Structural Integrity Analysis of Weld Overlay Repair on CRDM Upper Omega Weld

Literature Overview

This study by Lu Zhicheng and colleagues from the State Key Laboratory of Nuclear Power Safety Monitoring Technology and Shenzhen CGN Engineering Design Co., Ltd. addresses a critical nuclear safety issue: the structural integrity assessment of weld overlay repair (WOR) on the upper Ω (omega) seal weld of Control Rod Drive Mechanisms (CRDMs) in nuclear power plants. Published in Nuclear Power Engineering (Vol. 39, Issue 5, 2018, pp. 75–79), the research employs finite element analysis (FEA) to evaluate whether a WOR can restore the structural integrity of a degraded weld to meet regulatory requirements. This is a high-consequence engineering problem, as CRDMs are safety-related components in pressurized water reactors (PWRs), and the Ω weld provides the primary pressure boundary seal between the upper and lower housings.

Core Technical Methodology

The study follows a rigorous computational workflow that mirrors the regulatory framework for nuclear component repair evaluation:

1. Welding residual stress simulation: A 2D axisymmetric model was constructed with a Gaussian heat source equivalent to the WOR parameters. The ANSYS element birth-and-death technique was used to simulate the sequential weld pass deposition process.
2. Transient stress analysis: The repaired structure was subjected to all operational transients defined in the plant's safety analysis report, including normal operation, startup, shutdown, and emergency depressurization scenarios.
3. Fatigue analysis: Cycle-by-cycle fatigue assessment was performed using the transient stress results, considering the mean stress effect of welding residual stresses.
4. Fracture mechanics analysis: Stress intensity factor (SIF) calculations and crack growth predictions were conducted to evaluate the fitness-for-service of the repaired weld under postulated flaw sizes.

Results and Compliance Assessment

The study concludes that the WOR structure satisfies all relevant regulatory requirements in terms of fatigue life, stress intensity factor, and crack growth behavior. This is a significant finding, as it provides engineering justification for the in-service repair of CRDM Ω welds rather than requiring full component replacement, which would entail substantial outage time and cost.

The following table summarizes the analytical framework:

Engineering Practice Implications

For nuclear maintenance and engineering teams, this study provides a validated computational methodology for WOR integrity assessment. Several practical considerations emerge:

• Weld procedure qualification: The WOR parameters used in the FEA must correspond to a qualified welding procedure specification (WPS) that has been demonstrated on the actual component geometry. The Gaussian heat source model is an approximation, and its fidelity depends on accurate characterization of the actual weld thermal cycle.
• Post-repair inspection: The FEA assumes a specific defect population in the repaired weld. Post-WOR non-destructive examination (NDE), typically including ultrasonic testing (UT) and magnetic particle testing (MT), is essential to verify that the assumed defect state is not exceeded.
• In-service monitoring: Even after a successful integrity assessment, the repaired weld should be included in the plant's in-service inspection (ISI) program, with periodic UT surveys to detect any crack initiation or growth at the repair boundary.
• Regulatory acceptance: The study's methodology aligns with the ASME Code Case approach for repair evaluation, but specific national regulatory requirements (e.g., NRC in the US, CNSA in China) may impose additional criteria that must be addressed in the licensing basis.

Key Questions and Reflections

A critical question is whether the 2D axisymmetric model adequately captures the three-dimensional stress state at the Ω weld geometry. The Ω weld is a complex, non-axisymmetric feature with a distinctive double-curve profile, and the assumption of axisymmetry may introduce errors in the predicted residual stress distribution, particularly near the weld toe where stress concentrations are highest. Additionally, the study does not appear to address the metallurgical compatibility of the overlay material with the base metal, which is essential for ensuring long-term corrosion resistance and fatigue performance in the reactor coolant environment. The choice of overlay filler metal, its dilution with base metal, and the resulting microstructure and corrosion resistance are all factors that must be considered in a comprehensive repair qualification.

Study Insights and Implications

This study is a model example of how computational methods can be applied to support nuclear component repair decisions with defensible engineering rigor. The step-by-step approach—residual stress, transient stress, fatigue, fracture mechanics—mirrors the logical progression of a fitness-for-service assessment and provides a template that can be adapted to other repair scenarios in nuclear plants. For engineers working in nuclear maintenance, the key lesson is that WOR is not merely a welding exercise; it is a comprehensive engineering assessment that requires integration of welding metallurgy, structural mechanics, fracture mechanics, and regulatory compliance. The successful application of this methodology to CRDM Ω welds opens the door to similar assessments for other degraded welds in safety-related components, potentially extending the service life of critical plant equipment while maintaining safety margins.