Comment: It is apparent that the panel has not given appropriate credit to the IGSCC mitigation measures for the NSSS stainless steel piping that the BWR owners have implemented since the early 1980s. For example, the second paragraph from bottom on page xvii states, in part: “.. .the biggest frequency contributors for each LOCA size tend to be systems having the smallest pipes, or component, which can lead to that size LOCA. The exception to this general rule is the BWR recirculation system, which is important at all LOCA sizes due to lingering IGSCC concerns.” Since the largest pipe size in the recirculation piping system can be up to 28-inches, the preceding statement essentially implies that LB LOCA redefinition is not applicable to BWRs. The panel did not seem to give adequate credit for several effective mitigation measures in terms of better material (e. g., use of nuclear grade stainless steel in replacement lines), stress improvement (e. g., induction heating stress improvement [IHSI], last pass heat sink welding [LPHSW], and mechanical stress improvement process [MSIP]) and water environment (e. g., hydrogen water chemistry [HWC]) and repair measures such as the weld overlays and elimination of creviced geometries. The panel apparently did not consider the report GE-NEA41-00110-00-1, Rev. 0, A Review of NUREG/CR-5750 IGSCC Improvement Factor and Probability of Rupture Given a Through — Wall Crack, provided to NRC by the BWROG on April 25, 2002 (ADAMS Accession NO. ML021210417). In addition, the panel did not recognize the contribution of BWRVIP-75, which provides evidence that IGSCC is effectively managed at BWRs and provides the basis for revising examination frequencies. We consider these significant oversights, given their relevance to the panel’s conclusions. On the other hand, the panel did accept the future effectiveness of mitigating measures for PWSCC issue for the PWR small diameter piping (p. 6-5) in reducing failure rates for this piping. The NUREG should provide similar credit for the BWR IGSCC mitigation measures noted above with regard to break frequencies.

Response: The authors disagree with the contention that the panel has not given appropriate credit to the IGSCC mitigation measures for the NSSS stainless steel piping that the BWR plants have implemented since the early 1980s. The report referenced above (GE-NEA41-00110-00-1) was provided as background documentation to the peer review panel. Additionally, several of the panelists had extensive experience with the assessment of the IGSCC issue and the development of appropriate mitigation strategies. One of the BWR base cases specifically investigated the failure probability of primary recirculation piping due to IGSCC. The base case model plant was assumed to follow the Generic Letter 88-01 inspection technique, used weld overlay to reinforce the flawed piping, and utilized normal water chemistry.

This base case definition was chosen for convenience to improve the expected accuracy of the PFM analysis to be conducted by only considering one easily modeled mitigation strategy, i. e., weld overlay. It was well recognized and stressed during meetings that this base case was generally not representative of BWR plants. It was noted that most BWR plants employ hydrogenated water chemistry and may employ mitigation strategies other than weld overlays. Additional sensitivity analyses of the base case results were conducted to evaluate the effectiveness of BWR mitigation strategies for IGSCC. These sensitivity studies are summarized in Sections 4.3.3 and 4.3.4 of NUREG-1829, while more detail is provided in Appendices D — F. These results were presented to the panelists during the base case review meeting and copies were available to support the preparation of their individual elicitations. This information, the background information provided, and the experience of individual panelists was sufficient to ensure that the panelists were sufficiently informed about the effectiveness of IGSCC mitigation so that it was properly credited during the elicitation.

However, although the mitigation has been effective in reducing the associated LOCA frequencies, there is still risk associated with failure of BWR systems containing pre-existing flaws. As summarized in Section 6.3.2 of NUREG-1829, “The panel consensus is that the susceptibility of BWR piping systems to IGSCC is greatly reduced compared to what it was in the past. Measures such as improved HWC, weld overlay repairs, stress relief, and pipe replacement with more crack resistant materials have led to this reduction. Inspection quality has also improved such that the probability of crack detection is much better than in the past. However, as indicated earlier, there remains concern about the failure likelihood of the large recirculation piping and the RHR lines that have not been replaced. The original piping materials are much more susceptible to IGSCC and many lines retain preexisting cracks that initiated and grew before HWC was adopted.”

Additionally, at least one panelist was also concerned that the more IGSCC-resistant replacement piping materials may still crack under service conditions. This panelist cited the German plant experience with cracking in Type 347 stainless steel. Another panelist raised the possibility that cold work (e. g. due to grinding) could increase the IGSCC susceptibility of the low carbon (L grade) stainless steel that has been used as a replacement material in many U. S. plants. However, the U. S. BWR experience with L grade stainless steel piping was widely recognized by the panel as being very good thus far. For these reasons, many panelists believe that continued vigilance is required through the augmented inspection requirements in Generic Letter 88-01 and NUREG-0313.

Key elements of this response have been used to modify the Executive Summary and Sections 3.5, 4.3, and 6.3 of the revised NUREG.

Comment Number: ES3