A gradient method of identification of structural inhomogeneities in the objects of industrial equipment and products control based on the analysis of electric potential distribution inside a single reflex is developed. The computational and graphic method is applied to analyze the results of electrical control of NPP equipment during its manufacture. The research objective under consideration is to determine the degree of reproducibility of electrical inspection results and to develop for this purpose a universal digital identifier of structural inhomogeneities. Single reflexes are characterized by internal pressure and distribution of electric potential, which has a gradient. Single reflexes on potentiograms are isolated by electrophysical chromatography with the help of double amplitude discrimination using developed program codes. The emergence of potential distribution patterns on the surface of the controlled product is associated with the presence of inhomogeneous fields of internal stresses and deformations in it. To determine the local value of internal pressure in structural inhomogeneities, the value of energy density was estimated. This estimate for single reflexes was obtained using the value of electron density in metals and alloys. The magnitude of the gradient corresponds to the electric field strength around the reflex. On the surface, a single reflex represents a figure of concentric hexagons or other geometric figures. In the volumetric image, the reflex has the form of a pyramid with a certain figure at its base. The hexagonal shape of the reflex is associated with a quasi-equilibrium distribution of normal and tangential stresses around the point heterogeneity. The value of internal pressure in steels for the level of fixation in the interval (0 ≤ SLS ≤ 1) is close to the tensile strength; for the interval of negative values (–0.7 ≤ SLS ≤ –0.4), it is close to the yield strength.

The method of scanning contact potentiometry (SCP) was investigated in factory conditions and functional capabilities of electric non-destructive testing (NDT) method to detect structural heterogeneities and technological defects were demonstrated directly in the process of reactor equipment production.

Scanning contact potentiometry (SCP) method was used to detect structural inhomogeneities in a welded joint under the application of heat and constant electric current. For this purpose, a special sample was prepared, welded by manual arc welding from two halves of X8CrNiTi18-10 (Chromium-Nickel-titanium stainless Steel) austenitic steel, each with dimensions of 200×110×13.5mm. After welding, welded sample was inspected using X-ray radiography. The sample was stored for five months at room temperature, and the sample surface was scanned using different scanning speed ranging from 0.36 to 1.8 mm/sec. It was found that when scanning speed was increased, the signal values remained the same. In another experiment, a constant electric current was passed through the sample from 0.01 to 0.1A. Applying current led to the excitation of weak structural inhomogeneities from the welding joint butt, although it was machined. While heat led to an increase in diagnostic signal amplitude.

A method of electrophysical chromatography was proposed to visualize the results of electrical non-destructive testing for the first time. Surface potentiograms usually used to represent the results of scanning contact potentiometry are replaced by volumetric images of structural inhomogeneities. The alternative has obvious advantages as useful additional information can drastically change the results of testing which is demonstrated by the example of a weld overlay. Methodological recommendations for constructing volumetric images of defects are formulated and implemented. The causes of self-shielding of structural inhomogeneities at different levels of fixation which are due to the mutual spatial overlap of microregions emitting waves of elastic stresses are discussed. The biggest problems in identifying defects for NDT are their complete or partial screening. The electrophysical chromatography method requires at least one experimental array obtained from the following measurements: double scanning of the surfaces of the test object; double simultaneous scanning of the surfaces of the test object; double simultaneous scanning of the surfaces of the test object using a radiation synchronizer in time or frequency. In the case of scanning along one (external or internal) surface, the coordinates of defects are determined from the corresponding cross sections and the calculated values of their depth using frequency and time-frequency analysis

The electrophysical system of non-destructive testing is tested on the NPP operating process equipment. The welded joints of the process pipeline located in the turbine hall are selected as the object of control. The results of the study confirm that the base metal of the welded joints is in a plastically deformed state. A comparison of the results of testing welded joints by scanning contact potentiometry and magnetic anisotropy methods is presented.

The issue of tracking residual stresses initiation in welding copper under various affected conditions is essential in increasing safety through improving the welding quality in particular, in the nuclear industry. This study investigated the behavior of welded copper numerically and experimentally under contact-heating compression test with constant clamping force. The scanning contact potentiometry (SCP) method was used to track the initiation and development of residual stresses within the weld zone. Furthermore, the finite element analysis (FEA) method was used to simulate and study the effect of thermal variations, with a constant compressive force, on mechanical factors that contribute to residual stresses formation within the weld zone. Additionally, scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to get further information about the topography and composition of the specimen's surface. SCP results show that residual stress initiated from within the volume after 200 °C, and with further oxidation, its formation began on the surface after 250 °C. Using the relationship between maximum values of linearized von Mises stresses and maximum values of ADS at high SLS = 4.523, it was found that residual stresses generation began after 150 °C within the weld zone, and thermal stresses linearly increase with temperature due to further thermal expansion, which is associated with variation in linearized von Mises stresses and ADS maximum values. Comparison between potentiograms after 300 °C and FEA results have shown that the distribution of von Mises stress, normal stress, and total deformation are matching those in the distribution of ADS and are localized within the weld zone. HIGHLIGHTS • The behavior of welded copper under compression at medium-low temperature range • Residual stresses detection at an early stage localized within weldment zone • Residual stresses increase with increased oxidation at copper surface • Matched the SCP Experimental and FEA simulation results GRAPHICAL ABSTRACT

Scanning contact potentiometry (SCP) is used for defects detection and for determine the coordinates of the defects localized in welded joints. Studied samples are welded by manual arc welding of two halves austenitic steel 321H (Chromium-Nickel-titanium stainless Steel) dimension of each 200×110×13.5 mm. Thereafter welding, seam width is about 12 mm. Subsequently welded sample is inspected using X-ray radiographic testing system. In order to compare techniques capabilities, plotted potentiograms and weld radiograph are compared. Experimental outcomes prove that defects can be detected by SCP technique. SCP results are complied with mathematical signal analysis of defects. This method may substitute some destructive or nondestructive methods.

This study presents the results of electrophysical non-destructive testing of collectors welded joints to the body of steam generators PGV 1000Mof the reactor WWER-1000. The studied steam generators located in the Resource Сenter of National Research Nuclear University MEPhI at the site of reactors construction factory “Atommash” in Volgodonskcity-Russia. The primary loop collector was welded to the body of steam generators PGV 1000Mand the technical condition of welded joints was investigated by electrophysical non-destructive testing (EPhT). In steam generatorsmay appear corrosion and cracks due to corrosive environment and effect of stresses however,EPhTcan detect early stages of cracks. The study highlights the theoretical part of electrophysical diagnostics and control method, using modern mathematical signals analysis, and issues related to the practical implementation of EPhT in the factory. The novel alternative method of non-destructive testing can be used in diagnostic oil and gas pipelines, elements of complex technical structures and industrial equipment. It can be used in many industries where it is necessary to process control of the entire structure or its individual parts under operating conditions. Based on control results, potentiograms were constructed for various structural levels of the diagnostic signal. The results of controlled welded joints in steam generators showed their satisfactory condition.

Abstract The paper presents the results of nondestructive testing of process equipment of nuclear power plants a welding unit for the collector of steam generators PGV 1000M, the hull penetration and surfacing of the process pipe performed in the factory. The purpose of the study was to justify the possible use of contact potential difference at the stage of technological assembly and control of welded joints of steam generators in the factory, as well as to determine the prospects for using this method in operating conditions of process equipment.

The processes of formation and growth of an incipient crack in EI847 steel have been studied by uniaxial constant-load tension testing on an INSTRON-5982 tensile testing machine using scanning contact potentiometry. The nucleus was detected at the sample surface within the yield strength domain and was stably monitored based on the readings of apparatus under higher loads until the moment of destruction.

Conducted evaluation of technical state of welded joints, welding the collectors of the primary loop to the body of the sample of steam generator PGV 1000, located in Resource center National Research Nuclear University MEPhI at "AEM-Technologies" Atommash factory in Volgodonsk. The results of point-by-point scan shows that SLS 1000 µv and above, for almost all samples detected multiple superficial imperfections caused by the presence of certain rust spots and traces of a deep spot corrosion, and the presence of small nicks, scratches and other imperfections.

The results of physical and mechanical tests on the tensile strength of austenitic steel 12X18H10T, in the stress range from 100 MPa to the maximum value of 700 MPa, at which the sample was destroyed, are presented. Structural changes were registered synchronously by two methods: the method of scanning contact potentiometry and the method of diffraction of thermal neutrons. At loads above 650 MPa, the α-martensite phase was found in the austenite matrix, as well as the appearance of diffraction peaks characteristic of a cubic martensitic BCC lattice was observed in neutron spectra. On the potentiograms, this process corresponds to the appearance of local regions in which high values of electric potential gradients were observed. This is the case of discovery of the fatigue nucleus cracks in the tensile testing of steel ЭИ847 by the method of scanning contact potentiometry.