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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Tests and SEM studies of the samples of irradiated graphite after compression tests (the load applied along a generatrix, Fig. 4a) were carried out to confirm the proposed fracture mechanism of graphite blocks under stress. Test samples were cylinders with a height of 4-10 mm. The radiation dose of the tested samples corresponded to the maximum shrinkage stage and the onset of secondary swelling. a b c

Fig. 4. The loading scheme (a) and the image of the main crack (b, c) during compression tests on the generatrix

Fig. 4 shows that the main crack has a typical for compression-tested brittle materials and passes mainly along the boundaries between the filler and the binder, which indicates a lower strength of the filler-binder boundary compared to the filler body. Fractographic studies of the fracture surfaces of irradiated graphite samples after uniaxial static compression tests showed that in initial state and at the shrinkage stage the fracture occurs mainly along the filler-filler or binder binder type boundaries (Fig. 5a). In the area of the maximum shrinkage in the fractures of the tested samples the areas of fracture along the filler-binder type occur. Its fraction increases with the fast neutron fluence increase. At fluences of more than (2-2,4) х 10 26 m -2 , where a decrease of the material density compared to initial values is observed, the fracture of reactor graphite samples at uniaxial static compression tests occurs mainly along the filler binder boundaries (Fig. 5b). a b

Fig. 5. Typical images of the fracture surfaces of the irradiated graphite samples after uniaxial static compression tests: (a) in the shrinkage area, (b) in the area of intense secondary swelling

Thus, the results obtained confirm the assumption that it is the difference in the values of the radiation dimensional changes of the filler and binder crystallites are the sources of internal stresses at the filler-binder type boundaries under irradiation. Ultimately, these stresses lead to the crack formation at the filler-binder type