SSRT for Hydrogen Water Chemistry Verification in BWRs

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The slow strain rate test (SSRT) has been used as a laboratory tool to investigate the susceptibility of austenitic stainless steels and other materials to intergranular stress corrosion cracking (IGSCC). During operation, IGSCC has occurred in the recirculation piping of Boiling Water Reactors (BWRs). The SSRT has demonstrated that weld or thermally sensitized T-304 stainless is susceptible to IGSCC in simulated BWR environments. On the other hand, it has also been demonstrated that if the BWR environment can be modified, such that the electrochemical potential (ECP) is lowered sufficiently, IGSCC can be mitigated. A data base was developed indicating that the protection potential for T-304 stainless steel is ⩽-0.230 V(SHE). In application to operating BWR power plants hydrogen is injected into the reactor feedwater system, which eventually decreases the ECP in the recirculation system. The effect in the recirculation piping water is measured by delivering water from the piping to an external test system. The test system often consists of two autoclaves, one of which measures the ECP, and the second where SSRT is performed. Typically, highly sensitized T-304 stainless steel is tested in the SSRT autoclave before hydrogen addition. In the normal BWR water chemistry (∼270°C, 200 ppb O2) IGSCC occurs usually after >100 h of testing. Subsequently, a second SSRT is performed when the target ECP is achieved in the BWR water, now modified with hydrogen. This second SSRT verifies that indeed IGSCC mitigation has been achieved. Results from a number of operating BWRs show that ECP control verified by SSRT provides reactor operators with criteria for IGSCC control.