Evaluating the reliability of PWSCC resistance of TT Alloy 690 and associated welds to the end of anticipated PWR plant life

Abstract

In order to evaluate the primary water stress corrosion cracking (PWSCC) resistance and possible SCC growth rate (SCCGR) for thermally treated (TT) Alloy 690 to the end of anticipated PWR plant life, including various levels of cold work and associated welds, long-term PWSCC initiation tests using reverse U-bend specimens and SCCGR measurement tests were conducted at 360 °C in simulated PWR primary water. In addition, the possibility of long-range ordering (LRO) and/or short-range ordering (SRO) was evaluated by micro-Vickers hardness measurements, electron diffraction analysis and nano-scale EDS by high resolution TEM. The studied alloys were TT Alloy 690 and a Ni-33at%Cr model alloy after long-term ageing in air at 360, 400, 420 or 475 °C. No SCC was detected on any of the reverse U-bend specimens after testing for up to 45,000 h at 360 °C in simulated PWR primary water. Moreover, SCCGRs for the as-received TT Alloy 690 and with various levels of cold work or its associated welds did not increase when tested at 360 °C in simulated PWR primary water after thermal ageing at 360, 400, 420 or 475 °C in air for up to ∼20,000 h. In the case of the Ni-33at%Cr model alloy, LRO/SRO was clearly identified by electron diffraction analysis after thermal ageing at 475 °C in air. However, no evidence of LRO/SRO was observed in the case of the as-received TT Alloy 690 or after various levels of cold work or in its associated welds by any of the test methods after long-term thermal ageing in air at any temperature. From these test results, it is concluded that TT Alloy 690 and its associated welds in real components of PWRs have extremely high SCC resistance and extremely low SCCGRs should PWSCC be initiated, up to the end of any currently anticipated PWR plant life time.