PSI - Issue 28

L.A. Igumnov et al. / Procedia Structural Integrity 28 (2020) 2086–2098 L.A. Igumnov, I.A. Volkov/ Structural Integrity Procedia 00 (2019) 000–000

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Two values of temperature T on the region in the apex of the outer surface of the elliptical bottom, were used in the analysis:  T = 594 ◦ C (the first version of the analysis – Fig. 7a), taken from (Kazakov D.A. et al. (1994));  T = 800 ◦ C (the second version of the analysis – Fig. 7b), taken from (Frizen E.A. et al. (2014)). The geometrical dimensions and the temperature distribution over the reactor vessel surface (the temperature on the internal and external surfaces of the vessel) wereusedasboundaryconditionswhenanalysing the temperature fields in the reactor vessel cross section. In Fig. 7, temperature distributions are shown in yellow, and the reactor vessel in blue.

Fig. 7. a T = 594 ◦ C, taken from (Kazakov D.A. et al. (1994)), b T = 800 ◦ C, taken from (Frizen E.A. et al. (2014)). Temperature field distribution in the reactor vessel in the condition of a serious accident, obtained based on the corresponding thermal–physical analyses (numerically analysing the problem of heat-conduction) with increasing the temperature up to maximal values (the duration of the heating stage was 1 min) for the two above- mentioned versions of analysis, is depicted in Fig. 8 (close views of the most “critical” zones are given at the bottom of the pictures). Before directly modeling the emergency, the reliability of the developed MDM model was analyzed, and the material parameters of steel 15H2NMFA were deter- mined in the temperature range from 20 °C to 1200 °C. To this end, the experimental data published in (Drobyshevskiy N.I. et al. (2010), Loktionov V.D. et al. (2005)) was used. Papers (Drobyshevskiy N.I. et al. (2010), Loktionov V.D. et al. (2005)) present the results of experimentally studying the processes of short-range transient creep of steel 15H2NMFA in the temperature range up to 1200 °C. The creep process up to macrocracking was analyzed, using the physical–mechanical characteristics and material parameters of theMDMmodel for steel 15H2NMFA listed in Table 1. Figures 9 and 10 show creep curves for the following: temperature T = 900 ◦ C and stresses σ 11 = 20; 22 and 26.5 MPa, respectively (Fig. 9); temperature T = 1200 ◦ C and stresses σ 11 = 4 . 5; 5.4 and 6.3 MPa, respectively (Fig. 10). In the pictures, the solid lines correspond to the results of numerical modeling, usingdefining relationsofMDM(1)– (23), whereas the markers show the corresponding experimental data. Qualitative and quantitative agreement of the experimental and numerical data is observed, both in the values and in the character of the change of strains along all the three parts of the creep curve and in the time of macrocracking, which makes it possible to conclude that the modeling process is adequate and that the material parameters contained in the developed defining relations of MDM are determined accurately. The problem of evaluating the long-term strength of the NPP reactor vessel under thermal– mechanical loading was numerically analyzed in two stages. At the first stage, the stage of increasing the pressure and temperature up to maximal values during a short interval (the duration of the heating stage was 1 min) was analyzed. At the second stage, the pressure and temperature remained constant. A number of analyses were done that differed in the value of internal pressure p 2 . Four values of pressure p 2 were used in theanalyses:  p 2 1 . 25; 1.35; 1.5, and 2 MPa for the version of the temperature field depicted in Fig. 7a;  p 2 0 . 6; 0.7; 0.8, and 1 MPa for the version of the temperature field shown in Fig. 7b.