外揷形 磁氣카메라를 利用한 制御棒集合體의 非破壞檢査 및 評價

Abstract

The rod cluster control assembly (RCCA) in a nuclear reactor is a cluster of many control rods, which is used to control the operation and fast emergency stop of the reactor. Each control rod has a cladding tube rod with a diameter of 9–11 mm and thickness of 0.47–0.49 mm made of stainless steel (STS304) cover a silver (Ag-80%)–indium(In-15%)–cadmium(Cd-5%) alloy radiation absorber. An end blug is welded with the cladding tube; hence, the absorber is isolated from the coolant. The RCCA is located in the reactor vessel is supported by 8 guide cards. When the RCCA moves up/down in the reactor, it contacts with the guide cards cause of the vibration due to the coolant flow in the reactor vessel and causes fretting wear in the cladding tube. In addition, when the control rod assembly moves up/down or stop in the core reactor, integranular stress corrosion cracking (IGSCC) due to neutron radiation can occur at the weld of the end plug. If the control rod is damaged and breaked down, it can fall on the core, obstruct, and damage to the stable operation of the nuclear reactor. Therefore, it’s necessary to apply nondestructive testing methods to inspect the damages in early stage. This study aims to develop an encricling magnetic field camera that can detect circumferential cracking, fretting and sliding wear in control rods for the safe operation of a nuclear reactor. Furthermore, quantitative evaluation of the damage size will be performed. The system can inspect upto 24 control rods in 17×17 type RCCA at one scan by using 24 sensor probes. Each sensor probe consists of 16 Hall sensor elements arrayed in circumferential direction to detect damages by measuring the electromagnetic field distribution around the damages. The electromagnetic field is produced by a bobbin coil that is placed outside of the sensor array. The efficient of the proposed inspection system will be verified by testing artificial damages with different shapes and sizes on real control rods of the 17×17 type RCCA. The system operates at 15 kHz of an alternating current supply of the bobbin coil. The experiments show that the system can inspect 10-mm width artificial circumference grooves with a depth upto 10% of the cladding tube thickness (0.047 mm). And, 30% depth of artificial tapered wear (0.14 mm) could be detected. For circumferential cracks, the system can detect 0.2-mm width, 40% depth (0.19 mm), and 90o angle-length in circumferential direction. An automatic algorithm was proposed to automatically identify the shape of the defect. This algorithm can automatically identify circumferential grooves, tapered wear, and circumferential cracks and their angle. The width of the damages could be quantitatively evaluated with 0.37 mm error (3.7%). The depth of the damages also evaluated with 5.8%, 5.98%, 17.96% errors for circumferential grooves, tapered wear and circumferential cracks.