Doel 3 & Tihange 2 - Some Peer-reviewed Scientific Papers & Reports

- 30 -

Ruscak et al. (1991) studied CrMoV and CrNiMoV steels which were irradiated in the VVR-S reactor at 288°C reaching a dose of 8.7 x 10 23 n/m 2 (E > 0.5 MeV). The tests used pre-fatigued standard Charpy-V specimens (base material) and tensile test specimens (heat- affected zone, HAZ). After the irradiation, the test specimens were hydrogen charged in a 1 M H 2 SO 4 solution which contained 30 ppm As 2 O 3 . Figure 22 shows the effect of hydrogen (3.5-4.5 ppm H) on unirradiated and irradiated CrMoV steels. After the hydrogen charging, the shift of the transition temperature was 60-80°C for irradiated and unirradiated steels. The effect of hydrogen on the shift of the K Ic (= 100 MPa m 1/2 ) value was 30°C for irradiated steel and 60°C for unirradiated steel. Hydrogen also reduced significantly (40%) the upper plateau ductility of both irradiated and unirradiated steels. When the reduction of the upper plateau ductility was studied for different hydrogen contents, the lower hydrogen contents caused it in the irradiated steel (Figure 23). The critical hydrogen content for irradiated steel is 1.5 ppm H and for unirradiated CrMoV steel 2.5 ppm H, respectively. For high hydrogen contents, the fracture surface changed more and more into an intergranular fracture mode. The effect of hydrogen content on the crack growth rate was studied with unirradiated specimens at different strain rates, as shown in Figure 24. When the hydrogen content exceeds 2.5 ppm, the crack growth rate no longer increases significantly. This hydrogen content corresponds to the threshold for intergranular fracture toughness, according to Figure 23. Zdarek et al. (1992) used the same methods to study hydrogen charged CrNiMoV steel (irradiated in VVR-S reactor, 279°C, 6.9-10 x 10 23 n/m 2 , E > 0.5 MeV). Figure 25 shows the temperature dependence of the fracture toughness of the steels in question in different states. Irradiation and hydrogen charging favour the intergranular cracking mechanism which was seen in the fractography. Figure 25 shows that hydrogen (2- 5 ppm H) does not significantly increase the effects of irradiation embrittlement on this type of steel, although the effects of hydrogen exhibit a wide scatter of the results.