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

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4 SUMMARY

Hydrogen significantly affects the fracture properties of pressure vessel steel in both unirradiated and irradiated state at hydrogen contents above 2 ppm. Traditionally it has been assumed that the stainless cladding of the pressure vessel prevents hydrogen contents from approaching this high on the wall of the pressure vessel - however, this is something that has not been verified in experiments. Some studies have measured the hydrogen content of the cladding and found it to be 3-4 ppm in PWR water. This can be assumed to be the equilibrium content in the cladding/base material boundary, which suggests that hydrogen may play a major role in the mechanical properties of the reactor pressure vessel during operation. Although hydrogen diffuses quickly in the pressure vessel steel at high temperatures, some studies suggest that irradiated steel and the heat-affected zones of the weldments are efficient hydrogen traps and that hydrogen content my rise up to 2 ppm under the influence of irradiation. The effect of hydrogen seems to be the highest in the case of a ductile (upper plateau) fracture in both irradiated and irradiated steel. In the brittle fracture region of unirradiated steels, hydrogen seems to increase the transition temperature in some cases. Almost no test results exist with regard to irradiated specimens containing hydrogen. In steel, hydrogen promotes intergranular cracking jointly with intergranular segregation of impurities in particular. The anomalous fracture toughness results observed in survaillance specimens that are accompanied by a pronounced intergranular fracture may also be caused partially by hydrogen. The importance of hydrogen in the low fracture toughness values of the pressure vessel steel specimens and in the intergranular fracture requires further research.

BIBLIOGRAPHY

Alekseenko, N. N., Kuznetsov, A. A., Nikolaev, V. A. & Usatov, E. P. 1977. Delayed fracture of steel 15Kh3MFA under the influence of hydrogen after neutron bombardment. Fiziko-Khimicheskaya Mekhanika Materialov 13(1977)6. p. 6 - 18.