PSI - Issue 23

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

593

5

boundaries and, as a result, to the properties degradation of reactor graphite samples even in the absence of external stresses. An alternative approach to the properties degradation mechanisms of reactor graphite under operation was considered in [6-10]. According to this approach, the properties degradation of reactor graphite under irradiation is determined by the exhaustion of the reactor graphite accommodative ability to compensate the radiation dimensional changes of crystallites. This effect is due to the closure of the oriented porosity under irradiation (the so-called Mrozowski crack) as a result of both radiation dimensional change of the crystallites and also due to internal stresses that arise during the radiation dimensional change of crystallites with different orientations. Fig. 6 [1] presents data on the dose dependences of volume and elastic modulus changes under irradiation of the GR- 280 grade “two - phase” reactor graphite and MPG - 6 “single - phase” graphite under the same conditions. MPG -6 is manufactured based on uncalcined petroleum coke as the filler. These cokes usage facilitates the formation of a monolithic structure without distinct boundaries between the coke-filler and coke-binder areas, thus affecting the dimensional stability of a product made from such graphite [12,13]. MPG-6 has almost no noticeable difference in structure between the binder and filler regions, and also has smaller crystallite size dispersion. Such graphites can be called «single - phase» in contrary to conventional «two - phase» GR -280 graphite. Fig. 6 shows that for GR-280 graphite (where the crystallite sizes of the filler and binder differs significantly) at the irradiation temperature of 900° С the typical dose dependence of elastic modulus is observed (with a derivative sign change and the region where the absolute values of the elastic modulus decrease significantly to the initial values). For MPG-6 graphite with the close filler and binder crystallite sizes irradiation under the same conditions does not lead to the change of derivative sign on the dose dependences of elastic modulus, its absolute value grows monotonically. This behavior of the "single-phase" MPG-6 graphite under irradiation is observed at much larger volume changes (approximately 4 times) compared to GR-280 graphite. This contradicts the model according to which after the exhaustion of the accommodative ability of graphite to compensate for radiation dimensional changes, a decrease in density and strength characteristics should be observed (accommodation mechanism). It should be noted that a derivative sign change on the dose dependence of elastic modulus for GR-280 graphite at irradiation temperature of 900-1000 °C is observed upon reaching a neutron fluence of about 2 х 10 25 m -2 (Fig.6). At further increase of neutron fluence there is a pronounced decrease in mechanical properties. Moreover, for GR-280 graphite the derivative sign change on the dose dependence of the elastic modulus change at irradiation temperatures of 900-1000 °C occurs and at the fast neutron fluence 3 times lower than at irradiation temperature 500-600 °C ( Figs 1 and 6). This effect can be due to the higher influence of the difference between the filler and binder crystallite size on the rate of radiation dimensional change at higher irradiation temperatures and, as a consequence, the degradation rate is much higher.

V-V 0 /V 0 , %

E-E 0 /E 0 , %

1.E-24

Russian graphite variety EGCR graphite variety (American data) CGB graphite variety (American data) Data from [36]

3

60

1.E-26 Coefficient of creep, 10 -8 (kg/m 2 ) -1 (neutron/m 2 ) -1 1.E-25

MPG-6

2,5

50

MPG-6

2

40

1,5

30

1

20

GR-280

0,5

10

GR-280

1.E-27

0

200

400

600

800

1000

T irr , C

0

0

0

2

4

6

8

Neutron Fluence, 10 25 n/m 2 (E > 0.18 MeV) Fig. 6. – Dose dependences of the volume changes (solid lines) and elastic modulus (dashed lines) of «single - phase» MPG - 6 and “two phase” GR -280 graphites irradiated under the same conditions at the temperature 900- 1000°С [1]

Fig. 7. The temperature dependence of the creep coefficient according to different studies [14], [15]