Darlington NGS 'A' - Outlet Feeder Dissimilar Metal Weld Leak-Before-Break Assessments - Update 2

Transcription

1 ONTARIOFiiiiiER GENERATION Brian Duncan Senior Vice President Darlington Nuclear P.O. Box 4000 Bowmanville, ON L1 C3Z8 Tel: Fax: November 27,2012 NK P Mr. P.A. Webster Director, Darlington Regulatory Program Division Canadian Nuclear Safety Commission P.O. Box Slater Street OlTAWA, Ontario K1P 5S9 Dear Mr. Webster: Darlington NGS 'A' - Outlet Feeder Dissimilar Metal Weld Leak-Before-Break Assessments - Update 2 The purpose of this letter is to provide CNSC staff with the second update of the project activities associated with the Leak-Before-Break (LBB) assessments on Darlington feeder Dissimilar Metal Welds (DMW) regarding deterministic and probabilistic assessments as submitted in Reference 1. The update was also presented to CNSC Operational Engineering Assessment Division specialists and Licensing Staff at the meeting held on October 25, 2012 in Ottawa. As documented in Attachment 1 COG report, "Interim Deterministic Leak-Before-Break Assessment of DMWs in DNGS Outlet Feeders", sufficient margins on LBB in the base case are established for 40 out of 44 outlet feeders. The design loads at the flow elements are used together with an operational leak rate limit of 25 kg/h and a factor of five on detectable leak rate to account for the uncertainty in the leak rate calculation. Should the type of leaking through-wall crack studied in this report develop during an accident load, the crack growth due to the accident loading is not significant and could be detected with existing leakage detection capability and the reactor will be safely shutdown according to the operating procedure. The consequential leakage from the crack is bounded by the existing safety analysis. As documented in Attachment 2 COG report, "Probabilistic Leak-Before-Break Assessment of DMWS in DNGS Outlet Feeders", the probabilistic LBB assessment was carried out for 15 feeders, selected from Attachment 1 deterministic asse~sment and Ontario Power Generation Inc., This document has been produced and distributed for Ontario Power Generation Inc. purposes only. No part of this document may be reproduced. published, converted, or stored in any data retrieval system, or transmitted in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written of Ontario Power Generation Inc. Associated with OPG-STD-0031, Correspondence Standards OPG-TMP-0007-R004 (Microsoft 2007)

2 Mr. P.A. Webster November 27, 2012 including the four feeders with reduced margin. The assessment has demonstrated that DMW rupture is a very low probability event. The calculated rupture frequency is significantly low and shall meet CANDU feeder pipe acceptable rupture frequency currently under development. The assessment also shows that the current 25 kg/h operational leak rate limit is critical to detect a leakage and to ensure reactor safe shutdown to avoid potential feeder rupture. Nonetheless, inspection of DMW every two or three years, a high dose activity, has little effect in reducing the rupture probability. As documented in Attachment 3 COG report, "Natural PWSCC Crack Growth Analysis for Circumferential Crack in Darlington Feeder Dissimilar Metal Weld': AFEA (Advance Finite Element Analyses) is used to simulate "natural" crack growth. The methodology allows the progression of a planar crack subjected to typical stress corrosion cracking growth laws by calculating stress intensity factors at every nodal point along the entire crack front. The calculation is incrementally advancing the crack front in a more natural manner which overcomes overconservatism in the semi-elliptical crack fronts used in the conventional fracture mechanics. The Welding Residual Stress (WRS) profile from welding simulation/measurement results was implemented in AFEA models. The results of this initial study have demonstrated that the time intervals from the assumed initial flaw size to the incipient leakage and from the incipient leakage to 125 kg/h (operational action leak rate of 25 kg/h with safety factor of five), as well as from 125 to 1800 kg/h (immediate shutdown limit) are all sufficiently long to ensure reactor safe shutdown as per the current Darlington operation procedures. As documented in Attachment 4 COG report, "Residual Stress Measurements of Dissimilar Metal Welds in Feeder Piping by Neutron Diffraction", residual stress measurements were performed on two 3.5" NPS piping items consisting of DMW similar to Darlington feeders. The measured residual stresses using both neutron diffraction and X-Ray diffraction techniques agree reasonably well with the predicted values from finite element simulation of the dissimilar metal weld. Further to Reference 1, Attachments 1, 3 and 4 are the final reports of the three draft COG reports OPG committed to providing CNSC staff. Please note that there are slight title and reference number changes to these reports as to what was originally documented in Reference 1. Based on the improved knowledge from DMW engineering assessments made in the past two years, OPG proposes to modify the original project plan in order to effectively achieve final LBB deliverables in The current project status and justifications to change some of original activities are provided in Attachment 5. WRS is well recognized in the literature and Operating Experience as one of the main contributors of DMW stress cracking. In addition, both probabilistic and initial AFEA assessments have identified that WRS amplitude and profile are key input parameters. At present, there is no WRS data for repaired welds which are present at Darlington DMW. WRS on repaired welds could be very different from the current measurement and simulation data made on non-repaired welds. Nonetheless, with realistic WRS (Microsoft 2007) Page 2 of 4

3 Mr. P.A. Webster November 27, 2012 inputs, further AFEA studies could close the gap between deterministic and probabilistic assessments. Therefore, future DMW work will focus on weld residual stress and refined AFEA including: 1. WRS study: literature review or modeling or measurement 2. Refined AFEA: implement WRS results in AFEA OPG will provide CNSC staff with an update by November 29, 2013 of the project activities listed in Attachment 5, including completed project reports and an updated schedule. A Regulatory Management Action Request will be raised to track the project update. This submission completes Regulatory Management Action Request Should you have any further questions, please contact Mr. Garry Lam, Acting Section Manager - Major Components and Equipment Department - Feeders, at (905) ext Sincerely, ~c1~~ Brian Duncan Senior Vice President Darlington Nuclear Ontario Power Generation Inc. Attach. cc: Mr. A. Ling - CNSC (Darlington) OPG-TMP-0007-R004 (Microsoft 2007) Page 3 of4

4 Mr. P.A. Webster November Reference: 1. OPG letter, B. Duncan to P.A. Webster, "Darlington NGS 'A' Outlet Feeder Dissimilar Metal Weld Leak-Before-Break Assessments - Update 1", November 21, 2011, CD# NK38-CORR OPG-TMP-0007-R004 (Microsoft 2007) Page 4 of4

5 ATTACHMENT 5 OPG letter B. Duncan to P.A. Webster, "Darlington NGS 'A' - Outlet Feeder Dissimilar Metal Weld Leak-Before-Break Assessments - Update 2" CD# NK38-CORR Current Status of Project Activities and Justifications of Modification Prepared by: Checked by: M.Li R. Chelliah

6 Attachment 5 Status of Project Activities and Justifications for Modifications No. 1 1a Activity Residual stress measurements and modeling. AFEA analysis of PWSCC Crack Growth for Circumferential Cracks in Feeder Piping Girth Welds. Refined probabilistic LBB assessment. Characterization of tensile and fracture toughness properties. Refined deterministic LBB assessment. 5 EBSDIXRD analysis of DMW. 6 DMW component tests. Preliminary PWSCC growth 7 tests and plastic strain analysis. Status and Modification Justifications COG-JP-4401-V07 RO. Initial assessment report COG Completed. In deterministic assessment: elastic modulus (E), yield strength (S,) and tensile strength (Su) are code values. Flow strength is back calculated from the component test. Sensitivity study shows no effect on conclusions if flow strength of Alloy 600 flow strength code value is used (Table 15 in COG-JP-4401-V06 R1). In probabilistic assessment: elastic modulus (E), yield strength (Sy) and tensile strength (Su) are code values. Flow strength, and J-R curve are back calculated from the component test, Sensitivity study shows tensile properties and fracture toughness have little impact on conclusions (Table 16 in COG-JP-4401-V26 RO). Tensile & hardness test results: COG Completed. This work is aim to map the orientation of the crystallographic structure to very high resolutions. It may provide fundamental understanding of PWSCC in microscope level but it has no impact on the current enqineering assessments. The main purpose of tests is to support the net section collapse model be applicable to feeder pipes. Results from a 145' through wall hot commission and a 70' through-wall tests were all conservative and able to support the model application. Hot commission test: COG PWSCC crack growth rate for Alloy 82 and Alloy 600 became available in ASME Boiler and Pressure Vessel Section XI in Higher growth rates were also studied; results indicated no impact on the rupture frequency conclusions. Action Literature review or modeling or measurement on WRS is planned. Target Completion Date (TCD): June Further investigations are planned to implement WRS results in AFEA. TCD: October COG-JP-4401-V26 RO Update with WRS results if necessary. Completed fracture toughness test results will be documented in a COG report: TCD June COG-JP-4401-V06 R1. Preliminary EBSD results will be documented in a COG report: TCD June The 70' through wall test will be documented in a COG report: TCD June When US Dissimilar Metal Weld Pipe Fracture Program (N a 8" pipe test) results become available, OPG will review and assess. 8 9 Final PWSCC growth rate tests and post-ebsd analysis. Final engineering LBB assessments. Abbreviations: AFEA - Advanced Finite Element AnalysiS LBB - Leak-Before-Break DMW - Dissimilar Metal Weld J-R curve - Crack resistance fracture toughness curve JP-xxxx -Represents numbers assigned for projects by CANDU Owners Group EBSD - Electro Back Scatter Diffraction XRD X-ray Diffraction PWSCC - Primary Water Stress Corrosion Cracking WRS - Weld Residual Stress Same as items 5 and 7. November