[en] At the seminar held in Visalaje (Czechoslovakia) in September 1984, the participants heard 19 papers of which 13 have been inputted in INIS. They deal with the problems of thermodynamics and of diffusion in austenitic steels, of corrosion fatigue of metals, structural materials for fast reactor steam generators, of resistance and service life of structural materials, especially in the liquid sodium medium, and of the properties of welded joints and their effect on resistance to long-term stress. (M.D.)

[en] The corrosion effect is discussed of hot pressurized water on the service life of heat exchange tubes of the sodium evaporator of a fast reactor steam generator in the region of departure from nucleate boiling. The problems are discussed of corrosion fatigue of the tube material and of the propagation of small technological defects in the corrosion medium. (author)

[en] The instance is considered when austenitic steel of the Fe-C-Cr-Ni type comes in contact with flowing liquid sodium at temperatures of 500 - 600 degC. The self-diffusion coefficients of components were measured for this ternary system. The self-diffusion coefficients increase with iron content increasina from 0 to 15% and decline at an iron content of more than 35%. The ratio of self-diffusion coefficients of the individual components is constant for the entire studied temperature and concentration interval. Using determinations of thermodynamic activities in the binary systems Fe-Ni, Fe-Cr and Ni-Cr it is possible to assess the regrouping of elements in austenitic steels and thereby to estimate the possible long-term changes in the mechanical properties of these materials. (M.D.)

[en] The susceptibility of CrNi austenitic steels to the development of intergranular corrosion cracking is evaluated. Intergranular corrosion develops in dependence on carbon content after annealing at temperatures of 500 - 800 degC. An important role is played by impurities in the steel. They may modify the rate of corrosion or passivation or influence the C-Cr balance at the grain boundaries, which leads to changes in kinetics and in thermodynamic parameters of the precipitation of chromium carbides. Intergranular defects have also been found during the development of what are called warm cracks in the weld metal which are related to the segregation of phosphorus and sulfur. Methods are indicated of controlling the microsegregation processes of various additions and of eliminating the effects of annealing and other metallurgical parameters on the said processes. (M.D.)

[en] The rate of propagation of corrosion fatigue cracks results from the presence of the aggressive medium and from the synergic effects of cyclic load. The propagation of the fatigue crack under the corrosive medium is modelled by a change of constants in Paris' relationship. The variation of crack propagation rate with the amplitude of the change in the factor of stress intensity is given in diagrams. In austenitic steels of the 18-8 type, it was proved that there exists a threshold temperature of 130 degC of very pure water under which there is no intergranular stress corrosion cracking. Intergranular stress corrosion cracking has also been indicated in case maximum stress in the cycle is below the yield point if the temperature of the water was 288 degC. The rate of growth of the corrosion crack in very pure water is approximately by 1.5 orders higher than at room temperature and in the air. The increase in growth rate of the defect when simulating the chemical composition of water in a nuclear power plant, is relatively low. Algorithms have been processed for determining the service life of fast reactor steam generator pipes. A diagram is given representing the test loop which allows to experimentally identify the required characteristics of materials. (M.D.)

[en] The possibility is discussed of a quantitative approach to the evaluation of changes in Nb-stabilized low alloy steel due to the effect of liquid sodium. The numerical finite difference method was applied to diffusion processes using the variability of all thermodynamic values entering the calculation. At thermodynamic balance with existing carbides, it will be possible to take into account the temperature gradient, the effect of the size of carbides and the effect of applied strain. The fact that the diffusion coefficient of carbide-forming niobium is by seven orders lower than that of carbon led to the use of the model of quasi-steady state diffusion. The dissolution of carbides and the diffusion of both elements is studied and the activities are investigated of carbon and niobium. The dissolution of NbC is controlled by the diffusion of niobium; the diffusion of niobium and carbon is induced by the balancing of their activities at the steel/sodium boundary. The local activity of carbon is at any moment determined via the steady state constant from the activity of niobium considering the type of carbide. The Nb-stabilized steel is characterized by a great difference in carbide sizes. Small carbides are accompanied by high carbon and niobium activities. Spontaneous diffusion occurs from the near vicinity of the carbides and the carbides dissolve. Stress increases activity thereby accelerating the process of decarburization of the steel in sodium. (Pu)

[en] A special testing equipment was developed for studying the effects of superimposition of mechanical stress and the effect of liquid sodium at high temperatures on structural materials for nuclear power plants. The equipment consists of a dynamic two-circuit sodium loop with a temperature gradient in the main circuit and with forced sodium flow. It comprises a main circuit with the experimental section and with an absorber of corrosion products, and an auxiliary purification circuit with a cold trap. The circuits have a common storage tank, argon economy and vacuum system. The temperature difference between the test pieces and the absorber of corrosion products is at least 100 degC. The test section consists of six series-connected containers into which test samples of the material are inserted. A diagram is given of the testing equipment and the variation of temperature, velocity and size of active surface along the whole main circuit is shown. (M.D.)

[en] The service life is discussed of part of the evaporator of a sodium steam generator at the point of departure from nucleate boiling. An oxidized layer develops on the surface of the metal. This layer slows down the process of corrosion in the hot water medium, in the steam medium it accelerates it. In Cr2.25Mo1 steel in both steam and water a magnetite surface layer develops which in dependence on hydrogen diffusion decelerates the corrosion process. In case hydrogen diffusion is prevented a magnetite double layer develops consisting of an inner porous layer and a compact outer layer. Point metal corrosion then develops under the porous layer. In points of departure from nucleate boiling, cyclic heat fatigue moreover supports cracking of the oxide layer and concentration of impurities. The oxygen content and purity of water are also important factors. A facility is presented for the simulation of corrosion conditions, developed by VUHZ Dobra. Corrosion fatigue tests will be carried out on sample tubes strained with an electrohydraulic pulsator at a throughflow of corrosion medium. Maximum temperatures will be restricted to 343 degC, pressure to 17.2 MPa and the volume of transported water to 1590 l/h. (Pu)

[en] The technology of fitting tubes in collectors consists of two procedures: a front weld production between the end of the heat exchange tube and the opening of the tubeplate, and fixing the tube along the whole length of the opening in the tubeplate. In the WWER 440 steam generator, heat exchange tubes 16x1.4 mm in diameter from steel 08Cr18Ni10Ti are joined to tubeplates from the same steel in a thickness of 136 mm with openings 16.25 mm in dia. The technologies are discussed of hydraulic and explosion fixing. The latter takes 2 - 3 μs and the tube wall connects with the tubeplate at 150 m.s^-1. Following cyclic temperature stress the explosion-connected joints loosen and corrode. Surface explosion welding requires accurate configuration of joints with a gap size of ca. 0.7 mm and an explosive inducing the minimum tube impingement rate of 400 m.s^-1 at maximum cleanness of surfaces. These explosion welds are resistant to loosening. However, they must be tested for susceptibility to corrosion cracking. Hydraulic fixing is carried out by plastic deformation of the tube with a high pressure liquid. The joints are again susceptible to loosening. Further experiments to test the long-term properties of joints are being prepared by VUHZ Dobra. (Pu)

[en] With regard to the manufacture of fast reactor steam generators the following materials were tested: Cr20Ni35Ti (S31), Cr18Ni11 (3R12) and Cr2.25Mo1Nb (HT8X6). Mechanical properties are adequate to values of non-exposed steels. Creep tests showed that at operating temperatures of up to 500 degC no changes appeared in the refractory properties due to the effects of sodium or the water/steam mixture. The plastic properties (ductility) remained unchanged. Tests of the effect of the water/steam mixture on fatigue showed in Cr2.25Mo1Nb steel a decrease by one order of resistance to high cycle fatigue. This was caused by local corrosion of tubes. For an undisturbed internal surface with compact magnetite layer, the fatigue properties of tubes remained unchanged. Cracks near to the surface have a mixed character with prevalence of intergranular failure. (Pu)