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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fluence up to the critical values (up to 2,6 х10 26 m -2 ), the fraction of the open porosity increases accompanied by the significant decrease of the materials density. The increase in the open porosity fraction at this stage is mainly due to the formation of macrocracks in the material. a b c

Fig. 2. Typical SEM-images of nuclear graphite macrostructure (a) the shrinkage area F=1,0x10 26 m -2 , (b) beginning of the secondary swelling F=1,8x10 26 m -2 , (c) intense secondary swelling F=2,6x10 26 m -2

The authors earlier discussed one of the possible mechanisms of microcrack formation in [1, 11]. At the fluences corresponding to the graphite state after the shrinkage maximum, the crack formation at the filler-binder boundaries was observed, which occur in several stages [1]. Since, at irrad iation temperatures above ~ 400° C, the dimensional change of crystallites increases with the decrease of their size [11], and the binder crystallite sizes are significantly smaller than the filler crystallite sizes, the tensile stresses occur along the filler-binder boundary (wedging stress in relation to the filler crystallites). These stresses lead at the first stage to the split of the filler crystallites and then to the split of the binder crystallites along the a direction near the boundary. The microcracks are formed with an increase of the fast neutron fluence in areas with the splitting crystallites along the filler-binder boundary under tensile stresses along the c axis, caused by the compression of crystallites. Its appearance significantly reduces the cohesive strength at the boundaries between the filler and the binder as a whole. Under the further irradiation microcracks are combined into macrocracks, which determines the significant density decrease and, as a consequence, the compressive strength of graphite samples at supercritical fluences (Fig. 3).

Fig. 3. Dependence between strength and density of reactor graphite

Thus, within the framework of this mechanism, the main fracture cause of reactor graphite is the occurrence of internal stresses as a result of neutron irradiation due to the influence of crystallite size on the kinetics of radiation dimensional change and accumulation of radiation defects in reactor graphite.