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Array Eddy Current Testing of Nuclear Fuel Cladding Tubes
Release Date:
2022/09/29
Nuclear fuel elements are the core components of a reactor, operating under harsh conditions characterized by high temperature, high pressure, intense radiation, coolant erosion, and corrosive degradation. As the first safety barrier in a nuclear reactor, the cladding tube requires rigorous quality monitoring to ensure the safe operation of the reactor. Prior to fuel assembly loading, if defect parameters such as cracks, inclusions, and pitting in the cladding tube exceed established safety thresholds, the fuel rod will face a significantly elevated risk of failure. Therefore, it is essential to employ effective nondestructive testing methods to perform 100% inspection of cladding tubes. Currently, the primary NDT techniques for cladding tubes include ultrasonic testing, eddy current testing, and visual inspection.
Eddy-current testing of cladding tubes primarily encompasses mechanical rotary-scan techniques and array-based methods. Mechanical rotary scanning is predominantly employed for online inspection; however, it poses significant challenges in terms of equipment maintenance and exhibits relatively low inspection efficiency. Array eddy-current technology, on the other hand, is widely used for in-service inspection of nuclear power plant heat-transfer tubes. With the advancement of computer technology, electronic scanning technology, and signal-processing techniques, array eddy-current technology has gradually matured. This technique achieves rapid and effective inspection of materials and components by designing specialized eddy-current probe coil configurations and leveraging the powerful analytical, computational, and processing capabilities of eddy-current instruments. The sensor-array configurations are highly flexible and versatile, enabling convenient inspection of parts with complex surface geometries or large-area metallic surfaces. Moreover, the strategic arrangement of transmitting and receiving coils significantly enhances the penetration depth of eddy currents into the material.
As array-based inspection technology continues to mature, its applications are expanding across an increasingly broad range of fields. Nuclear fuel rod cladding tubes typically have diameters ranging from 6 to 16 mm and wall thicknesses of about 0.5 mm; during manufacturing, defects such as scratches and folds of varying depths and orientations may occur, placing stringent requirements on the detection tolerance for defect orientation and depth. Array eddy current inspection technology, which integrates cutting-edge sensor technology, computer technology, and digital signal processing, offers a new approach for inspecting cladding tubes.
Technical personnel at China National Nuclear Corporation Northern Nuclear Fuel Elements Co., Ltd. employed array eddy-current testing to inspect artificial defects on 316 stainless steel cladding tubes. The test results demonstrate that, under a signal-to-noise ratio of 11 dB—meeting the requirement stipulated in the standard YB/T 4083-2020 “Comprehensive Performance Test Method for Automatic Eddy-Current Flaw Detection Systems for Steel Pipes and Steel Bars” that the signal-to-noise ratio shall exceed 10 dB—the method can accurately detect internal and external surface defects of various types and sizes, thereby providing important guidance for eddy-current inspection of cladding tubes.
Principle of Array Eddy Current Testing
The two most common T–R (transmit–receive) configurations in array eddy-current testing equipment are shown in Figure 1. In the figure, LSD denotes a single-coil excitation with multiple-receiver coils; in this configuration, the receiver coils are spaced relatively far from the excitation coil, making it particularly suitable for detecting large surface defects and less susceptible to lift-off effects. SDD, on the other hand, is a dual-coil excitation–dual-coil reception configuration, which offers high sensitivity for detecting small defects. Both excitation–reception modes can simultaneously detect defects oriented in different directions, enabling comprehensive defect characterization.
The experiment employed an external through‑coil array probe (arranged in two concentric rings, with 23 probes per ring) and utilized SDD excitation–reception mode. The scanning direction of the array eddy‑current probe is shown in Figure 2.
2. Test Procedure
Artificial Defect Comparison Specimen Pipe
Artificial defect comparison test tubes are fabricated in accordance with relevant standards and approved by the competent authorities to meet the inspection requirements of the object under examination, serving as reference specimens for assessing the conformity of the object’s quality. The forms of artificial defects are not standardized; they are typically determined by the nature and morphology of defects that may arise during product manufacturing or service use.
The reference test specimens were fabricated by introducing grooves and through-holes to simulate common defects in 316 stainless steel cladding tubes (groove-type defects were used to simulate cracks, folds, and similar imperfections, while through-hole defects were used to simulate delamination and inclusions). For the artificial defects, cladding tubes with dimensions of 15.7 mm × 0.55 mm (diameter × wall thickness, the same applies hereinafter) were selected and machined to create through-holes, internal circumferential grooves, internal longitudinal grooves, external circumferential grooves, and external longitudinal grooves. The structure of the artificial-defect reference specimens is shown in Figure 3.
The excitation voltage and gain of the array eddy-current probe are adjustable; the higher the voltage and gain, the greater the induced emf in the detection coil, but also the larger the amplitude of the noise signal. With the noise signal kept within a specified level, the voltage and gain should be increased as much as possible. In this experiment, a dual-channel detection system (axial and circumferential channels) was used, with the voltage set at 6 V and the gain at 54 dB. According to eddy-current testing theory, the higher the frequency, the stronger the skin effect, resulting in higher sensitivity for detecting surface defects; however, penetration depth decreases, and sensitivity for near-surface defects also declines. After comprehensive consideration, the test frequency was selected as 350 kHz.
Test Results
The lift-off effect is a critical factor in eddy current testing, as it degrades the signal-to-noise ratio of the inspection signals for thin-walled tubes, thereby adversely affecting detection performance. Therefore, the impact of the lift-off effect on inspection results must be evaluated first.
The relationship curve between the signal-to-noise ratio of through-holes and lift-off is shown in Figure 4. As can be seen, at a inspection speed of 50 mm/s, the system can maintain a signal-to-noise ratio of 11 dB under a lift-off condition of 1 mm.
The array eddy current inspection results for the stainless steel test tubes with artificial defects are shown in Figure 5. As can be seen from Figure 5, the defect detectability and spatial distribution can be directly determined.
As shown in Figure 5, both channels can accurately detect defects such as through-holes, outer annular grooves, and outer longitudinal grooves; however, the image recognition accuracy for inner annular grooves and inner longitudinal grooves is relatively lower.
The quantitative detection results for different artificial defects are shown in Table 1. By comparing the phase and amplitude of the detection signals, the type and burial depth of the defects can be determined.
Table 1: Quantitative Detection Results for Different Artificial Defects
As shown by the quantitative results in Table 1, the signals from through-holes, outer annular grooves, and outer longitudinal grooves exhibit relatively high signal-to-noise ratios, with amplitudes all exceeding 0.3 V. In contrast, the defect signals from inner annular grooves and inner longitudinal grooves have lower amplitudes, each less than 0.2 V. For defects of the same type, the response amplitude for external defects is more than twice that of internal defects; moreover, phase analysis reveals that the phase of internal-surface defects is approximately 110°, whereas that of external-surface defects is about 70°. These findings indicate that the detection rate of defects is strongly influenced by defect depth and location. By appropriately adjusting the relevant parameters, it is possible to accurately detect internal annular grooves, internal longitudinal grooves, external annular grooves, external longitudinal grooves, and through-holes with a diameter of 0.2 mm, each having dimensions of 3 mm × 0.2 mm × 0.05 mm (length × width × depth).
3 Conclusion
The above work has conducted research on the application of array eddy current technology in the inspection of nuclear fuel cladding tubes, using a 316 stainless steel reference tube with dimensions of 15.7 mm × 0.55 mm for verification of detection sensitivity. The test results demonstrate that array eddy current technology can be effectively applied to the inspection of nuclear fuel cladding tubes. Moreover, owing to the flexibility of the probe design, this inspection technique exhibits strong adaptability and can be extended to quality control of other related nuclear fuel components.
Authors: Yu Dongbao, Tang Hui, Zhang Xiaogang, and Cheng Zhifei Affiliation: China National Nuclear Corporation Northern Nuclear Fuel Elements Co., Ltd.
Brief biography of the first author: Yu Dongbao is a senior engineer whose primary work involves the manufacturing of nuclear fuel components.
Source: Nondestructive Testing, Issue 5, 2022
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