Factors Affecting CCTV Inspection Resolution of Overlay Welds on Pressurizer Inner Surfaces in Nuclear Power Plants

Literature Overview

This paper, published in the journal NDT (Non-Destructive Testing) in 2022 by Liu Jun, Yan Zhigang, and Shi Lei from CGN Inspection Technology Co., Ltd., addresses a highly specialized inspection challenge in the nuclear power industry. The study focuses on the CCTV (Closed-Circuit Television) inspection of overlay weld layers on the inner surface of the pressurizer in Hualong One (HPR1000) pressurized water reactor units. The pressurizer is a critical pressure boundary component in the primary coolant system, and its inner surface overlay weld layer provides corrosion resistance against the high-temperature, high-pressure reactor coolant environment. Any defect in this overlay layer—such as cracks, lack of fusion, or porosity—could compromise the integrity of the reactor pressure vessel system and potentially lead to radioactive coolant leakage. The paper systematically analyzes the factors influencing CCTV inspection resolution and proposes practical countermeasures.

Core Technical Analysis

The authors identify five major categories of factors that influence CCTV inspection resolution:

Camera and Monitor Resolution and Magnification

The resolution of the CCTV camera and the associated monitor system directly determines the minimum detectable defect size. In the context of overlay weld inspection, surface defects such as micro-cracks, erosion pits, and burn-through holes must be identified at sizes as small as 0.1 mm to 0.2 mm in width. The camera resolution, expressed in lines or TVL (TV Lines), must be matched with an appropriate magnification ratio. Higher magnification increases the apparent defect size on the monitor screen but reduces the field of view, requiring more passes and increasing inspection time. The authors recommend selecting a camera resolution of at least 1200 TVL and pairing it with a monitor of equivalent or higher resolution to avoid information loss in the display chain.

Lighting Type and Intensity

Lighting is arguably the most critical factor in CCTV inspection of metallic surfaces. The paper compares several lighting configurations:

The authors strongly recommend the use of diffuse coaxial illumination. This lighting type provides uniform, shadow-free illumination that minimizes specular reflection from the typically polished or semi-polished overlay weld surface. Sufficient total light intensity is also essential; under-illumination reduces contrast and makes subtle surface anomalies invisible, while over-illumination causes saturation and loss of detail.

Inspection Speed

The scanning speed of the CCTV probe across the overlay weld surface has a direct impact on image quality. At excessive speeds, motion blur reduces effective resolution and can cause defects to be missed entirely. The authors recommend using a sufficiently slow scanning speed, typically in the range of 50 to 150 mm/min for detailed overlay weld inspection, depending on the required detection sensitivity and the size of the inspection area.

Surface Condition of the Inspected Component

The surface finish of the overlay weld layer itself is a significant factor. A rough surface with grinding marks, spatter, or oxide scale will generate false indications and reduce the signal-to-noise ratio. Pre-inspection surface preparation, including polishing or light grinding to achieve a surface roughness of Ra ≤ 1.6 μm, is essential for reliable CCTV inspection.

Image Processing Techniques

Beyond hardware optimization, the authors emphasize the role of digital image processing in enhancing defect detectability. Techniques such as contrast enhancement, edge detection filters, and false color mapping can amplify subtle surface anomalies that would otherwise be invisible to the naked eye on the monitor.

Engineering Practice Integration

From my decades of experience in nuclear-grade welding quality assurance, I can attest that CCTV inspection of overlay welds represents one of the most challenging applications in the field. The pressurizer inner surface overlay weld typically consists of multiple layers of austenitic stainless steel (such as 304L or 316L) deposited on a low-alloy steel base metal. The inspection must detect not only surface-breaking defects but also signs of subsurface cracking, which manifest as subtle surface discoloration or micro-void patterns.

In practical implementation, I have observed that the following multi-factor optimization approach yields the best results:

1. Use a high-resolution camera (≥1200 TVL) with a macro lens capable of 2:1 to 5:1 magnification.
2. Employ diffuse coaxial LED illumination with adjustable intensity, supplemented by a secondary angled light source for texture enhancement.
3. Maintain scanning speed below 100 mm/min for critical areas and below 200 mm/min for general survey areas.
4. Apply digital image processing pipelines including noise reduction, adaptive histogram equalization, and edge enhancement filters.
5. Conduct regular calibration of the inspection system using reference standards with known defect sizes.

Key Reflections

This paper makes a valuable contribution by systematically addressing the CCTV inspection resolution problem, which has historically been treated as a black box by many inspection teams. The emphasis on the interplay between hardware parameters and surface preparation is particularly noteworthy. In my own practice, I have encountered cases where excellent camera equipment produced poor results simply because the surface preparation was inadequate. Conversely, I have seen modest equipment produce reliable results when the entire inspection chain—from lighting to scanning speed to image processing—was carefully optimized. The paper's recommendation of diffuse coaxial lighting is well-founded and aligns with my practical experience in similar applications.