Investigation of the protection potential against IASCC

Environmentally induced cracking of components exposed to the radiation and high-temperature aqueous environment of a light water reactor (LWR) is known as irradiation-assisted stress corrosion cracking (IASCC). In boiling water reactors (BWRs) annealed austenitic stainless steels have a threshold level of irradiation damage. Above a fluence level of 5 x 10{sup 20}n/cm{sup 2}, these alloys are susceptible to cracking in the normal BWR core environment. Below this threshold value, laboratory stress corrosion tests of irradiated material have not indicated cracking over a range of corrosion potentials. Recent electrochemical measurements in the core bypass region of several operating BWRs have indicated that the corrosion potentials of Type 304 stainless steel can be as high as 0.250 {+-} 0.025 V (SHE). The potential depends on the specific measurement location. Testing indicates that at high potentials Type 304 stainless steel irradiated above the threshold fluence suffers IASCC, but, if the potential can be decreased, the phenomenon does not occur. An effective environmental modification for eliminating cracking of sensitized Type 304 stainless steel in BWR piping systems is the addition of hydrogen to the reactor feedwater. Hydrogen injection results in a decrease in the electrochemical potential (ECP). Below -0.230 V(SHE), pipe cracking known asmore » intergranular stress corrosion cracking (IGSCC) is effectively mitigated. The identification of a threshold potential for IGSCC was established by performing laboratory stress corrosion tests on thermally sensitized Type 304 stainless steel using the constant extension rate technique (CERT) at controlled ECPs. Similar logic was applied to the experimental program described in this paper.« less