SUMMARY OF THE LOW-ALLOY STEELS SESSION – Chaired by Hans-Peter Seifert

This session on Thursday after lunch consisted of four presentations on EAC and high-temperature water effects on the fracture behaviour of low-alloy RPV steels as well as one talk about containment liner plate corrosion incidents in several Korean NPPs.

With the use of more SCC resistant alloys in LWRs such as high-Cr Alloy 690 and Alloy 152/52/52i weld metals, areas of intermediate susceptibility to SCC may become important in LTO in such welds like the dilution zone (first one or two passes of weld metal), the fusion line interface, or in the case of the presentation of Peter Andresen (Honorary Member, formerly GE-GRC, USA), the heat-affected zone (HAZ) of the low-alloy steel (LAS). The SCC crack growth behavior of the LAS HAZ close to the fusion line of Alloy 52/LAS welds was investigated under PWR primary water at 360°C close to the Ni/NiO boundary. In untempered welds whose hardness is clearly not representative of plant welds, rather fast SCC was observed in the HAZ at a rate of ~2·10-10 m/s. In a temper bead weld HAZ, sustained stationary SCC was observed under constant K conditions at a KI of 33 MPa·m1/2 over a period of 5500 h with a crack advance of 0.33 mm at a rate of ~2·10-11 m/s. Quite high local peak hardness values up to 350 HV were observed after temper bead welding. These results under reducing PWR conditions at low ECP are remarkable and clearly demonstrate the need for further investigations on the effect of yield stress on the SCC behavior of LAS (weld HAZs of homogeneous and dissimilar metal welds, irradiated RPV steels, CW carbon steel pipe bents, etc.).

Bob Carter (EPRI, USA) gave an update on the development of new SCC disposition curves for low-alloy RPV steels in BWR environment. Such disposition curves are needed for flaw tolerance evaluations, e.g., for the various SCC incidents in Alloy 182 shroud support and RPV instrument penetration welds and various RPV nozzles. Based on recent SCC lab test data under well-controlled conditions in high-purity and chloride containing water, major revisions to the BWRVIP-60-A SCC growth disposition lines were recommended in BWRVIP-233 (EPRI report no. 1019061) and subsequently in BWRVIP-233, Revision 1 (EPRI report no. 1022841). Based on the observed decay of SCC crack growth behavior in chloride-free (≤ 1 ppb) high-purity water at KI values below 70 MPa·m1/2, the SCC crack growth rates in short-term tests (< 2000 h) or experiments with PPU were extrapolated to a constant load period of 4380 h (corresponding to 2 transients per year) leading to a factor of 2 lower SCC rate of 10-11 m/s in the KI range below 55 MPa·m1/2. Furthermore, a SCC threshold at 15 MPa·m1/2 and a gradual increase of the SCC crack growth rates to the low- and high-sulphur lines of the GE model  above 55 MPa·m1/2 was introduced in NWC environment for low- (< 0.015 % S) and high-sulphur steels (≥ 0.015 % S), respectively. This transition starts at 75 MPa·m1/2 in case of HWC. Additionally, new SCC disposition curves were defined for NWC with chloride transients  (> 3 ppb) that are based on the GE high-sulphur line leading to very fast SCC during chloride transients. Due to the observed SCC behavior during chloride transients, no extra time has to be added to the transient period to consider potential delay time effects. Furthermore, the chloride tolerance is very high in HWC environment, and the high-purity water SCC curves can be applied up to at least 100 ppb of chloride. An ASME Code Case draft has been submitted for review and approval by the ASME BPV Committee. The basic document for the revisions, BWRVIP-233 Rev. 2, will be available as a public report by Q2 2018 and as an ASME BPV 2018 Conference paper.

Size effects and the SCC behavior of LAS in the high KI region were discussed in the presentation of Hans-Peter Seifert (PSI, Switzerland). EAC test validity criteria are usually based on pure mechanical considerations (e.g., small scale yielding (SSY) criteria for plane strain linear fracture mechanics) only, but in reality sub-critical EAC crack growth of LAS in HTW is synergistically controlled by environmental, material and mechanical parameters that govern the crack-tip strain rate and crack crevice chemistry and not by KI or DK per se. There are thus many mechanical (dK/da, constraint (loss)/stress state, LT creep), environmental (crevice chemistry, mass transport) or material (MnS) reasons for size, geometry and constraint effects and some of them may have opposite effects (e.g., violation of SSY in small specimens may increase crack-tip strain rate, but lower availability of MnS and shorter diffusion distance in small specimens may reduce sulphur concentration in the crack crevice environment) and even partially compensate each other. After a brief recapitulation of the EAC crack growth mechanism/model and control factors for EAC in LAS, several examples for the absence or unusual size effects (with faster EAC in bigger specimens) were given. Under conditions where high-sulpur crack crevice chemistry conditions prevail (e.g., high ECP and steel S content) and crack-tip strain rate is easily maintained (cyclic loading, high KI), i.e., when there are usually strong environmental effects and fast EAC, specimen size or constraints usually have very little effect and SSY limit can be significantly exceeded without change in crack growth rate. Tremendous effects may occur in the transition region from low-sulphur to high-sulphur EAC rates in a relatively narrow range only, where small changes can have a big impact. A special emphasis of the talk was placed to the SCC behavior at high KI-values that is of high practical significance, e.g., due to SCC in (highly constrained) Alloy 182 reactor internal attachment and RPV penetrations dissimilar metal welds or for highly stressed (and sometimes unclad) RPV feedwater nozzle corners (which are at high ECP in HWC also). Most of the underlying tests in this region involved moderate to severe violation of SSY in too small specimens. The experimentally observed transition of slow to fast SCC at high KI values is related to the difficulties to sustain SCC and maintain a crack-tip strain rate at KI < ~50 MPa·m1/2 and the mutual interrelation between crack growth rate and crack-tip strain rate and crack crevice chemistry and vice-versa. It was clearly shown, e.g., by the SCC crack growth with chloride, or in weld HAZ, in steels with high DSA susceptibility or in 2T C(T) specimens with high-sulphur steels, that the violation of SSY conditions is not the root cause for this transition, although this may affect the exact position of the transition region through, e.g., dK/da or LT creep effects. There is a clear practical need for more SCC tests in the high KI range from 40 to 100 MPa·m1/2 with sufficiently large specimens and high-sulphur steels to evaluate the role (and synergism) of

  • dK/da,
  • yield stress (weld HAZ, irradiation hardening, etc.) and DSA,
  • sulphur content,
  • very low chloride concentration

on the SCC transition behavior in this region to assess the real safety concerns with SCC.

Zaiqing Que (PSI, Switzerland) gave an update on his investigations on the effect of PWR and BWR environments on the fracture behavior of RPV steels. The special emphasis of the talk was placed to two low-sulphur steels with high DSA and one high-sulphur steel with high EAC, but low DSA susceptibility. In the RPV steel with an increased DSA susceptibility, a clear reduction of initiation and tearing fracture resistance was observed in high-temperature water in synergy with DSA. The toughness reduction was more severe at slower loading rates and at 288°C compared to 250°C. Hydrogenated water resulted in the strongest environmental effects on the fracture behavior. The localization of plastic deformation and increase in strength by DSA favour local hydrogen enrichment, which in turn can further amplify the localisation of plastic deformation (e.g., by the HELP mechanism). In the high-sulphur steel, on the other hand, the environmental reduction of fracture resistance was more pronounced under oxidizing BWR/NWC conditions, where the aggressive crevice chemistry leads to a higher hydrogen availability and uptake. Within his PhD project, a wide range of RPV steels (MnMoNi and CrMoV, forgings and hot-rolled plates) with different sulphur contents and EAC, DSA and temper embrittlement (TE) susceptibilities and of a simulated CG HAZ is evaluated by EPFM tests in the upper shelf region. These tests revealed the following important interim conclusions: All investigated materials showed stable ductile TG tearing by MVC in air and high-temperature water (HTW) with additional various and varying, but small amounts (a few %) of secondary cracking, macrovoids, quasi-cleavage and IG cracking in HTW. No unstable cracking and no change in the dominant fracture mechanism was observed in HTW. Only a moderate reduction in fracture initiation and tearing resistance occurred in both PWR and BWR environments. A high strength, DSA susceptibility or an aggressive occluded crevice chemistry (O2, S, Cl–, EAC) may cause a moderate reduction (< 50 %) in upper shelf toughness. In low-sulpur steels with high DSA susceptibility, the effect of DSA seems to dominate and results in a stronger reduction of fracture resistance in hydrogenated HTW. In high-sulphur steels with moderate DSA, the occluded crevice chemistry seems to dominate, which results in a stronger reduction in oxygenated HTW. At very slow strain rates, sub-critical SICC may occur, particularly in oxygenated HTW, that appears as an apparent environmental reduction of fracture resistance, but has to be clearly separated from environmental effects on the fracture behavior. The main reason for the moderate effects are the low hydrogen availability in HTW in connection with a very high density of various (fine-dispersed & strong) hydrogen traps in RPV steels. HELP and HESIV are the most probable mechanisms (with potential contributions of HEDE) for the observed moderate effects. Although moderate so far, the effects and concerns could be more critical for materials with low initial upper shelf toughness (e.g., high-sulphur steels) or with unfavorable combinations of material parameters (high S, DSA and EAC susceptiblity and increased strength). The ongoing investigations are focused on the identification of the underlying mechanism, the effect of temperature and the potential synergy with temper embrittlement in a high-P steel. The potential synergy with irradiation embrittlement will be evaluated in the new PSI LEAD project.

The last presentation in this session by Kyung-Hwan Na (KHNP, Korea) was a root cause analysis of containment liner plate corrosion incidents in some Korean NPPs. From destructive examination it was confirmed that the liner plate was penetrated by corrosion. The locations of corrosion were mainly distributed along the uppermost cold joint between concrete shell and dome. The international service experience and possible initiating causes for liner plate corrosion were discussed and summarized. Due to the high pH in concrete, the CS or LAS liner are usually in the passive and thus protected state. In most cases, a pH decrease and/or presence of contaminants like chloride are necessary for liner corrosion initiation. Thereafter, the performed failure analysis was discussed in detail. The material was in the proper specified condition. It was concluded that ingress of chlorides from marine atmosphere played a major role for passivity breakdown. Corrosion initiation occurred by typical crevice corrosion mechanism in the presence of chlorides during the construction phase. In the limited area at this specific location an active surface can sustain for a considerable time period after construction completion and corrosion may still proceed during operation, resulting in liner penetration. Finally, the applied maintenance process was briefly introduced.

Concluding remarks from the session chairman:

The possibility of fast SCC of many cm’s per year in RPV steels at low KI values in BWR/NWC represents a serious potential safety concern, but is usually overseen or even ignored and RPV steels are often (erroneously) regarded as inherently resistant to SCC.  Furthermore, there is new evidence that SCC might also occur under primary PWR conditions in weld HAZ materials of DMWs at low KI values. The good service record is primarily based on the stainless steel cladding and the high EAC initiation resistance from smooth surfaces, particularly under turbulent flow conditions. In fact, very fast SCC may occur in RPV steels in case of incipient cracks under conditions that are not very far away from typical operating and material conditions. The exact threshold conditions are not sufficiently known. Although service record is excellent and SCC appears as unlikely, we currently do not know enough to qualify SCC as a neglectable residual risk, in particular, in the context of the high SCC rates, LTO and SCC DMW incidents.

So far, tests were mainly done with good or optimal quality base materials and welds and rarely involved weld HAZs and interface regions (fusion region, dilution zones, HAZ). The quality of RPV material and welds may deviate from specifications and is not always as high as usually claimed or assumed for class 1 components, as shown by recent service experience (e.g., H-flakes, Al2O3 inclusions, carbon segregation, cladding defects, weld repairs, etc.). Many old (US) plants have RPV steels with high S content that were never covered in EAC test programs so far. Similarly, there are almost no EAC tests with irradiated or thermally-aged RPV steels or carbon piping steels with high DSA susceptibility (e.g., Si-killed high free N steels) or CW (pipe bents). There can be many synergies with related phenomena (e.g., between EAC, temper embrittlement, DSA and hydrogen embrittlement) that were not covered so far.

There are thus still several open and important practical issues in the context of the safe LTO of LWRs, e.g.:

  • SCC behavior in the transition region of Alloy 182/82 RPV attachment and penetrations welds. This is probably the biggest practical EAC concern for the RPV and it is stressed that the SCC rates in Alloy 182/82, although lower than in NWC, are still quite high in (moderate) HWC or PWR at higher temperatures and thus could potentially approach the fusion boundary during LTO;
  • SCC behavior of irradiated RPV steels and effect of yield stress;
  • effect of dK/da, YS, S and DSA on the transition of slow to fast SCC at high KI values;
  • HTW and hydrogen effects on the fracture behavior and potential interactions with irradiation and temper embrittlement, DSA or EAC.

Further investigations are thus highly recommended and encouraged for this more safety-related than field incident driven issue.