Irradiation assisted stress corrosion cracking of 347 stainless steel in simulated PWR primary water containing lithium hydroxide or potassium hydroxide

Abstract

The susceptibility of type 347 stainless steel to irradiation assisted stress corrosion cracking (IASCC) in pressurized water reactor (PWR) primary water with two water chemistries was evaluated. Miniature four-point bend tests were used to assess the effect of potassium hydroxide (KOH) in comparison to lithium hydroxide (LiOH) on the stress to initiate IASCC. Four-point bend specimens were machined from baffle-former bolts removed from the Salem unit 1 and D.C. Cook unit 2 reactors with accumulative doses ranging from 7.8 to 26.4 dpa. It was found that the pseudo-threshold stress to initiate IASCC in this alloy was 50–60% of the irradiated yield strength irrespective of water chemistry. Intergranular cracking was the dominant fracture mode in all specimens and the fracture morphology was characterized by a tortuous fracture path through the oxide and was common to both water chemistries. Grain boundary oxidation combined with localized stress enhancement at deformation band – grain boundary sites were precursors for IASCC initiation in both water chemistries. The pseudo-threshold stress for IASCC initiation is in good agreement with O-ring test results on both 316SS and 304SS. Overall, the stress to initiate cracks in neutron-irradiated 347SS was indistinguishable between simulated primary water containing LiOH or KOH.