of circular cylindrical shells are presented for modes in which transverse deflection dominates. Based on the Donnell-Mushtari thin shell theory the equations of motion of the circular cylindrical shell are introduced, using series expansion for axial coordinate and Fourier series for the circumferential direction, a simple explicit solution is obtained. Also, the influence of deformation component is investigated, it is shown that it can be neglected. Good agreement with experimental data and FEM is shown. The advantage of a current approach over the existing formulas is simplicity in programming.

Volume 5: Operations, Applications, and Components; Seismic Engineering; Non-Destructive Examination

Volume 4: Student Paper Competition