Zhuojin Pipe Fitting Co., LtdFactors Influencing CCTV Inspection Resolution of Pressurizer Internal Overlay Weld Layer in HPR1000 Nuclear Power Plants
Literature Overview
This paper by Liu Jun, Yan Zhigang, and Shi Lei from CGN Inspection Technology Co., Ltd., published in the journal NDT (Non-Destructive Testing) in 2022, addresses a highly specialized inspection challenge in nuclear power plant maintenance. The HPR1000 (Hualong One) pressurized water reactor pressurizer is a critical pressure boundary component, and its internal surface is clad with a nickel-based overlay weld layer to provide corrosion resistance and neutron absorption capability. Over the operational life of the reactor, this overlay layer may develop defects such as micro-cracks, spalling, and erosion, which must be detected with high fidelity to ensure continued safe operation. The authors systematically analyze the factors that influence the resolution of Closed-Circuit Television (CCTV) inspection systems used to examine the internal surface of the pressurizer.
Core Technical Analysis
CCTV inspection is a visual-based non-destructive testing method that relies on camera imaging to detect surface defects. The resolution of such a system is not a single fixed value but rather a function of multiple interacting variables. The paper identifies the following key factors:
| Factor | Influence on Resolution | Recommended Practice |
|---|---|---|
| Camera resolution and magnification | Directly determines pixel density on the image | Use high-resolution cameras with appropriate magnification optics |
| Lighting type | Affects contrast and shadow formation | Employ diffuse coaxial illumination to minimize specular reflection |
| Inspection object characteristics | Surface geometry and material affect reflectivity | Account for curvature and alloy reflectance in system calibration |
| Surface condition | Fouling, oxide scale, or residual deposits degrade image quality | Clean and prepare the surface before inspection |
| Scan speed | Faster scanning reduces frame capture time per area | Use sufficiently slow scanning speed to ensure adequate frame rate |
The authors emphasize that resolution improvement requires a holistic approach. Simply increasing camera pixel count without addressing lighting and scan speed may not yield proportional gains. The concept of effective resolution—what can actually be discriminated on the displayed image—differs from theoretical optical resolution.
Engineering Practice Implications
In nuclear power plant operations, pressurizer internal inspection is typically performed during refueling outages when the vessel is accessible. The inspection must comply with regulatory requirements, and any defect found must be assessed according to established acceptance criteria. The paper proposes several practical measures to enhance detection capability:
- Upgrading both camera and monitor resolution to ensure the full resolution chain is matched, avoiding bottlenecks at either end of the imaging pipeline.
- Adopting diffuse coaxial lighting, which illuminates the target surface uniformly and returns light along the same axis as the camera lens, thereby reducing specular highlights and revealing fine surface features.
- Providing adequate overall illumination levels to ensure sufficient signal-to-noise ratio in the captured images.
- Controlling scan speed to a level that allows the camera to capture a sufficient number of frames per unit area, ensuring that transient defects are not missed.
- Improving surface condition through cleaning and preparation procedures prior to inspection.
- Applying image processing techniques such as contrast enhancement, edge detection, and noise filtering to amplify defect signatures and improve visual discrimination.
From a quality assurance perspective, the inspection system should be validated using reference standards that simulate known defect sizes and types. The minimum detectable defect size (MDDS) should be established and documented as part of the inspection procedure. This approach aligns with the PDCA cycle: plan the inspection parameters, carry out the inspection, check the results against acceptance criteria, and act on any non-conformances by adjusting parameters.
Key Questions and Reflections
A critical question arises regarding the applicability of CCTV inspection for subsurface or near-surface defects in the overlay weld layer. CCTV is inherently a surface inspection technique and cannot detect internal porosity, lack of fusion, or subsurface cracking. For these defect types, complementary methods such as ultrasonic testing (UT) or eddy current testing (ECT) should be employed. The paper implicitly acknowledges this limitation by focusing on surface defect detection resolution.
Another consideration is the geometric challenge of inspecting the curved internal surface of a pressurizer. The camera must maintain consistent standoff distance and focus across varying curvatures, which may require specialized borescope design with auto-focus capability or a mechanical scanning head that maintains constant clearance.
The use of image processing to enhance defect features raises questions about false indications. Post-processing algorithms may amplify noise patterns that resemble real defects, leading to unnecessary repair or shutdown time. A well-designed procedure should include a second-review step where an experienced inspector evaluates processed images independently.
Study Insights and Outlook
This paper provides a valuable engineering reference for practitioners involved in nuclear component inspection. The systematic identification of resolution-influencing factors and the proposed mitigation strategies offer a clear roadmap for optimizing CCTV inspection performance. The emphasis on a multi-parameter optimization approach rather than a single-variable fix reflects mature engineering thinking. Future work could explore the integration of structured light scanning or 3D imaging to provide quantitative defect sizing, moving beyond qualitative visual assessment. For nuclear power plant operators, adopting the recommendations in this paper can contribute to improved inspection reliability and, ultimately, to the safe and efficient operation of HPR1000 reactors.