ML23054A057
| ML23054A057 | |
| Person / Time | |
|---|---|
| Issue date: | 01/20/2023 |
| From: | Jay Collins NRC/NRR/DNRL/NPHP |
| To: | Matthew Mitchell NRC/NRR/DNRL/NPHP |
| References | |
| Download: ML23054A057 (4) | |
Text
US Nuclear Regulatory Commission (NRC) Staff Comments on Proposed Changes to Bare Metal Visual Acceptance Criteria for American Society of Mechanical Engineers Boiler and Pressure Vessel Code Case N-729-10, Alternative Examination Requirements for PWR Reactor Vessel Upper Heads With Nozzles Having Pressure-Retaining Partial-Penetration Welds,Section XI, Division 1 The current bare metal visual (BMV) acceptance criteria for the American Society of Mechanical Engineers Boiler and Pressure Vessel (ASME) Code Case N-729-6, as mandated by Title 10 of the Code of Federal Regulations Part (10 CFR) 50.55a(g)(6)(ii)(D), with conditions, is to ensure an effective BMV examination of each penetration nozzle to reactor pressure vessel upper head (RPVH) for indications of nozzle leakage. If an effective BMV cannot be performed due to relevant conditions indicative of possible nozzle leakage, subsequent volumetric examinations of the penetration nozzle are to be performed to identify any leakage from the penetration nozzle or associated partial penetration weld. The NRC has authorized proposed alternatives to the volumetric examination requirements through 10 CFR 50.55a(z)(2) on a hardship basis after NRC staff individual case review of the licensees BMV results and conclusion that any remaining relevant conditions have a reasonable assurance of not being indicative of nozzle or weld leakage. Further, additional defense-in-depth activities were considered in the NRC staffs assessment that reasonable assurance of the structural integrity of the RPVH would be maintained during the subsequent cycle of operation, when further examinations could be performed.
The ASME staff proposed changes to the BMV acceptance criteria of ASME Code Case N-729-10, which is identical to that of ASME Code Case N-729-6, into a proposed N-729-11.
Through N-729-11, the code case authors seek to provide a clear process for evaluating the BMV relevant conditions and apply what the code case authors consider a more reasonable standard for weighing the evidence. The authors of the current proposed changes utilized the NRC authorized alternatives in the development of their proposal. However, the NRC staff has several concerns regarding the proposed changes. The primary concerns can be summarized as follows.
- 1. The ambiguity created by the use of the reasonable confidence standard versus the relevant condition of possible nozzle leakage standard,
- 2. The allowance of masked conditions for RPVHs utilizing Alloy 600 materials to not require volumetric examinations,
- 3. The proposed assessment of relevant conditions not being developed within the Code Case itself, and the process highlighted in the technical basis document being open to various interpretations,
- 4. Repeated application of the masked condition to defer effective visual examination,
- 5. The allowance of the term essentially 100% for examination coverage to consider masking deposits as obstructions in Note 1(f) of Table 1, and
- 6. Allowing superficial discoloration or superficial deposits in the annulus region of the nozzle and RPVH penetration location to not be evaluated for consideration as relevant conditions.
Reasonable Confidence Issue The NRC staff notes the use of reasonable confidence is a lower bar than the current regulatory requirement to ensure the relevant condition is not indicative of possible nozzle leakage.
Further it is not well defined in the proposed N-729-11 or Section 3.0 of the technical basis document, Technical Basis for Proposed ASME Code Case N-729-11 with Revised VE
2 Acceptance Criteria (-3140), November 14, 2022. From the technical basis document, the use of the reasonable confidence bar is justified, in part, based on the condition of a BMV, being performed in the next refueling outage, as sufficiently timely to avoid a consequential challenge to nuclear safety. While the NRC staff agrees this could be a condition in some cases, see the previously authorized proposed alternatives for examples, this is not true in all cases. The potential allowable condition could be an active leak, of undetermined age. Further, there remains uncertainty in the timeline of any possible boric acid corrosion event of the RPVH. The NRC staff does not see the preferred regulatory approach to allow the possibility of an active leak from the reactor coolant pressure boundary due to an inadequate BMV examination and removal of the requirement for a volumetric examination during the same outage, without NRC review.
The NRC staff would prefer the ASME Code Case N-729 to more clearly define the process to analyze a relevant condition to determine if it is indicative of possible nozzle leakage. This process may be similar to the current proposed changes to evaluate the relevant conditions, but with modified criteria. These criteria are expected to be similar to those described in Section 3.0 of the technical basis document and expected examples within a future revision of EPRI MRP-60, Materials Reliability Program: Visual Examination for Leakage of PWR Reactor Vessel Upper Head Nozzles.
The NRC staff also finds the use and methodology of reasonable confidence will lead to regulatory uncertainty for licensees and a debate about adequate scoring of a masking condition as provided in Table 1 of the technical basis document. Ideally the code case should be clear in the requirements to ensure the most effective BMV can be performed and evaluated with defined acceptance criteria. Finally, the NRC notes that the term reasonable confidence is utilized by the NRC in the development of 10 CFR 50.69 in the application of low safety significance items. This rule is specifically restricted from the use on inservice inspection and repair replacement activities on ASME Class 1 structures, systems and components, of which the RPVH is categorized.
The NRC staff recognizes that use of the reasonable confidence term is a cornerstone of the proposed changes for ASME Code Case N-729. However, the proposed evaluation criteria could be utilized to define the acceptance criteria for relevant conditions that are not indicative of possible nozzle leakage. This is NRC staff recommended path forward.
Masked Condition use on RPVH with Alloy 600 Penetrations The NRC staff notes that the changes identified -3142.1(c) allow for the disposition of a relevant condition to be a masking condition with reasonable confidence that the indication is not the result of head penetration leakage. Further, N-729-11 defines a head category of reduced concern. As noted in the discussion on reasonable confidence, the technical basis document bases this change, in part, based on the condition of a BMV, being performed in the next refueling outage, as sufficiently timely to avoid a consequential challenge to nuclear safety. The NRC staff finds this position provides reasonable assurance for RPVH utilizing Alloy 690 head penetration nozzle and weld materials due to the extensive primary water stress corrosion cracking initiation and crack growth rate testing performed by Industry and the NRC. However, for RPVH utilizing Alloy 600 head penetration nozzle and welds, there are no effective changes from the current required examination frequency and leakage monitoring controls, while allowing the exemption of currently required volumetric examination of the specific penetration nozzle with a relevant condition. The current regulatory requirements ensure either an effective BMV examination or a volumetric examination is performed to verify no indication of reactor coolant
3 pressure boundary leakage. Operating experience has shown that cold leg temperature Alloy 600 components and welds have increased cracking as they continue to age in operating time.
Additionally, as noted previously there is uncertainty in the corrosion timeline for the RPVH. In the case of peened RPVH penetration nozzles and welds, the BMV examination requirement each refueling outage is a cornerstone defense-in-depth measure specifically conditioned by the NRC to allow the extended volumetric examination frequency. The NRC staff notes that effective BMV examinations are required for these components to maintain the currently approved volumetric examination frequencies. NRC staff does not believe reliance on the potential for leakage detection alone to identify reactor coolant pressure boundary leakage for a masked nozzle as defined by the current proposed changes in N-729-11 to be adequate without prior NRC approval on a case by case basis.
However, the NRC staff does see a path forward to better define the options to evaluate relevant conditions to determine if they are indicative of possible nozzle leakage. As included in Note 1(d) of Table 1, use of pressurized air can be included. NRC staff note that only low pressure air should be used, and that pressure should be defined. Further, the criteria of -3142.1(b)(1)(-b) should be better defined in an accessible and publicly available way to provide regulatory clarity for the licensee and NRC inspectors. The proposed discussion of these items in the future revision of MRP-60 would be useful for the NRC staff to gain confidence in proposed changes.
Proposed Assessment of Relevant Conditions As noted above, the NRC staff sees significant value in developing and clarifying the methodology to assess relevant conditions to determine if they are indicative of possible nozzle leakage. As provided in NRC Regulatory Issue Summary 2018-06, Clarification of the Requirements for Reactor Pressure Vessel Upper Head Bare Metal Visual Examinations, While Subsection 3142.1(b)(1) requires the licensee to determine the source of leakage, the methodology for identifying the source of leakage is not specified in ASME Code Case N-729-4. It is the NRCs position that relevant conditions (e.g., boron deposits) should be examined in the as-found condition. If leakage through a nozzle cannot be excluded by examining the as-found condition of the relevant conditions at a nozzle annulus (e.g., by assessment of boron deposit tenacity using light cleaning methods or by boron deposit chemical analysis), the requirements of ASME Code Case N-729-4, Subsection 3142.1, Subsection 3142.2, or Subsection 3142.3(b) must be met.
However, the limited language currently in the proposed code case is insufficient to provide that clarity. If a reference document is used, it should be cited and publicly available.
Repeated Application of Masked Condition The NRC staff notes that there is no strict restriction on the use of the masked condition to exempt volumetric examination. This could allow an area of the head or a penetration nozzle(s) to have repeated leakage from other sources masking the inspection and preventing an effective BMV to be performed for multiple cycles of operation. While this item is listed as a category for consideration under -3142.1(b)(1)(-b)(-6), it is only listed as an item of consideration and not an automatic disqualification for the deferral of the volumetric examination. NRC staff finds a subsequent masked condition in a follow-up BMV examination should require a supplemental volumetric examination to verify no leakage from the penetration nozzle or weld.
4 Essentially 100% Examination Coverage The NRC staff finds the proposed change to include Note 1(f) of Table 1 of proposed ASME Code Case N-729-11 could be interpreted to imply that the area of missed coverage could be material that should be considered a masked condition. The NRC staff finds this position unacceptable as all relevant conditions should be evaluated.
Superficial Discoloration and Superficial Deposits The NRC staff finds the clarification proposed for superficial discoloration and superficial deposits having no depth is a reasonable clarification for evaluating relevant conditions as being indicative of possible nozzle leakage. However, in the area of the annulus, the NRC staff questions the ability to verify the depth of discoloration that might be indicative of corrosion or the sizing of deposits without their removal and evaluation. Therefore, the NRC staff recommends clarification of this option be limited to outside the annulus region between the penetration nozzle and RPVH surface.