Three-dimension flaw detection for the welded seams of pressure vessels

This article deals with the three-dimensional analysis of internal flaws occurring in the welded seams of pressure vessels so as to determine the real location of the defects. For this purpose stereoscopic twin radiographs were used. Owing to the fact that radiographs are taken with films in circular form whereas the films have to be assessed in the flat condition, it is necessary to establish the relevant analytic relation between the circular and the flat film. To this end some equations were first established for the image points of the assumed flat film and that of the circular film, leading to the relevant formulae for ascertaining the spacial point location of the defect in the welded seam. Defects of different locations bring about different photocouple-points. The real location of the defect can be determined through its coordinates, Z_A, X_A and Y_A obtained through film observation and calculation.     

On the trustworthiness of safety assessments for nuclear pressure vessels

The progress made in improving the safety against failure of nuclear pressure vessels has relied, among other things, on intangible factors like good workmanship, tacit knowledge, sense of responsibility, etc. This has long been recognized by those for whom this effort is made. The trust of the public and decision-makers is based on an appreciation of the awareness of the technological community also of the importance of non-technical, critical aspects. In order to maintain the trustworthiness of safety assessments in this respect it is therefore necessary to follow up previous research and development work with studies of problems of modelling for probabilistic calculations, collection of data on defect detection, intelligence concerning inconceivable events, competence and managerial aspects.     

Simulation of JR-curves for reactor pressure vessels steels on the basis of a ductile fracture model

A procedure for the prediction of J_R-curves for reactor pressure vessels (RPVs) steels in the initial and embrittled states is presented. Prediction of the J_R-curves is performed over the ductile fracture temperature range on the basis of a ductile fracture model. A procedure for the determination of ductile fracture model parameters based on tests of smooth and notched cylindrical specimens is proposed. The stress and strain fields near the stationary and growing crack tip are analyzed by FEM. Approximate analytical solutions for stress and strain fields near a growing crack tip are proposed. Comparison of the predicted J_R-curves and experimental 2T–CT data for the initial and embrittled RPV 2Cr–Ni–Mo–V steels for WWER-1000 is performed.     

Overview of JSME flaw evaluation code for nuclear power plants

A Japanese flaw evaluation code for nuclear power plant components has been developed at the Japan Society of Mechanical Engineers (JSME). The code prescribes methods for the evaluation of flaws, which are detected during inservice inspection for pressure vessels and pipes in nuclear power plants. This paper describes the basic flow chart, methods of evaluation and allowable flaw sizes for acceptance standards and criteria, including comparisons with the ASME Code Section XI.     

Review of a procedure for performing constraint and attenuation-corrected fracture mechanics safety case calculations for Magnox reactor steel pressure vessels

An assessment methodology put forward by BNFL Magnox Generation to take account of effects of constraint and dose attenuation in determining the margin between onset of upper shelf temperature (OUST) and normal steady state service temperature has been examined in detail by the UK Technical Advisory Group on Structural Integrity of Nuclear Plant (TAGSI). The standard methodology assumes that the constraint at a service flaw corresponds to that in a deeply notched three point bend or compact tension fracture toughness specimen. It also assumes that the maximum level of neutron and thermal ageing embrittlement at the vessel inner surface applies throughout the thickness. The revised procedure uses the local approach with detailed finite element analysis of specific defect geometries to determine constraint effects and allows for attenuation of embrittlement effects through the thickness. Demonstration examples of the analysis procedures were provided by BNFL Magnox Generation and AEA Technology Ltd.TAGSI endorses the proposed methodology, subject to validation in two specific areas, and to sensitivity studies being undertaken on the effects of other related factors. The key areas requiring validation are: (a) the use of start-of-life yield strength and work hardening properties for estimating end-of-life constraint benefits, and (b) the transferability of local approach model parameters to situations of differing constraint.TAGSI has given consideration to circumstances in which the methodology could be used. In carrying out this review, it has been recognised that OUST is sensitive to constraint and attenuation adjustments and it can no longer be regarded as a material property. The value of OUST determined from highly constrained fracture toughness specimens should be regarded as a ‘reference’ value for a given material but will not necessarily represent the value of OUST for a service application.     

Master curve techniques to evaluate an irradiation embrittlement of nuclear reactor pressure vessels for a long-term operation

Irradiation embrittlement is a limiting condition for the long-term safety of a nuclear reactor pressure vessel (RPV). The first PWR in Korea is approaching its initial licensing life of 30 years. In order to operate the reactor for another 10 years and more, it should be demonstrated that the irradiation embrittlement of the reactor will be adequately managed by ensuring that the fracture toughness properties are above a certain level of the required safety margin. The RPV was designed by an old construction code and its beltline circumferential welds have suffered from an irradiation shift problem like other Linde 80 welds. The master curve method is considered as the most promising tool to characterize irradiated structural steels by using a fracture mechanics basis. In order to implement the master curve method for the assessment of an irradiation embrittlement of old power reactors for a continued long-term operation, three practical issues were emphasized in this investigation, which are the specimen geometry effects on the master curve results, the specimen reconstitution techniques in an old existing surveillance program, and the index temperatures of an irradiation embrittlement when compared with the conventional Charpy data.     

Structural integrity evaluation for corrosion in spherical pressure vessels

Corrosion is typical of the damage that occurs in ageing pressure vessels and pipelines used in industrial processes as a result of reactive products inside or harsh environmental conditions on the outside. Structural integrity of such components needs to be evaluated periodically to establish the continued suitability of the vessels under operating conditions. The present paper develops a method for Level 2 (as categorized by API 579) fitness-for-service (FFS) evaluation of spherical pressure vessels with localized corrosion. The decay lengths for spherical shells subject to different sizes of corroded areas are calculated based on elastic effects in shells so as to identify the reference volume participating in plastic action. Lower bound “remaining strength factors” of spherical pressure vessels containing corrosion damage are formulated by the application of Mura's variational formulation and the m-multiplier method. Three alternative design recommendations are given. The effectiveness of the proposed methods is evaluated and demonstrated through illustrative examples and comparison with Level 3 inelastic finite element analyses.     

Fatigue failure criteria for thick-walled cylindrical pressure vessels

The fatigue failure of simple thick-walled cylinders under repeated internal pressure is considered with a view to establishing general criteria for failure which can be of use in design. The factors controlling the endurance limit and the limited life range are considered separately, utilising current thinking on the fatigue process. It is found that the fatigue behaviour of cylinders can be adequately predicted from conventional material fatigue data when the complexities of the elastic-plastic stress-strain state in a pressurised cylinder are taken into account. The importance of pressurising medium in assisting crack development is noted.     

Modeling for ductile-to-brittle transition under ductile crack growth for reactor pressure vessel steels

The local stress–strain state (SSS) near the crack tip is investigated by the finite element method in the finite strain statement (with regard to a change of the crack tip blunting) for both stationary cracks and crack growing by a ductile mechanism. Using the revealed particularities of SSS near the stationary and growing crack tips and the local cleavage fracture criterion the phenomenon of the ductile-to-brittle transition is explained for reactor pressure vessel steels. The model is proposed to predict the amount of ductile crack extension preceding the ductile-to-brittle transition as a function of the test temperature. The procedure for calculation of the cleavage fracture toughness is also elaborated with regard to ductile crack extensions. Analysis of the obtained calculated results and available experimental data is made. Alternative approaches for the interpretation of the ductile-to-brittle transition are discussed.     

木质素结构分析方法研究进展

木质素是由愈创木基丙烷、紫丁香基丙烷和对羟苯基丙烷3种单体组成的三维网状天然高分子聚合物,广泛存在于自然界中,它在现代能源和化工行业都具有广阔的应用前景。文章从木质素的结构出发,对木质素分离纯化和结构研究的主要方法(化学降解法、紫外可见光谱、傅立叶红外光谱、核磁共振光谱等)进行总结,并提出了木质素结构研究方法所面临的难点问题。     

A computer program for nonlinear analysis of pressure vessels

This paper describes the nonlinear analysis of pressure vessels necessary for taking into account the large deformations that take place at the junctions of shells of different geometries. Specifically, a computer program has been developed, based on both the linear and nonlinear theories of shells, which obtains numerical solutions for the most commonly used types of pressure vessels, namely those with spherical, ellipsoidal or conical heads and also flat-end pressure vessels. A multisegment integration technique has been used to obtain the solutions of the governing equations. The computed solutions are found to be highly accurate when compared with the known results of simple shells, as no nonlinear analysis is reported in the literature on the shell junctions in pressure vessels.     

Design stresses in probabilistic form for ellipsoidal and toroidal pressure vessels

Design has customarily been based on applied loading, geometry and handbook values for strength to give a deterministic solution. The engineering profession, however, has become increasingly concerned with the adequacy of design calculations. This concern indicates a need for critical evaluation of designs based on arbitrary multipliers, such as factors of safety or worst-case treatment.Ellipsoids are frequently used for end closure of cylindrical pressure shells. Toroids of elliptical or circular cross-section are widely used—for example, for connecting two parallel legs in a U-shape. This paper gives equations for means and standard deviations of stresses developed in ellipsoids and toroids with internal pressure. Inherent are: (1) design variables generally characterised by spectra of values (assumed to be normally distributed), rather than by unique values and (2) the fact that a small, but finite, probability of failure must be recognised in any design. By coupling stresses due to applied loading, as calculated by the given equations, with strength available in a material, reliability (or the alternative probability of failure) can be calculated. Conversely, for a given reliability, the appropriate size can be determined.Appropriate illustrations of the application of these equations are provided. The difficulty of relying on a factor of safety is demonstrated. As an example, consider a torus of circular cross-section with internal pressure. In one configuration with a given set of parameters using mean values and deterministic equations, the factor of safety is 1·70. The probabilistic equations indicate two probable failures in 100,000 vessels. If the standard deviation of the pressure is doubled, there are twelve probable failures in 100,000 vessels, although the factor of safety is still 1·70. In other words, if the range, but not the mean, of any parameter is changed, the failure rate changes—a change not predicted by the factor of safety.     

Estimation of peak pressure for sonic-vented hydrocarbon explosions in spherical vessels

Two closed-form approximate solutions are presented for the final pressure produced by a hydrocarbon explosion in a spherical vessel with sonic venting. A constant factor which multiplies the ideal spherical flame velocity is used to describe the effect of flame acceleration. One of the solutions is a simple, easy-to-use equation which may appeal to vent designers; it agrees well with reported results from a comprehensive computer model and correlates available experimental data as well as previous models involving several variable turbulence factors.     

Vehicular storage of hydrogen in insulated pressure vessels

This paper describes an alternative technology for storing hydrogen fuel onboard vehicles. Insulated pressure vessels are cryogenic capable vessels that can accept cryogenic liquid hydrogen, cryogenic compressed gas or compressed hydrogen gas at ambient temperature. Insulated pressure vessels offer advantages over conventional storage approaches. Insulated pressure vessels are more compact and require less carbon fiber than compressed hydrogen vessels. They have lower evaporative losses than liquid hydrogen tanks, and are lighter than metal hydrides.

The paper outlines the advantages of insulated pressure vessels and describes the experimental and analytical work conducted to verify that insulated pressure vessels can be safely used for vehicular hydrogen storage. Insulated pressure vessels have successfully completed a series of certification tests. A series of tests have been selected as a starting point toward developing a certification procedure. An insulated pressure vessel has been installed in a hydrogen fueled truck and tested over a six month period.     

The design of thickness transition regions for GRP pressure vessels

A computer program (BOSOR4), which allows stress analysis of thin shells of linear elastic material, was used to investigate the design of thickness transitions between the end closure and cylindrical body of GRP pressure vessels. Attention is focussed on the junction of a hemispherical dome and cylinder. A simple geometry and tapered transition is proposed together with a simple but safe design procedure which gives a possible material saving of 16% for hemispherical domes compared with the recommendations of the GRP pressure vessel design code BS4994, 1973.     

Weibull statistical models of KIc/KIa fracture toughness databases for pressure vessel steels with an application to pressurized thermal shock assessments of nuclear reactor pressure vessels

This paper presents new statistical representations of recently extended fracture toughness K_Ic and K_Ia databases for pressure vessel steels. These models were developed by the Heavy Section Steel Technology program at Oak Ridge National Laboratory in support of the current effort by the U.S. Nuclear Regulatory Commission to update its regulatory guidance for pressurized-thermal-shock (PTS) transients in nuclear reactor pressure vessels. The Weibull distribution, with two of its parameters calculated by the Method of Moments point-estimation technique, forms the basis for the new statistical models. An application of the new K_Ic/K_Ia models, as implemented in the favor probabilistic fracture mechanics computer program, is also presented for three PTS transients.     

A design procedure for pad reinforced flush nozzles in spherical pressure vessels

Charts have been produced for the design of pad reinforced nozzles in spherical pressure vessels. They are based on previously published theoretical and experimental work and on the limitation of maximum elastic stress in the pad or vessel near the intersection to 2·25 times the membrane stress in the main spherical shell. The maximum principal stress and maximum shear stress yield criteria have both been used as a basis for the preparation of separate charts.     

Application of ultrasonic inspection data in strength calculations for nuclear power plant equipment

Several kinds of test specimens were produced with three types of defects of defined sizes and positions in the particular localities of weld joints. Such specimens have been used for the assessment of defect parameter characterization by ultrasonic testing. The principles for schematization of such defects and the formulae for the stress intensity factor calculations for elliptical and semielliptical cracks have been worked out. The methods for defining the sizes of defect which are acceptable have been designed for use on the nuclear power plant equipment in the operational stage and take account of the mutual effects of the force, thermal and residual stresses. The method can be used in the brittle, transitional and tough material state.     

Dynamics of cryogenic hydrogen storage in insulated pressure vessels for automotive applications

A dynamic model is used to characterize cryogenic H₂ storage in an insulated pressure vessel that can flexibly hold liquid H₂ and compressed H₂ at 350 bar. A double-flow refueling device is needed to ensure that the tank can be consistently refueled to its theoretical capacity regardless of the initial conditions. Liquid H₂ charged into the tank is stored as supercritical fluid if the initial tank temperature is >120 K and as a subcooled liquid if it is <100 K. An in-tank heater is needed to maintain the tank pressure above the minimum delivery pressure. Even if H₂ is stored as a supercritical fluid, liquid H₂ will form as H₂ is withdrawn and will further transform to a two-phase mixture and ultimately to a superheated gas. The recoverable fraction of the total stored inventory depends on the minimum H₂ delivery pressure and the power rating of the heater. The dormancy of cryogenic H₂ is a function of the maximum allowable pressure and the pressure of stored H₂; the evaporative losses cannot deplete H₂ from the tank beyond 64% of the theoretical storage capacity.