ML17228A409
| ML17228A409 | |
| Person / Time | |
|---|---|
| Issue date: | 02/22/2018 |
| From: | Mcginty T, Chris Miller Division of Construction Inspection and Operational Programs, Division of Inspection and Regional Support |
| To: | |
| Mensah, T.M. | |
| References | |
| TAC MF9558 | |
| Download: ML17228A409 (5) | |
Text
ML17228A409 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION OFFICE OF NEW REACTORS WASHINGTON, DC 20555-0001 Month XX, 2018 DRAFT NRC REGULATORY ISSUE
SUMMARY
2018-XX CLARIFICATION OF THE REQUIREMENTS FOR REACTOR PRESSURE VESSEL UPPER HEAD BARE METAL VISUAL EXAMINATIONS ADDRESSEES All holders of, or applicants for, a pressurized-water reactor (PWR) operating license or construction permit under Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic Licensing of Production and Utilization Facilities, and 10 CFR Part 52, Licenses, Certifications, and Approvals for Nuclear Power Plants, except those that have certified that they have permanently ceased operations and have permanently removed all fuel from the reactor vessel.
INTENT The U.S. Nuclear Regulatory Commission (NRC) is issuing this regulatory issue summary (RIS) to clarify the requirements for bare-metal visual examination, which can be either a visual examination of the bare metal of the upper head or a visual testing (VT)-2 examination under the insulation to meet the requirements of notes 1 and 4 in Table 1 of American Society of Mechanical Engineers (ASME) Code Case N-729-4, Alternative Examination Requirements for PWR Reactor Vessel Upper Heads with Nozzles Having Pressure-Retaining Partial-Penetration WeldsSection XI, Division 1. This RIS requires no action or written response on the part of an addressee.
BACKGROUND INFORMATION Pressure-boundary leakage of borated water from upper head penetrations can cause corrosion of the low-alloy steel, which can compromise the structural integrity of the head. PWR licensees are required by 10 CFR 50.55a(g)(6)(ii)(D) to augment their inservice inspection program to implement ASME Code Case N-729-4. This Code Case provides alternative examination requirements and acceptance standards for PWR reactor vessel upper heads with nozzles having pressure-retaining partial-penetration welds to those standards found in the ASME Code,Section XI, Table IWB-2500-1, Examination Category B-P, All Pressure Retaining Components. Code Case N-729-4 requires licensees to perform a visual examination of the reactor vessel upper head at an interval that depends on the material from which the head, penetration nozzles, and welds are fabricated.
In 2002, at the Davis-Besse Nuclear Power Station, Unit 1, a significant amount of reactor coolant leaked from above, obscuring the leakage through the nozzle and the resulting corrosion of the upper head.
Draft RIS 2018-XX Page 2 of 5 Two recent events illustrate how the requirements of Code Case N-729-4 can be misinterpreted by licensees.
In 2015, boric acid deposits, including boric acid at the annulus of several nozzles, were observed on the upper head of a PWR. In accordance with ASME Code Case N-729-4, Subsection 3142.1(b), the licensee is required to identify the deposits as a relevant condition of possible nozzle leakage. In this event, the licensee did not believe the deposits to be relevant conditions of possible nozzle leakage and cleaned the surface of the head using aggressive cleaning methods, without further analysis. Upon further consideration, the licensee recognized that because the deposits were removed, they could not conclusively attribute the leakage to a source other than the nozzle. Therefore, the licensee was required to meet the acceptance criteria for supplemental examination in Subsection 3142.2, Acceptance by Supplemental Examination, or those for repair or replacement in paragraph (b) of Subsection 3142.3, Acceptance by Corrective Measures or Repair/Replacement Activity.
A similar sequence of events took place at a different PWR in 2016 when boric acid deposits were detected by the licensee on the upper head. A large amount of boric acid leaked from above, covering several penetration tubes and lightly coating almost the entire upper head. The licensee performed the required supplemental examinations on the heavily affected nozzles, but the lighter boric acid deposits were not identified as relevant conditions of possible nozzle leakage in accordance with ASME Code Case N-729-4, Subsection 3142.1(b). The licensee cleaned the surface of the head using aggressive cleaning methods before performing subsequent visual examination, or other analyses needed, to determine the source of leakage.
SUMMARY
OF ISSUE Subsection 3141(c) defines Relevant conditions for the purposes of the VE include evidence of reactor coolant leakage, such as corrosion, boric acid deposits, and discoloration.
Subsection 3142.2 of ASME Code Case N-729-4 states, A nozzle with relevant conditions indicative of possible nozzle leakage shall be acceptable for continued service if the results of supplemental examinations [-3200(b)] meet the requirements of -3130. Alternatively, Subsection 3142.3(a) of ASME Code Case N-729-4 states, A component with relevant conditions not indicative of possible nozzle leakage is acceptable for continued service if the source of the relevant condition is corrected by a repair/replacement activity or by corrective measures necessary to preclude degradation. Therefore, correctly assessing whether the boric acid present constitutes possible nozzle leakage is critical to the determination of whether supplemental examinations are required.
Consistent with the definition of relevant conditions, areas of corrosion, boric acid deposits, or discoloration at or near a nozzle annulus in all cases, and irrespective of the material from which the affected components are fabricated, are relevant conditions of possible nozzle leakage according to ASME Code Case N-729-4, Subsection 3142.1(b). The affected components are unacceptable for continued service until the requirements of Subsections 3142.1(b)(1), (b)(2),
and (c) are met. An indication that the leakage may have come from a component not subject to the visual examination does not remove the possibility that some or all of the leakage possibly came from a nozzle. If the licensee fully removes the deposits using aggressive cleaning methods, such as high-pressure water/steam or power washing, an adequate examination and evaluation to determine the source of leakage is no longer possible.
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;
Draft RIS 2018-XX Page 3 of 5 however, relevant conditions (e.g., boron deposits) should be examined in the as-found condition. If the source of relevant condition of possible nozzle leakage cannot be determined by using light cleaning methods or by chemical analysis, either because the boric acid deposits were tightly adhered to the surface, or because evidence of the relevant condition was removed by aggressive cleaning methods, the requirements of ASME Code Case N-729-4, Subsection 3142.2 or Subsection 3142.3 must be met.
The NRC expects that licensees will review this information for applicability to their inservice inspection program.
BACKFITTING AND ISSUE FINALITY DISCUSSION This RIS provides clarification regarding the appropriate methods for bare-metal visual examination under ASME Code Case N-729-4, Alternative Examination Requirements for PWR Reactor Vessel Upper Heads with Nozzles Having Pressure-Retaining Partial-Penetration WeldsSection XI, Division 1. This RIS requires no action or written response beyond that already required by the NRC regulations. Therefore, this RIS does not represent backfitting as defined in 10 CFR 72.62(a) or 50.109(a)(1), nor is it otherwise inconsistent with any issue finality provision in 10 CFR Part 52. Consequently, for this RIS, the NRC staff did not perform a backfit analysis or further address the issue finality criteria.
FEDERAL REGISTER NOTIFICATION The NRC will publish a notice of opportunity for public comment on this draft RIS in the Federal Register.
CONGRESSIONAL REVIEW ACT Discussion to be provided in the final RIS.
PAPERWORK REDUCTION ACT STATEMENT This RIS does not contain new or amended information collection requirements that are subject to the Paperwork Reduction Act of 1995 (44 U.S.C. 3501 et seq.). Existing information collection requirements were approved by the Office of Management and Budget (OMB),
approval number 3150-0011.
Public Protection Notification The NRC may not conduct or sponsor, and a person is not required to respond to, a collection of information unless the document requesting or requiring the collection displays a currently valid OMB control number.
Draft RIS 2018-XX Page 4 of 5 CONTACT Please direct any questions about this matter to the technical contacts or the Lead Project Manager listed below, Timothy J. McGinty, Director Christopher G. Miller, Director Division of Construction Inspection Division of Inspection and Regional Support and Operational Programs Office of Nuclear Reactor Regulation Office of New Reactors Technical Contacts:
Greg Oberson, RES 301-415-2183 e-mail: Greg.Oberson@nrc.gov Stephen Cumblidge, NRR 301-415-2823 e-mail: Stephen.Cumblidge@nrc.gov Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under NRC Library/Document Collections.