PSI - Issue 23

B.A. Gurovich et al. / Procedia Structural Integrity 23 (2019) 589–594 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Upon reaching a certain critical stress value, the micro- and macrocracks begin to form. The value of the critical stress depends on many factors (elastic modulus, pore size distribution, creep coefficient, crystallite and coke particle sizes, anisotropy coefficient, etc.). Formation of the micro- and macrocracks corresponds to the maximum shrinkage and beginning of the secondary swelling stage of graphite. At the secondary swelling stage, significant structural changes occur that determine the mechanical properties degradation of graphite as a structural material. a b

1 0 0

n m

Fig. 1. (а) The typical TEM -image of reactor graphite microstructure: the area with the filler-binder boundary [1] (b) changes of elastic modulus (square markers) and volume (round markers) of GR-280 reactor graphite samples after irradiation at the temperature 5 00÷600 °С [ 2]

The causes of the properties degradation of graphite materials under irradiation were previously discussed in a number of papers [1, 5-11], but are still not completely clear. The main goal of this work was to obtain the new additional and direct experimental data, which are necessary for a more reliable justification of the radiation degradation mechanism of the graphite properties during operation in uranium-graphite reactors conditions. 2. Materials and Methods The studies were carried out on graphite samples trepanned from graphite block stacks of operating RBMK type at various stages of operation (irradiated to fluences corresponding to different areas on the dose dependence of volume change at a temperature of 500- 700 ° C). Trepanned reactor graphite samples were cut by hollow mill with 10 mm diameter from graphite block stacks in perpendicular to the extrusion axis of the block blank direction. Samples were further machined to a 8-9 mm diameter of and cut to a length of 15-20 mm to determine the tensile strength in uniaxial compression. The ultimate strength (fracture resistance) was determined in accordance with ASTM C695 standard using the Hegewald & Peschke Inspect 50kN universal testing machine at a 1 mm per minute speed of the upper crosshead to the specimen fracture (50% load drop of the maximum achieved value). Scanning electron microscopy (SEM) studies were carried out using scanning Merlin (Zeiss) electron microscope to determine the size of crystallites and to study the fracture nature of trepanned graphite samples after uniaxial compression tests in the initial state and after neutron irradiation. To study the crystallite sizes the sample surface was polished on a grinding paper with the following 3 minutes cleaning in an ultrasonic bath. The sample etching was carried out in a glow discharge at 1.1-1.4 kV voltage and 2-4 mA current for 8-20 minutes. The image sample surface was obtained in secondary electrons at accelerating voltages of 2 – 20 kV. 3. Results and discussion Graphite samples irradiated to the different fast neutron fluences (that correspond to the different parts of the dimensional change dose dependence, Fig. 2) were studied to analyze the structural changes of graphite stack under irradiation. Figs 2 ( а -c) show that the macrostructure of the studied graphite samples consists of the filler grains separated by the binder and the coarse open porosity. Herewith, for the graphite samples irradiated to the fluences, (1,0-1,8 )х10 26 m -2 , (that correspond to the shrinkage stage and the beginning of the secondary swelling on the dose dependence of graphite dimensional changes) the fractions of the open porosity are similar. With an increase of the fast neutron