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Irradiation-assisted stress corrosion cracking (IASCC) is a significant materials issue for the light water reactor (LWR) industry and may also pose a problem for fusion power reactors that will use water as coolant. A new metallurgical process is proposed that involves the radiation-induced release into solution of minor impurity elements not usually thought to participate in IASCC. MnS-type precipitates, which contain most of the sulfur in stainless steels, are thought to be unstable under irradiation. First, Mn transmutes strongly to Fe in thermalized neutron spectra. Second, cascade-induced disordering and the inverse Kirkendall effect operating at the incoherent interfaces of MnS precipitates are thought to act as a pump to export Mn from the precipitate into the alloy matrix. Both of these processes will most likely allow S, which is known to exert a deleterious influence on intergranular cracking, to re-enter the matrix. To test this hypothesis, compositions of MnS-type precipitates contained in several unirradiated and irradiated heats of Type 304, 316, and 348 stainless steels (SSs) were analyzed by Auger electron spectroscopy. Evidence is presented that shows a progressive compositional modification of MnS precipitates as exposure to neutrons increases in boiling water reactors (BWRs). As the fluence increases, the Mn level in MnS decreases, whereas Fe level increases. MnS precipitates were also found to be a reservoir of F, an impurity implicated with cracking of F-contaminated shielded-metal- or submerged-arc welds such as BWR core shroud weld.
austenitic stainless steel, irradiation-assisted stress corrosion cracking, manganese sulfide, transmutation, compositional instability, sulfur, fluorine
Metallurgist, Argonne National Laboratory, Argonne, IL
Engineering Specialist, Argonne National Laboratory, Argonne, IL
Senior Scientist, Pacific Northwest National Laboratory, Richland, WA