Radiation-Induced Instability of MnS Precipitates and its Possible Consequences on Irradiation-Induced Stress Corrosion Cracking of Austenitic Stainless Steels

Abstract

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, cascadeinduced 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 sulfur, 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. A s the fluence increases, the Mn level in MnS decreases, whereas the Fe level increases. The S level also decreases relative to the combined level of Mn and Fe. MnS precipitates were also found to be a reservoir of other deleterious impurities such as F and 0, which could be also released due to radiation-induced instability of the precipitates.

Assessment of XM-19 as a Substitute for AISI 348 in ATR Service

“…This process has been proposed as one contribution to irradiation-assisted stress corrosion cracking observed in 300 series steels in water-cooled reactors. 14,15 The stability of MnS precipitates is also strongly influenced by a phenomenon designated the Inverse-Kirkendall effect. 16 This is a radiation-driven process that strongly segregates elements which diffuse by a vacancy exchange mechanism.…”

“…This provides another driving force to dissolve MnS by pumping Mn away from MnS precipitates that are also becoming rich in Fe via transmutation. 14,15 In the modern production of steels, the concentration of S and various halides are usually much lower, so the problem of MnS dissolution is not as pronounced. As will be shown later, the Inverse Kirkendall effect strongly changes the composition at grain boundaries with some potential consequences of concern to the current study.…”