PSI - Issue 16

Andrii Kotliarenko et al. / Procedia Structural Integrity 16 (2019) 223–229 Andrii Kotliarenko et al. / Structural Integrity Procedia 00 (2019) 000 – 000

225

3

2. Experimental and Analysis Methods

2.1. Test Setup

For the investigation of the behavior of the material, standard smooth cylindrical specimens of 15Kh2NMFA steel with a diameter of the gauge length of 5 mm were used. The test was carried out on the Instron 8802 servo-hydraulic test machine equipped with a high-temperature furnace (up to +1100°C) and the Instron extensometer 2632-057 with a gauge length of 25 mm. Experimental tests were divided into the following stages:

 tension test  creep test  relaxation test  deformation of specimens with different strain rates test.

At the first stage of testing, in order to determine physical and mechanical properties of the material, uniaxial tension tests were conducted. The tests were carried out at the following temperatures: 20°C, 800°C, 1000°C. The loading rate of the specimen at testing was 2 mm/min or ~ 1.3 · 10 -3 sec -1 . At the second stage, determination of the properties of high-temperature creep was carried out. The test was conducted in conditions of short-term static loading on the basis of 60 minutes. The tests were carried out at a temperature of +1000 °C at constant stress of 0.8 σ 0,2 . During the tests, the original creep curve was recorded. In the third stage of testing an estimation of relaxation of stresses was carried out. To do this, two consecutive tests were conducted. At first, the specimen, preheated to a temperature T = + 650 °C , tension at a rate v = 0.01mm/s (the rate was measured by the movement of the piston of the test machine). After reaching (in 33 seconds) strain of  = 0.64%, which corresponds to maximum stress, the piston was held in this position for 6 hours. To evaluate how mechanical properties of the metal have changed after exposure at stresses at + 650 °C , the specimen (C012) was then tested on tension at temperature + 650 °C , obtained results were compared with the data obtained for tension test of the specimen (C06) in the initial state. At the fourth stage of testing of specimens at severe accidents temperature conditions results for two identical specimens were compared. Strain of the specimens C010 and C011 was measured by an extensometer. At the fourth stage of experimental research, a comparison between the fracture behavior for the different strain rates at a temperature of + 650 °C was made. The comparison was performed for two main strain rates v = 0,01 mm/s and v = 0,0001 mm/s. Taking into account the gauge length of the specimen, chosen strain rates correspond to έ  3 · 10 -6 s -1 , and έ  3 · 10 -4 s -1 , respectively. For the numerical simulation an axisymmetric model of reactor vessel WWER-1000 was used. The melting area, as a result of the accident, is modeled as reduction of wall thickness in the zone of connection of the elliptical lower plenum of the reactor vessel with its cylindrical part. In analysis the residual thickness of the wall and the temperature of cooling of its outer surface were set as variable parameters. Simulation of the thermal and stress strain state of the vessel was carried out using the SPACE-RELAX analysis complex by Chirkov (2012). In the simulation of the stress-strain state of the reactor vessel, a model of elastic-plastic material behavior was used. The material properties obtained during the tests at typical for a severe accident temperature were used. In the simulation it was assumed that due to the occurrence of the accident, the residual thickness of the reactor vessel wall in the zone of the melting was 50%, 25% and 10% of the original thickness. While determining the thermal state of the wall of the vessel, three levels of temperature on the outer surface of the lower part of the reactor vessel 100 °C , 300 °C , and 600 °C were considered. On the inner surface of the vessel, the temperature at the melting area and the entire bottom of the vessel was 1250 °C , the upper part of the vessel was 280 °C for all three cases considered. 2.2. Numerical simulation