Session: CS-17-03 Environmental Fatigue Issues (Joint with M&F)-3: INCEFA SCALE & International Studies

Paper Number: 154611

154611 - Transferability of Eaf Data and Models to Plant Applications 

Abstract: 

Gravity and transferability of the penalty factors (F_en) endorsed for assessment of Environment Assisted Fatigue (EAF) in stainless primary circuit components have been contested for decades. Wide research efforts have not revealed the underlying mechanisms but observed reductions in endurance of laboratory specimens have been regression fitted as function of temperature and strain rate. In addition, the water environment is parametrized according to the amount of dissolved oxygen. Notable environmental effects in oxygen containing (low hydrogen) waters are reported at very low strain rates for steel batches sensitive for stress corrosion cracking. Otherwise, rather moderate F_en factors are predicted when DO>0.1ppm, e.g., in normal BWR chemistry, but removing the dissolved oxygen by addition of hydrogen for “hydrogen water chemistry” or PWR water makes a remarkable shift in predicted F_en factors.

Degradation of fatigue performance in hydrogen gas is widely studied, but the role of hydrogen in EAF of reactor piping steels is ignored. We suggest that hydrogen content in water might fit as an environmental parameter for stainless steels, even better than oxygen. Electrochemical reactions on the metal surface result to absorption of hydrogen and at temperatures round 300°C the rate of diffusion is sufficient to feed hydrogen in bulk of the specimen already during a week or two of presoaking typically required to stabilize the corrosion potential before starting an EAF experiment. We have confirmed the absorption and diffusion of hydrogen together with changes in cyclic response of typical stainless piping grades (304L, 347) in PWR water. These changes can be attributed to relative rates of strain (moving dislocations) and diffusion of hydrogen. Accelerated cyclic softening promotes localization of strain and initiation of fatigue cracks.

Plant operation experience and true strain – life data suggest that excessive conservatism was embedded in the environmental penalty factors (F_en per NUREG/CR-6909) determined through stroke-controlled testing. This paper reports our ongoing research aiming to develop improved F_en models which are compatible with the codified fatigue analyses. Coffin and Manson showed in 1954 how fatigue endurance is a function of plastic strain amplitude. We are confident that plastic strain and its rate are key factors also in EAF. Transferability of laboratory EAF results to fatigue assessment of primary circuit components can be improved through mechanism informed parametric modelling of environmental effects.

Presenting Author: Tommi Seppanen VTT Technical Research Centre of Finland Ltd.

Presenting Author Biography: To be added later.

Authors:

Tommi Seppanen VTT Technical Research Centre of Finland Ltd.
Jussi Solin VTT Technical Research Centre of Finland Ltd.