V. V. Kharchenko

Stress-Strain State Evaluation of a Welded Joint of Hot Collector to Nozzle of NPP Steam Generator PGV-1000

The paper describes a method and main results of a refined stress-strain state evaluation for a welded joint of the hot collector to steam generator nozzle of a nuclear power plant with a WWÉR-1000 reactor, which takes into account a fuller range of the actual loading conditions in operation. In calculations, we considered the stresses induced by uncompensated thermal expansion of the reactor elements. The results of 3D finite element calculations suggest that the level of local stresses in the area of the joint exceeds the metal yield stress. The calculated stresses for the outer nozzle surface agree satisfactorily with the in-situ strain measurement data.

Elastoplastic Fracture Resistance Analysis of NPP Primary Circuit Equipment Elements

Ñurrent calculation analysis approaches to the fracture resistance assessment for main equipment elements of the WWER NPP primary circuit are considered. It is noted that the solution of the problems of justifying safe operation, assessment of the integrity and extension of the lifetime of the reactor pressure vessel is essentially dependent on the results of calculations of the kinetics of the stress strain state, a proper consideration of the residual welding stress and strain fields, reliable evaluation of non-linear fracture mechanics parameters. A general methodology is developed for a calculated analysis of the fracture resistance of WWER reactor pressure vessels in the simulation of emergency cooling conditions. The main statements in the elastoplastic calculation of the kinetics of the stress strain state for reactor pressure vessels are formulated with consideration of the residual welding stress and strain fields, and the calculation procedure for determining the fracture parameters of the postulated crack is presented. Using the calculation methods and software product developed, a significant influence of such factors as consideration of the thermomechanical loading history and elastoplastic deformation of the metal in the vicinity of the postulated crack front, different ways of taking into consideration the residual welding stress level, the regularity and density of the finite element mesh in the vicinity of the calculated crack front, the procedure for calculating the elastoplastic fracture parameters in discrete finite element method models on the calculated fracture resistance assessment of WWER-1000 reactor pressure vessels is shown. It is found that the neglect of the loading history and residual stress level after heat treatment results in a non-conservative assessment of the fracture resistance for the collector-to-nozzle welded joint in the PGV-1000 steam generator in the simulation of the operating cycle of loading.

Experimental Determination of the Moment of Fracture Initiation in Standard Charpy Specimens and Specimens with Edge Notches

The results of investigations of the moment of crack initiation in the course of impact bending testing are presented. The procedure for recording the moment of crack initiation using resistance strain gauges has been developed. This procedure has been tested on standard Charpy specimens and specimens with edge notches. The diagrams of gauge signal and the loading diagrams are compared with the results of fractography investigations of the specimen fracture.

Impact Deformation and Fracture Mechanisms of Plates of Brittle Materials

We have studied the deformation and fracture mechanisms of plates of brittle materials under impact loading. The paper presents the results of numerical simulation of impact interaction between a projectile and a multilayer plate with a brittle layer. Methods of compression and perforation testing of brittle materials have been elaborated. The results of numerical simulation are compared with the experimental data.

Stress-Strain State of the Coolant Collector-to-Nozzle Welded Joint in the PGV-1000M Steam Generator Under the Operating Loads with Consideration of Residual Manufacturing Stresses

Results are presented for both the calculated assessment of the residual manufacturing stress fields in the simulation of the heat treatment in the vicinity of welded joint No. 111, and the calculation analysis of operating stresses and the fracture resistance of the collector-to-nozzle welded joint in the PGV-1000M steam generator in the simulation of the operating loading cycle.

Determination of Stress Intensity Factors in the Welded Joint Between the Header and Shell of Pgv-1000m Steam Generator with a Defect in the Form of a Cavity with a Crack

The stress intensity factors in the welded joint between the header and shell of steam generator with a defect in the form of a cavity with a crack has been investigated. The influence on the history of thermomechanical loading on the stress intensity factor at the crack tip has been analyzed. A comparative analysis of the stress intensity factors for the crack and cavity with a crack has been performed.

Effect of Pulsed Magnetic Field Treatment on the Fracture Resistance of a Cracked Specimen

We present the results of experimental investigations on the evaluation of the effect of pulsed magnetic field treatment of 45 steel cracked specimens on their fracture resistance. Before loading, the specimens were mechanically loaded to the stress level at the crack tip at which the stress intensity factor value is below the critical one. As a result of the treatment by pulsed magnetic field, the stress relaxation occurs in the specimen due to plastic flow at the tip of the crack, which results in its blunting. The test data for the specimens after treatment are indicative of a significant increase in the fracture resistance as compared to the specimens before treatment (i.e., in the initial state).

Some Aspects of Inclusion of Loading History in the Fracture Strength Analysis of Reactor Pressure Vessels Under Thermal Shock Conditions

The paper addresses some aspects of inclusion of loading history in the fracture strength analysis of reactor pressure vessels under thermal shock conditions. The strength assessment of a reactor pressure vessel and its lifetime extension are shown to essentially depend on reliability of results of elastic-plastic modeling of stress-strain kinetics and determination of design values of the fracture mechanics parameters. The present findings of the finite-element analysis demonstrate that the loading history must be taken into account in the determination of the temperature dependence of stress intensity factor for a postulated specified crack. It is shown that, in the fracture toughness analysis of a reactor pressure vessel, the temperature dependence of the stress intensity factor has to be derived from the results of the analysis of kinetics and mode of loading of metal at the crack tip under thermal shock conditions. For construction of the above dependence, we proposed approach, which accounts for the active loading and unloading processes occurring in the metal in the crack front vicinity. Incorporation of the loading history into the analysis of the temperature dependence of the stress intensity factor using the proposed approach forcan permit revealing some additional margin of strength during the substantiation of lifetime extension for a reactor pressure vessel under thermal shock conditions.

Experimental Investigation of Mechanical Behavior of Materials by Small Punch Testing of Miniature Disk Specimens

A procedure and experimental equipment are developed to perform small punch testing of miniature disk specimens. The procedure is experimentally tested on specimens of 45 steel. A comparative analysis is made of the known procedures for determining the yield stress from the small punch load–displacement curves for miniature disk specimens.

Investigation on Strength Characteristics of Steam Generator Welded Joint After Operational Life Using Instrumented Indentation Test Method

The paper presents the distribution of the strength characteristics of weld metal in heat-resistant steel 10GN2MFA over the thickness and height of the hot collector-to-nozzle welded joint in NPP steam generator determined by the instrumented indentation test method.