ML23067A324

Presentation - Code Case N-894 Alternative Rules for Repair of Classes 1, 2, and 3 Austenitic Stainless Steel Piping with Thermal Fatigue Cracking

ML23067A324
Person / Time
Issue date:	03/07/2023
From:	Marlette S, Jeffrey Poehler NRC/RES/DE, Westinghouse
To:
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Download: ML23067A324 (1)

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CODE CASE N-894 ALTERNATIVE RULES FOR REPAIR OF CLASSES 1, 2, AND 3 AUSTENITIC STAINLESS STEEL PIPING WITH THERMAL FATIGUE CRACKING Record Number 17-3243 Stephen Marlette, P.E. Westinghouse Electric Company 1

Presentation Overview

• Background

• Sections of Case N-894

- 1000 Scope

- 2000 Prerequisites

- 3000 Flaw growth analysis and design

- 4000 Examination and Testing

• -4100 Acceptance Examination

• -4200 Inservice Examination

• -4300 Pressure Testing

- Table 1 Inservice Examination Categories 2

Background and Purpose

• Thermal fatigue cracking is a potential issue at nuclear plants within normally stagnant lines connected to the reactor coolant system.

• Recent cases have led to repairs using structural weld overlays (SWOL)

• MRP-146 provides guidance to identify susceptible locations in PWRs and recommends examinations

• Current ASME SWOL repair documents like N-504-4, N-740-2 and ASME Section XI Appendix Q were written to address stress corrosion cracking (SCC) not fatigue cracking New Code Case needed for SWOL repair of thermal fatigue cracking Work began on new Case in 2017.

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-1000 Scope

• Case is intended to apply to stainless steel piping in both PWRs and BWRs

• The Code Case mainly addresses thermal fatigue cracking

• The Case also requires consideration of SCC crack growth when uncertain e.g.,

- Stagnant stainless steel lines in PWRs

- BWR locations where hydrogen water chemistry may change

• The repairs rules apply to both stainless steel welds and base metal (N-504 and Appendix Q focus on weldments only)

Consideration of SCC was added to cover situations where the potential is unknown to address NRC Staff concerns.

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-2000 PREREQUISITES

• The overlay material must be SCC resistant for the applicable environment

• SWOL material may be low carbon (0.035% max) stainless steel

• Alternatively nickel alloy with at least 26% Cr may be used for PWRs and at least 20% Cr for BWRs

- Higher yield strength of nickel alloy can provide more compression with less weld metal

• For BWR stainless steel SWOL applications an as-deposited delta ferrite content of at least 7.5 FN is required

- Alternatively, the first layers of at least 5 FN are acceptable, provided the carbon content of the deposited weld metal is determined by chemical analysis to be less than 0.02%.

SCC resistant material is required for the SWOL 5

-3000 FLAW GROWTH ANALYSIS AND DESIGN

• Full structural weld overlay (FSWOL) design is required

- Flaws in the original wall is assumed to 100% through-wall for sizing

• The as-found flaw size is analyzed for fatigue crack growth (FCG) to determine the design life of the FSWOL

• If SCC is uncertain then SCC crack growth is included in the analyses

- If SCC rates for material and environment is not available in Section XI then experimental data may be used for crack growth curve

• The FSWOL is centered over the flaw rather than the weld as in N-504, N-740 and Appendix Q since thermal fatigue flaws could be entirely in base metal

• Maximum 30° end slope unless specifically analyzed FSWOL Max 30° End Slope

• FSWOL design

• FCG is the main analysis required

• SCC is required if uncertain As-found flaw Overlay centered over flaw 6

-4100 ACCEPTANCE EXAMINATION

• Only the structural volume (G-H-I-J) needs to be examined for planar and laminar flaws

• The non-structural portion of the overlay (C-G-J-F and H-D-E-I) only needs to be examined for laminar flaws that could obstruct UT coverage

• PT is also required for overlay and adjacent material

• First approved in August 2020 @ WGPQVE and SGNDE.

• Currently Ballot 23-512 is out to WGPQVE and SGNDE for review and approval.

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-4100 (g) ADJACENT MATERIAL EXAMINATION

• UT examination of adjacent material is unique to N-894 (Figure -4200-1)

- Verifies flaws were not opened after overlay

- Sets baseline for future exams

- Exam distance set to 2Rt and 1/3 t from ID surface

- Sentinel location for future thermal fatigue flaws

• Adjacent material exam is unique to N-894 8

-4200 INSERVICE EXAMINATION

• Inservice exam volume of outer 25% of original material (Figure -4100-2)

• Adjacent Material also examined (Figure -4200-1) 9

-4200 INSERVICE EXAMINATION (Cont.)

• UT exam required during first refueling outage after installation of overlay (more restrictive than Appendix Q)

• Acceptance standards for planar flaws are in Table IWB-3514-1 for Class 1 piping and IWC-3514-1 for Class 2 and 3

• Alternatively acceptance by analysis per IWB-3600, IWC-3600, or IWD-3600 for Class 1, 2 or 3 piping

• If flaw growth greater than predicated in -3000 analysis is detected the overlaid pipe must be repaired

• If no flaw growth or new flaws are detected the examination schedule follows the inspection plan per IWB-2411, IWC-2411, or IWD-2411 applicable to class of piping (different schedule for >75% and <75% flaws)

• NRC inputs incorporated for exam frequencies 10

-4200 INSERVICE EXAMINATION (Cont.)

• If new flaws meeting acceptance standard or flaw growth within the predicted rate per -3000 analysis are detected, the overlay must be examined during next outage

- Examinations shall continue every outage until no growth is detected

• If the source of fatigue cracking has not been identified, the examination of -4200 shall be performed during subsequent refueling outages until the source is identified More restrictive inservice acceptance criteria and schedule than N-504-4 or Appendix Q to guard against unpredicted crack growth 11

-4300 PRESSURE TESTING

• Pressure testing shall be conducted in accordance with IWA-4540. Pressure testing is required only when flaws are through-wall or become through-wall during weld operations.

Same pressure testing requirements asSection XI, Appendix Q, -4400 12

Table 1 Inservice Examination Categories Table 1 was added per NRC Staff Comments (July 2021) 13

Questions/Comments/Discussion 14