1.0 Background
The Dissimilar Metal Welds (DMWs) between carbon steel and alloy 600 flow devices are used in a number of outlet feeders in some CANDU reactors. World-wide PWR operating experience has proved welds containing alloy 600 and its weld equivalents to be potentially susceptible to primary water stress corrosion cracking (PWSCC). However, the volumetric inspection on the DMWs in accordance with the requirements of CSA N285.4 is difficult to perform due to high dose rates and access limitations.
A licensee has requested an alternative approach from the volumetric inspection of the DMWs. It has proposed leak-before-break (LBB) assessments for the DMWs instead of performing the inspections. The licensee provided a preliminary LBB assessment in 2010, and committed to further laboratory studies and more analytical assessments to improve the assessment methods and validate the assessment results. The exemption from the volumetric inspection of feeder DMWs is subject to the CNSC staff’s acceptance of the final LBB assessment of the DMWs.
The licensee is demonstrating the LBB methodology for DMWs using deterministic and probabilistic approaches which supplement and complement each other. In addition to the preliminary LBB assessment in 2010, the licensee has provided refined LBB assessments and proposed future activities to further validate the refined assessments. The licensee is planning to make a final submission on the LBB assessment by December, 2015.
This is the first time that the LBB methodology has been applied in an assessment for fitness for service for feeders. Due to the complexity of the LBB assessment, a third party’s opinion is necessary to improve the quality of the overall LBB assessment and to increase the confidence in the position that the CNSC will take on receiving the licensee’s final submission.
2.0 Objectives
To obtain an independent , third-party evaluation of the interim and final reports of LBB assessment of the DMWs in specific CANDU units. This is to improve the quality of the LBB assessment and to increase the confidence in the position that the CNSC will take on the final submission.
3.0 Scope of Work
The successful contractor will carry out a technical evaluation of the interim assessments and the final LBB assessment submitted by a licensee. These documents are listed in section 4.
The contractor will evaluate the assessment approaches used and identify any gaps to be filled by reviewing the interim documents listed in Section 4.1. The contractor will also provide a comparison between the licensee proposed approaches and LBB implementations made under other nuclear regulatory authorities.
After reviewing the final versions of the LBB assessment reports, the contractor, will provide final conclusions on whether the LBB assessment for the DMWs in the outlet feeders has been appropriately performed and identify gaps and improvements, if any.
4.0 Tasks to be Performed
4.1 Conduct a technical review of the following interim documents submitted:
i. Deterministic Leak-Before-Break Assessment of DMWs in (the) Outlet Feeders, COG-JP-4401-V06 R1, October 2012.
ii. Probabilistic Leak-Before-Break Assessment of DMWs in (the) Outlet Feeders, COG-JP-4401-V26-R0, October 2012
iii. Natural PWSCC Crack Growth Analysis for Circumferential Crack in (the) Feeder Dissimilar Metal Weld, COG-11-4056, August 2012 (interim report for Advanced Finite Element Analysis)
iv. Welding Residual Stress Modeling of Dissimilar Metal Welds in Outlet Feeders OP-12-4081, April 2013 (interim report for WRS characterization and modeling)
v. Current Status of Project Activities and tentative delivery schedules
Following a review of these documents, the contractor shall provide comments on:
- whether the LBB assessment using each of the approaches has been properly conducted
- whether the collective use of these approaches is sufficient to demonstrate LBB for the CANDU outlet feeder DMWs
- Compare the licensee proposed approaches with LBB implementations made under other nuclear regulatory authorities
- any gaps to be addressed in the final LBB assessment
This task is scheduled to start on contract award and complete within 3 months.
4.2 Conduct a technical review of the licensee’s final submissions on LBB assessment. The final package of the LBB assessment will include the final version of documentations i to iv under Section 4.1. Note, the titles and numbers of the final version documents i to iv may not be identical with those of the interim ones.
The review of this package will include an overall assessment of the application of the LBB method to the DMWs on the outlet feeders of CANDU reactors. The contractor shall provide a conclusion on whether the LBB assessment has appropriately been performed, and identify gaps and improvements if there are still any.
The final package of the LBB assessment will be submitted to the CNSC in December 2015, based on the current schedule. This task is planned to begin when the licensee’s final package of the LBB assessment is available to the contractor, and to be completed 3.5 months later.
Before finalising its report, the contractor will make a presentation to CNSC staff.
Note: Bidders should make a request to the Contracting Authority for the redacted summaries of the above-referenced reports for the purpose of bid preparation. Un-redacted reports will be provided to the winning bidder once the security requirements have been met. A non-disclosure agreement may also be required at that time.
