X-ray absorption study of oxide film formed on Alloy 600 in PWR primary water at 290°C with a decreased concentration of dissolved hydrogen

ABSTRACT

Primary water stress corrosion cracking (PWSCC) of Alloy 600 remains an important degradation phenomenon in PWRs. In particular, the susceptibility to the PWSCC initiation at 290°C is of recent interest because that is one of the typical temperatures of the primary water in contact with Alloy 600 which remains in PWRs. In this paper, oxide film formed on Alloy 600 in simulated primary water at 290°C was analyzed by X-ray absorption fine structure (XAFS) measurements at the Fe K-edge. For the preparation of the oxide film, Alloy 600 was immersed in simulated primary water with dissolved hydrogen (DH) concentration of 5 or 30 ml/kg-H₂O. The near edge region of the XAFS suggested that the oxide film for 5 ml/kg-H₂O DH had a larger Fe³⁺/Fe²⁺ ratio than for 30 ml/kg-H₂O DH. Additionally, the extended region of the XAFS revealed that Fe³⁺ mainly occupied the octahedral site of the spinel oxide. Previous reports indicate that an Fe³⁺-rich spinel oxide is less defective and protective against PWSCC initiation. Therefore, our results suggest that the lower DH concentration (5 ml/kg-H₂O) could potentially mitigate the susceptibility of Alloy 600 to the PWSCC initiation at 290°C.

KEYWORDS:

• PWR type reactor
• Alloy 600
• oxide film
• water chemistry
• dissolved hydrogen
• XAFS
• stress corrosion cracking
• PWSCC

Disclosure statement

No potential conflict of interest was reported by the author(s).