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Igor Orynyak

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Proceedings Papers

*Proc. ASME*. PVP2018, Volume 3B: Design and Analysis, V03BT03A030, July 15–20, 2018

Paper No: PVP2018-85033

Abstract

The determination of stress intensity factors (SIF) and crack opening area or displacements (COA or COD) is important constituent when performing the “leak before break” analysis of piping systems in NPPs. The tabulated parametrical results of their calculation are widely presented in modern scientific and normative literature. Nevertheless, there is one aspect of crack behavior, at least in thin walled pipes, which still had not obtained its due attention. We mean here the geometrically nonlinear effect, which can be the big enough to be accounted for in practical applications. It is considered in geometrically linear analysis that only the inner pressure opens the crack, and COA and SIF are directly proportional to it. SIF is presented usually as solution for infinite plate multiplied by so-called bulging factor, BF, which depends on dimensionless crack length, i.e. ratio of crack length divided on square root of product of radius, R , and wall thickness, t . Two loading factors in thin walled pipes can contribute to geometrically nonlinear behavior. The first one is axial stresses induced by value of axial force or bending moment. The second one – is the inner pressure itself. The most attention in present paper is given to influence of the axial force. With this goal the numerical models were created for pipes with different ratios of R/t (20, 30, 40, 50) and different dimensionless crack length (2, 4, 6, 8). To exclude the nonlinearity due to circumferential stress the inner pressure is kept as a very small value and dimensionless SIF and COD values are calculated with respect to axial force. To prove the correctness of choosing the finite element types, meshing, number of elements along the thickness, loading steps the auxiliary problem of nonlinear modeling of transverse beam loaded additionally by very big axial force is considered. The very good correspondence was attained. For the pipe with axial crack the careful verification of numerical model was performed by comparison with linear results existing in literature. The results obtained are presented as a percentage of difference between the linear and nonlinear results. They show that influence of geometrical nonlinearity is fairly essential to be accounted in practice and can reach for practically real cases almost 3–10%. The change of SIF in percentages due to geometrical nonlinearity for different axial stress levels and for different crack lengths can be fairly well presented as unique dependence from product of stresses, radius to thickness ratio, and square root of dimensionless crack length. The change COD in central point of crack is slightly bigger than for SIF and the same unique dependence can be formulated for COD with only exception for small cracks λ < 3.

Proceedings Papers

*Proc. ASME*. PVP2018, Volume 8: Seismic Engineering, V008T08A050, July 15–20, 2018

Paper No: PVP2018-84932

Abstract

Simple approximate 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 Vlasov assumptions and Fourier series for the circumferential direction, an exact solution in the axial direction is obtained. To improve the results assumptions of Vlasov’s semimomentless theory are enhanced, i.e. we have used only the hypothesis of middle surface inextensibility to obtain a solution in axial direction. Nonlinear characteristic equations and natural mode shapes, are derived for all type of boundary conditions. Good agreement with experimental data and FEM is shown and advantage over the existing formulas for a variety of boundary conditions is presented.

Proceedings Papers

*Proc. ASME*. PVP2018, Volume 3A: Design and Analysis, V03AT03A033, July 15–20, 2018

Paper No: PVP2018-85032

Abstract

There is the general feeling among the scientists that everything what could be performed by theoretical analysis for cylindrical shell was already done in last century, or at least, would require so tremendous efforts, that it will have a little practical significance in our era of domination of powerful and simple to use commercial software. Present authors partly support this point of view. Nevertheless there is one significant mission of theory which is not exhausted yet, but conversely is increasingly required for engineering community. We mean the educational one, which would provide by rather simple means the general understanding of the patterns of deformational behavior, the load transmission mechanisms, and the dimensionless combinations of physical and geometrical parameters which governs these patterns. From practical consideration it is important for avoiding of unnecessary duplicate calculations, for reasonable restriction of the geometrical computer model for long structures, for choosing the correct boundary conditions, for quick evaluation of the correctness of results obtained. The main idea of work is expansion of solution in Fourier series in circumferential direction and subsequent consideration of two simplified differential equations of 4 th order (biquadratic ones) instead of one equation of 8 th order. The first equation is derived in assumption that all variables change more quickly in axial direction than in circumferential one ( short solution), and the second solution is based on the opposite assumption ( long solution). One of the most novelties of the work consists in modification of long solution which in fact is well known Vlasov’s semi-membrane theory. Two principal distinctions are suggested: a) hypothesis of inextensibility in circumferential direction is applied only after the elimination of axial force; b) instead of hypothesis zero shear deformation the differential dependence between circumferential displacement and axial one is obtained from equilibrium equation of circumferential forces by neglecting the forth order derivative. The axial force is transmitted to shell by means of short solution which gives rise (as main variables in it) to a radial displacement, its angle of rotation, bending radial moment and radial force. The shear force is also generated by it. The latter one is equilibrated by long solution, which operates by circumferential displacement, axial one, axial force and shear force. The comparison of simplified approach consisted from short solution and enhanced Vlasov’s (long) solution with FEA results for a variety of radius to wall thickness ratio from big values and up to 20 shows a good accuracy of this approach. So, this rather simple approach can be used for solution of different problems for cylindrical shells.

Proceedings Papers

*Proc. ASME*. PVP2018, Volume 4: Fluid-Structure Interaction, V004T04A026, July 15–20, 2018

Paper No: PVP2018-84933

Abstract

The approach for quick and effective search of natural vibration frequencies of multibranched beam systems with distributed mass, developed by F.W. Williams and W.H. Wittrick, is adopted for complex pipe systems with fluid-structure interaction. The general principle of the matrix composition is formulated for the case of beam systems of arbitrary complexity. The flexibility of such principle allows to extend it and to take into account the fluid free vibration. The special approach is developed to consider the fluid boundary conditions at the ends of the pipeline and a volumetric balance equation in the junctions. The formulated methods are implemented in an effective calculation procedure for the search of natural frequencies of coupled vibrations of the system with fluid-structure interaction, i.e. taking into account junction coupling as well as Poisson coupling. The procedure accuracy is demonstrated for a number of comparison examples.

Proceedings Papers

*Proc. ASME*. PVP2018, Volume 4: Fluid-Structure Interaction, V004T04A035, July 15–20, 2018

Paper No: PVP2018-84762

Abstract

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.

Proceedings Papers

*Proc. ASME*. PVP2017, Volume 7: Operations, Applications and Components, V007T07A019, July 16–20, 2017

Paper No: PVP2017-65769

Abstract

Traditionally, the brittle strength evaluation of reactor pressure vessel was the central issue in lifetime assessment of Ukrainian nuclear power plants (NPPs). The problem of swelling of the reactor core baffle only recently got due attention from the side of operator. Here the most efforts were given on numerical modeling of austenitic steel 08Kh18N10T swelling and its effect on induced stresses in core baffle and distortion of its geometry. The calculation shows that essential changing of core baffle dimensions is expected after 35–40 years of operation. Eventually this can lead to the contact with the core barrel. Yet, these predictions contain the big number of uncertainties related to the input data used in analysis: fluence distribution; temperature variation due to heat release induced by neutron and gamma radiation; thermal-hydraulic boundary condition between the baffle and coolant; and, especially, the adopted law of swelling in dependence with above factors as well as mechanical stresses. So, the second task was to measure the real geometry of baffle after 27 years of operation, to determine its change and compare these results with the numerically calculated data with accounting for the design tolerances. Thus, the spatial measurement system (SMS) equipped with ultrasonic gages was designed. It contains the central vertical beam which can move in vertical direction and rotate. To the lower end of the beam four horizontal levels are attached, which are equipped with device resistant to the hot water and radiation. The gages are used to measure the shortest distances to the edges of baffle. Two types of results were obtained. The first one are the measurements in the different horizontal planes obtained by rotation the SMS around the vertical axis with angular steps equal to 1 degree. These results were difficult to handle with and required a special mathematical treatment due to the possible shift of the centre of measurement. The second set of measurements was performed by moving the SMS in vertical direction. These data demonstrate the change of distance with the height. The results clearly show that problem of swelling do exists, and, in general, the measured patterns of the distortions along the vertical and angular coordinates correspond to numerically obtained results. Further work on baffle integrity is however needed.

Journal Articles

Article Type: Research-Article

*. June 2017, 139(3): 031208.*

*J. Pressure Vessel Technol*Paper No: PVT-16-1071

Published Online: February 3, 2017

Abstract

A physically and statistically based method for steam generator (SG) heat exchanger tubes (HET) integrity assessment is proposed. The method based is on the stochastic laws of crack dimensions distribution with taking into account its growth, limit load-model of cracked tube, and SG plugging statistics. Based on the history of the tubes, plugging of the specific SG three statistical parameters has to be found: initial number of defects, stochastic parameter of defect depth, and the defect growth rate. The developed method was used for the prediction of HET failure for all Ukrainian SG. It is also used for the justification of pressure reduction of hydrostatic test (HT) for primary circuit of WWER NPPs. It is shown that the pressure reduction from 24.5 to 19.6 MPa for WWER-1000 s and from 19.1 to 15.7 MPa for the WWER-440 s does not practically increase the HET failure probability during operation.

Journal Articles

Article Type: Research-Article

*. April 2017, 139(2): 021210.*

*J. Pressure Vessel Technol*Paper No: PVT-16-1012

Published Online: January 11, 2017

Abstract

Consideration of a geometrical nonlinearity is a common practice for thin-walled pressurized structures, especially when their cross section is not a perfectly circular one due to either initial imperfections or distortions caused by the nonsymmetrical loading. The application of inner pressure leads to so-called rerounding effect when decreasing of local flexibilities takes place. The crack can be also treated as the concentrated flexibility, so the goal of this work is the investigation of dependence of stress intensity factor (SIF) on applied pressure. Two cases of SIF calculation for 1D long axial surface crack in a pipe loaded by inner pressure are considered here: (a) cross section of pipe has an ideal circular form and (b) the form has a small distortion and crack is located at the place of maximal additional bending stresses. The theoretical analysis is based on: (a) well-known crack compliance method (CCM) (Cheng, W., and Finnie, I., 1986, “Measurement of Residual Hoop Stresses in Cylinders Using the Compliance Method,” ASME J. Eng. Mater. Technol., 108 (2), pp. 87–92) and (b) analytically linearized solution for deformation of the curved beam in the case of action of uniform longitudinal stresses. It is shown that for moderately deep crack (crack depth to the wall thickness ratio of 0.5 and bigger) in thin-walled pipe (radius to thickness ratio of 25–40) and inner pressure which induce hoop stress up to 300 MPa, the effect investigated can be quite noticeable and can lead to 5–15% reduction of calculated SIF as compared with the linear case. The analytical results are supported by the geometrically nonlinear finite element method (FEM) calculations.

Proceedings Papers

*Proc. ASME*. PVP2016, Volume 3: Design and Analysis, V003T03A045, July 17–21, 2016

Paper No: PVP2016-63710

Abstract

After the replacement due to the well known problem of weld seam #111 the steam generators (SG) of the Ukrainian WWER-1000 type nuclear power plants (NPP) have been in operation for almost 25 years. So now, the main practical problem related to the SG is the leakage/breaking of heat exchanger tubes (HET). If the leakage criterion is not satisfied the NPP Unit has to be stopped until the corresponding HET is plugged, which leads to unexpected huge financial losses. Every year at least 12.5% of the SG HETs are controlled by eddy current inspection, after which some of the HETs are plugged according to the appropriate plugging criteria (in most cases – 65% loss of tube wall thickness). Thus, every SG has its own statistics of plugging. From the practical point of view the Utility is interested in the following questions: whether the failure of the HET happens during the normal operation mode (NOM) up to the next NPP stop or not; what is the probability of this event; what is the defect growth rate? Answers on these questions could give us the opportunity to increase the effectiveness of the HET in-service inspection and to improve the plugging criterion. A simple statistically-based method for HET integrity assessment is proposed. The method based on an exponent distribution law and HET plugging statistics of specific SG, taking into account defect growth. Based on the history of the tubes plugging (year of operation versus quantity of pluggings) the three statistical parameters inherent to this specific SG have to be found: initial number of defects, parameter of the exponential distribution (initial defect size) and the defect growth rate. The developed method was used for the prediction of HET leak/break number for all Ukrainian WWER-1000 SG. It is shown that for those SG which has less than 2% HET pluggings the accumulated pluggings as of the end of the next year of NOM will not exceed this value. Based on the Ukrainian history of WWER-1000 SG operation the database of HET plugging is presented. This method is also used for justification of pressure reduction of periodic hydrostatic test (HT) for primary circuit of WWER-1000 and WWER-440 NPPs from 24.5 MPa to 20.3 MPa and from 19.1 MPa to 15.7 MPa, correspondingly. To justify a reduction of the HT pressure, a quantitative, risk-informed assessment of HT effectiveness has been performed with taking into account the HT pressures change. The HET failure probability is calculated as the proportion of defects which exceed critical sizes for NOM and HT. Limit load models are used for the determination of the critical defects size. The variation of reliability is calculated as the difference between fracture probability during NOM after HT at routine and reduced pressures. It is shown that HT pressure reduction does not practically increase the fracture probability during operation, and satisfies the criteria of risk change.

Proceedings Papers

*Proc. ASME*. PVP2016, Volume 4: Fluid-Structure Interaction, V004T04A021, July 17–21, 2016

Paper No: PVP2016-63589

Abstract

In this article we have analyzed the most severe design accident for the reactor facility — sudden rupture of the primary cooling loop. This rupture causes a water hammer event for the reactor core barrel, which is a significant but short term load. Assuming that core barrel is a thin shell, we have performed dynamic stress and strain calculations in the frequency domain. The Duhamel integral was used to calculate the transient response of a shell to an impulse load caused by the water hammer event. The results obtained were used to estimate structural stability of the core barrel.

Proceedings Papers

*Proc. ASME*. PVP2016, Volume 1A: Codes and Standards, V01AT01A001, July 17–21, 2016

Paper No: PVP2016-63598

Abstract

The continuous pipe bend behavior is well elaborated in literature. It is characterized by local ovalization of each cross section during bending which results in enhanced flexibility of it as compared to straight pipe. When pipe bend approaches some other structural elements of a piping system the end effect take place which can be described by so called long shell solution. This long solution is, in fact, a semi-membrane Vlasov’s solution when the derivative of any geometrical or force function in axial direction is much smaller than in the circumferential one [1]. Mitred bend is formed by conjunction by welding of two oblique sections of initially straight pipes. Its behavior during loading by pressure or bending moment is not evident and poorly described in standards. The goal of this paper is to give a set of general functions within a thin cylindrical shell theory which will give the opportunity to consider the mitred bend as an element of a piping system. Here we additionally introduce the so called short solution when the derivative of any parameter in axial direction is much bigger than that in circumferential one. Its main goal is to give the local behavior of stress in the vicinity of the oblique weld. Each of these two solutions satisfy by differential equations of forth order. The complete theoretical solution for a particular mitred bend is compared with a) existing analytical solutions and formulas; b) numerical results obtained by FEM with distinction of the zones of influence of a long as well as short shell solution; c) experimental data on real mitred bends given in the literature.

Proceedings Papers

*Proc. ASME*. PVP2016, Volume 6A: Materials and Fabrication, V06AT06A003, July 17–21, 2016

Paper No: PVP2016-63304

Abstract

The general approach of numerical treatment of integro-differential equation of the flat crack problem is considered. It consists in presenting the crack surface loading as the set of the polynomial functions of two Cartesian coordinates while the corresponding crack surface displacements are chosen as the similar polynomials multiplied by the function of form (FoF) which reflects the required singularity of their behavior. To find the relations matrixes between these two sets a new effective numerical procedure for the integration over the area of arbitrary shape crack is developed. In based on the classical hyper-singular method, i.e. Laplace operator is initially analytically applied to the integral part of equation and the resulting hyper singular equation is subsequently considered. The presented approach can be implemented with any variant of FoF, but Oore-Burns FoF, which was earlier suggested in their famous 3D weight function method, is supposed to be the most accurate and universal. It takes into account all points of crack contour, which provides perfect physical conditionality of the solution, but such FoF is relatively heavy in implementation and of low computational speed. The special procedure is developed for the approximation of the crack contour of arbitrary shape by the circular and straight segments. It allows to easily obtain analytical expression for Oore-Burns FoF, which greatly increases the calculation speed and accuracy. The accuracy of the considered method is confirmed by the examples of the circular, elliptic, semicircular and square cracks at different polynomial laws of loading. The developed methods are used in the implemented procedure for crack growth simulation. It allows to model growth of crack of arbitrary shape at arbitrary polynomial loading, at that all contour points are taken into account and can expand with their own speeds each. Procedure has high accuracy and don’t need complex and high-cost re-meshing process between the iterations unlike FEM or other numerical methods. At that usage of Oore-Burns FoF provides high flexibility of the presented approach: unlike similar theoretical methods, where FoF calculation procedure is rigidly connected with the crack shape, which complicates the adequate crack growth modeling, the used FoF automatically takes into account all points of crack contour, even if its shape became complex during the growth. Presented crack growth procedure can be effectively used to test accuracy and correctness of correspondent numerical methods, including the newest XFEM approach.

Proceedings Papers

*Proc. ASME*. PVP2016, Volume 5: High-Pressure Technology; Rudy Scavuzzo Student Paper Symposium and 24th Annual Student Paper Competition; ASME Nondestructive Evaluation, Diagnosis and Prognosis Division (NDPD); Electric Power Research Institute (EPRI) Creep Fatigue Workshop, V005T05A024, July 17–21, 2016

Paper No: PVP2016-63826

Abstract

In this article we have analyzed a water hammer event that occurred during the hydrostatic test of an oil pipeline. As a result of a significant dynamic loading event the piping was thrown off of its supports. We have performed dynamic calculations in the frequency domain with two models. The first model is a 2-DOF mass-spring model, used to perform an engineering assessment of the pipeline. The second model is a complex piping model of the aboveground section, which matches the real pipeline geometry and physical properties as well as possible. The Duhamel integral was used to calculate the transient response of the pipeline to the local water hammer impulse loading. The analysis results were used to estimate the pipeline displacement, and to develop certain practical recommendations to prevent such damage in the future.

Proceedings Papers

*Proc. ASME*. IPC2016, Volume 2: Pipeline Safety Management Systems; Project Management, Design, Construction and Environmental Issues; Strain Based Design; Risk and Reliability; Northern Offshore and Production Pipelines, V002T02A010, September 26–30, 2016

Paper No: IPC2016-64197

Abstract

Designing and maintenance of pipeline cable bridge with dynamic loads is complex because this problem belongs to the geometrically nonlinear problems. Analysis shown that existing mathematics models of cables have restrictions in use and we can’t use these cable models for dynamic loads calculations of cable-suspended pipeline bridge. Movement, produced by motion of inspection pig inside pipeline is an example of such dynamic loads. During its motion through the pipeline cable bridge the inspection pig induces additional stresses in pipeline due its weight and finite velocity which induces the vibration of the bridge. Its stress state assessment requires a lot of modeling, measuring and calculating actions to be done. First of all the initial static stress state of the cable bridge should be evaluated. It depends on the existing tension forces in the cable elements. They approximately were derived from the optical measurement of their geometrical curvatures with accounting for known weight density of the cables. Then, existing software tool for piping stress calculation “3D Pipe Master”, which operates by 12 degrees of freedom in pipe elements, was modernized to be able to take into account the geometrically nonlinear behavior of 6 d.o.f. cable elements. The equations which relate the elongations and rotations of cable elements with tension forces in cables are written in the form convenient for application of the transfer matrix method in the linearized iteration procedure which adjusts the measured displacements of the elements of the bridge with calculated one. In this way the initial tension forces in cables, in particular, and the bridge state, in general were determined. The dynamic part of the problem is solved by expansion in terms of natural frequencies eigenfunctions. Given inspection pig velocity calculation allows to determine the time dependence of generalized loads for each of natural vibration mode as product of the pig weight multiplied by mode shape displacement in point of pig position at the given time moment. Eventually the technique of Duhamel integral is used to calculate the dynamic behavior of the bridge for each natural mode of vibration. Two examples of dynamic stress calculation are considered. First is primitive one and relate to calculation joint interaction pipeline and cable system at dynamic loading. The second example concerns dynamic calculation pipeline cable bridge through the river Svicha during movement inspection pig. This bridge consists of two support, two parallel pipelines (1220×15) with bends and cable system. Analysis shown possibility uses “3D Pipe Master” software for the solving problems of durability pipeline cable bridge any complexity in the conditions of static and dynamic loading.

Proceedings Papers

*Proc. ASME*. PVP2015, Volume 4: Fluid-Structure Interaction, V004T04A028, July 19–23, 2015

Paper No: PVP2015-45289

Abstract

This article suggests calculation method for frequency and amplitude of acoustic vibration in piping with closed side branches, caused by gaseous running flow. The calculation algorithm consists of following steps: i) local excitation system is defined; ii) different combinations of boundary conditions are formed; iii) for fixed pair of boundary conditions ratio of stored in system energy and radiated from boundaries energy is written; iv) for every frequency energy functional is maximized to find boundary conditions; v) resonance frequencies are determined from plotting a curve of maximal energy ratio vs. frequency. Energy approach was further developed to analyze amplitude of vibration. For amplitude determine balance between injected energy (which depends on the Strouhal number and is defined from experimental data for laboratory geometries), and radiated from boundaries energy is written.

Proceedings Papers

*Proc. ASME*. PVP2015, Volume 4: Fluid-Structure Interaction, V004T04A029, July 19–23, 2015

Paper No: PVP2015-45291

Abstract

This article presents vibrations analysis of the reactor core barrel caused by pressure pulsations induced by the main coolant pump. For this purpose, the calculations of the pressure distribution in the annulus between the core barrel and the reactor pressure vessel, bounded above by a separating ring were performed. Using transfer matrix method is obtained the solution of two-dimensional problem of pressure pulsations in the annulus between reactor core barrel and reactor vessel. The calculation results are compared with the pulsation pressure measurements made at commissioning unit 2 of the South Ukraine Nuclear Power Station. The distribution of pressure over the height of core barrel was obtained, which makes possible to estimate its strength for variant deformation of the core barrel as a beam, and in the case of deformation of the core barrel as a shell. The calculation results are used to assess the reliability of core barrel pre-load, which clamps the core barrel flange in place at the top, at full power operating.

Proceedings Papers

*Proc. ASME*. PVP2015, Volume 1A: Codes and Standards, V01AT01A008, July 19–23, 2015

Paper No: PVP2015-45838

Abstract

The development of powerful commercial computer programs made the concept of J-integral as computational parameter of fracture mechanics to be a very attractive one. It is equivalent to SIF in linear case, it converges in numerical calculation and the same results are obtained by different codes (programs). Besides, it is widely thought that elastic-plastic analysis gives bigger values than elastic SIF ones what is good from regulatory point of view. Such stand was reflected in the recommended by IAEA TECDOC 1627 (February 2010) devoted to pressurized thermal shock analysis of reactor pressure vessels, where the embedded crack in FEM mesh, elastic-plastic analysis with simultaneous determination of J-integral was stated as the best option of analysis. But at that time all the most widely used software were not able to treat the residual stresses, the thermal stresses in case of two different materials. Such a contradiction between requirements and the possibilities made a lot of problems for honest contractors especially in countries where the regulator had no own experience in calculation and completely relied on the authority of international documents. This means that at that time the said recommendations were harmful. The main reason of such a situation was the absence of the carefully elaborated examples. Now the capabilities and accuracy of such software are increasing. Nevertheless, some principal ambiguities and divergences of computations results in various J-integral contours around the crack tip still exist. They are exhibited when the large plastic zone emerges at the crack tip. Other problem is influence of the history of loading and the specification of the time of crack insertion in the mesh including the time of emergence of residual stress. This paper is invitation for discussion of the accuracy and restriction of computational J-integral. With this aim the detailed analysis of some simplified 2D examples of calculation of elastic -plastic J-integral for surface crack with accounting for residual stress, thermal stress and inner pressure are performed and commented. The attainment of consensus among the engineering society for treating the outcome results is the prerequisite for practical application of computational elastic plastic J-integral.

Proceedings Papers

*Proc. ASME*. PVP2015, Volume 1B: Codes and Standards, V01BT01A005, July 19–23, 2015

Paper No: PVP2015-45276

Abstract

The consideration of a geometrical nonlinearity is a common practice for the thin-walled structures. The relevance here are two well-known cases treated in ASME codes. First one is accounting for reduction of the pipe bends flexibility due to the inner pressure. The second one is the retarded increasing (and subsequent saturation) of additional local bending stress with increasing of inner pressure in a pipe with initial cross section form distortion. In both cases the rerounding effect and decreasing of local flexibilities take place. The crack can be treated as the concentrated flexibility and it is quite natural to expect that the stress intensity factor should grow nonlinearly with applied load. Two cases of SIF calculation for 1-D long axial surface crack in a pipe loaded by inner pressure are considered here: a) cross section has an ideal circular form: b) the form has a small distortion and crack is located in the place of maximal additional bending stresses. The theoretical analysis is based on: a) the well known crack compliance method [1] and b) analytical linearized solution obtained for deformation of the curved beam in case of action of fixed circumferential stress due to pressure written in the form convenient for transfer matrix method application. It was shown that for moderately deep crack (crack depth to the wall thickness ratio is 0.5 and bigger) and typical dimensions of pipes used for oil and gas transportation (radius to thickness ratio is 25–40) and loading which can reach up to 200 to 300 MPa, the effect investigated can be quite noticeable and can lead to 5–15 percent reduction of calculated SIF as compared with linear calculation. The analytical results are supported by nonlinear FEM calculation.

Proceedings Papers

*Proc. ASME*. PVP2015, Volume 3: Design and Analysis, V003T03A068, July 19–23, 2015

Paper No: PVP2015-45679

Abstract

For WWER-1000 reactor pressure vessel (RPV) the metal embrittlement monitoring is performed with the usage of the radiation and the temperature surveillance specimens (SS) sets. For the fracture toughness curve indexing the critical temperature of brittleness (CTB) is used, CTB is obtained by the processing of Charpy impact energy data. Modern codes that regulate WWER RPVs CTB prediction are based on the CTB shift ideology. According to this ideology the CTB is defined as the initial CTB value plus CTB shift due to the radiation, plus CTB shift due to the thermal ageing, plus certain margin. This margin includes the scatter of CTB in the initial state, scatter of CTB shift due to the radiation and scatter of CTB shift due to the thermal ageing. A tendency of modern WWER RPV integrity assessment codes to provide conservatism with: choosing the CTB in the initial state for the CTB shifts determination; choosing the CTB in the initial state for the CTB determination; taking into account the scatter of CTB in the initial state, scatter of CTB shift due to the radiation and scatter of CTB shift due to the thermal ageing, leads to huge predictive values of CTB and to formal unfulfillment of brittle fracture criterion for RPVs. This paper demonstrates that the usage of actual CTB data and their scatter (obtained directly from Charpy V-notched impact tests) instead of the shift ideology can be the one of the possible ways of adequate CTB prediction. The CTB database for welds is created on the base of the results of WWER-1000 RPV surveillance program of all Ukrainian NPPs. The article presents the results of the thermal aged Charpy V-notched SS tests. The thermal aging has been shown to be practically absent. The CTB thermal shift values, which were obtained in some laboratories, are caused by the CTB scattering in initial and thermal aged states only. According to the processing results of the irradiated SS tests data, the chromium, manganese and silicon have been found to have the greatest impact on the CTB value. The CTB dependence on the neutron fluence and chemical composition of WWER-1000 RPV beltline welds has been obtained. A new methodology of CTB curve prediction for WWER-1000 RPV welds is developed. This methodology is expected to be the basis in the modern Ukrainian normative of RPV integrity assessment since it reduces the unreasonably high conservatism inherent to the RPV project, to a more reasonable level.

Proceedings Papers

*Proc. ASME*. PVP2015, Volume 7: Operations, Applications and Components, V007T07A034, July 19–23, 2015

Paper No: PVP2015-45573

Abstract

An analysis is formulated to determine how reductions in test pressures change the effectiveness of hydraulic strength tests (HT). The analysis is applied to Series 1000 Water-Water Energetic Reactor (WWER-1000) nuclear power plants (NPP) operating in countries of the former Soviet Union. During periodic HT for these reactors, the applied pressure exceeds the pressure for the normal operation mode (NOM) by a factor of more than 1.5. These HT pressures are among the highest in the nuclear industry. It is desirable to reduce the HT pressure in order to minimize potential damage to equipment due to pressurization during the tests. To justify a reduction in the HT pressure, a quantitative, risk-informed assessment of HT effectiveness for changes in HT pressures has been performed. This assessment follows the guidelines of INSAG-25 [1]. A deterministic analysis is used to calculate the HT minimum temperature, based on the estimation of the damage caused by HT and the evaluation of the warm prestressing effect. A probabilistic analysis is used to estimate the change in the probability of equipment fracture caused by a reduction in the HT pressure. The probabilistic analysis assumes that HT is performed as a destructive control method for replaceable components, so that HT combined with replacement or repair of defective components increases their reliability. A simple probabilistic analysis method, based on an exponent distribution law for defect depth and a lognormal law for the defect aspect ratio, taking into account laws for defect growth, has been proposed. The fracture probability is calculated as the proportion of defects that exceed critical sizes for NOM and HT. Limit load models are used for the determination of the critical size of defects. The variation in reliability is calculated as the difference between fracture probability during NOM after HT at routine and reduced pressures. The calculation results make it possible to analyze the effect of HT pressure reduction for WWER-1000 NPPs.