2023, Maltsev, D. A., Исаенкова, Маргарита Геннадьевна, Крымская, Ольга Александровна, Федотов, Иван Владимирович, Кулешова, Евгения Анатольевна, Минушкин, Роман Александрович, Kuleshova, E. A., Fedotov, I. V., Isaenkova, M. G., Krymskaya, O. A., Minushkin, R. A.

The paper considers the results of structural studies and mechanical tests after a long-term thermal exposure of laboratory heats of the metallurgically improved 15Kh2NMFA steel and steel with an increased content of nickel considered as materials for the pressure vessels of advanced VVER-type reactors of various designs. It has been shown that, both for the improved 15Kh2NMFA steel and the high-nickel steel, there are no signs of grain boundary embrittlement after an segregation provoking embrittlement heat treatment. This is explained by the extremely low grain boundary segregation of phosphorus in the initial state caused by a high degree of the structure dispersity as well as by rather a low content of impurities. Besides, no changes have been found in the yield strength value for the improved 15Kh2NMFA steel, which agrees with the structure investigation results. For the high-nickel steel, a tendency towards a minor yield strength decrease by 5 to 10% and a regular reduction of the critical brittleness temperature has been revealed. A decrease in the mechanical properties has been caused by a relatively low temperature of tempering for the high-nickel steel and, accordingly, by the potential occurrence of the structure recovery during long-term thermal exposure, as evidenced by the results of an X-ray diffraction analysis. Despite the structure recovery in the high-nickel steel under the long-term thermal exposure, the main strengthening carbide phases remain stable. Due to this, the yield strength value remains at a relatively high level that exceeds the values for the modern VVER-type vessel steels, even in the case of a thermal exposure much in excess of the expected operating conditions for advanced VVER reactors. The observed decrease of critical brittleness temperature during the long-term thermal exposure contributes to an increase in the steel resistance to brittle fracture.

2019, Frolov, A. S., Zhuchkov, G. M., Fedotov, I. V., Kuleshova, E. A., Кулешова, Евгения Анатольевна

© 2019, Pleiades Publishing, Ltd.Abstract: The radiation-induced structural elements in the materials of water-moderated water-cooled reactor pressure vessels have been studied by TEM and atomic probe tomography at the National Research Center Kurchatov Institute. The effect of Ni concentration in the range from 0.34 to 1.28 wt % on the formation of phases under fast neutron irradiation has been analyzed. The volume density of radiation-induced phases has been shown to depend on the Ni concentration in steel; the phase sizes and compositions remain almost the same. The volume density of these precipitations affects the radiation hardening, one of the radiation embrittlement mechanisms. The quantitative parameters of radiation-induced phases have been shown to control the service life growth, the enhancement of the thermal stability, and the melting technology of reactor vessel steels bearing nickel in the 0.3–0.7 wt % range.

2019, Gurovich, B. A., Frolov, A. S., Maltsev, D. A., Safonov, D. V., Kuleshova, E. A., Кулешова, Евгения Анатольевна

© 2019 Elsevier Inc.Microstructural studies of fuel cladding (made of E110 alloy based on sponge zirconium) after operation in VVER-1000 conditions (up to ~13 dpa) were carried out. The essential microstructural elements, which are responsible for the degradation of this material under the influence of operational factors, are established. It is known that in the process of irradiation, beta‑niobium needle-shaped precipitates are formed (in addition to the dislocation structures), which, upon reaching a specific fluence of fast neutrons, form periodic structures (with a bulk density of ~2–3∙1022 m−3). However, for the first time in this work, it was shown that relatively low doses of irradiation (~1.5 dpa) are characterized by the formation of needle-like phases with the high bulk density of ~4∙1022 m−3, which decreases with increasing of irradiation dose. This indicates the coagulation process of these particles during further operation of the fuel rods as a part of the VVER-1000 Fuel Assembly (FA). It is also shown in this work that the threshold dose for the formation of -type dislocation loops in alloy E110 (under the specified irradiation conditions) does not exceed 5.6 dpa.

2023, Maltsev, D. A. , Frolov, A. S. , Stepanov, N. V. , Safonov, D. V. , Кулешова, Евгения Анатольевна, Kuleshova, E. A., Fedotov, I. V.

Nickel is an essential alloying element in steels used as structural materials in the most common nuclear power reactors of the VVER type. The paper considers reviews the results of structural studies of traditional and advanced materials of the vessels and internals of VVER-type reactors with high nickel contents in their compositions. It is shown that an increased nickel content (up to 5 wt.%) in the steels of VVER pressure vessels contributes to the formation of a more dispersed structure with a smaller size of substructural elements and an increased density of dislocations, as well as a higher volume density of carbide phases. The revealed features of the structure of the reactor pressure vessel steel with high nickel content have the prerequisites for improving the strength and viscoplastic properties due to the increased number of barriers both for the dislocation motion and brittle crack propagation. Using the example of materials for VVER internals, it is shown that the nickel content increased in them up to 25 wt.% contributes to an increase in the volume density of radiation defects (dislocation loops of various types) and radiation-induced phase precipitates (G-phase). As nickel increases from 10 to 25 wt.%, there is a tendency to reduce swelling, which contributes to less shape change of the components of the reactor vessel internals. At the same time, in the steel with the highest nickel content, the highest nickel content was found in the near-boundary regions of the matrix, which contributes to greater austenite stability and a lower probability of the formation of an embrittling α-phase. The data obtained in the work on the effect of nickel alloying on the steel structural phase state and service characteristics were used in the development of new materials for the vessels and internals of advanced reactors.

2019, Fedotov, I. V., Kuleshova, E. A., Кулешова, Евгения Анатольевна

© 2019, Pleiades Publishing, Ltd.Abstract: The annealing technique is used to estimate the effect of radiation-induced structural elements on the service properties of the materials used in nuclear power plants: reactor pressure vessel (RPV) and in-vessel internals steels. The WWER-1000 RPV weld seam is affected by a hardening mechanism, the contribution of which is 60–65%, and a nonhardening mechanism resulting in grain-boundary segregation, the contribution of which is 30–35%. Radiation defects 40–45% and G-phase precipitates 35–40% are mainly responsible for the radiation-induced hardening of WWER-1000 internals materials. Radiation-induced structural changes should be eliminated for these materials to be reused.

2019, Mal'tsev, D. A., Fedotova, S. V., Kuleshova, E. A., Кулешова, Евгения Анатольевна

© 2019, Springer Science+Business Media, LLC, part of Springer Nature.Results of fractographic analysis and Auger electron spectroscopy of materials of vessels of VVER-1000 reactors obtained at the “Kurchatov Institute” Research Center are systematized. Comparative analysis of the effect of the operating factors on the level of grain boundary embrittlement in the matrix metal and welded joints is performed. The contribution of grain boundary embrittlement into the total radiation-induced embrittlement of the weld metal of the vessels is determined.

2019, Frolov, A. S., Gurovich, B. A., Maltsev, D. A., Safonov, D. V., Kuleshova, E. A., Кулешова, Евгения Анатольевна

© 2019, Pleiades Publishing, Ltd.Abstract: The paper presents microstructural studies of specimens cut from fuel elements made of E110 spongy zirconium-based alloy after operation in a VVER-1000 before reaching the burnup of ~35 MWd/kg U. As a result of exposure to high temperatures and neutron irradiation, significant changes in the phase composition of fuel cladding materials appear: change in the size, density, and composition of β-Nb particles; change in the composition of the Laves phase; formation of dislocation loops of α type, as well as δ and γ hydrides. The main structural elements determining the degradation of the mechanical properties of the E110 alloy under irradiation are dislocation loops and fine-phase precipitates owing to their relatively large density. The data obtained can be used to construct dose dependences of microstructural changes with the aim of predicting the residual life of claddings and fuel assemblies as a whole.